PH bond activation of primary phosphine-boranes: Access to α-hydroxy and α,α′-dihydroxyphosphine-borane adducts by uncatalyzed hydrophosphination of carbonyl derivatives

UV induced hydrophosphination of dimethyl 2-vinylcyclopropane-1,1-dicarboxylate towards phosphine chalcogenides

Dimethyl 2-vinylcyclopropane-1,1-dicarboxylate underwent a hydrophosphination reaction with either a primary or secondary phosphine under photolytic conditions. Notably, a free radical initiator was not required. The resulting tertiary phosphines were derivatized using S₈ to afford moisture and air stable yellow or colorless oils in a 27%-73% isolated yield. A series of control reactions were performed, and we propose that this UV induced hydrophosphination reaction proceeds through a radical mechanism.

Molecular Motion in the Interconverting σ-H2, Di-, and Tri-hydride Regimes: Mo(PH3)5H2

The transition-metal complex Mo(PH3)5H2 is known to exist in three possible isomeric forms, including a nonclassical, σ-bound dihydrogen complex and two classical dihydride isomers. As such, it has served as a model complex for the energies of conversion between these limiting structural regimes. In the present study, ab initio molecular dynamics computer simulations, combined with enhanced sampling techniques, were utilized to directly assess the degree of motion and isomerization of the dihydrogen/dihydride moieties in this complex. Ligand rotations (for both the H₂ unit and the phosphine units) were found to be dominant in the low-temperature (298 K) regime, and the classical thermodynamic distribution showed no probability of thermally accessing dihydride forms, although unrestrained molecular dynamics trajectories showed fleeting configurations outside of the σ-H₂ configuration. Simulations at higher temperatures surprisingly revealed new tri-hydride isomers that are energetically competitive with the σ-H₂ and cis-/trans-dihydride isomers. Low-energy pathways to hydrogen/hydride transfer and phosphine dissociation were readily accessible, which considerably expands the known isomeric flexibility of this complex.

Easy Access to Phosphine-Borane Building Blocks

In this paper, we highlight the synthesis of a variety of primary phosphine-boranes (RPH2 ⋅BH3 ) from the corresponding dichlorophosphines, simply by using Li[BH4 ] as reductant and provider of the BH3 protecting group. The method offers facile access not only to alkyl- and arylphosphine-boranes, but also to aminophosphine-boranes (R2 NPH2 ⋅BH3 ) that are convenient building blocks but without the protecting BH3 moiety thermally labile and notoriously difficult to handle. The borane-protected primary phosphines can be doubly deprotonated using n-butyllithium to provide soluble phosphanediides Li2 [RP⋅BH3 ] of which the phenyl-derivative Li2 [PhP⋅BH3 ] was structurally characterized in the solid state.

Increasing steric demand through flexible bulk - primary phosphanes with 2,6-bis(benzhydryl)phenyl backbones

Sterically demanding primary phosphanes of the type RBhp-PH2 (Bhp = 2,6-bis(benzhydryl)-4-R-phenyl; R = Me, tBu) could be prepared in high yields by modification of synthetic protocols of established bulky phosphanes. The Bhp substituents simultaneously exhibit extensive steric expansiveness and high degrees of flexibility compared to other 2,6-substituted phenyl backbones, enabling a range of different-sized atoms and functionalities to be located between the flanking benzhydryl moieties. Therefore, it was also possible to isolate and fully characterize several halogenated precursors, a diazonium intermediate, as well as a dihalostibane.

