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DOI: 10.1055/sos-SD-027-00847

Schobert, R.; Gordon, G. J.Science of Synthesis, (2004271043.

In the following account the chief methods for the preparation of phosphacumulene ylides are described. Two strategies have been successfully applied; either the chain elongation of phosphonium methylides by the addition of a suitable C1-building block, followed by an elimination step to generate the Cα=Cβ bond, or the direct use of C2-ylides (e.g., ester or acyl ylides), in which the Cα=Cβ bond can be established either by elimination reactions or by O-alkylation of the acyl group with Meerwein salts. Other methods, which are now obsolete due to a lack of generality and applicability to large scale syntheses, are: (1) The pyrolysis of betaines obtained from the reaction of carbodiphosphoranes (Ph3P=C=PPh3) with carbon dioxide, carbon disulfide, or isothiocyanates, respectively, giving phosphacumulene ylides 137 (X=O),[‌562‌] (X=S),[‌562‌] or (X=NR1)[‌53‌] as appropriate. (2) The reaction of allenyl bromides with triphenylphosphine and subsequent deprotonation of the resulting salt leading to ylides 138 or 139.[‌560‌] (3) The deprotonation of 2-haloallyl phosphonium salts yielding ylides 139.[‌563‌] (4) The interconversion of cumulated ylides [e.g., 137 (X=O to X=S) by the action of carbon disulfide].[‌564‌]