Carbon shift reactions, ring expansions

Cu(hfacac)2-catalyzed elimination of N2 from aroyl(a-diazoacyl)hydra-zines 653 yields 2//-l,3,4-oxadiazin-2-ones 654 (70%). The reaction occurs consecutively via intramolecular carbonyl ylide formation, 1,3-cyclization into oxirane 655, and ring opening by the carbon-oxygen bond followed by a C- 0 shift with ring expansion (88CB887). 

Silacyclobutanes as well as silacyclopropanes undergo aldehyde insertion under catalysis by /-BuOK (Equation (77)).292 The reaction of silacyclobutanes with lithium carbenoids such as dihalomethyllithium and oxiranyllithium gives 2-substituted silacyclopentanes (Equation (78)). Treatment of l-(l-iodoalkyl)- and 1-oxiranyl-silacyclobutanes with a stoichiometric amount of an alkali alkoxide leads to silacyclopentanes by anionic 1,2-shift of the ring carbon adjacent to silicon. These ring-expansion reactions proceed probably via a pentacoordinate silane intermediate. 

A ring expansion by two carbon atoms was discovered on heating (Z)-l-vinyl-cyclonon-3-en-l-yl-trimethylsilyl ether (V/62), Scheme V/10 [31] [32], Two types of trimethylsiloxy enol ethers were observed, each formed by a [1.3] or a [3.3] sigmatropic shift which, after hydrolysis, gave the ketones V/63, V/64, and V/65. The protection of the tertiary alcohol function is not necessary, if the reaction is done in the presence of potassium hydride [33] [34]. The reaction proceeds when the cyclic 1-vinyl alcohols have either a double bond or a benzo group at the 3-position. Other examples of this reaction type are given in Scheme V/10. 

Whilst reaction of acyl chloride 303 with Na[Co(CO)3(PEt3)] affords the oxocyclobut-enyl complex 306" by ring expansion and CO loss, analogous treatment with NaRe(CO)5 delivers the non-fluxional // -cyclopropenylrhenium compound 305" . In the latter case, compound 304 loses carbon monoxide with concomitant migration of the cyclopropenyl moiety from carbonyl to rhenium as an allylic rearrangement rather than a 1,2-shift. 

A modern method of expanding medium and large rings by two carbon atoms uses flash vacuum pyrolysis (FVP) of aUyl alcohols. Under the reaction conditions, only traces of the expected [l,5]-hydrogen migration are observed the main product arises presumably from a [l,3]-carbon shift, a process which involves a diradical intermediate. This ring-expansion sequence may also be repeated successfully. [181]... 

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