Controlling both ground- and excited-state thermal barriers to Bergman cyclization with alkyne termini substitution

The cross-coupling reaction of 2,3-dibromo-5,10,15,20-tetraphenylporphyrin with corresponding organostannanes in the presence of a Pd0 catalyst in THF at reflux temperature yields free base 2,3-dialkynylporphyrins 1a,c-e. The subsequent deprotection of trimethylsilyl group of 1a with TBAF in THF under aqueous conditions produces the 2,3-diethynyl-5,10,15,20-tetraphenylporphyrins 1b in 87% yield. Compounds 1a-d undergo zinc insertion upon treatment with Zn(OAc)2.2H2O in CHCl3/MeOH to give zinc(II) 2,3-dialkynyl-5,10,15,20-tetraphenylporphyrins (2a-d) in 70-92% yields. Thermal Bergman cyclization of 1a-e and 2a-d was studied in chlorobenzene and approximately 35-fold 1,4-cyclohexadiene at 120-210 degrees C. Compounds 1b and 2b with R = H react at lower temperature (120 degrees C) and produce cyclized products 3b and 4b in higher yields (65-70%) than their propyl, isopropyl, and phenyl analogues, with R = Ph being the most stable. Continuing in this trend, the -TMS derivatives 1a and 2a exhibit no reactivity even after heating at 190 degrees C in chlorobenzene/CHD for 24 h. Photolysis (at lambda >/= 395 nm) of 1b and 2b at 10 degrees C leads the formation of isolable picenoporphyrin products in 15 and 35% yields, respectively, in 72 h, whereas these compounds are stable in solution under same reaction conditions at 25 degrees C in the dark. Unlike thermolysis at 125 degrees C, which did not yield Bergman cyclized product for R = Ph, photolysis generated very small amounts of picenoporphyrin products (3c: 5%; 4c: 8% based on 1H NMR) as well as a mixture of reduced porphyrin products that were not separable. Thus, trends in the barrier to Bergman cyclization in the excited state exhibit the same trend as those observed in the ground state as a function of R-group. Finally, photolysis of 2b at 10 degrees C with lambda >/= 515 or 590 nm in benzene/iPrOH (4:1, 72 h) produces 4b in 15 and 6% isolated yields, indicating that conjugation of the enediyne unit into the porphyrin electronic transitions leads to sufficient distortion to generate photoproduct even with long wavelength excitation.