Electronic structure of compound I in human isoforms of cytochrome P450 from QM/MM modeling

Human cytochromes P450 play a vital role in drug metabolism. The key step in substrate oxidation involves hydrogen atom abstraction or C=C bond addition by the oxygen atom of the Compound I intermediate. The latter has three unpaired electrons, two on the Fe-O center and one shared between the porphyrin ring and the proximal cysteinyl sulfur atom. Changes in its electronic structure have been suggested to affect reactivity. The electronic and geometric structure of Compound I in three important human subfamilies of cytochrome P450 (P450, 2C, 2B, and 3A) that are major contributors to drug metabolism is characterized here using combined quantum mechanical/molecular mechanical (QM/MM) calculations at the B3LYP:CHARMM27 level. Compound I is remarkably similar in all isoforms, with the third unpaired electron located mainly on the porphyrin ring, and this prediction is not very sensitive to details of the QM/MM methodology, such as the DFT functional, the basis set, or the size of the QM region. The presence of substrate also has no effect. The main source of variability in spin density on the cysteinyl sulfur (from 26 to 50%) is the details of the system setup, such as the starting protein geometry used for QM/MM minimization. This conformational effect is larger than the differences between human isoforms, which are therefore not distinguishable on electronic grounds, so it is unlikely that observed large differences in substrate selectivity can be explained to a large extent in these terms.

When and how do diaminocarbenes dimerize?

No example of a simple uncatalyzed dimerization of a diaminocarbene has been clearly established, so it is timely to ask what factors control the thermodynamics of this reaction, and what mechanisms are responsible for the observed dimerizations? In agreement with qualitative experimental observations, the dimerizations of simple five- and six-membered-ring diaminocarbenes are calculated to be 100 kJ mol(-1) less favorable than those of acyclic counterparts. This large difference is semiquantitatively accounted for by bond and torsional angle changes around the carbene centers. Carbenes such as (Et(2)N)(2)C are kinetically stable in THF at 25 degrees C in agreement with calculated energy barriers, but they rapidly dimerize in the presence of the corresponding formamidinium ion. This proton-catalyzed process is probably the most common mechanism for dimer formation, and involves formation of C-protonated dimers, which can be observed in suitable cases. The possibility of alkali-metal-promoted dimerization is raised, and circumstantial evidence for this is presented.

On the Origin of HighESelectivity in the Wittig Reaction of Stabilized Ylides: Importance of Dipole−Dipole Interactions

The salt-free Wittig reaction of stabilized ylides Ph3PCHCO2Me 1 and (MeO)3PCHCO2Me 2 has been investigated using DFT method including solvation. This analysis shows that TS structures and E/Z selectivity in the phosphine stabilized ylide 1, which gives high E selectivity with PhCHO, are predominantly controlled by a dipole-dipole interaction between the two reactants at the TS (as well as the well-known 1,2 and 1,3 steric interactions). The surprisingly different behavior of the phosphite ylide 2, which gives only 69:31 E/Z ratio with PhCHO, is accounted for by its much smaller overall dipole. The introduction of this new parameter (dipole-dipole interactions), which has not previously been invoked in discussions of this important reaction, accommodates all the experimental observations relating to selectivity in the Wittig reaction of stabilized ylides under salt-free conditions.

Multiple high-level QM/MM reaction paths demonstrate transition-state stabilization in chorismate mutase: correlation of barrier height with transition-state stabilization

Multiple profiles for the reaction from chorismate to prephenate in the enzyme chorismate mutase calculated with hybrid density functional combined quantum mechanics/molecular mechanics methods (B3LYP/6-31G(d)-CHARMM27) agree well with experiment, and provide direct evidence of transition-state stabilization by this important enzyme, which is at the centre of current debates about the nature of enzyme catalysis.

QM/MM studies of the electronic structure of the compound I intermediate in cytochrome c peroxidase and ascorbate peroxidase

Cytochrome c peroxidase (CcP) and ascorbate peroxidase (APX) both involve reactive haem oxoferryl intermediates known as 'compound I' species. These two enzymes also have a very similar structure, especially in the vicinity of the haem group. Despite this similarity, the electronic structure of compound I in the two enzymes is known to be very different. Compound I intermediates have three unpaired electrons, two of which are always situated on the Fe-O core, whilst the third is located in a porphyrin orbital in APX and many other compound I species. In CcP, however, this third unpaired electron is positioned on a tryptophan residue lying close to the haem ring. The same residue is present in the same position in APX, yet it is not oxidized in that case. We report QM/MM calculations, using accurate B3LYP density functional theory for the QM region, on the active intermediate for both enzymes. We reproduce the observed difference in electronic structure, and show that it arises as a result of subtle electrostatic effects which affect the ionization potential of both the tryptophan and porphyrin groups. The computed structures of both enzymes do not involve deprotonation of the tryptophan group, or protonation of the oxoferryl oxygen.

