A NEW METHOD FOR THE CONSTRUCTION OF THE PYRROLE RING BY THE CARBONYLCOBALT-CATALYZED REACTION OF TRIMETHYLSILYL CYANIDE WITH ACETYLENES - ITS SCOPE AND LIMITATIONS

The carbonylcobalt-catalyzed reaction of acetylenes with trimethylsilyl cyanide (1) to give 5-amino-1H-pyrrole-2-carbonitriles is described. Reaction of the symmetric internal acetylenes, 4-octyne (2a) and 2,9-dimethyl-5-decyne (2b), with 1 in the presence of a cobalt complex gave 3a and 3b, respectively, in high yield. From unsymmetric acetylenes were obtained mixtures of regioisomeric pyrroles. The regioisomeric ratio depended on the substitution pattern of the acetylenes and on the kind of substituents possessed by the acetylenes. In the case of the internal unsymmetric acetylenes, 2c-2f, isomers in which the bulkier of the two substituents appeared at the 4-position of the pyrrole ring were obtained as the main products. However, the reaction of internal acetylenes bearing an ester group (2g and 2h) gave a reversed selectively. In case of the terminal acetylenes, 1-octyne (2k), 21, and 2m, isomers in which the alkyl group was located at the 3-position of the pyrrole ring were obtained highly selectively. A new cobalt complex (8), prepared by the reaction of CO2(CO)8 with excess 1, catalyzed the formation of pyrrole 3a from 2a and 1. The stoichiometric reaction of complex 8 with 2a also gave 3a in a good yield.