Switching of stereochemistry. Dramatic effect of HMPA on the stereoselectivity of the cyclopropanation reaction of telluronium allylides with alpha,beta-unsaturated esters and amides

First Application of Ionic Liquid to Reactions Involving Organotellurium Compounds as Intermediates

The condensation reaction of telluronium salts 1 with aldehydes and dibutyl telluride 4, bromide 5 with aldehyde 2 proceeded smoothly in the ionic solvent [bmim][BF4], affording a novel method for the stereoselective synthesis of (E)-α,β-unsaturated compounds 3 in high purity, excellent yields and high stereoselectivity.

Synthesis of spiroaminals by bimetallic Au/Sc relay catalysis: TMS as a traceless controlling group

An efficient synthesis of spiroaminals over fused aminals has been successfully developed by bimetallic Au/Sc catalysis by using TMS as a traceless controlling group.

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.

Ylide-initiated michael addition-cyclization reactions beyond cyclopropanes

[Reaction: see text]. Ylides are nucleophiles that bear a unique leaving group, L n M, and can attack aldehydes, ketones, imines, and electron-deficient alkenes. Over the course of the reaction, they react with CX (X = C, N, O, etc.) double bonds to form betaine or oxetane intermediates, which further eliminate the heteroatom-containing group in one of two ways to give the corresponding olefination or cyclization product. Since the discovery of the Wittig reaction, ylide olefination has developed as one of the most useful approaches in constructing carbon-carbon double bonds. These reactions provide unambiguous positioning of the C-C double bond and good stereoselectivity. Researchers have also widely used ylides for the synthesis of small ring compounds such as epoxides, cyclopropanes, and aziridines. However, the use of ylides to prepare larger cyclic structures was very limited. This Account outlines our recent work on ylide-initiated Michael addition/cyclization reactions. By altering the heteroatoms and the ligands of the ylides, we have modulated the reactivity of ylides. These modified ylides provide easy access to diverse cyclic compounds with the ability to control regioselectivity, chemoselectivity, diastereoselectivity, and enantioselectivity. Reactions using these ylides produce the structural components of many biologically active compounds and valuable intermediates in organic synthesis. Allylic telluronium and sulfonium ylides can react with alpha,beta-unsaturated esters, ketones, amides, and nitriles to afford multisubstituted vinylcyclopropanes with high selectivities. Telluronium allylides react with aromatic N-phenyl aldimines to give trans-vinylaziridines and with chiral N- tert-butylsulfinylimines to afford the optically active cis-2-substituted vinylaziridines, both with high diastereoselectivities. We also used sulfonium and telluronium allylides to prepare vinylepoxides. In addition, ylides are good reagents for the synthesis of five-membered heterocyclic compounds. By treatment of stable camphor-derived sulfur ylides with alpha-ylidene-beta-diketones, we obtained multisubstituted dihydrofurans with high diastereo- and enantioselectivities. Ammonium salts derived from cinchonidine and cinchonine react smoothly with 3-aryl and 3-heteroaryl-2-nitro acrylates, affording both enantiomers of isoxazoline N-oxides with up to 99% ee. Ylides can initiate tandem cyclizations for the synthesis of chromenes, bicyclic compounds, and cyclohexadiene epoxides. Varying the choice of base allows access to 2 H-chromenes and 4 H-chromenes from 3-(2-(bromomethyl)phenoxy)acrylates via a tandem ylide Michael addition/elimination/substitution reaction. Phosphines can catalyze an intramolecular ylide [3 + 2] annulation constructing bicyclo[ n.3.0] ring systems with three contiguous stereogenic centers. The reaction of crotonate-derived sulfur ylides with alpha,beta-unsaturated ketones affords cyclohexadiene epoxides with excellent diastereoselectivities (>99/1) in good to high yields. Using a camphor-derived sulfonium salt, we have produced asymmetric cyclohexadiene epoxides with high ee's. Overall, these results illustrate the versatility and tunability of ylides for the preparation of cyclic systems containing more than three atoms.

