Bismuthonium ylides structure

A stable bismuthonium ylide is formed by the reaction between triphenyl-bismuth carbonate and dimedone [85JCS(P1)2667]. The cyclic a,a -dioxo structure is likely to be the main origin of high stability. 

The first bismuthonium ylide reported by Lloyd is a thermally stable colored substance, but the literature lacks details of its characterization. Several bismuthonium ylides containing a cyclic a,a -dicarbonyl or a,a -disulfonyl framework have been isolated as stable crystalline solids, and 4,4-dimethyl-2,6-dioxo-l-triphenylbismuthoniocyclohexane has been characterized structurally by X-ray crystallographic analysis, where the bismuth atom possesses a distorted tetrahedral geometry and interacts with one of the carbonyl oxygen atoms [90JCS(P1)3367]. The Bi-Cyude bond (2.156 A) is a bit shorter than the Bi-Cph bond (2.21-2.22 A), suggesting little or no double bond nature of the bismuth and ylidic carbon bond (Fig. 3.5). The ylidic carbon of this class of stabilized ylide appears at 8 100-113. 

The carbenes or carbenoids can be generated in a variety of ways. It is not always clear whether the reaction is concerted or stepwise and whether the carbene behaves as an electrophilic, nucleophilic or radical species. For instance, a carbenoid generated from bismuthonium ylide (19) in the presence of copper(I) chloride would behave as a triplet and add as a radical to a terminal alkyne" (equation 17). The dicarbonyl structure and the absence of reaction with methyl propionate to a furane might well indicate electrophilic character of this carbene. 

The bismuthonium acyl- and sulphonyl-ylides have a completely different appearance and electronic spectra, all of them being yellowish in colour. This is because in their cases, as in those of similar arsonium and stibonium ylides, interaction between an oxygen atom and the bismuth atom is possible, leading to structures such as 48 in accord with this... 

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