Benzoin stable carbenes

Dichloroindium hydride (Cl2InH), generated by the reaction of InCl3 with tributyltin hydride, is also successfully used for the reduction of carbonyl compounds and for the debromination of alkyl bromides.366 This reductant has features such as the chemoselective reduction of functionalized benzaldehydes, chelation-controlled reduction of benzoin methyl ether, and 1,4-reduction of chalcone. The stable carbene and tertiary phosphine adducts of indium trihydride, InH3 CN(Mes)CH=CHN(Mes) and InH3 P(c-G6H11)3, reduce ketones to alcohols (Equation (90)).367... 

Teles JH, Melder JP, Ebel K, Schneider R, Gehrer E, Harder W, Brode S, Enders D, Breuer K, Raabe G (1996) The chemistry of stable carbenes. Part 2. Benzoin-type condensations of fromaldehyde catalyzed by stable carbenes. Helv Chim Acta 79 61-83... 

The synthesis of y-lactams has been achieved under similar reaction conditions (Table 18) [124]. Initially, Bode and co-workers screened a variety of acyl imines in order to find suitable electrophiles. Control experiments provided evidence for carbene addition to the acyl imine, yielding a stable complex with complete inhibition of the desired reactivity. Reversibility of this addition was key to the success of the reaction. A -4-Methoxybenzenesulfonyl imines 212 proved to be the most efficient partners for lactamization with cinnamaldehydes 228 to provide y-lactams 229 in moderate yields and good diastereoselectivities. Notably, no benzoin or S tetter products or their corresponding derivatives were observed during this reaction. 

In benzoin condensations using benzaldehyde as starting material, the initial adduct, i.e. the C2a-hydroxybenzylthiazolium (HBT) compound or its alkoxide form, is readily tautomeiized to the enol, or enolate, that then condenses with a second benzaldehyde molecule. In a series of reports it was suggested that this is the result of carbene insertion behavior of the ylide " , rather than the result of nucleophilic addition. In aprotic media several mechanistic probes tended to disprove this thesis The need for a protic solvent to complete the condensation reaction (since compounds such as HBT are rather stable in DMSO ) prompted the suggestion of the intermediacy of the charged species... 

Finally, other important contributions that should be cited as very relevant with regard to the evolution of the organocatalysis field are those related to the use of stable A-heterocyclic carbenes as catalysts. The basis of this chemistry also finds its roots at the very early beginnings of organic synthesis, in particular in the well-known cyanide-catalyzed benzoin condensation by Liebig and Wholer, and... 

The benzoin condensation has been examined computationally as a function of the structure of three carbene organocatalysts, that is, imidazol-2-, triazol-3-, and thiazol-2-ylidenes. DFT studies have featured three aldehydes and the historical mechanism of Breslow appeared to be consistent with the calculated reaction energy profiles whatever the aldehydexarbene combination was. It is noteworthy that the Breslow intermediate (118) arising from thiazol-2-ylidene proved to be much more stable than (119)/(120)... 

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