A theoretical analysis of substituent electronic effects on phosphine-borane bonds

Phosphine-borane adducts are a well-known moiety in synthetic and coordination chemistry. These complexes form a dative bond in which the Lewis basic phosphorus atom donates electron density into an empty p-orbital of the Lewis acidic boron atom. However, donation of the phosphorus lone pair is not the only stabilizing interaction, as hyperconjugation and electrostatic interaction also play important roles in bonding. This paper describes a detailed density functional theory level (B3LYP) study completed to determine the impact electron-donating and withdrawing substituents have on phosphine-borane bonds through the investigation of a series of para-substituted PAr₃-BH₃ and PH₃-BAr₃ phosphine-borane adducts. Natural bond orbital (NBO) partitioning was used to calculate the distribution of electron density between the phosphine and borane fragments. Extended transition state and natural orbitals for chemical valence (ETS-NOCV) analysis was used to isolate contributions to the overall electronic interaction of the phosphine-borane adducts. Molecular orbital composition and charge donation was calculated using AOMix. The resulting data was correlated with Hammett σ constants.

Mechanistic aspects of the stereospecific reduction of chiral hydroxyalkyl phosphinates and phosphine oxides

We present recent advances in the understanding of the reduction of optically pure hydroxyalkylphosphinates and phosphine oxides, which represent key intermediates for the preparation of P-stereogenic ligands. Their reduction leads to P-chiral phosphinites and phosphines, respectively, and occurs stereospecifically with inversion of configuration using BH3·THF, which plays three roles: activating, reducing and protecting agent. The formation of by-products as hydroxyalkyl secondary phosphine–boranes has also been studied.

Zirconium-catalyzed intermolecular hydrophosphination using a chiral, air-stable primary phosphine

Catalytic hydrophosphination of alkenes using a chiral, air-stable primary phosphine, (R)-MeO-MOPH₂, proceeds under mild conditions with a zirconium catalyst to selectively furnish anti-Markovnikov, air-stable secondary phosphine or tertiary phosphine prodcuts with slight modification of the protocol.

Mechanism of phosphine borane deprotection with amines: the effects of phosphine, solvent and amine on rate and efficiency

The kinetics of borane transfer from simple tertiary phosphine borane adducts to a wide range of amines have been determined. All data obtained, including second-order kinetics, lack of cross-over, and negative entropies of activation for reaction of triphenylphosphine borane with quinuclidine and triethylamine, are consistent with a direct (SN 2-like) transfer process, rather than a dissociative (SN 1-like) process. The identities of the amine, phosphine, and solvent all impact substantially on the rate (k) and equilibrium (K) of the transfer, which in some cases vary by many orders of magnitude. P-to-N transfer is more efficient with cyclic amines in apolar solvents due to reduced entropic costs and ground-state destabilisation. Taken as a whole, the data allow informed optimisation of the deprotection step from the stand-point of rate, or synthetic convenience. In all cases, both reactants should be present at high initial concentration to gain kinetic benefit from the bimolecularity of the process. Ultimately, the choice of amine is dictated by the identity of the phosphine borane complex. Aryl-rich phosphine boranes are sufficiently reactive to allow use of diethylamine or pyrrolidine as a volatile low polarity solvent and reactant, whereas more alkyl-rich phosphines benefit from the use of more reactive amines, such as 1,4-diaza[2.2.2]bicyclooctane (DABCO), in apolar solvents at higher temperatures.

A Tandem Olefin Migration and Prins Cyclization Using Cu(OTf)(2)-Bisphosphine Complexes: An Improved Synthesis of Functionalized Tetrahydropyrans

In situ-generated (bis-DPPMB)-Cu(OTf)(2) complex has been examined to catalyze a tandem olefin migration and Prins cyclization of an alkenol with various aldehydes. The reaction proceeded with electron-rich aromatic aldehydes at room temperature and provided functionalized tetrahydropyrans in good yields. An efficient synthesis of the bis-DPPMB ligand has also been described.