Substituent effects and the mechanism of alkene metathesis catalyzed by ruthenium dichloride catalysts

Density functional theory calculations are reported concerning the dissociative mechanism for alkene metathesis by ruthenium dichloride catalysts, including both bisphosphine and diaminocarbene/phosphine complexes. The calculations use a hierarchy of models, ranging from [(L)(PH(3))Ru(Cl)(2)(CH(2))](L=PH(3) or diaminocarbene) through the larger [(L)(PMe(3))Ru(Cl)(2)(CHPh)] to the "real"[(L)(PCy(3))Ru(Cl)(2)(CHPh)]. Calculations show that the rate-limiting step for metathesis is either ring closing from an alkene complex to form a ruthena-cyclobutane, or ring-opening of the latter intermediate to form an isomeric alkene complex. The higher efficiency of the diaminocarbene based catalysts is due to the stabilization of the formal +iv oxidation state of the ruthenium centre in the metallacycle. This effect is partly masked in the smaller model systems due to a previously unnoticed stereoelectronic effect. The calculations do not reproduce the experimental observation whereby the initiation step, phosphine dissociation, is more energetically demanding and hence slower for the diaminocarbene-containing catalyst system than for the bisphosphine. Further calculations on the corresponding bond energies using a variety of DFT and hybrid DFT/molecular mechanics methods all find instead a larger phosphine dissociation energy for the bisphosphine catalyst. This reversed order of binding energies would in fact be the one expected based on the stronger trans influence of the diaminocarbene ligand. The discrepancy with experiment is small and could have a number of causes which are discussed here.

Do Spin State Changes Matter in Organometallic Chemistry? A Computational Study

Spin changes occur often in organometallic chemistry, and their effect on kinetics is not well understood. We report computations on the singlet and triplet potential energy surfaces of several processes of this type and show that the topology of the individual surfaces, as well as of the crossing regions between them, can be used to rationalize the observed reactivity in all cases. In particular, the slow addition of dihydrogen to W[N(CH(2)CH(2)NSiMe(3))(3)]H (Schrock, R. R.; Shih, K. Y.; Dobbs, D. A.; Davis, W. M. J. Am. Chem. Soc. 1995, 117, 6609) is shown to be a "spin-blocked" reaction with a high barrier due to the crossing between reactant triplet and product singlet surfaces. In contrast, addition of CO to TpCo(CO) (Detrich, J. L.; Reinaud, O. M.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 1995, 117, 11745) is fast because the triplet and singlet surfaces cross at low energy. Particular care is taken to use DFT methods which are in adequate agreement with experimental and high-level computational energetics for these systems.

The effect of oral folic acid on glutathione, glycaemia and lipids in Type 2 diabetes

Plasma homocysteine is an established risk factor for vascular disease and precursor of the anti-oxidant glutathione. This study was designed to investigate the relationship of changes in homocysteine (Hcy) induced by oral folate to glutathione and measures of glycaemia and lipid metabolism in Type 2 diabetes (T2DM). Twenty-seven patients (26 male, 1 female, aged 48-68 years) with T2DM and microalbuminuria were treated with folic acid 10 mg daily for 3 months. During the study, diastolic blood pressure (p=0.04), HbA1c (p=0.04), serum triglycerides (p=0.04) and serum total/HDL-cholesterol ratio (p=0.004) all increased and serum HDL-cholesterol fell (p=0.006). The increased red cell folate correlated with a reduction in microalbuminuria (p=0.001). Overall, plasma glutathione increased (p=0.016) despite reduction in its precursor Hcy (p<0.001). Change in glutathione correlated inversely with change in HbA1c (p<0.02), total cholesterol (p=0.003) and triglycerides (p<0.02) and positively with HDL-cholesterol (p=0.033). Increase in glutathione correlated with levels of vitamin B6 (p<0.05). Metformin treatment protected against the rise in blood pressure (BP) (p=0.02), independently of changes in plasma glutathione. In summary, oral folic acid supplementation in T2DM reduced plasma Hcy and increased glutathione levels. HbA1c, triglycerides and HDL-cholesterol deteriorated during the trial: their levels correlated inversely with changes in glutathione. The increase in glutathione may depend on an adequate supply of B6, as changes in glutathione correlated with vitamin B6 levels. Reduced Hcy and increased glutathione may both mediate improvement in vascular function and outcome. Some aspects of the response to folate may be different in patients on metformin.

Radical-Promoted Stone−Wales Rearrangements

The mechanism of the known Stone-Wales rearrangement of bifluorenylidene to dibenzo[g,p]chrysene is assessed with the aid of B3LYP/6-31G(d) density functional calculations, and it is shown that a radical-promoted mechanism involving a sequence of homoallyl-cyclopropylcarbinyl rearrangement steps gives a realistic activation energy and can explain experimental observations, whereas a unimolecular mechanism has an improbably high activation energy. Radical-promoted mechanisms are then applied to the hypothetical Stone-Wales rearrangements of diindeno[1,2,3,4-defg;1',2',3',4'-mnop]chrysene and C(60) itself. Severe steric constraints in these cases raise the activation energy for the radical-promoted pathways substantially, but they are still strongly preferred to uncatalyzed, unimolecular pathways

The long-term renal and retinal outcome of childhood-onset Type 1 diabetes

AIMS

To quantify the influence of childhood onset on long-term renal and retinal outcome in Type 1 diabetes.

