Density Functional Theory Investigations on Sulfur Ylide Promoted Cyclopropanation Reactions: Insights on Mechanism and Diastereoselection Issues

Synthesis of methylene cyclopropane-fused chromenes and dihydroquinolines by sequential [4 + 2]- and [1 + 2]-annulation

A base promoted sequential [4 + 2]- and [1 + 2]-annulation of 2-hydroxychalcones or 2-tosylaminochalcones with prop-2-ynylsulfonium salts was developed.

Mild and Diazo-Free Synthesis of Trifluoromethyl-Cyclopropanes Using Sulfonium Ylides

The synthesis of several 1,1-disubstituted trifluoromethyl-cyclopropanes (TFCPs), known as tert-butyl bioisosteres, has been achieved from the reaction between trifluoromethylalkenes and unstabilized sulfonium ylides in yields of ≤97%. This method offers practical access to this cyclopropyl moiety of pharmacological interest, employing a commercially available reagent at low temperatures. The synthesis of cyclopropanes bearing other electron-withdrawing groups as well as trisubstituted TFCPs was also accomplished.

Directed electrostatic activation in enantioselective organocatalytic cyclopropanation reactions: a computational study

Cyclopropane rings are versatile building blocks in organic chemistry. Their synthesis, by the reaction of sulfur ylides with α,β-unsaturated carbonyl compounds, has recently aroused renewed interest after the discovery of efficient catalysis by using (S)-indoline-2-carboxylic acid. In order to rationalize the behavior of this catalyst, MacMillan proposed a directed electrostatic activation (DEA) mechanism, in which the negative carboxylate group interacts with the positive thionium moiety, thus reducing the activation energy and increasing the reaction rate. More recently, Mayr refuted some of MacMillan conclusions, but accepted the DEA mechanism as a justification for the experimental high reaction rates. In contrast, our results indicate that the selectivity obtained in the process seems to result from several strong hydrogen bond interactions between the two reacting species, while no strong evidence for a DEA mechanism was found. We also concluded that the hydrogen bonds don't improve the reaction rate by lowering the activation energy of the rate-determining step, but can do it by promoting efficient reaction trajectories due to long-range complexation of the reagents. Finally, our results confirm that the cyclopropanation reaction occurs by a two-step mechanism, and that the overall enantioselectivity depends on the relative energies of the two steps, averaged by the relative populations of the iminium intermediates that are initially formed in the reaction.

Diastereoselective Johnson-Corey-Chaykovsky trifluoroethylidenation

(2,2,2-Trifluoroethyl)diphenylsulfonium triflate was found to be an efficient ylide reagent for the Johnson-Corey-Chaykovsky reaction to afford trifluoromethyl epoxides, cyclopropanes and aziridines. Interestingly, excellent but different diastereoselectivity was observed for these transformations. Both trifluoromethyl epoxides and cyclopropanes were obtained with trans-selectivity, whereas aziridines were obtained with cis-selectivity.

Rational design of catalysts for asymmetric diamination reaction using transition state modeling

DFT calculations have been used to design chiral phosphoramidite ligands for the asymmetric diamination of vicinal diamines. The substituents at both the 3,3′ positions of the binol framework and the amido nitrogen play a vital role in the stereochemical outcome.

One-pot Synthesis of Cyclopropane Derivatives with a Cis:Trans Stereoselectivity by Wittig Olefination-Sulfur Ylide Cyclopropanation Sequence

Cyclopropane derivatives were synthesised in one pot from aromatic aldehydes, phenacyltriphenylphosphonium bromide, and S-phenacylthiolanium bromide by a Wittig olefination-sulfur ylide cyclopropanation sequence. When Cs2CO3 was used as the base and CH3CN/water (8:2, v/v) as the solvent, the major product was the cis-1,2-dibenzoyl cyclopropane. In contrast, when using DBU as the base and MeOH as the solvent, the major product was trans-dibenzoyl cyclopropane. In some cases, the major isomer was obtained in high purity and good yield by simple filtration.

On the origin of regio- and stereoselectivity in singlet oxygen addition to enecarbamates

The reactions of excited state singlet molecular oxygen ((1)Δ(g),(1)O(2)) continue to witness interesting new developments. In the most recent manifestation, (1)O(2) is tamed to react with enecarbamates in a stereoselective manner, which is remarkable, in view of its inherently high reactivity (Acc. Chem. Res. 2008, 41, 387). Herein, we employed the CAS-MP2(8,7)/6-31G* as well as the CAS-MP2(10,8)/6-31G* computations to unravel the origin of (i) diastereoselectivities in dioxetane or hydroperoxide formation and (ii) regioselectivity leading to a [2 + 2] cycloadduct or an ene product when (1)O(2) reacts with an oxazolidinone tethered 2-phenyl-1-propenyl system. The computed Gibbs free energy profiles for E- and Z-isomers when (1)O(2) approaches through the hindered and nonhindered diastereotopic faces (by virtue of chiral oxazolidinone) of the enecarbamates exhibit distinct differences. In the case of E-isomer, the relative energies of the transition structures responsible for hydroperoxide (ene product) are lower than that for dioxetane formation. On the other hand, the ene pathway is predicted to involve higher barriers as compared to the corresponding dioxetane pathway for Z-isomer. The energy difference between the rate-determining diastereomeric transition structures involved in the most favored ene reaction for E-enecarbamate suggests high diastereoselectivity. In contrast, the corresponding energy difference for Z-enecarbamate in the ene pathway is found to be diminishingly close, implying low diastereoselectivity. However, the dioxetane formation from Z-enecarbamate is predicted to exhibit high diastereoselectivity. The application of activation strain model as well as the differences in stereoelectronic effects in the stereocontrolling transition structures is found to be effective toward rationalizing the origin of selectivities reported herein. These predictions are found to be in excellent agreement with the experimental observations.

