Kinetic Evidence for the Formation of Oxazolidinones in the Stereogenic Step of Proline-Catalyzed Reactions

Bond Energies of Enamines

Energetics of reactive intermediates underlies their reactivity. The availability of these data provides a rational basis for understanding and predicting a chemical reaction. We reported here a comprehensive computational study on the energetics of enamine intermediates that are fundamental in carbonyl chemistry. Accurate density functional theory (DFT) calculations were performed to determine the bond energies of enamines and their derived radical intermediates. These efforts led to the compilation of a database of enamine energetics including a thermodynamic index such as free-energy stability, bond dissociation energy (BDE), and acid dissociation constant (pK _a) as well as a kinetic index such as nucleophilicity and electrophilicity. These data were validated by relating to experimentally determined parameters and their relevance and utility were discussed in the context of modern enamine catalysis. It was found that pK _a values of enamine radical cations correlated well with redox potentials of their parent enamines, the former could be used to rationalize the proton-transfer behavior of enamine radical cations. An analysis of the BDE of enamine radical cations indicated that these species underwent facile β-C-H hydrogen transfer, in line with the known oxidative enamine catalysis. The enamine energetics offers the possibility of a systematic evaluation of the reactivities of enamines and related radicals, which would provide useful guidance in exploring new enamine transformations.

Catalytic properties of 4,5-bridged proline methano- and ethanologues in the Hajos–Parrish intramolecular aldol reaction

The catalysis of the Hajos–Parrish reaction by cis- and trans-4,5-ethano-proline was explored experimentally and computationally with DFT (ωB97X-D and MN15) and DLPNO-CCSD(T).

Mechanistic studies of an L-proline-catalyzed pyridazine formation involving a Diels-Alder reaction with inverse electron demand

The mechanism of an L-proline-catalyzed pyridazine formation from acetone and aryl-substituted tetrazines via a Diels-Alder reaction with inverse electron demand has been studied with NMR and with electrospray ionization mass spectrometry. A catalytic cycle with three intermediates has been proposed. An enamine derived from L-proline and acetone acts as an electron-rich dienophile in a [4 + 2] cycloaddition with the electron-poor tetrazine forming a tetraazabicyclo[2.2.2]octadiene derivative which then eliminates N2 in a retro-Diels-Alder reaction to yield a 4,5-dihydropyridazine species. The reaction was studied in three variants: unmodified, with a charge-tagged substrate, and with a charge-tagged proline catalyst. The charge-tagging technique strongly increases the ESI response of the respective species and therefore enables to capture otherwise undetected reaction components. With the first two reaction variants, only small intensities of intermediates were found, but the temporal progress of reactants and products could be monitored very well. In experiments with the charge-tagged L-proline-derived catalyst, all three intermediates of the proposed catalytic cycle were detected and characterized by collision-induced dissociation (CID) experiments. Some of the CID pathways of intermediates mimic single steps of the proposed catalytic cycle in the gas phase. Thus, the charge-tagged catalyst proved one more time its superior effectiveness for the detection and study of reactive intermediates at low concentrations.

