Buchner rearrangement

Treatment of Grubbs second-generation metathesis catalyst with carbon monoxide results in a Buchner rearrangement in which the alkylidene inserted into one of the aryl substituents of the NHC (Scheme 3.18)."" It was proposed that CO, being a strong 71-acceptor, decreases the electron density available for back donation and stabilization of the alkylidene ligand. The alkylidene thus becomes more electrophilic, and is attacked by the adjacent aryl ring. 

Scheme 6.9 Four pericyclic reaction steps proposed for the formation of tricyclic heterocycles, including a dipolar cycloaddition, a Buchner reaction sequence, and a final sigmatropic rearrangement of a C—O bond.

The intramolecular Buchner reaction of aryl diazoketones has been carried out using both copper(I) and rhodium(II) catalysts. For example, 1-diazo-4-phenylbutan-2-one 27a cyclizes in bromobenzene with copper(I) chloride catalysis, furnishing 3,4-dihydroazulen-l(2//)-one 30 in 50% yield after purification by chromatography over alumina. Trienone 30 is not the primary cyclization product, and the less conjugated isomeric trienone 29a is first produced, but contact with alumina causes isomerization to 30. The yield of this cyclization is further improved when rhodium(II) acetate is used as the catalyst instead of copper(I) chloride. Thus a catalytic amount of rhodium(II) acetate brings about the nearly quantitative conversion of 27a to 29a within minutes in hot dichloromethane. Compound 29a isomerizes to 30 on treatment with triethylamine, and rearranges to 2-tetralone 31a when exposed to silica gel or acid. 

The effect of ortho- and weto-substitution in the above-mentioned intramolecular Buchner reactions has been examined. When the 2-methoxy-substituted diazoketone 32 is subjected to rhodium(II) acetate catalysis, a single cycloheptatrienone 34 is obtained in 94% yield.This result is consistent with the outcome of the rhodium(II) trifluoroacetate-catalyzed intermolecular reaction of ethyl diazoacetate with anisole, which yields no product arising from addition of the ketocarbenoid on the most hindered site of the anisole. Dihydroazulenone 34 rearranges to tetralone 36 under acidic conditions, and isomerizes to the conjugated ketone 35 under basic conditions. It is interesting that the catalyzed decomposition of the para-methoxy derivative 37 provides exclusively 6-methoxy-2-tetralone 40 with no trace of the putative trienone 39. ... 

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