The ever-surprising chemistry of boron: enhanced acidity of phosphine.boranes

The acidity-enhancing effect of BH(3) in gas-phase phosphineboranes compared to the corresponding free phosphines is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. Thus, the enhancement of the acidity of protic acids by Lewis acids usually observed in solution is also observed in the gas phase. For example, the gas-phase acidities (GA) of MePH(2) and MePH(2)BH(3) differ by about 118 kJ mol(-1) (see picture).The gas-phase acidity of a series of phosphines and their corresponding phosphineborane derivatives was measured by FT-ICR techniques. BH(3) attachment leads to a substantial increase of the intrinsic acidity of the system (from 80 to 110 kJ mol(-1)). This acidity-enhancing effect of BH(3) is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. This indicates that the enhancement of the acidity of protic acids by Lewis acids usually observed in solution also occurs in the gas phase. High-level DFT calculations reveal that this acidity enhancement is essentially due to stronger stabilization of the anion with respect to the neutral species on BH(3) association, due to a stronger electron donor ability of P in the anion and better dispersion of the negative charge in the system when the BH(3) group is present. Our study also shows that deprotonation of ClCH(2)PH(2) and ClCH(2)PH(2)BH(3) is followed by chloride departure. For the latter compound deprotonation at the BH(3) group is found to be more favorable than PH(2) deprotonation, and the subsequent loss of Cl(-) is kinetically favored with respect to loss of Cl(-) in a typical S(N)2 process. Hence, ClCH(2)PH(2)BH(3) is the only phosphineborane adduct included in this study which behaves as a boron acid rather than as a phosphorus acid.

High atom-economical one-pot synthesis of secondary phosphines and their borane complexes using recycling phosphorus donor reagent

[reaction: see text] A general new method for the one-pot preparation of secondary phosphines 11 and in situ generation of their borane complexes 12 is described. This method consists of the sequential addition, at room temperature, of equivalent amounts of R(1)MgBr and R(2)MgBr to 1 equiv of the phosphorus atom donor reagent 1. Final treatment with water gives secondary phosphines R(1)R(2)PH (or the corresponding phosphine-borane complexes if treated with BH(3).THF) and the end product 6, which can be recycled.

Rhodium-catalyzed dehydrocoupling of fluorinated phosphine-borane adducts: synthesis, characterization, and properties of cyclic and polymeric phosphinoboranes with electron-withdrawing substituents at phosphorus

The dehydrocoupling of the fluorinated secondary phosphine-borane adduct R2PH.BH3 (R = p-CF3C6H4) at 60 degrees C is catalyzed by the rhodium complex [{Rh(mu-Cl)(1,5-cod)}2] to give the four-membered chain R2PH-BH2-R2P-BH3. A mixture of the cyclic trimer [R2P-BH2]3 and tetramer [R2P-BH2]4 was obtained from the same reaction at a more elevated temperature of 100 degrees C. The analogous rhodium-catalyzed dehydrocoupling of the primary phosphine-borane adduct RPH2.BH3 at 60 degrees C gave the high molecular weight polyphosphinoborane polymer [RPH-BH2]n (Mw = 56,170, PDI = 1.67). The molecular weight was investigated by gel permeation chromatography and the compound characterized by multinuclear NMR spectroscopy. Interestingly, the electron-withdrawing fluorinated aryl substituents have an important influence on the reactivity as the dehydrocoupling process occurred efficiently at the mildest temperatures observed for phosphine-borane adducts to date. Thin films of polymeric [RPH-BH2]n (R = p-CF3C6H4) have also been shown to function as effective negative-tone resists towards electron beam (e-beam) lithography (EBL). The resultant patterned bars were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and time-of-flight secondary ion mass spectrometry (TOF-SIMS).

Heterogeneous or Homogeneous Catalysis? Mechanistic Studies of the Rhodium-Catalyzed Dehydrocoupling of Amine-Borane and Phosphine-Borane Adducts

