Carbenoids rearrangements

Here again, all these reactions have been widely reviewed and just general trends will be discussed here, with a special focus on the mechanisms. These mechanisms can involve a carbenoid rearrangement or a free carbene rearrangement. The question of the carbenoid or carbene intermediacy has been largely addressed and, in most cases. 

Thus, a-sulfinyl lithium carbanion of 1-chloroethyl p-tolyl sulfoxide was reacted with 1,4-cyclohexanedione mono ethylene ketal (195) to afford the adduct (196) in quantitative yield. The adduct was treated with ferf-butylmagnesium chloride (magnesium alkoxide was initially formed) followed by isopropylmagnesium chloride to result in the formation of magnesium /3-oxido carbenoid 197. The /3-oxido carbenoid rearrangement then takes place to give one-carbon expanded magnesium enolate 198. Finally, an electrophile was... 

The /3-oxido carbenoid rearrangement then took place to give one-carbon expanded magnesium enolate having a chlorine atom (217), which was treated with water to afford a-chloroketone (218) in moderate yield. Unfortunately, this method could not be applied to larger cycloalkanones and acyclic ketones. Application of this method to aldehydes gave chloromethyl aryl ketones and chloromethyl alkyl ketones in moderate yields. ... 

Aldehyde -> Alkyne Elongation via Carbene and Carbenoid Rearrangements... 

A one-pot synthesis and derivatization of diynes and triynes is reported. The polyyne framework was formed from a dibromoolefm precursor 96 based on a carbenoid rearrangement, and the resulting Li-acetylide is then transmetallated with zinc chloride which then allowed for the divergent preparation of aryl polyynes 97 via Negishi palladium-catalyzed cross-coupling reactions. 

A one-carbcMi ring enlargement approach for the synthesis of 8-lactones has been developed by Satoh and Kurihara [123] (Scheme 76). Reaction of the lithium carbanion of chloromethyl phenyl sulfoxide with lactone 336 afforded a diastereo-meric mixture of the hemiacetal 337. Treatment of this adduct with three equivalents of KH in THF generated potassium enolate 338, which on addition of four equivalents of t-BuLi underwent an alkylidene carbenoid rearrangement to give alkynolate 339. Protonation of 339 by the addition of sulfuric acid facilitated the formation of m-hydroxy ketene 340, which underwent an intramolecular cyclization to afford 8-lactone 341. 

The various outcomes of the rearrangement of dihalotriangulanes promoted by methyl-lithium have been mechanistically discussed, showing the very unique nature of these carbenoid rearrangements. ... 

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