Phosphonium ylides conformation

FIGURE 3. Possible conformations of monophosphino-substituted phosphonium ylides. The preferred conformations is syn... 

Fig. 11.1. Parent compound of phosphonium ylides (A) and a typical phosphonium ylide (B). Below the Ph3P -CH2 conformer, which is capable of maximum anomeric stabilization, and the corresponding MO diagram.

The spectra of highly coloured phosphoniovinamidinium salts (68 Y or Z = PhsP+) have been recorded. The energy levels and conformations of an extensive series of phosphonium ylides have been deduced from their u.v. spectra. In addition there have been studies of phosphonium polyiodides, phosphinyl styrenes, and solvent effects on the spectra of thioamides (69). ... 

The preferred conformations of ketodiester phosphonium ylides were investigated. Acyclic diphosphonium bis-ylides from the corresponding diphosphonium precursors were systematically analysed with respect to the steric demand at the phosphorus centers and the substitution pattern of the phenylene bridge. In the o-phenylene series, bis-ylides were not observed due to the instability of the mono-ylide intermediates. In contrast, in the m-phenylene series, in which the two phosphonium centers were in remote positions, the bis-ylide was fully characterized. ... 

Phosphorus ylides have been succinctly described23 as an easily pyramidalized car-banion stabilized by an adjacent tetrahedral phosphonium centre. They usually adopt the trans-bent conformation (Figure 2) but the barrier to rotation about P=C is very low (4-5 kJ mol 1). 

Macrocycllc Thiophens, - Wittig reaction between 1,3,5-benzene-tricarbaldehyde and the ylide prepared from the bis-phosphonium salts of 2,5-bis(chloromethyl)thiophen gave (193) in about 1.5% yield. The conformation was analyzed by n.m.r. ... 

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