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

The mechanism and diastereoselectivity of synthetically useful sulfur ylide promoted cyclopropanation reactions have been studied using the density functional theory method. Addition of different substituted ylides (Me2S+CH-R) to enone ((E)-pent-3-en-2-one, MeHC=CH-COMe) has been investigated. The nature of the substituent on the ylidic carbon brings about subtle changes in the reaction profile. The stabilized (R=COMe) and semistabilized (R=Ph) ylides follow a cisoid addition mode, leading to 1,2-trans and 1,2-cis cyclopropanes, respectively, via syn and anti betaine intermediates. The simplest and highly reactive model ylide (R=H) prefers a transoid addition mode. Diastereoselectivity is controlled by the barrier for cisoid-transoid rotation in the case of stabilized ylides, whereas the initial electrophilic addition is found to be the diastereoselectivity-determining step for semistabilized ylides. High selectivity toward trans cyclopropanes with stabilized ylides are predicted on the basis of the relative activation energies of diastereomeric torsional transition states. The energy differences between these transition states could be rationalized with the help of weak intramolecular as well as other stereoelectronic interactions.

Enantioselective Synthesis of Vinylcyclopropanes and Vinylepoxides Mediated by Camphor-Derived Sulfur Ylides: Rationale of Enantioselectivity, Scope, and Limitation

By a sidearm approach, camphor-derived sulfur ylides 1 were designed and synthesized for the cyclopropanation of electron-deficient alkenes and epoxidation of aldehydes. Under the optimal conditions, the exo-type sulfonium salts 4a and 4b reacted with beta-aryl-alpha,beta-unsaturated esters, amides, ketones, and nitriles to give 1,3-disubstituted-2-vinylcyclopropanes with high diastereoselectivities and enantioselectivities. When the endo-type sulfonium salts 5a and 5b were used, the diastereoselectivities were not changed, whereas the absolute configurations of the products became the opposite to those of the reactions of 4a and 4b. An ylide cyclopropanation of chalcone derivatives with phenylvinyl bromide in the presence of catalytic amount of chiral sulfonium salts 4b and 5b has been developed. The sidearmed hydroxyl group was found to play a key role in the control of enantioselectivity and diastereoselectivity. The origins of the high diastereoselectivity and enantioselectivity were also studied by density functional theory calculations, which reveal the importance of the hydrogen-bonding between the sidearmed hydroxyl group and the substrate in determining the diastereoselectivity and enantioselectivity. The ylides 1 were also successfully applied for the epoxidation of aromatic aldehydes.

Enantioselective synthesis of allenic esters via an ylide route

Pseudo-C2-symmetric chiral phosphorus ylides have been designed and synthesized for the enantioselective preparation of allenic esters, and up to 92% ee has been achieved.

Scandium triflate catalyzed cycloaddition of imines with 1,1-cyclopropanediesters: efficient and diastereoselective synthesis of multisubstituted pyrrolidines

A tandem ring-opening-cyclization reaction of cyclopropanes with imines in the presence of 5 mol% of scandium triflate was developed for the highly diastereoselective synthesis of multi-substituted pyrrolidines.

Telluronium Salts Mediated Aziridination of Chiral N-tert-Butylsulfinylimines: Highly Stereoselective Synthesis of Optically Active Vinylaziridines

[reaction: see text] Optically active cis-2-substituted vinylaziridines are synthesized by the reaction of N-tert-butylsulfinylimines with telluronium ylides with excellent diastereoselectivity in good to excellent yields.

Highly Stereoselective Synthesis of Trisubstituted Vinylcyclopropane Derivatives via Arsonium Ylides

[reaction: see text] Alkylidene or arylidene malonates reacted with arsonium allylides to give trans-disubstituted cyclopropane-1,1-dicarboxylates with high stereoselectivity in high yields. The mechanism of the cyclopropanation reactions has also been investigated.

Influence of HMPA on the Stereochemical Outcome of the Addition of a Racemic Allenylzinc onto Enantiopure N-tert-Butanesulfinimines: Stereoselective Access to Enantiopurecis-Ethynylaziridines

In the presence of 60 equivalents of HMPA, the condensation of the racemic allenylzinc derived from 1-chloro-3-trimethylsilylpropyne onto enantiopure non-alpha-branched N-tert-butanesulfinimines was proven to give access to the corresponding cis-ethynylaziridines as the major products. The good cis selectivity observed presumably resulted from a high kinetic resolution with the allenylzinc being partially configurationally labile with respect to the time scale defined by the rate of the reaction. Both single crystal X-ray analysis and semiempirical calculations conducted at the MM2 and AM1 levels of theory proved that the reaction certainly occurred through a preferred synclinal transition state in a supra- or antarafacial S(E)2' process. In all cases, chromatographic purification over silica gel allowed the cis-ethynyl-N-tert-butanesulfinylaziridines to be obtained as diastereo- and enantiomerically pure compounds (dr >98:2 and ee >99%).

