Syntheses and some chemistry of 1,2- and 1,1-bis(2-pyrrolyl)ethenes

trans-1,2-Bis(2-pyrrolyl)ethenes (e.g. 18-20) are prepared by McMurry-type reductive coupling of the corresponding 2-formylpyrroles. The isomeric 1,1-bis(2-pyrrolyl)ethenes (e.g. 24) are prepared as a minor byproduct in the reaction of 2-unsubstituted pyrroles (e.g. 22) with acetic anhydride under Friedel-Crafts conditions; the major product, as expected is the 2-acetylpyrrole 23. However, 5-(chloromethyl)dipyrromethanes (e.g. 35) can be obtained in high yield by reaction of 2-unsubstituted pyrroles 22 with chloroacetaldehyde diethyl acetal. Base-catalyzed elimination of HCl from 35 affords the 1,1-bis(2-pyrrolyl)ethene 24 along with the trans- and cis-1,2-bis(2-pyrrolyl)-ethenes 18 and 36, respectively. Conditions are optimized to afford a 66% yield of the 1,1-bis(2-pyrrolyl)ethene 24. In neutral organic solvents, 1,1-bis(2-pyrrolyl)ethenes exist in the ethene tautomeric form 2, rather than as the corresponding 5-methyldipyrromethene isomer 3; however, under acidic conditions, the 5-methyldipyrromethene salt 38 is observed, and the 5-methyl group undergoes acid-catalyzed exchange in deuterated solvents. 1,1-Bis(2-pyrrolyl)ethenes (e.g. 24) undergo standard chemistry, such as catalytic hydrogenation (Adams catalyst) of the alkene bond (to give 5-methyldipyrromethane 44), Vilsmeier formylation [to give 2-formyl-1,1-bis(2-pyrrolyl)-ethene 57], and reaction with Eschenmoser\u27s salt (N,N-dimethyl(methylene)ammonium iodide) [to give 2-((N,N-dimethylamino)methyl)-1,1-bis(2-pyrrolyl)ethene 59]. Both the 5-methyldipyrromethane-1,9-dicarboxylic acid 45 and the 1,1-bis(5-carboxy-3,4-dimethyl-2-pyrrolyl)ethene 53 react with the 1,9-diformyldipyrromethane 46, under standard MacDonald conditions, to give 3,5,8-trimethyldeuteroporphyrin IX dimethyl ester 47