METHODS

We used a population-based diabetes register to identify all Type 1 patients diagnosed before age 15 from 1960 to 1982 and resident in a defined catchment area in 1999. Those diagnosed before age 5, aged 5-9 and 10-14 years were compared with a reference group diagnosed at age 21-25 years over the same period.

RESULTS

Compared with adult-onset controls, proteinuria occurred earlier (P = 0.02) and nephropathy outcome was worse (P = 0.008) in childhood-onset diabetes. The risk of developing microalbuminuria was greater in childhood-onset diabetes: odds ratio 2.6 (95% confidence interval 1.4-4.9, P = 0.003). The relative risk of established nephropathy was 3.8 (1.5-9.4, P = 0.005) with childhood onset. The number developing background retinopathy did not differ with age at onset but younger onset patients were more likely to need laser treatment: relative risk 2.1 (1.1-3.8, P = 0.02). This maintained visual outcome which was not significantly different between the various age at onset groups.

CONCLUSIONS

Patients with onset of Type 1 diabetes before age 15 have substantially worse renal outcome and require more laser treatment than adult-onset patients. Differences between those with onset before age 5, onset at 5-9 and 10-14 years are small compared with the difference between childhood onset and adult onset. Events in the teenage years therefore appear to have a major adverse effect on the risk of developing long-term microvascular complications.

On-the-flyab initiotrajectory calculations of the dynamics of Cl atom reactions with methane, ethane and methanol

The dynamics of Cl atom reactions with methane, ethane, and methanol have been studied by calculation of quasi-classical trajectories, with computation of potential energies and gradients only at the geometries through which the trajectories pass. Trajectories were started from the transition state, with 2 kcal mol(-1) of energy given to the mode with an imaginary frequency (representing the reaction coordinate at the transition state) and inclusion of zero-point energy in some or all of the remaining vibrational modes. The trajectories were propagated as far as separated products, with the majority of potential energy calculations performed at the HF/6-31G level of theory. The rotational quantum state population distributions of the HCl products from the reactions of Cl atoms with methane, ethane and methanol peaked at J'=1, 2, and 6, respectively. The calculations thereby exhibit somewhat greater rotational excitation than is found experimentally, but correctly describe the trend of increasing HCl product rotation for the three respective reactions. In agreement with previous observations, only 4% of the energy available to the products of the reaction of Cl atoms with methane was channeled into CH3 radical internal energy, and 1% into HCl rotation, with 92% ending up as translational energy. For the reaction of Cl atoms with ethane and with methanol, the corresponding values for radical internal energy, HCl rotation and product translation are 21, 3, and 78%, and 46, 13, and 42%, respectively. For the latter two reactions, the radical internal energy is mostly accounted for by rotational motion. The clear increase in rotational excitation of the HCl products from the Cl atom reaction with methanol is explained in terms of a dipole-dipole interaction between the departing polar fragments. A smaller set of more computationally expensive trajectory calculations using potentials and gradients from the MP2/6-311G(d,p) level of theory were performed for reactions of Cl atoms with methanol, and give results in better agreement with experimentally measured HCl rotational excitation, consistent with the model of dipole-induced product rotation because the MP2/6-311G(d,p) calculations give smaller dipole moments for both products than the HF/6-31G calculations. The calculated angles between the rotational angular momentum vectors and recoil velocities of the radical peak sharply at 90 degrees for the reactions of Cl atoms with ethane and methanol, but exhibit a much broader distribution for reaction with methane.

The reaction of M(CO)3(Ph2PCH2CH2PPh2) (M = Fe, Ru) with parahydrogen: probing the electronic structure of reaction intermediates and the internal rearrangement mechanism for the dihydride products

The photochemical reaction of Ru(CO)(3)(dppe) and Fe(CO)(3)(dppe)(dppe = Ph(2)PCH(2)CH(2)PPh(2)) with parahydrogen has been studied by in situ-photochemistry resulting in NMR spectra of Ru(CO)(2)(dppe)(H)(2) that show significant enhancement of the hydride resonances while normal signals are seen in Fe(CO)(2)(dppe)(H)(2). This effect is associated with a singlet electronic state for the key intermediate Ru(CO)(2)(dppe) while Fe(CO)(2)(dppe) is a triplet. DFT calculations reveal electronic ground states consistent with this picture. The fluxionality of Ru(CO)(2)(dppe)(H)(2) and Fe(CO)(2)(dppe)(H)(2) has been examined by NMR spectroscopy and rationalised by theoretical methods which show that two pathways for ligand exchange exist. In the first, the phosphorus and carbonyl centres interchange positions while the two hydride ligands are unaffected. A second pathway involving interchange of all three ligand sets was found at slightly higher energy. The H-H distances in the transition states are consistent with metal-bonded dihydrogen ligands. However, no local minimum (intermediate) was found along the rearrangement pathways.

