Vilsmeier reactions of porphyrins and chlorins with 3-(dimethylamino)acrolein to give meso-(2-formylvinyl)porphyrins: new syntheses of benzochlorins, benzoisobacteriochlorins, and benzobacteriochlorins and reductive coupling of porphyrins and chlorins using low-valent titanium complexes

Vilsmeier reactions between nickel(II) or copper(II) porphyrins or chlorins and 3-(dimethylamino)- acrolein/phosphoryl chloride (3-DMA/POCI3) are described. For example, copper(II) octaethylporphyrin 5 affords the meso-(2-formylvinyl) derivative 7, which cyclizes in strong acid to give the benzochlorin 10. Treatment of the nickel(II) benzochlorin 9 with 3-DMA/POCl3 gives the meso-(2-formylvinyl) derivative 14, which is cyclized in acid to give the dibenzoisobacteriochlorin 16. Prolonged treatment of nickel(II) octaethylporphyrin (4) with 3-DMA/POCl3 gives the disubstituted compound 17 in which the acrolein substituents are on adjacent rather than opposite meso positions. Acid-promoted cyclization of 17 afforded the monocyclized product 15 as well as the dibenzobacteriochlorin 18. The reaction is extended to obtain spiro benzochlorins (37,38) from (tetrabutano- and -pentanoporphyrinato)nickel(II) complexes, as well as benzoisobacteriochlorins (e.g., 28,29) from nickel(II) mesochlorin e6 trimethyl ester (19) and nickel(II) octaethylchlorin (26), respectively. Nickel(II) deuteroporphyrin IX dimethyl ester (11) also affords benzochlorins (12, 13) resulting from the corresponding meso-(2-formylvinyl)porphyrin. A centrally chelated metal is shown to be essential in order to accomplish cyclization of the 2-formylvinyl substituents to afford benzo derivatives. Porphyrin and chlorin dimers joined by one or three carbon-carbon double bond linkages are formed in good yields via reductive coupling by low valent titanium complexes of nickel(II) or copper(II) porphyrins or chlorins containing a formyl or an acrolein side chain. For example, nickel(II) α-(formylvinyl)octaethylporphyrin (6) reacts with the active titanium reagent to produce dimer 48 in 96% yield. When two different porphyrins or chlorins are cross-reacted under the same conditions, a mixture of products is obtained. The acrolein group seems to be more reactive in reductive coupling reactions than does the corresponding formyl substituent. © 1991, American Chemical Society. All rights reserved.