REACTION OF AMINOCARBENE COMPLEXES OF CHROMIUM WITH ALKYNES 9 - FROM NITROGEN YLID COMPLEXES TOWARD ALKALOID FRAMEWORKS

Aminocarbene complexes of chromium having the general structure (CO)(5 )Cr=C(R)NR1R2 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 alpha or gamma with res pect to the nitrogen atom. The mechanism of this new reaction has been thoroughly investigated: a nitrogen ylid originating from the interac tion of the nitrogen atom of the starting aminocarbene complex with th e central carbon of the ketene formed by insertion of the alkyne and o f CO into the aminocarbene complex, is a crucial intermediate in these reactions. This ylid complex, the structure of which could be establi shed as 25, leads to the observed pyrrolinones upon thermolysis. Mecha nisms involving radicals have been discarded on the grounds of the rea ction of cyclopropylcarbinyl-substituted aminocarbene complexes 45a,b: no rearrangement of the cyclopropylcarbinyl group is observed upon it s migration, as shown by the X-ray structure of the pyrrolinone 46b. M echanisms involving ion pairs or the participation of the metal have a lso been eliminated. For that purpose, the X-ray structures of two com plexes, 51 and 52, in which the metal is not bound to the phenyl ring of the migrating groups, have been established. Finally, concerted (1, 5) sigmatropic migrations of the alkyl groups from nitrogen to the car bons of the five-membered heterocycle in 25 account best for the obser ved results. The role of the metal could also be determined by the exa mination of the reactivity of the metal-free N-ylides. No rearrangemen t similar to that observed for complexes 25 is observed; only products arising from the cleavage of the bond between nitrogen and the centra l carbon of the ketene were obtained. As an application of this origin al reaction of carbene complexes, the synthesis of 102, 103, 105 and 1 06 which are derivatives of the lycorane alkaloid will be described: t he keypoint is the use of intramolecular insertions of alkynes into su itably substituted aminocarbene complexes of chromium 101 and 104. (C) 1998 Elsevier Science S.A. All rights reserved.