Aza-alkynals undergo a cyclization reaction with diazo compounds in the presence of catalytic amounts of Cp*RuCl(cod) to afford vinyl epoxypyrrolidines, valuable building blocks for the synthesis of biologically active molecules. Ruthenium vinyl carbene intermediates have been invoked to explain the overall [2+1] cycloaddition (epoxy-annulation reaction). The reaction proceeds under mild conditions and in short reaction times (5-80 min) with complete (Z)or (E)-stereoselectivity on the vinyl substituent depending on the nature of the diazo compound used. Theoretical calculations support a mechanistic rationale to explain this controlled process.
The Cp*RuCl-based catalyst enables expedient access to a variety of benzofused six-membered azaheterocycles from unprotected oalkynylanilines and trimethylsilyldiazomethane through an unprecedent tandem carbene/alkyne metathesis/N−H insertion reaction. The transformation takes place under mild reaction conditions (room temperature, <15 min) and with excellent functional group tolerance. The synthetic utility of the final products and a mechanistic rationale are also discussed.
Ruthenium vinyl carbenes derived from Cp/Cp*RuCl‐based complexes (Cp=cyclopentadiene, Cp*=1,2,3,4,5‐pentamethylcyclopentadiene) have been routinely invoked as key intermediates in tandem reactions involving a carbene/alkyne metathesis (CAM). A priori, these intermediates resemble the Grubbs‐type family of catalysts, but they exhibit a completely different reactivity pattern that few, if any, other catalytic system can reproduce so far. The reactivity of these species with α‐unsubstituted and α‐substituted alkynals showcases the peculiarities of these intermediates. Although Z‐vinyl dihydrooxazines are preferentially obtained with the former, Z‐vinyl epoxypyrrolidines are obtained with the latter. A combination of spectroscopic and computational data now prove that a η3‐coordination mode of the ruthenium vinyl carbene and the presence of a Lewis basic chloride ligand give rise to two markedly different stereoelectronic faces, which are responsible for the unconventional reactivity of these species.
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