Carbenes, acyl, rearrangement

It has been proposed that these carbyne-acyl rearrangements start with the protonation of the car-byne carbon atom to initially give a carbene complex. For the next step two different pathways have been proposed i) An intramolecular carbene carbonyl coupling to yield a 7C-ketene complex,... 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Ketenes react with tertiary allylic amines in the presence of Lewis acids to give zwitterionic intermediates which undergo [3,3]-sigmatropic rearrangement [119]. Photolysis of chromium carbene complexes in the presence of tertiary amines results in similar chemistry [120]. Cyclic (Table 21) and strained allylic amines (Eq. 34) work best, while acylic amines are less reactive (Eq. 35). 

The main synthetic application of the Wolff rearrangement is for the one-carbon homologation of carboxylic acids.242 In this procedure, a diazomethyl ketone is synthesized from an acyl chloride. The rearrangement is then carried out in a nucleophilic solvent that traps the ketene to form a carboxylic acid (in water) or an ester (in alcohols). Silver oxide is often used as a catalyst, since it seems to promote the rearrangement over carbene formation.243... 

An jV-heterocyclic carbene 110 catalyzed the rearrangement of the 0-acyl carbonates 109 into their corresponding C-acylated isomers 111, generating a C-C bond and a quaternary stereocenter with high efficiency <06OL3785> The same reaction can be performed enantioselectively using TADMAP 112 <06JA925>. 

For this reason unstable cyclopropanes or only rearrangement products are obtained when donor-substituted alkenes react with acceptor-substituted carbene complexes [1409-1416]. In reactions of acyl- and vinylcarbene complexes with enol ethers the most common types of rearrangement observed are those shown in Figure 4.23. 

In this context, it is worth mentioning that there is only one other, clear-cut example for the simultaneous occurrence of the acyl(silyl)carbene-to-acylsilene and the acylcarbene-to-silylketene rearrangement of an acylcarbene bearing a Si—Si substituent. Carbene 57, generated by photolysis of diazoketone 56 in benzene, isomerized to both 58 and 59 in about equal amounts44. While the acylsilene cyclized to 1,2-silaoxetene 60, the ketene was isolated and structurally characterized by X-ray diffraction analysis of the derived... 

Acyl silanes have been obtained by the pyrolytic rearrangement of silyloxy carbenes derived from a-keto silyl esters (Scheme 19)91. The pyrolysis takes place in high yield for aroyl silanes, but is less effective for other substrates. 

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