Chiral Ancillaries in the Benzannulations of Alkoxy and Amino Carbene Complexes with Alkynes

Abstract

The asymmetric benzannulation of chiral Fischer car- bene complexes were investigated with a series of complexes bear- ing either chiral alcohols or chiral amines at the heteroatom stabilizing substituent. A set of seven alkenyl complexes and one aryl complex were prepared from five chiral alcohols; (1S,2R,5S)- (+)-menthol, (1R,2S)-(−)-trans-2-phenylcyclohexanol, (±)-trans-2- methylcyclopentanol, (±)-isoborneol, and (−)-8-phenylmenthol. The reaction of all seven of the alkenyl complexes with pent-1-yne and 3,3-dimethylbut-1-yne gave (arene)chromium tricarbonyl com- plexes with very low asymmetric inductions (0−20% de). The phe- nyl carbene complex derived from (+)-menthol gave good asymmetric induction with 3,3-dimethylbut-1-yne as had been pre- viously reported, but the reaction with pent-1-yne gave lower induc- tion and a much lower chemical yield. Alkenyl complexes bearing prolinol derivatives as the heteroatom stabilizing substituent also gave very low induction in reactions with pent-1-yne and this held true for both E- and Z-isomers of these complexes. An alkenyl car- bene complex derived from the C-2 symmetrical amine, 3,5-dime- thylmorpholine, reacted with pent-1-yne to give moderate yields of the (arene)chromium tricarbonyl complex but only in 10% de. High asymmetric inductions were observed for the cyclohexadienone an- nulation of atropisomeric 2-(N-methyl-3-methylindolyl) carbene complexes prepared from the chiral auxiliary, (4R,5S)-5-phenyl-4- methylimidazolidinone. Each atropisomer gives a single and differ- ent diastereomer of the same 4H-carbazol-4-one where it is the chiral axis of each atropisomer that controls the sense of induction and not the chirality of the auxiliary. Thus it is likely that these com- plexes, unlike all of the others examined in this work, react with acetylenes in a process in which the chiral auxiliary is not free to ro- tate about the carbene carbon in any of the reaction intermediates leading to product formation.