Spin-forbidden CO ligand recombination in myoglobin

The reaction of small ligands within the distal pocket of haem proteins such as myoglobin, to form ligated, low-spin iron complexes is an archetypal spin-forbidden process in bioinorganic chemistry, because the initial, "deoxy" iron complex has a high-spin ground state. Density functional theory (DFT), transition-state theory (TST), and hybrid DFT/molecular mechanics (QM/MM) calculations are reported on the carbon monoxide reaction. Using DFT data for a model compound, TST rate calculations at room temperature are carried out which give fair agreement with experiment, and suggest a highly non-adiabatic nature to the reaction. QM/MM calculations on the whole protein are reported, which are in qualitative agreement with the gas-phase model results, but suggest that protein matrix effects on the reaction rate may be important.

Mechanism and structure–reactivity relationships for aromatic hydroxylation by cytochrome P450

Cytochrome P450 enzymes play a central role in drug metabolism, and models of their mechanism could contribute significantly to pharmaceutical research and development of new drugs. The mechanism of cytochrome P450 mediated hydroxylation of aromatics and the effects of substituents on reactivity have been investigated using B3LYP density functional theory computations in a realistic porphyrin model system. Two different orientations of substrate approach for addition of Compound I to benzene, and also possible subsequent rearrangement pathways have been explored. The rate-limiting Compound I addition to an aromatic carbon atom proceeds on the doublet potential energy surface via a transition state with mixed radical and cationic character. Subsequent formation of epoxide, ketone and phenol products is shown to occur with low barriers, especially starting from a cation-like rather than a radical-like tetrahedral adduct of Compound I with benzene. Effects of ring substituents were explored by calculating the activation barriers for Compound I addition in the meta and para-position for a range of monosubstituted benzenes and for more complex polysubstituted benzenes. Two structure-reactivity relationships including 8 and 10 different substituted benzenes have been determined using (i) experimentally derived Hammett sigma-constants and (ii) a theoretical scale based on bond dissociation energies of hydroxyl adducts of the substrates, respectively. In both cases a dual-parameter approach that employs a combination of radical and cationic electronic descriptors gave good relationships with correlation coefficients R2 of 0.96 and 0.82, respectively. These relationships can be extended to predict the reactivity of other substituted aromatics, and thus can potentially be used in predictive drug metabolism models.

Aromatic Hydroxylation by Cytochrome P450: Model Calculations of Mechanism and Substituent Effects

The mechanism and selectivity of aromatic hydroxylation by cytochrome P450 enzymes is explored using new B3LYP density functional theory computations. The calculations, using a realistic porphyrin model system, show that rate-determining addition of compound I to an aromatic carbon atom proceeds via a transition state with partial radical and cationic character. Reactivity is shown to depend strongly on ring substituents, with both electron-withdrawing and -donating groups strongly decreasing the addition barrier in the para position, and it is shown that the calculated barrier heights can be reproduced by a new dual-parameter equation based on radical and cationic Hammett sigma parameters.

Scalar Coupling between the15N Centres in Methylated 1,8-Diaminonaphthalenes and 1,6-Diazacyclodecane: To What Extent is2HJNN a Reliable Indicator of NN Distance?

The scalar couplings between hydrogen bonded nitrogen centres ((2H)J(NN)) in the free-base and protonated forms of the complete series of [(15)N(2)]-N-methylated 1,8-diamino naphthalenes in [D(7)]DMF solution have been determined, either directly (15N[1H] NMR), or, indirectly (13C[1H] NMR and simulation of the X part of the ABX spectrum (X=13C, A,B=15N)). Additionally, the (2H)J(NN) value in the HBF(4) salt of [(15)N(2)]-1,6-dimethyl-1,6-diazacyclodecane was determined, indirectly by 13C[(1H] NMR spectroscopy. As confirmed by DFT calculations and by reference to CSD, the rigid nature of the naphthalene scaffold results in rather low deviations in N,N distance or H-N,N angle within each series, apart from the free base of the permethylated compound (proton sponge) where the naphthalene ring is severely distorted to relieve strain. Despite such restrictions, the (2H)J(NN) values increase smoothly from 1.5 to 8.5 Hz in the protonated series as the degree of methylation increases. The effect in the free-base forms is much less pronounced (2.9 to 3.7 Hz) with no scalar N,N coupling detected in the permethylated compound (proton sponge) due to the lack of hydrogen bond between the N,N centres. Neither the pK(a) nor the N-N distance in the protonated forms correlates with (2H)J(NN). However, the sum of the (13)C NMR shifts of the naphthalene ring C(1,8) carbons which are attached directly to the nitrogen centres correlates linearly with (2H)J(NN) and with the degree of methylation. The gas-phase computed (2H)J(NN) is almost constant throughout the homologous series, and close to the experimental value for the tetramethylated ion. However, the computed coupling constant is attenuated in structures involving microsolvation of each N-H unit, and the trend then matches experiment. These experimental and computational observations suggest that Fermi contact between the two N centres is decreased upon formation of strong charge-dispersing intermolecular hydrogen bonds of the free N-H groups with the solvent.

