Silvia Martinez

Benzannulation and Cyclopentannulation Reactions of Electron-Poor Amino-Stabilized Alkenyl Fischer Carbene Complexes

Abstract The alkenyl(amino)carbene complexes of chromium and tungsten bearing an electron-withdrawing group at the Cβ-position 5–6 were prepared by condensation of methyl(amino)carbene complexes 1–2 and esters of glyoxylic acid. The complexes 5–6 were readily α-metallated with n-BuLi affording the deuteriated metal carbene complexes 7–8. The chromium complexes 5, 7 display a remarkable reactivity toward different types of alkynes. These complexes reacted with both unactivated alkynes and terminal electron-poor alkynes to give exclusively the phenol products 11–13 via the benzannulation reaction (Dotz reaction). In contrast, the reaction of carbene complexes 5 with internal electron-poor alkynes yielded solely the cyclopentadiene derivatives 15 via the cyclopentannulation reaction. The tetracarbonyl carbene intermediates 16 were isolated and fully characterized.

One-pot, two-step synthesis of substituted anthraquinones from chromium(0) alkynyl carbenes and isobenzofurans.

The reaction of alkynyl Fischer carbenes and isobenzofurans gives rise to the corresponding [4 + 2] cycloadducts. The newly formed carbene adducts are suitable for benzannulation processes in the presence of tert-butylisocyanide or carbon monoxide to yield a variety of new highly substituted polycyclic structures having the anthraquinone framework. The whole two-step process is conducted in a one-pot fashion from easily available 1,4-dihydro-1,4-epoxynaphthalenes.

[8+2] and [8+3] Cyclization Reactions of Alkenyl Carbenes and 8-Azaheptafulvenes: Direct Access to Tetrahydro-1-azaazulene and Cyclohepta[b]pyridinone Derivatives

The reactivity of Fischer alkenyl carbenes toward 8-azaheptafulvenes is examined. Alkenyl carbenes react with 8-azaheptafulvenes with complete regio- and stereoselectivity through formal [8+3] and [8+2] heterocyclization reactions, which show an unprecedented dependence on the C(beta) substituent at the alkenyl carbene complex. Thus, the formal [8+3] heterocyclization reaction is completely favored in carbene complexes that bear a coordinating moiety to give tetrahydrocyclohepta[b]pyridin-2-ones. Otherwise, alkenyl carbenes that lack appropriate coordinating groups undergo a formal [8+2] cyclization with 8-azaheptafulvenes to give compounds that bear a tetrahydroazaazulene structure. A likely mechanism for these reactions would follow well-established models and would involve a 1,4-addition/cyclization in the case of the [8+2] cyclization or a 1,2-addition/[1,2] shift-metal-promoted cyclization for the [8+3] reaction. The presence of a coordinating moiety in the carbene would favor the [1,2] metal shift through transition-state stabilization to lead to the [8+3] product. All these processes provide an entry into the tetrahydroazaazulene and cycloheptapyridone frameworks present in the structure of biologically active molecules.