Metal-Catalyzed Oxidative Cyclizations of a,c-Biladiene Salts Bearing 1- and/or 19-Arylmethyl Substituents: Macrocyclic Products and Their Chemistry.

Several 1-mono- and 1,19-bis(p-arylmethyl)-a,c-biladiene salts were prepared and subjected to either copper(II)- or chromium(III)-assisted oxidative cyclization to yield numerous products in which the 1- or 19- substituent is adapted, eliminated, or rearranged to other points on the tetrapyrrole. For example, cyclization using copper(II) acetate of 19-((ethoxycarbonyl)methyl)-2,3,7,8,12,13,17,18-octamethyl-19-(p-tolylmethyl)-a,c-biladiene dihydrobromide (25) yielded the copper(II) 20-((ethoxycarbonyl)methyl)-1-(p-tolylmethyl)-2,3,7,8,12,13,17,18-octamethyl-1,20-dihydroporphyrin (42), copper(II) 20-(ethoxycarbonyl)-3-methylidene-2,3,7,8,12,13,17,18-octamethyl-2-(p-tolylmethyl)chlorin (44), copper(II) 20-(ethoxycarbonyl)-2-methylidene-2,3,7,8,12,13,17,18-octamethyl-3-(p-tolylmethyl)chlorin (45), and three porphyrins: copper(II) 20-(ethoxycarbonyl)-2,3,7,8,12,13,17,18-octamethylporphyrin (3), copper(II) 2,3,7,8,12,13,17,18-octamethyl-20-p-tolylporphyrin (50), and copper(II) 20-(ethoxycarbonyl)-2,3,7,8,12,13,17,18-octamethyl-5-(p-tolylmethyl)porphyrin (47). Formation of porphyrin 47 and the intermediate chlorins 44 and 45 suggests the stepwise migration of the arylmethyl group from the 1-position in compound 42. The isolation of products from cyclization reactions of various 1,19-arylmethyl-substituted a,c-biladiene salts provides further insight into the mechanisms of metal-assisted oxidative cyclization of a,c-biladiene salts to give cyclic tetrapyrroles. Macrocyclizations of a,c-biladienes such as 25 using chromium(III) afford good yields of the metal-free 1-substituted compounds such as 43.