The seventeen- and eighteen-electron metallacarbaboranes [1,1,1-(CO)3-2-Ph-closo-1,2-MnCB9H9]n- (n = 1, 2): a structurally characterized, redox-related pair

An eleven-vertex manganese-monocarbaborane dianion, upon one-electron oxidation, gives a stable radical monoanion in which the unpaired electron is delocalized over the cluster.

The azulene-to-naphthalene rearrangement revisited: a DFT study of intramolecular and radical-promoted mechanisms

Intramolecular and radical-promoted mechanisms for the rearrangement of azulene to naphthalene are assessed with the aid of density functional calculations. All intramolecular mechanisms have very high activation energies (>/=350 kJ mol(-1) from azulene) and so can only be competitive at temperatures above 1000 degrees C. Two radical-promoted mechanisms, the methylene walk and spiran pathways, dominate the reaction below this temperature. The activation energy for an orbital symmetry-allowed mechanism via a bicyclobutane intermediate is 382 kJ mol(-1). The norcaradiene-vinylidene mechanism that has been proposed in order to explain the formation of small amounts of 1-phenyl-1-buten-3-ynes from flash thermolysis of azulene has an activation energy of 360 kJ mol(-1); subtle features of the B3LYP/6-31G(d) energy surface for this mechanism are discussed. All intermediates and transition states on the spiran and methylene walk radical-promoted pathways have been located at the B3LYP/6-31G(d) level. Interconversion of all n-H-azulyl radicals via hydrogen shifts was also examined, and hydrogen shifts around the five-membered ring are competitive with the mechanisms leading to rearrangement to naphthalene, but those around the seven-membered ring are not. Conversion of a tricyclic radical to the 9-H-naphthyl radical is the rate-limiting transition state on the spiran pathway, and lies 164.0 kJ mol(-1) above that of the 1-H-azulyl radical. The transition state for the degenerate hydrogen shift between the 9-H-azulyl and 10-H-azulyl radicals is 7.4 kJ mol(-1) lower. Partial equilibration of the intermediates in the spiran pathway via this shift may therefore occur, and this can account for the surprising formation of 1-methylnaphthalene from 2-methylazulene. The rate-limiting transition state for the methylene walk pathway involves the concerted transfer of a methylene group from one ring to the other and lies 182.3 kJ mol(-1) above that of the 1-H-azulyl radical. It is shown that rearrangement via a combination of 31% methylene walk and 69% spiran pathways can account semiquantitatively for all the products from 1-(13)C-azulene, 9-(13)C-azulene, and 4,7-(13)C(2)-azulene, in addition to accounting for the products from methylazulenes, and the formation of naphthalene-d(0) and -d(2) from azulene-4-d. It is also pointed out that a small extension to the spiran pathway could provide an alternative explanation for the formation of 1-phenyl-1-buten-3-ynes.

Factors controlling the relative stabilities of cis- and trans-[PtX2L2] isomers: Chatt and Wilkins--50 years on

DFT computations reveal that cis-trans preferences in [PtX2(PR3)2] species are due to a combination of electrostatics, pi-backbonding, antisymbiosis, and solvation effects.

Spin forbidden chemical reactions of transition metal compounds. New ideas and new computational challenges

Many reactions of transition metal compounds involve a change in spin. These reactions may proceed faster, slower--or at the same rate as--otherwise equivalent processes in which spin is conserved. For example, ligand substitution in [CpMo(Cl)2(PR3)2] is faster than expected, whereas addition of dinitrogen to [Cp*Mo(Cl)(PMe3)2] is slow. Spin-forbidden oxidative addition of ethylene to [Cp*Ir(PMe3)] occurs competitively with ligand association. To explain these observations, we discuss the shape of the different potential energy surfaces (PESs) involved, and the energy of the minimum energy crossing points (MECPs) between them. This computational approach is of great help in understanding the mechanisms of spin-forbidden reactions, provided that accurate calculations can be used to predict the relevant PESs. Density functional theory, especially using gradient-corrected and hybrid functionals, performs reasonably well for the difficult problem of predicting the energy splitting between different spin states of transition metal complexes, although careful calibration is needed.

Modelling spin-forbidden reactions: recombination of carbon monoxide with iron tetracarbonyl

New density functional theory and ab initio computations on the [Fe(CO)5] system are reported. Careful exploration of basis set and correlation effects leads to "best" values for the difference in energy deltaE(1,3) between ground state 3[Fe(CO)4] and the singlet excited state of ca. 8 kcal mol(-1), and for the bond dissociation energy BDE(3) of [Fe(CO)5] with respect to ground state fragments 3[Fe(CO)4] + CO of ca. 40 kcal mol(-1). A modified form of the B3PW91 functional is used to explore the potential energy surface for the spin-forbidden recombination reaction of CO with 3[Fe(CO)4]. A Cs-symmetric minimum energy crossing point (MECP) between the reactant (triplet) and product (singlet) potential energy surfaces is found, lying 0.43 kcal mol(-1) above the reactants. The rate coefficient for recombination is computed using a non-adiabatic form of transition state theory, in which the MECP is treated as the critical point in the reaction. Semi-quantitative agreement with experiment is obtained: the predicted rate coefficient, 8.8 x 10(-15) cm3 molecule(-1) s(-1), is only six times smaller than the experimental rate. This is the first computation from first principles of a rate coefficient for a spin-forbidden reaction of a transition metal compound.