Chemo-, regio-, and diastereoselectivity preferences in the reaction of a sulfur ylide with a dienal and an enone

Mechanistic insights into an interesting class of reaction between sulfur ylides with (i) a dienal, and (ii) an enone, obtained by using density functional theory, is reported. The kinetic and thermodynamic factors responsible for chemo-, regio-, and diastereoselectivities are established by identifying all key transition states and intermediates along the reaction pathway for 1,2-, 1,4-, and 1,6- modes of attack of dimethylsulfonium benzylide to 5-phenylpenta-2,4-dienal. The reaction profiles for 1,2- and 1,4- modes of addition are also evaluated for the reaction between dimethylsulfonium benzylide and pent-3-en-2-one. Our results show that the final outcome of the reaction with both these substrates would be decided by the interplay between kinetic and thermodynamic factors. It is found that the addition of a semi-stabilized ylide to conjugated carbonyl compounds prefers to proceed through a 1,4- (conjugate) pathway under thermodynamic conditions, which is in accordance with the available experimental reports. However, the formation of epoxides via a 1,2- (direct) addition pathway is computed to be equally competitive, which could be the favored pathway under kinetic conditions. Even though the lower barrier for the initial addition step is kinetically advantageous for the direct (or 1,2-) addition pathway, the higher energy of the betaine intermediates--as well as the reversibility of the accompanying elementary step--may disfavor product formation in this route. Thus, high diastereoselectivity in favor of 2,3-trans cyclopropanecarbaldehyde is predicted in the case of the dienal, using the most favored conjugate addition (1,4-addition) pathway. Along similar lines, ylide addition to the enone is identified to exhibit a preference toward conjugate addition over direct (1,2-) addition. The importance of transition state analysis in delineating the controlling factors towards product distribution and diastereoselectivity is established.

Enantio- and diastereoselectivities in chiral sulfur ylide promoted asymmetric aziridination reactions

Density functional theory investigation on the factors controlling enantio- and diastereoselection in asymmetric aziridination reaction by the addition of chiral bicyclic sulfur ylides to substituted aldimines is presented. High levels of enantioselection are predicted toward the formation of (2S,3S)-cis and (2R,3S)-trans aziridines by the addition of stabilized ylide (R = COMe) respectively to SO2Me and CO2Me protected aldimines. Similarly, high %ee is predicted for the formation of (2S,3R)-cis aziridines from semistabilized (R = Ph) ylide. Moderate to high levels of diastereoselectivity is noticed as well. The present study highlights that a correct prediction on extent of enantioselection requires the knowledge of the activation barriers for elementary steps beyond the initial addition step. In the case of stabilized ylides the ring-closure (or elimination of sulfur compound) is found to be crucial in controlling enantio- and diastereoselection. A cumulative effect of electronic as well as other weak interactions is identified as factors contributing to the relative energies of transition states leading to enantio- and diastereomeric products for the stabilized ylide addition to aldimines. On the contrary, steric control appears quite dominant with semistabilized ylide addition. With the smallest substituent on ylide (R = Me), high enantioselectivity is predicted for the formation of (2R,3R)-trans aziridines although the %de in this case is found to be very low.

Computational investigations on the general reaction profile and diastereoselectivity in sulfur ylide promoted aziridination

Mechanism and diastereoselectivity of sulfur ylide promoted aziridination reactions were studied by density functional theory with inclusion of solvent effects through the continuum solvation model. The general reaction pathway was modeled for the addition of substituted sulfur ylides (Me(2)S(+)CH(-)R) to an aldimine ((E)-methyl ethylidenecarbamate, MeHC=NCO(2)Me). The nature of the substituents on the ylidic carbon atom substantially affects the reaction profile. The stabilized (R=COMe) and semistabilized (R=Ph) ylides follow a cisoid addition mode leading to trans aziridines via anti betaine intermediates. The simplest model ylide (unstabilized, R=H) underwent cisoid addition in a similar fashion. In the case of stabilized ylides product diastereoselectivity is controlled by the barriers of the elimination step leading to the 2,3-trans aziridine, whereas it is decided in the addition step in the case of semistabilized ylides. The importance of steric and electronic factors in diastereoselective addition (2 and 5) and elimination (5) transition states was established. Comparison of results obtained with the gas-phase optimized geometries and with the fully optimized solvent-phase geometries reveals that the inclusion of solvent effects does not bring about any dramatic changes in the reaction profiles for all three kinds of ylides. In particular, diastereoselectivity for both kinds of ylides was found to be nearly the same in both these approaches.