Neighboring Component Effect in a Tri-stable [2]Rotaxane

The redox properties of cyclobis(paraquat- p-phenylene)cyclophane (CBPQT⁴⁺) render it a uniquely variable source of recognition in the context of mechanically interlocked molecules, through aromatic donor-acceptor interactions in its fully oxidized state (CPBQT⁴⁺) and radical-pairing interactions in its partially reduced state (CBPQT^2(•+)). Although it is expected that the fully reduced neutral state (CBPQT⁽⁰⁾) might behave as a π-donating recognition unit, resulting in a dramatic change in its binding properties when compared with the other two redox states, its role in rotaxanes has not yet been investigated. To address this challenge, we report herein the synthesis of a tri-stable [2]rotaxane in which a CBPQT⁴⁺ ring is mechanically interlocked with a dumbbell component containing five recognition sites-(i) a bipyridinium radical cation (BIPY^(•+)) located centrally along the axis of the dumbbell, straddled by (ii) two tetrafluorophenylene units linked to (iii) two triazole rings. In addition to the selective recognition between (iv) the CBPQT⁴⁺ ring and the triazole units, and (v) the CBPQT^2(•+) ring and the reduced BIPY^(•+) unit in the dumbbell component, investigations in solution have now confirmed the presence of additional non-covalent bonding interactions between the CBPQT⁽⁰⁾ ring, acting as a donor in its neutral state, and the two tetrafluorophenylene acceptors in the dumbbell component. The unveiling of this piece of molecular recognition in a [2]rotaxane is reminiscent of the existence in much simpler, covalently linked, organic molecules of neighboring group participation (anchimeric assistance giving way to transannular interactions) in small-, medium-, and large-membered rings.

Importance of the Electron Correlation and Dispersion Corrections in Calculations Involving Enamines, Hemiaminals, and Aminals. Comparison of B3LYP, M06-2X, MP2, and CCSD Results with Experimental Data

While B3LYP, M06-2X, and MP2 calculations predict the ΔG° values for exchange equilibria between enamines and ketones with similar acceptable accuracy, the M06-2X/6-311+G(d,p) and MP2/6-311+G(d,p) methods are required for enamine formation reactions (for example, for enamine 5a, arising from 3-methylbutanal and pyrrolidine). Stronger disagreement was observed when calculated energies of hemiaminals (N,O-acetals) and aminals (N,N-acetals) were compared with experimental equilibrium constants, which are reported here for the first time. Although it is known that the B3LYP method does not provide a good description of the London dispersion forces, while M06-2X and MP2 may overestimate them, it is shown here how large the gaps are and that at least single-point calculations at the CCSD(T)/6-31+G(d) level should be used for these reaction intermediates; CCSD(T)/6-31+G(d) and CCSD(T)/6-311+G(d,p) calculations afford ΔG° values in some cases quite close to MP2/6-311+G(d,p) while in others closer to M06-2X/6-311+G(d,p). The effect of solvents is similarly predicted by the SMD, CPCM, and IEFPCM approaches (with energy differences below 1 kcal/mol).

Cross-Aldol Reaction of Isatin with Acetone Catalyzed by Leucinol: A Mechanistic Investigation

Comprehensive mechanistic studies on the enantioselective aldol reaction between isatin (1 a) and acetone, catalyzed by L-leucinol (3 a), unraveled that isatin, apart from being a substrate, also plays an active catalytic role. Conversion of the intermediate oxazolidine 4 into the reactive syn-enamine 6, catalyzed by isatin, was identified as the rate-determining step by both the calculations (ΔG(≠) =26.1 kcal mol(-1) for the analogous L-alaninol, 3 b) and the kinetic isotope effect (kH /kD =2.7 observed for the reaction using [D6 ]acetone). The subsequent reaction of the syn-enamine 6 with isatin produces (S)-2 a (calculated ΔG(≠) =11.6 kcal mol(-1) ). The calculations suggest that the overall stereochemistry is controlled by two key events: 1) the isatin-catalyzed formation of the syn-enamine 6, which is thermodynamically favored over its anti-rotamer 7 by 2.3 kcal mol(-1) ; and 2) the high preference of the syn-enamine 6 to produce (S)-2 a on reaction with isatin (1 a) rather than its enantiomer (ΔΔG(≠) =2.6 kcal mol(-1) ).