In depth, comparative studies on the catalytic dehydrocoupling of the amine-borane adduct Me(2)NH.BH(3) (to form [Me(2)N-BH(2)](2)) and the phosphine-borane adduct Ph(2)PH.BH(3) (to form Ph(2)PH-BH(2)-PPh(2)-BH(3)) with a variety of Rh (pre)catalysts such as [[Rh(1,5-cod)(micro-Cl)](2)], Rh/Al(2)O(3), Rh(colloid)/[Oct(4)N]Cl, and [Rh(1,5-cod)(2)]OTf have been performed in order to determine whether the dehydrocoupling proceeds by a homogeneous or heterogeneous mechanism. The results obtained suggest that the catalytic dehydrocoupling of Me(2)NH.BH(3) is heterogeneous in nature involving Rh(0) colloids, while that of Ph(2)PH.BH(3) proceeds by a homogeneous mechanism even when starting with Rh(0) precursors such as Rh/Al(2)O(3). The catalytic dehydrocoupling reactions are thought to proceed by different mechanisms due to a combination of factors such as (i) the greater reducing strength of amine-borane adducts, (ii) the increased ease of dissociation of phosphine-borane adducts, and (iii) phosphine ligation and/or poisoning of active catalytic sites on metal colloids.

t-BuOK-Mediated Hydrophosphination of Functionalized Alkenes: A Novel Synthesis of Chiral P,N- and P,P-Ligands

A novel synthesis of effective chiral P,N- and P,P-ligands has been developed by using a t-BuOK-mediated hydrophosphination of chiral alkenylpyridines and alkenylphosphine oxides. Ir complexes of chiral P,N-ligands 1 and 3 gave high enantioselectivities for the hydrogenation of (Z)-alpha-(acetamido)cinnamate 25 and (E)-1,2-diphenylpropene leading to the hydrogenated products with up to 97% ee.

Regio- and stereoselective hydrophosphination reactions of alkynes with phosphine-boranes: access to stereodefined vinylphosphine derivatives

Vinylphosphine-borane complexes are easily synthesized by regio- and stereoselective hydrophosphination of terminal alkynes with use of secondary phosphine-boranes as hydrophosphinating agent. The regioselectivity of the reaction is efficiently controlled by the choice of the activation process: thermal or metal catalyst activation. The vinylphosphine derivatives are purified by chromatography on silica gel. Scalability of the process is demonstrated on a gram scale. This simple route should promote the use of vinylphosphines as building blocks for organic and organometallic chemistry.

Synthesis of both enantiomers of a P-chirogenic 1,2-bisphospholanoethane ligand via convergent routes and application to rhodium-catalyzed asymmetric hydrogenation of CI-1008 (pregabalin)

Both enantiomers of a P-chirogenic 1,2-bisphospholanoethane ligand are synthesized via two convergent methods. The first method relies on the chiral alkylation of 1-((-)-menthoxy)phospholaneborane using a s-BuLi/(-)-sparteine derived chiral base. Only one enantiomer of the catalyst could be synthesized via this method because only one antipode of sparteine is available in nature. The second route relies on the combination of methylphosphine borane and a chiral 1,4-diol. Either enantiomer of the ligand can be synthesized via the second route from the appropriate enantiomer of the 1,4-diol. Asymmetric hydrogenation using catalyst precursor 36 on acetamidoacrylic acid derivatives provided modest to good enantioselectivity (77-95% ee) under low H(2) pressure (30 psi). Asymmetric hydrogenation of CI-1008 (pregabalin) precursors, 39 and 40, provided good enantioselectivities (92%) at high catalyst loading (1 mol %) and low pressure (30 psi). Enantiomeric excesses dropped sharply with catalyst loading at this pressure. Increasing the pressure of H(2) caused a significant increase in enantiomeric excess for low catalyst loading reactions. Several studies were undertaken to further investigate the enantioselectivity dependence on both pressure and catalyst loading.

Nonvolatile Me(3)P-like P-donor ligand: synthesis and properties of 4-phenyl-1-phospha-4-silabicyclo[2.2.2]octane

[reaction: see text] A new trialkylphosphine ligand with Me(3)P-like steric and electronic properties, 4-phenyl-1-phospha-4-silabicyclo[2.2.2]octane (Ph-SMAP), was synthesized. Given a phenyl group at the silicon atom, the Ph-SMAP ligand displayed nonvolatility with retention of Me(3)P-like properties. The new ligand was air-stable, crystalline, and easy to handle.