Ketenes, carbenes from dimerization

At the present, the most straightforward mechanism for the formation of J5 from 1 is via insertion of CO into the Th-C(acyl) bond to form a ketene (H, (eq. (4)) which subsequently dimerizes. Presumably, initial CO interaction could involve coordination either to the metal ion as shown or to the electrophilic vacant "carbene p atomic orbital. Considering the affinity of the Th(IV) ion for oxygenated ligands, interaction of the ketene oxygen atom with the metal ion seems reason-... 

Ketenes are useful sources of carbenes in the gas phase but are rarely applied in solution, owing to their tendency to dimerize and to accept nucleophiles. However, the sterically encumbered ketene 36, derived from cyclogeranium... 

The metal-free eyclobutane-1,2-dioxime can be generated by oxidative displacement. It is interesting to note that, unlike ketene dimerization, head-to-head dimerization takes place here. The chromium ketenimine complex 20 is prepared by reaction of the Fischer-type chromium carbene complex with alkyl isocyanides.60 A cyclobutane-1,2,3,4-tetraimine 24 has been reported from the reaction of the ketenimine phosphonium ylide 22.61 Bisimine 23 has been proposed as the intermediate in this transformation. 

In light of the above results it is interesting to note that the reaction of diphenylcyclo-propenone dimer spirolactone with ironenneacarbonyl yields a mixture of ring-opened vinyl carbene and -vinylketene complexes, and these interconvert under addition (or removal) of CO (equation 225) . A possible pathwav to vinylketene Fe-complexes, prepared earlier from cyclopropenes and ironcarbonyls " , may thus involve initial f -coordination, followed by ring cleavage to vinyl carbene and finally carbonylation to the ketene iron // -complexes. An analogous // -manganese complex is prepared similarly by the reaction of CpMn(CO),THF with 3,3-dimethylcyclopropene complex (equation 226) . ... 

A non-Kekule molecule, a /n-xylylene (88), has been generated by the Bamford-Stevens reaction of a fulvene-ketene adduct (85), via the homofulvene (87) from the intermediate carbene (86). This unstable intermediate readily dimerizes to octamethyl[2.2]metacyclophane (89 Scheme 9). ... 

Insertion of carbenoid species into the azetidinone N-H bond is the most straightforward route to 2-oxocarbapenams, and thence to the vast majority of carbapenem antibiotics [186]. Unfortunately, synthesis of the 2-oxopenam 281a (R" = Me) from diazomalonic azetidinyl thioester 323 was abortive [172,187]. The prescribed carbene was generated (rhodium acetate dimer or hv), but it underwent Wolff rearrangement to a ketene. 

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