The Multifaceted Chemistry of Variously Substituted α,β-Unsaturated Fischer Metalcarbenes

Abstract

The insertion of an alkyne into an α,β-unsaturated Fischer metalcarbene complex leads to a 1-metalla-1,3,5-hexatriene. This usually undergoes subsequent insertion of a carbon monoxide molecule, and the resulting dienylketene complex, in a 6π-electrocyclization, yields an alkoxycyclohexadienone or its tautomeric hydroquinone monoether. The overall process is a [3+2+1] cocyclization and constitutes the so-called Dötz reaction. With a dialkylamino instead of the alkoxy group on the carbene center, or an additional dialkylamino group on C3 of an alkoxycarbene complex, the 1-metalla-1,3,5-hexatrienes resulting from alkyne insertion more generally do not undergo CO insertion, but direct 6π-electrocyclization and subsequent reductive elimination to yield five- rather than six-membered rings. 1-Dialkylamino-1-arylcarbenemetals thus yield indenes, and 1-alkoxy-3-dialkylaminopropenylidenemetal complexes with alkynes furnish 3-alkoxy-5-dialkylaminocyclopentadienes, which essentially are protected cyclopentenones and even doubly protected cyclopentadienones. The multifunctionality of these cyclopentadienes makes them highly versatile building blocks for organic synthesis. Synthetically useful cyclopentenones are also obtained from 1-cyclopropyl-1-alkoxycarbenemetals with alkynes. Yet, with certain combinations of substituents and conditions, the amino-substituted metallatrienes can also undergo CO insertion with subsequent cyclization to five-membered rings, twofold alkyne and CO insertion with subsequent intramolecular [4+2] cycloaddition to yield cyclopenta[b]pyranes, or even threefold alkyne insertion with subsequent twofold cyclization to yield spiro[4.4]nonatrienes. Variously amino-substituted α,β-unsaturated Fischer carbenes can also give rise to pyrrolidines, pyridines, and pyrroles. Normal, i.e., 1-alkoxy-substituted, a,b-unsaturated Fischer carbene complexes react with acceptor-substituted alkenes and alkadienes to yield donor-acceptor-substituted vinylcyclopropanes or cyclopentenes and cycloheptadienes, respectively. Enantiocontrolled formal [3+2] cycloadditions of chirally modified alkoxycarbenemetals with imines can be achieved to yield, after hydrolysis of the alkoxypyrrolines, 1,2,5-trisubstituted pyrrolidin-3-ones with high enantiomeric excesses.Fischer carbenesTemplate synthesisCocyclizationCycloadditionCyclopentadienesCyclopentenonesDomino reactions