Stabilized telluronium ylides

3 From telluronium ylides 4.7.3.1 Stabilized telluronium ylides 

Stabilized telluronium ylides such as dibutyltelluronium carbethoxy, phenacyl/ cyano- and carbamoylmethylide (easily prepared by the reaction of dibutyl teUurides with the appropriate substituted methyl hahdes, followed by treatment with a base), undergo Wittig-type olefmation reactions with a variety of carbonyl compounds, giving the expected olefins in satisfactory yields (method A). This behaviour is in sharp contrast to that of stabilized sulphonium yhdes, which are inert towards carbonyl compounds. 

C02Et Br KOf-Bu/THF/-20°C aliphatic and aromatic aldehydes aliphatic, cycloaliphatic and aromatic ketone, a,p-unsaturated aldehydes and ketones 52-90 

CONHBu-/ Br NaH/THF/HMPA/-50°C,r.t aliphatic and cycloaliphatic aldehydes a,p-unsaturated aldehydes 40-71 

Noteworthy features of these reactions are the high predominance of E stereochemistry in the case of aldehydes and the good results obtained even with highly enolizable ketones (such as cyclopentanone), a, -epoxy ketones (isophorone oxide) and a,j8-unsaturated compounds (benzalacetophenone and cinnamaldehyde). 

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Reaction of stabilized telluronium ylide with aldehydes. Method A - using telluronium ylide (typical procedure).A solution of carbethoxymethyldibutyltelluronium bromide (1.23 g, 3 mmol) in dry THF (3 mL) is added dropwise to a solution of KOf-Bu (0.337 g, 3 mmol) in THF (4 mL) at -20°C. After a few minutes, fran -cinnamaldehyde (0.264 g, 2 mmol) is added over 1 min. The reaction mixture is then stirred for 1 h at -20°C. After the usual work-up, the product is purified by Si02 column chromatography, to give ethyl 5-phenylpenta-(2 , 4 )-dienoate (0.291 g (72%)). GC and NMR of the product reveals a purity of 98%. 

In sharp contrast to the olefmation reaction employing stabilized telluronium ylides, semi-and non-stabilized ylides react with carbonyl compounds to give epoxides (like non-stabilized sulphonium and selenonium ylides). 

The semi-stabilized telluronium ylides, generated in situ from the corresponding telluronium salts (156 R1 = CH=CHSiMe3, CH=CH2, CH=CHMe, CH=CHPh, Ph), have been reported to react with a,/(-unsaturated carbonyl compounds (157 R2 = Ph, OR, NR)) to afford 2-vmylcyclopropyl derivatives (158) with high... 

This was the first-described, non-stabilized ylide, obtained by treatment of the corresponding telluronium tetrafluoroborate with hthium 2,2,6,6-tetramethylpiperidide (LiTMP). Epoxides are obtained by reaction with both aldehydes and ketones. ... 

Dibutyltelluronium benzyhde, generated by treatment of the telluronium salt with potassium /-butoxide, behaves similarly to the above-stabilized ylides, undergoing Wittig-type olefinations with aromatic aldehydes. ... 

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