Reaction of aminocarbene complexes of chromium with alkynes 9.: From nitrogen ylid complexes toward alkaloid frameworks

Abstract

Aminocarbene complexes of chromium having the general structure (CO) 5CrC(R)NR 1R 2 react with diphenylacetylene to give pyrrolinones as the result of the insertions of the alkyne, of CO and the migration of an alkyl group from nitrogen to a carbon atom in α or γ with respect to the nitrogen atom. The mechanism of this new reaction has been thoroughly investigated: a nitrogen ylid originating from the interaction of the nitrogen atom of the starting aminocarbene complex with the central carbon of the ketene formed by insertion of the alkyne and of CO into the aminocarbene complex, is a crucial intermediate in these reactions. This ylid complex, the structure of which could be established as 25, leads to the observed pyrrolinones upon thermolysis. Mechanisms involving radicals have been discarded on the grounds of the reaction of cyclopropylcarbinyl-substituted aminocarbene complexes 45a,b: no rearrangement of the cyclopropylcarbinyl group is observed upon its migration, as shown by the X-ray structure of the pyrrolinone 46b. Mechanisms involving ion pairs or the participation of the metal have also been eliminated. For that purpose, the X-ray structures of two complexes, 51 and 52, in which the metal is not bound to the phenyl ring of the migrating groups, have been established. Finally, concerted (l,5) sigmatropic migrations of the alkyl groups from nitrogen to the carbons of the five-membered heterocycle in 25 account best for the observed results. The role of the metal could also be determined by the examination of the reactivity of the metal-free N-ylides. No rearrangement similar to that observed for complexes 25 is observed; only products arising from the cleavage of the bond between nitrogen and the central carbon of the ketene were obtained. As an application of this original reaction of carbene complexes, the synthesis of 102, 103, 105 and 106 which are derivatives of the lycorane alkaloid will be described: the keypoint is the use of intramolecular insertions of alkynes into suitably substituted aminocarbene complexes of chromium 101 and 104.