| The reaction of the cyclopropylcarbene-molybdenum complexes with alkynes to give cycloheptadienones has been investigated. The reaction is general for internal alkynes, but fails for terminal alkynes and substituted cyclopropyl carbene complexes. With some alkynes, furanone formation is a significant competing reaction pathway, but this can be suppressed by addition of triphenylphosphine. A mechanism for the formation of cycloheptadienones has been proposed involving metallacyclobutene formation, electrocyclic ring opening, electrocyclic ring closure, CO insertion and reductive elimination.; The reaction of Fischer carbene complexes with cyclobutenediones and cyclopropenones has been investigated. Alkylcarbene-chromium complexes react with cyclobutenediones to give 2-alkoxy-4-cyclopentene-1,3-diones in moderate yields, accompanied by varying amounts of 4-cyclopentene-1,3-dione derivatives. The reaction is general for alkoxy-, alkyl- and aryl-substituted cyclobutenediones. The reaction of arylcarbene complexes and cyclobutenediones produces 5-alkylidenefuranones as major products. A mechanism has been proposed involving oxidative addition of the acyl-acyl C-C bond, followed by carbene insertion and reductive elimination.; The reaction of diphenylcyclopropenone and alkylcarbene-chromium complexes produces a mixture of diphenylacetylene and cyclobutenones. A mechanism has been proposed involving metallacyclobutene formation, followed by either carbene insertion and reductive elimination or fragmentation. When {dollar}alpha{dollar},{dollar}beta{dollar}-unsaturated carbene complexes are employed, benzannulation products and diphenylacetylene are formed. To some extent benzannulation reactions appear to proceed via 1,3-shift of intermediate metal complexes rather than through electrocyclization of vinylketene intermediates.; A new ring expansion methodology for the synthesis of cyclopentenediones has been developed. The reaction of 4-hydroxy-4-alkynyl-2-cyclobutenones and chromium carbene complexes produces 2-alkenyl-4-cyclopentene-1,3-diones in good yields. The reaction is general for a variety of alkynols and carbene complexes. A mechanism has been proposed involving insertion of alkyne into carbene complex, giving an intermediate electrophilic vinylcarbene complex, which undergoes a semipinacol rearrangement to give cyclopentenediones. This reaction occurs in preference to other well-established reaction processes for alkynes and carbene complexes, such as benzannulation and cyclopentenone formation. |
