Carbonyl ylide, metal carbene synthesis

The carbonyl ylide generated from metal carbene can also add to C=0 or C=N bonds. The [2 + 3]-cycloaddition of carbonyl ylide with G=0 bond has been used by Hodgson and co-workers in their study toward the synthesis of zaragozic acid as shown in Scheme n 27a,27d Recently, a three-component reaction approach to syn-a-hydroxy-f3-amino ester based on the trapping of the carbonyl ylide by imine has been reported.The reaction of carbonyl ylide with aldehyde or ketone generally gives l,3-dioxolanes. Hu and co-workers have reported a remarkable chemoselective Rh2(OAc)4-catalyzed reaction of phenyl diazoacetate with a mixture of electron-rich and electron-deficient aryl aldehydes. The Rh(ii) carbene intermediate reacts selectively with electron-rich aldehyde 95 to give a carbonyl ylide, which was chemospecifically trapped by the electron-deficient aldehyde 96 to afford 1,3-dioxolane in a one-pot reaction (Equation (12)). 

A few years later, Tebbe and co-workers found that the methylene-bridged metallacycle 3, which has become known as the Tebbe reagent, is useful for the methylenation of ketones and aldehydes [5]. Titanocene-methylidene 4, the active species of this olefination, also transforms carboxylic acid derivatives into heteroatom-substituted olefins. Because the carbene complex 4 is much less basic than conventional olefination reagents such as phosphorus ylides, it can be employed for the olefination of carbonyl compounds possessing highly acidic a-protons or of highly hindered ketones, and has become an indispensable tool in organic synthesis. Various methods for the preparation of titaniumcarbonyl olefination. This chapter focuses on the use of metal-carbene complexes and some related species in carbonyl olefination (Scheme 4.2). 

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