Carbonyl ylides open-chain

Open-chain Carbonyl Ylide-Alkene Cyclizations 1161... 

Any attempt to rationalize the complex stereochemistry of olefin formation will have to be concerned with the mechanism of the Wittg reaction. Unfortunately many mechanistic details of this reaction have not been satisfactorily elucidated. Thus, it is even uncertain whether the reaction system (ylide plus carbonyl compound) passes, on its way to phosphine oxide and olefin, through an open chain zwitterion, i.e. a betaine, through a cyclic oxaphosphetane, or through both of them consecutively. The reaction sequence depicted in Figure I has no other merit than that of being believed to be the most probable one (1). 

S.2.2.6. Tetrahydrofurans from Carbonyl Ylides. Carbonyl ylides are unusual species that must be generated as needed. One method is to break the C-C bond of certain epoxides thermally or photochemically, which yields an open chain. Thereby one carbon is positively charged and the other is negatively charged, which is the structural feature of carbonyl ylides. This is shown in Scheme 5.33 A. Cycloaddition with an alkene follows immediately to give tetrahydrofuran derivatives (Scheme 5.33). 

The first X-ray structure of a thiabenzene derivative, i.e. l-benzoyl-2-methyl-2-thianaphthalene, has been reported. The geometry is similar to that of open-chain benzoyl-stabilized ylides with pyramidal sulphur. Other studies point to the ylidic character of 9,10-diphenyl-10-thia-anthracene . N.m.r. data, including C n.m.r. chemical shifts, have been presented for indole-3-sulphonium ylides and carbonyl-stabilized sulphonium ylides and their Pd" complexes. Infrared spectra of cr-nitrosulphonium ylides have been analysed. Further details have been provided of a study of the pyramidal inversion of unstabilized sulphonium ylides (see Chap. 5, pp. 243 and 251). 

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