Mechanism of the hydrogenation of ketones catalyzed by trans-dihydrido(diamine)ruthenium II complexes

The complexes trans-RuH(Cl)(tmen)(R-binap) (1) and (OC-6-43)-RuH(Cl)(tmen)(PPh(3))(2) (2) are prepared by the reaction of the diamine NH(2)CMe(2)CMe(2)NH(2) (tmen) with RuH(Cl)(PPh(3))(R-binap) and RuH(Cl)(PPh(3))(3), respectively. Reaction of KHB(sec)Bu(3) with 1 yields trans-Ru(H)(2)(R-binap)(tmen) (5) while reaction of KHB(sec)Bu(3) or KO(t)Bu with 2 under Ar yields the new hydridoamido complex RuH(PPh(3))(2)(NH(2)CMe(2)CMe(2)NH) (4). Complex 4 has a distorted trigonal bipyramidal geometry with the amido nitrogen in the equatorial plane. Loss of H(2) from 5 results in the related complex RuH(R-binap)(NH(2)CMe(2)CMe(2)NH) (3). Reaction of H(2) with 4 yields the trans-dihydride (OC-6-22)-Ru(H)(2)(PPh(3))(2)(tmen)(6). Calculations support the assignment of the structures. The hydrogenation of acetophenone is catalyzed by 5 or 4 in benzene or 2-propanol without the need for added base. For 5 in benzene at 293 K over the ranges of concentrations [5] = 10(-)(4) to 10(-)(3) M, [ketone] = 0.1 to 0.5 M, and of pressures of H(2) = 8 to 23 atm, the rate law is rate = k[5][H(2)] with k = 3.3 M(-1) s(1), DeltaH++ = 8.5 +/- 0.5 kcal mol(-1), DeltaS++ = -28 +/- 2 cal mol(-1) K(-1). For 4 in benzene at 293 K over the ranges of concentrations [4] = 10(-4) to 10(-3) M, [ketone] 0.1 to 0.7 M, and of pressures of H(2) = 1 to 6 atm, the preliminary rate law is rate = k[4][H(2)] with k = 1.1 x 10(2) M(-1) s(-1), DeltaH++ = 7.6 +/- 0.3 kcal mol(-1), DeltaS++ = -23 +/- 1 cal mol(-1) K(-1). Both theory and experiment suggest that the intramolecular heterolytic splitting of dihydrogen across the polar Ru=N bond of the amido complexes 3 and 4 is the turn-over limiting step. A transition state structure and reaction energy profile is calculated. The transfer of H(delta+)/H(delta-) to the ketone from the RuH and NH groups of 5 in a Noyori metal-ligand bifunctional mechanism is a fast process and it sets the chirality as (R)-1-phenylethanol (62-68% ee) in the hydrogenation of acetophenone. The rate of hydrogenation of acetophenone catalyzed by 5 is slower and the ee of the product is low (14% S) when 2-propanol is used as the solvent, but both the rate and ee (up to 55% R) increase when excess KO(t)Bu is added. The formation of ruthenium alkoxide complexes in 2-propanol might explain these observations. Alkoxide complexes [RuP(2)]H(OR)(tmen), [RuP(2)] = Ru(R-binap) or Ru(PPh(3))(2), R= (i) Pr, CHPhMe, (t)Bu, are observed by reacting the alcohols (i)PrOH, phenylethanol, and (t)BuOH with the dihydrides 5 and 6, respectively, under Ar. In the absence of H(2), the amido complexes 3 and 4 react with acetophenone to give the ketone adducts [RuP(2)]H(O=CPhMe)(NH(2)CMe(2)CMe(2)NH) in equilibrium with the enolate complexes trans- [RuP(2)](H)(OCPh=CH(2))(tmen) and eventually the decomposition products [RuP(2)]H(eta(5)-CH(2)CPhCHCPhO), with the binap complex characterized crystallographically. In general, proton transfer from the weakly acidic molecules dihydrogen, alcohol, or acetophenone to the amido nitrogen of complexes 3 and 4 is favored in two ways when the molecule coordinates to ruthenium: (1) an increase in acidity of the molecule by the Lewis acidic metal and (2) an increase in the basicity of the amido nitrogen caused by its pyramidalization. The formato complexes trans-[RuP(2)]H(OCHO)(tmen) were prepared by reacting the respective complex 4 or 5 with formic acid. The crystal structure of RuH(OCHO)(PPh(3))(2)(tmen) displays similar features to the calculated transition state for H(delta+)/H(delta-) transfer to the ketone in the catalytic cycle.