Electrophilicities of benzaldehyde-derived iminium ions: quantification of the electrophilic activation of aldehydes by iminium formation

Rate constants for the reactions of benzaldehyde-derived iminium ions with C-nucleophiles (enamines, silylated ketene acetals, and enol ethers) have been determined photometrically in CH3CN solution and used to determine the electrophilicity parameters E of the cations defined by the correlation log k(20°C) = s(N)(E + N) (Mayr, H.; et al. J. Am. Chem. Soc. 2001, 123, 9500-9512). With electrophilicity parameters from E = -10.69 (Ar = p-MeOC6H4) to E = -8.34 (Ar = p-CF3), the iminium ions Ar-CH═NMe2(+) have almost the same reactivities as analogously substituted arylidenemalononitriles Ar-CH═C(CN)2 and are 10 orders of magnitude more reactive than the corresponding aldehydes. The rate constants for the reactions of iminium ions with amines and water in acetonitrile are 10(3)-10(5) times faster than predicted by the quoted correlation, which is explained by the transition states which already experience the anomeric stabilization of the resulting N,N- and O,N-acetals.

A quantitative approach to nucleophilic organocatalysis

The key steps in most organocatalytic cyclizations are the reactions of electrophiles with nucleophiles. Their rates can be calculated by the linear free-energy relationship log k(20 °C) = s(N)(E + N), where electrophiles are characterized by one parameter (E) and nucleophiles are characterized by the solvent-dependent nucleophilicity (N) and sensitivity (s(N)) parameters.Electrophilicity parameters in the range -10 < E < -5 were determined for iminium ions derived from cinnamaldehyde and common organocatalysts, such as pyrrolidines and imidazolidinones, by studying the rates of their reactions with reference nucleophiles. Iminium activated reactions of α,β-unsaturated aldehydes can, therefore, be expected to proceed with nucleophiles of 2 < N < 14, because such nucleophiles are strong enough to react with iminium ions but weak enough not to react with their precursor aldehydes. With the N parameters of enamines derived from phenylacetaldehyde and MacMillan's imidazolidinones one can rationalize why only strong electrophiles, such as stabilized carbenium ions (-8 < E < -2) or hexachlorocyclohexadienone (E = -6.75), are suitable electrophiles for enamine activated reactions with imidazolidinones. Several mechanistic controversies concerning iminium and enamine activated reactions could thus be settled by studying the reactivities of independently synthesized intermediates.Kinetic investigations of the reactions of N-heterocyclic carbenes (NHCs) with benzhydrylium ions showed that they have similar nucleophilicities to common organocatalysts (e.g., PPh(3), DMAP, DABCO) but are much stronger (100-200 kJ mol(-1)) Lewis bases. While structurally analogous imidazolylidenes and imidazolidinylidenes have comparable nucleophilicities and Lewis basicities, the corresponding deoxy Breslow intermediates differ dramatically in reactivity. The thousand-fold higher nucleophilicity of 2-benzylidene-imidazoline relative to 2-benzylidene-imidazolidine is explained by the gain of aromaticity during electrophilic additions to the imidazoline derivatives. O-Methylated Breslow intermediates are a hundred-fold less nucleophilic than deoxy Breslow intermediates.

Electrophilicities of bissulfonyl ethylenes

Kinetics of the reactions of bissulfonyl ethylenes with various carbanions, a sulfur ylide, and siloxyalkenes have been investigated photometrically at 20 °C. The second-order rate constants have been combined with the known nucleophile- specific parameters N and s(N) for the nucleophiles to calculate the empirical electrophilicity parameters E of bissulfonyl ethylenes according to the linear free energy relationship log k(20 °C)=s(N)(N+E). Structure-reactivity relationships are discussed, and it is shown that the electrophilicity parameters E derived in this work can be employed to define the synthetic potential of bissulfonyl ethylenes as Michael acceptors.

Fluorine conformational effects in organocatalysis: an emerging strategy for molecular design

Molecular design strategies that profit from the intrinsic stereoelectronic and electrostatic effects of fluorinated organic molecules have mainly been restricted to bio-organic chemistry. Indeed, many fluorine conformational effects remain academic curiosities with no immediate application. However, the renaissance of organocatalysis offers the possibility to exploit many of these well-described phenomena for molecular preorganization. In this minireview, we highlight examples of catalyst refinement by introduction of an aliphatic C-F bond which functions as a chemically inert steering group for conformational control.