Alkenes From carbonyl groups using Wittig

Alkylidenephosphoranes (a.La. phosphorus yUdes) of the general formula Ph3P=CR R (1) have been frequently used in key steps of heterocycle synthesis. Numerous papers and review articles [1-4] testify their versatility in the construction of rings with sizes ranging from three to well beyond 20 and with virtually any number, kind and distribution of heteroatoms. The Wittig alkenation of carbonyl groups is doubtless the most common, though not the only, reaction of P-ylides that has been employed in the cyclization of bifunctional precursors. The cycloaddition between acyl ylides (1 = H,... 

In any chain reaction, apart from initiation steps, the termination steps are also important. In metathesis there are many possibilities for termination reactions. Besides the reverse of the initiation step, the reaction between two carbene species is also a possibility (eq. (17)). The observation that, when using the Me4SnAVCl6 system, as well as methane traces of ethylene are also observed [26] is in agreement with this reaction. Further reactions which lead to loss of catalytic activity are (1) the destruction of the metallacyclobutane intermediate resulting in the formation of cyclopropanes or alkenes, and (2) the reaction of the metallacycle or metal carbene with impurities in the system or with the functional group in the case of a functionally substituted alkene (e. g., Wittig-type reactions of the metal carbene with carbonyl groups). 

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