Energetics of the ligated vanadium dications VO2+, VOH2+, and [V,O,H2]2+

Charge-stripping mass spectrometry and electronic structure calculations are combined to examine the energetics of the vanadium-containing dications VO2+, VOH2+, [HVOH]2+, and [V(OH2)]2+. Energy-resolved measurements, in conjunction with quantum-mechanical calculations, led to ionization energies (IE) of (VO+) = 16.6 +/- 0.3 eV and IE(VOH+) = 16.0 +/- 0.4 eV. Born-Haber cycles reveal that VO2+ and VOH2+ are both thermochemically stable dications with respect to their charge-separation asymptotes. Depending on the mode of ion generation, the [V,O,H2]+ monocations are formed as variable mixtures of the structural isomers [HVOH]+ and [V(OH2)]+. The measured IE([V,O,H2]+) = 13.7 +/- 0.3 eV is between the theoretical predictions for these isomers, indicating that mixtures of both isomers are sampled in the experiments. According to the theoretical results, [V(OH2)]2+ is also a thermochemically stable dication, whereas [HVOH]2+ is suggested to be metastable.

Unraveling the mechanism of epoxide formation from sulfur ylides and aldehydes

Sulfur ylides R(2)S(+)-C(-)HR' react with aldehydes R' '-CHO to form epoxides, predominantly as the trans isomers, in a synthetically useful reaction which is increasingly used in its asymmetric variant with chiral sulfides. The mechanisms of the "model" reaction (R = Me, R' = R' ' = H) and the reaction forming stilbene oxide (R = Me, R' = R' ' = Ph) have been studied in detail using density functional theory, the B3LYP density functional, and flexible basis sets. It has been shown that for this reaction involving highly polar intermediates, continuum solvation models need to be used throughout to obtain reasonable results. For the reaction of benzaldehyde with dimethylsulfonium benzylide, the key steps are shown to be quasi [2 + 2] addition of the ylide to the aldehyde to form a betaine R'-CH(S(+)Me(2))-CH(O(-))-R' ' in which the charged groups are gauche to one another, and torsional rotation around the C-C single bond of the betaine to form its rotamer with the two charged groups anti. The final step, elimination of sulfide from this second rotamer of the betaine, is found to be facile. In the case of the anti pathway, leading to trans-stilbene epoxide, the initial addition is found to be rate-determining, whereas for the diastereomeric syn pathway, leading to the cis-epoxide, it is instead the torsional rotation which is slowest. These results are in excellent agreement with experiment, unlike previous computational work. The unexpected and apparently unprecedented (for C-C bond-forming reactions) importance of the torsional rotation step, especially in the syn case, is due to the fact that all the barriers involved are low-lying. This novel picture of the mechanism provides a sound basis for the future development of chiral sulfides for enantioselective epoxide synthesis.

Aromatic 4-tetrahydropyranyl and 4-quinuclidinyl cations. Linking Prins with Cope and Grob

The chair 4-tetrahydropyranyl cation and the 4-quinuclidinyl cation are shown to be energy minima and to be delocalized, with exceptionally long CH2CH2 bonds, according to B3LYP/6-31G* calculations; the implications for Prins cyclizations, Cope rearrangements, and Grob fragmentations are discussed.

Aromatic 4-tetrahydropyranyl and 4-quinuclidinyl cations. Linking Prins with Cope and Grob

The chair 4-tetrahydropyranyl cation and the 4-quinuclidinyl cation are shown to be energy minima and to be delocalized, with exceptionally long CH2CH2 bonds, according to B3LYP/6-31G* calculations; the implications for Prins cyclizations, Cope rearrangements, and Grob fragmentations are discussed.

Estimation of the prevalence of diagnosed diabetes from primary care and secondary care source data: comparison of record linkage with capture-recapture analysis

STUDY OBJECTIVE

To compare multiple source linkage and capture-recapture analysis in determining the current age and gender specific prevalence of type 1 and type 2 diabetes in a UK white population. To assess whole population trends in diabetes prevalence and treatment by comparison with previous studies.

DESIGN

Data were obtained from hospital sources and all 74 general practices in the study population. Analyses were carried out both by record linkage and by use of a two source capture-recapture model to correct for incomplete ascertainment.

SETTING

County of Clwyd, North Wales: total population 418,200.

MAIN RESULTS

By record linkage the age adjusted prevalence of all diabetes was 2.04 (95% confidence intervals 2.00 to 2.09)%. Using the capture-recapture method it was 2.29 (2.24 to 2.33)%. From capture-recapture data the age adjusted prevalence of type 1 diabetes was 0.40 (0.37 to 0.43)% in men and 0.28 (0.25 to 0.30)% in women; the prevalence of type 2 was 2.03 (1.97 to 2.09)% in men and 1.67 (1.62 to 1.72)% in women. These figures represent an increase compared with previous surveys. The age specific prevalence of type 2 diabetes was greater in men in a ratio of approximately 1.5:1 and there were more patients treated by diet alone.

CONCLUSIONS

Record linkage using multiple sources underestimates the prevalence of diabetes compared with capture-recapture estimates. The results suggest the prevalence of known diabetes in the UK has approximately doubled in less than 20 years. There is an increasing preponderance of male patients and of patients treated currently with diet alone.

