TETRAHEDRON Pergamon Tetrahedron 54 (1998) 2021-2030 The Reductive Coupling of 21Cyanopyrroles: A Study Pertaining to the Mechanism of Formation of Porphocyanines Christian Briicher’, Lily Y. Xie and David Dolphin* Department of Chemistry. University of British Columbia 2036 Main Mall, Vancouver, B.C., V6T lZ1, CANADA Received 27 June 1997; accepted 12 September 1997 Abstract: 2-Cyanopyrrole was found to form (2-pyrrolyhnetheoe)-(2-pyxmlyhnethyl)imine when treated witi lithium ahuninum hydride (LAH). followed by a mild work-up. A plausible mechanism of this reductive coupling was inferred from a series of experiments, including ~ ~ AI-NMR, deuteration experiments, and the reduction of variously substituted cyawpynn les. The mechanism, a metal chelate mediateddimekation, may be the key to understanding porphocyanine synthesis via the LAH reduction of 1.9dicyanodipyrmmeUmnes. 0 1998 Elsevier Science Ltd. All rights reserved. INTRODUCTION The synthesis of long-wavelength absorbing pyrrolic pigments for potential use in photodynamic therapy (PDT) is currently an active area of research.2 We contributed to this field recently by reporting on the synthesis of a novel class of expanded porphyrins. 3-5 These tetrapyrmlic, imine containing pigments were named porphocyanines (1) and two alternative methods for their synthesis have been found since the initial report. In particular, the synthesis by an in siru oxidation of the lithium aluminum hydride &AI-I) reduction product of 1,9-dicyano substituted dipyrromethanes is simple and efficient (yields over 30% are commonly observed). However, details of the mechanism of this effective formation of the macrocycle remained unclear (Scheme 1, Pathway A).3 . . Reaction (i) 1. LAH, 2. H20,3. oxidation (DDQ or 02) Sc he m e 1 e-mail: david@dolphin.chem.ubc.ca 0040-4020/98/$19.00 0 1998 Elsevier Science Ltd. All rights reserved. HI: S0040-4020(97)10424-0