Four-membered carbocycles

The past 30 years have seen the development of a broad spectrum of widely applicable preparative methods for three- and four-membered carbocycles and the accumulation of detailed knowledge about their structure-reactivity relationships. Nowadays, more and more synthetic methodology is being developed, which utilizes the potential of small ring compounds as reactive entities. Cyclopropyl and cyclobutyl moieties in a molecule can be regarded as unique functional groups they allow transformations which are far more difficult or impossible with any of the more conventional functional groups. 

Photochemical reactions of quinones with allenes have also been studied and in some cases cyclobutane formation occurs, although in competition with products derived from attack of the allene on the carbonyl oxygen. Thus, photocycloaddition of tetramethyl-l,4-benzoquinone with 1,1-dimethylallene affords the four-membered carbocycle 6 in good yield.12... 

Four-membered carbocyclic ring systems are commonly formed by cycloaddition of electrophilic alkenes, ketenes and arynes to enamines. Since cycloaddition reactions of enamines are dealt with in Chapter 18 these reactions will only be mentioned briefly here. 

The procedure described here illustrates a practical and convenient method for the generation of thio-substituted ketenes which participate in surprisingly facile cycloadditions with both activated and unactivated alkenes, alkynes, and imines to form four-membered carbocycles and heterocycles. As... 

As illustrated below, desulfurization of a-thiocyclobutanone cycloadducts is easily achieved in high yield upon exposure to either tributyltin hydride or activated zinc dust, and the overall sequence thus represents a useful new alternative to the existing dichloroketene-based methodology for the synthesis of a variety of four-membered carbocycles and heterocycles . 

The results of these model studies have shown that transition metal catalyzed [2+21-cycloadditions of methylenecyclopropanes are — at least at the moment — not very useful for the controlled formation of four-membered carbocycles. 

The available C— H coupling data for four-membered carbocyclic ring systems have been summarised by Fleming and Williams. The values of coupling constants can provide a qualitative... 

Four-membered carbocycles Compounds with sufficiently electron-rich olefinic double bonds react with TCNE to yield tetracyanocyclobutyl derivatives by [2 + 2] cycloadditions. Thus 1-vinylpyrazoles 369" as well as 4-vinyl-pyrazoles 371" react to yield the corresponding substituted tetracyanocyclobutanes 370 and 372, respectively. The same reaction has been observed with 1-vinylindazoles". ... 

All these Ni(0) catalyzed [2- -2]-cycloadditions proceed through nickelacyclopentane derivatives as intermediates. This has been supported by the isolation of model complexes and the study of their reactivity The results of these model studies have shown that transition metal catalyzed [2 -)- 2]-cycloadditions of methylenecyclopropanes are — at least at the moment — not very useful for the controlled formation of four-membered carbocycles. 

P-carbon elimination which is facilitated by the release of the ring strain of a four-membered carbocycle via C—C a-bond cleavage, yielding an alkyl-Rh species. The key aryl-Rh intermediate was next delivered by a [1,4]-Rh migration. " Finally, the insertion of the C—Rh bond into the ketone moiety and protonation of the Rh—O bond furnishes the desired substituted indanol products. 

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