Rhodium(II)-Stabilized Vinylcarbenes: Synthetic Applications in Ylide, Cyclopropanation, and C-H Insertion Reactions

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A previously unknown cascade reaction in which two units of a rhodium(II)-stabilized vinylcarbene obtained from methyl trans-styryldiazoacetate react with benzylideneaniline is described; the products of the cascade reaction are complex bicyclic pyrrolidines. An evaluation of reaction conditions for the formation of bicyclic pyrrolidine determined that slow addition of two equivalents of methyl trans-styryldiazoacetate to a refluxing dichloromethane solution of benzylideneaniline and 1 mol% Rh2(OAc)4 provided optimal yields of bicyclic pyrrolidine products. A study was initiated to determine the scope of this cascade process, it was found that the only metal vinylcarbene which amendable to the cascade reaction was trans-styryldiazoacetate.

A series of endocyclic vinyldiazocarbonyl compounds were synthesized using one of two synthetic protocols, direct diazo transfer reaction with an unsaturated cyclic carbonyl compound or a reduction-dehydration of a diazo -dicarbonyl compound. The direct diazo transfer reaction with an unsaturated cyclic carbonyl was the most general and commonly applied route to endocyclic vinyldiazocarbonyl compounds. Five endocyclic vinyldiazocarbonyl compounds were prepared using both strategies, including vinyldiazolactones of varying ring size and substitution, a cyclic vinyldiazoketone, and a vinyldiazolactam.

The endocyclic vinyldiazocarbonyl compounds which were prepared were evaluated as metal vinylcarbene precursors in asymmetric cyclopropanation and intermolecular C-H insertion reactions. The vinyldiazolactone derived from 5,6-dihydro-2H-pyran-2-one provided good enantioselectivities (>80% ee) in an intermolecular C-H insertion reaction with 1,4-cyclohexadiene and cyclopropanation reactions with styrene and terminal diene olefins using the catalyst Rh2(S,R-MenthAZ)4. The azetidinate ligated dirhodium catalysts were the only dirhodium catalysts surveyed which provided enantioselectivities >40% ee in either intermolecular C-H insertion or cyclopropanation reactions.