Ylides stibonium—

It is, therefore, to be expected that a compound such as trimethyl-methylene stiborane, (CHs SbCtE, should be relatively unstable and attempts to synthesize this species would have to be carried out at low temperature. Another difficulty arises from the experimental fact (104) that all conventional methods of synthesis for stibonium ylides lead to pentaalkylstiboranes instead of ylides (see Introduction). Thus mcthyl-ation of (CH3)4SbX or (CHY) sHbX2 halides by organometallic reagents of lithium, magnesium, aluminum, or zinc invariably yield (CHY) 5Sb as the sole product. 

This section is not complete without mentioning the hydrogen-deuterium exchange experiment by Doehring and Hoffmann (12), which indicated an enhanced acidity of the hydrogens in a tetramethylstibonium salt. In the corresponding equilibrium, stibonium ylides play the role of strong bases ... 

The first preparation of a stibonium ylide was probably achieved in 1953. When dibenzyldimethylstibonium bromide was treated with phenyllithium, the product obtained was dimethyl(l,2-diphenylethyl)stibine which could arise from a Stephens rearrangement of the expected ylide (see equation 45) addition of the phenyllithium first of all afforded a yellow colour which rapidly disappeared. 

The first stibonium ylide to be isolated was triphenylstibonium tetraphenylcyclopen-tadienylide, obtained by thermal decomposition of diazotetraphenylcyclopentadiene in the presence of triphenylstibine. A problem which arose initially in the preparation of stibonium ylides from thermal decomposition reactions of diazo compounds was that the... 

The preparation of a stibonium ylide in good yield from indanetrione has been reported (equation 46). ... 

An interesting property of the stibonium ylides with two electron-withdrawing substituents is that they very readily gain static and adhere to glass surfaces and to spatulae ... 

Stibonium ylides bearing two electron-withdrawing substituents did not undergo Wittig reactions ... 

It has also proved possible to make alkenes in a one-pot process from diazo compounds N2CR2, where R is an electron-withdrawing group, and aldehydes or ketones by heating them in the presence of tributylstibine no base was required Reaction was assumed to proceed through a stibonium ylide as intermediate, although it proved impossible to isolate this ylide, which probably reacted further too rapidly to allow this. 

When the stibonium ylides (39) became available, they provided a more complicated picture In their IR spectra the frequencies of the CO and SO2 groups become steadily lower on going from phosphonium to arsonium to stibonium ylides, consistent with an expected increase in polarity of ylides on going down the Periodic Table. 

However, the electronic spectra of the stibonium ylides were very similar to those of their arsonium analogues, both in solution and in the solid state, and their dipole moments were almost the same, indicating an overall similarity in the electronic structure of the two... 

The one physical feature which shows greater polarity of the stibonium ylides, namely their IR spectra, does so just because it is concerned with the CO or SO groups. 

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... 

In parallel with this is the ease with which the ylides are proton ated to form heteronium salts. Thus again taking the ylides (XII) as examples, the sulphonium and selenonium ylides do not form salts readily, the phosphonium, arsonium and stibonium ylides are readily protonated with increasing basicity Ptelluronium ylides are destroyed by mineral acids. [149,156]. Protonation may take place at different sites in the molecule depending partly on.the substitution pattern, and the site appears to depend on a balance of electronic and steric factors. [126]. Thus dimethylsulphonium cyclopentadienylide in trifluoroacetic acid is protonated at the 1-, 2- and 3-positions in the ratios of 11 56 33 [157]. 

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