The Michael Addition−Elimination of Ylides to α,β-Unsaturated Imines. Highly Stereoselective Synthesis of Vinylcyclopropanecarbaldehydes and Vinylcyclopropylaziridines

The Michael addition-elimination of ylide to alpha,beta-unsaturated imines leads to a highly diastereoselective and enantioselective synthesis of vinylcyclopropanecarbaldehydes in good to high yields for the first time. A sequential cyclopropanation-aziridination protocol for the preparation of cyclopropylaziridines is also developed with good diastereoselectivity in good to high yields.

Preparation and Regioselective SN2‘ Reaction of Novel gem-Difluorinated Vinyloxiranes with RLi

A series of hitherto unknown 3,4-epoxy-1,1-difluorobutenes were prepared from the readily accessible alpha,beta-epoxy ketones and these compounds were found to undergo regioselective S(N)2' reactions with hard RLi nucleophiles occurring at the highly positively charged terminal fluorine-possessing sp(2) carbon atom in quite sharp contrast to the cases of the corresponding nonfluorinated vinyloxiranes which only attained a low level of regioselectivity. Addition of HMPA substantially improved the products' olefinic stereoselectivity. Theoretical calculations were used to qualitatively explore the nature of selectivity in these reactions.

Synthesis and Characterization of Pentaerythritol-Derived Oligoglycol and Their Application to Catalytic Wittig-Type Reactions

Several pentaerythritol-derived oligoglycols 1 with free hydroxyl groups are readily prepared by a convergent approach. Quantitative (13)C NMR proves to be an efficient tool for the characterization of oligoglycols. The corresponding telluride of oligoglycol 17 is synthesized and used as a good catalyst for Wittig-type reactions in preparing both disubstituted and trisubstituted olefins in good to high yields.

Controllable Diastereoselective Cyclopropanation. Enantioselective Synthesis of Vinylcyclopropanes via Chiral Telluronium Ylides

Novel chiral telluronium salts 1 are designed for asymmetric synthesis of 1,3-disubstituted 2-vinylcyclopropanes. The allylides, generated in situ from the corresponding telluronium salt in the presence of different base, reacted with alpha,beta-unsaturated esters, ketones, and amides to afford cis-2-silylvinyl-trans-3-substituted or trans-2-silylvinyl-trans-3-substituted cyclopropane derivatives with high diastereoselectivity and excellent enantioselectivity in good to high yields. Thus, either one of the two diastereomers could be enantioselectively synthesized at will just by the choice of LiTMP/HMPA or LDA/LiBr. The first examples of catalytic ylide reaction for enantioselective synthesis of 1,3-disubstituted 2-vinylcyclopropanes with high distereoselectivity is also achieved.

Unexpected catalyst for Wittig-type and dehalogenation reactions

A novel catalyst 2 for Wittig-type and dehalogenation reactions was developed. In the presence of triphenyl phosphite, a wide variety of aldehydes could react with alpha-bromoacetates to afford alpha,beta-unsaturated esters or ketones in high yields with excellent E-stereoselectivity when 1-2 mol % of compound 2 was used. Compound 2 was also an effective catalyst for reductive dehalogenation of alpha-bromocarbonyl compounds. The mechanisms for the above reactions were also proposed.

Wittig-type olefination catalyzed by PEG-telluride

Soluble poly(ethylene glycol) (PEG)-supported telluride 2 was designed and synthesized for catalytic Wittig-type reactions. It was found that the catalytic loading could be reduced from 20 to 2 mol % by the introduction of PEG (even to 0.5 mol % when some telluride salts were used as the catalyst). Under the catalytic reaction conditions, a wide variety of aldehydes with different structures could react with bromoacetate to afford beta-substituted or alpha,beta-disubstituted unsaturated esters in high yields with excellent E-stereoselectivity. The modified process, by using sodium bisulfite instead of triphenyl phosphite, represented a very simple product isolation procedure. The roles of PEG for promoting the ylide formation and stabilizing the catalytic species were disclosed. The mechanism was also studied.

A novel chiral sulfonium yilde: highly enantioselective synthesis of vinylcyclopropanes

A newly designed chiral sulfonium allylide, generated in situ from the corresponding sulfonium salt in the presence of KOBu(t), reacted with alpha,beta-unsaturated esters, ketones, amides, and nitriles to afford trans-2-silylvinyl-trans-3-substituted cyclopropyl esters, ketones, amides, and nitriles with outstanding diastereoselectivity and excellent enantioselectivity in good to high yields. A mechanistic rationale is proposed.