Population-based survey and analysis of trends in the prevalence of diabetic nephropathy in Type 1 diabetes

AIMS

To determine the prevalence of the various stages of diabetic nephropathy in Type 1 diabetes in a population-based survey. To make direct comparison with results from previous published studies to assess current trends.

METHODS

Identification of all Type 1 patients using a population-based diabetes register. Urine samples for albumin assay were obtained at clinic visit and by postal request. Prevalence rates were calculated specifically for direct comparisons with previously published surveys.

RESULTS

This study and European data from the 1990s show a clear reduction in the cumulative prevalence of microalbuminuria and established nephropathy compared with surveys from Copenhagen (1985), Pittsburgh (1986-8) and Boston (1992). In North Wales in 1999 the overall cumulative prevalence of microalbuminuria was 27.2% (95% confidence interval (CI) 24.3-30.1%) and established nephropathy was 9.6% (7.7-11.5%). Comparisons with data from EURODIAB and from Spain indicated similar results, although the prevalence of microalbuminuria was lower in North Wales than in the EURODIAB study. Significantly lower rates of nephropathy were seen in more recent Swedish cohorts. Diabetic nephropathy remains more common in males. Microalbuminuria before 10 years duration of diabetes was seen at all post-pubertal ages.

CONCLUSIONS

Rates of nephropathy in Europe in the 1990s are lower than a decade ago. Modern methods of management are therefore associated with demonstrable benefit at the population level. The lowest rates of nephropathy are associated with optimum glycaemic control in Swedish data, indicating the importance of metabolic in addition to haemodynamic factors.

T(h)-symmetrical N8(C=C)6 and P8(C=C)6; an extraordinary contrast in heterofullerene stability

[structure: see text]. T(h)-symmetrical P8(C=C)6, 1, is predicted to be a remarkably stable small heterofullerene with carbon atoms less pyramidal than in C60. T(h)-N8(C=C)6, 2, in sharp contrast, is strongly destabilized relative to T(h)-(HC)8(C=C)6. The causes of this extraordinarily large difference (nearly 1000 kJ x mol(-1) between 1 and 2 are explained.

A Computational Study of Ethylene C−H Bond Activation by [Cp*Ir(PR3)]

It has previously been demonstrated that both [(C5Me5)Ir(PMe3)(CH=CH2)H] and [(C5Me5)Ir(PMe3)(H2C=CH2)] are formed when [(C5Me5)Ir(PMe3)] is thermolytically generated in the presence of ethylene. At higher temperatures, the vinyl hydride is converted to the eta2-ethylene adduct. Density functional theory has now been used to investigate this reaction, using the B3LYP functional, two types of basis sets (LanL2DZ and TZV*), and two models of the [(C5R5)Ir(PR3)] species (R=H and CH3). The study consists of full optimizations of local minima, first-order saddle points, and minimum energy crossing points (MECP). The experimental results are best accounted for by considering both singlet and triplet spin surfaces. The relative energies of singlet [(C5R5)Ir(PR3)(CH3)H], [(C5R5)Ir(PR3)(CH=CH2)H], and [(C5R5)Ir(PR3)(H2C=CH2)] are in good agreement with experiment, as is the calculated barrier for the conversion from the vinyl hydride to the eta2-alkene complex. However, the singlet surface alone fails to explain the experimentally observed product ratio, or the intermediate inferred from experimental isotope effect studies. Locating the MECP between singlet and triplet surfaces indicates that the thermolysis of the singlet alkyl hydride precursor directly forms triplet [(C5R5)Ir(PR3)]. The weak vanderWaals adduct of triplet [(C5R5)Ir(PR3)] and ethylene is proposed to be the key intermediate in the overall reaction. The interchanging of the available ethylene C-H bonds in this triplet sigma complex accounts for the observed kinetic isotope effects, and partitioning between alkene pi-complexation and C-H bond activation may also occur from this common intermediate. The possible role of steric factors and molecular dynamics are also discussed.

Platinum dioxide cation: easy to generate experimentally but difficult to describe theoretically

A formal platinum(V) dioxide cation [Pt,O2](+) can be generated in the gas phase by successive oxidation of Pt(+) with N2O. The ion's reactivity is in keeping with the dioxide structure OPtO(+), rather than with [Pt,O2](+) isomers having intact O-O bonds, e.g., the dioxygen complex Pt(O2)(+) and peroxo species PtOO(+). Inter alia due to the high ionization energy of the neutral counterpart (11.2 eV), the [Pt,O2](+) cation is a rather aggressive reagent toward oxidizable neutrals. [Pt,O2](+) is even capable of activating inert substrates such as H2, CO, and CH4. Further, a sequence for the catalytic conversion CO + N(2)O --> CO2 + N2 is described with a turnover number of >100 for the catalytically active species PtOn(+) (n = 0-2). As a consequence of the high reactivity, however, the observed selectivities with most substrates are rather poor. For example, the reaction of PtO2(+) with ethane gives rise to 10 different product channels. In an attempt to analyze the structural features and different minima of the [Pt,O2](+) system, extensive ab initio studies are performed. While correlated ab initio methods describe the system reasonably well, density functional theory turns out to be much less accurate in terms of both structural and energetic descriptions.