Aldehydes carbenes, reversible

Breslow and co-workers elucidated the currently accepted mechanism of the benzoin reaction in 1958 using thiamin 8. The mechanism is closely related to Lapworth s mechanism for cyanide anion catalyzed benzoin reaction (Scheme 2) [28, 29], The carbene, formed in situ by deprotonation of the corresponding thiazolium salt, undergoes nucleophilic addition to the aldehyde. A subsequent proton transfer generates a nucleophilic acyl anion equivalent known as the Breslow intermediate IX. Subsequent attack of the acyl anion equivalent into another molecule of aldehyde generates a new carbon - carbon bond XI. A proton transfer forms tetrahedral intermediate XII, allowing for collapse to produce the a-hydroxy ketone accompanied by liberation of the active catalyst. As with the cyanide catalyzed benzoin reaction, the thiazolylidene catalyzed benzoin reaction is reversible [30]. 

The acyl cation 2a or acylium ion 2b is a familiar intermediate in the Friedel-Crafts reaction. It is easy to make (acid chloride + Lewis acid 1) and it can be observed by NMR as it expresses the natural reactivity pattern of the acyl group. The acyl anion by contrast has umpolung or reverse polarity.1 One might imagine making it from an aldehyde by deprotonation 3 and that it would be trigonal 4a or possibly an oxy-carbene 4b. Such species are (probably) unknown and their rarity as well as their potential in synthesis has led to many synthetic equivalents for this elusive synthon. The acyl anion, the d1 synthon, is the parent of all synthons with umpolung2 and should perhaps have been treated before the homoenolates dealt with in the previous chapter. 

The cis-1,2-addition of M-X bonds to unsaturated A=B bonds and its reverse, the -elimination of X from M-B-A-X, are fundamental elementary steps of catalytic reactions such as hydrogenation, hydroformylation, oligomerization, polymerization, hydrosilation, hydrocyanation, or alkene isomerization processes, as well as the Heck reaction. Most of the reactions described in the literature involve M-H or M-C bonds, and alkenes or alkynes. Besides them there are processes where the unsaturated substrate is different from alkene or alkyne This includes CO2, CS2, aldehydes and ketones, imine, or nitrile. Also, there are processes involving M-Si, M-Sn, M-B, M-N, M-P, or M-M bonds. The insertion of alkenes into M-carbene bonds is not essentially different in their intimate mechanism, but it is not discussed in this chapter. 

The carbenes Zr[C(H)CH2R]PPh3Cp2 react with aldehydes giving preferential 1 alkenation but imines give reverse selectivity. 

In a series of publications beginning in 1973, Hermann Stetter and coworkers reported that activated olefins could intercept the putative acylanion intermediate of the benzoin reaction. Typical catalysts for the benzoin reaction, sodium cyanide and thiazolylidine carbenes, were found to perform well in this new reaction. Stetter also established that the success of the reaction is due to the reversible nature of the benzoin condensation relative to the irreversible formation of 1,4-dicarbonyl products. As a consequence, benzoins or aldehydes can be used interchangeably as reactants. The reaction has proven to be a highly efficient method for the synthesis of 1,4-dicarbonyl compounds and 4-oxonitriles. A resurgence of interest in acyl anion chemistry has resulted in many new discoveries, including alternative acyl donors, as well as catalysts capable of highly enantioselective intra- and intermolecular Stetter reactions. ... 

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