BENEFICIAL-EFFECTS OF SALTS ON AN ACID-CATALYZED CONDENSATION LEADINGTO PORPHYRIN FORMATION

Addition of one of a variety of salts to the room temperature, two-ste p, one flask reaction at 0.1 M forming tetraphenylporphyrin (TPP) gave yield increases of up to 2-fold. Among 21 insoluble salts, 12 gave in creased yields, 6 had no effect, and 3 gave diminished yields. The sal ts that gave increases encompassed diverse cations but were restricted to the anions Cl-, Br, I-, and Ph4B- while SO42-, F-, or BF4- did not give improved yields. All 7 soluble tetraalkylammonium or tetraphenyl phosphonium salts (F-, Cl-, Ph4B-, PF6-, or HSO4- counterions) that we re surveyed gave yield increases of > 1.5 fold. Thus a 10(-1) M pyrrol e-benzaldehyde condensation catalyzed with 10(-2) M BF3 . O(Et)(2) in CH2Cl2 containing 0.1 equiv of NaCl (5.85 mg/10 mL CH2Cl2) or 0.0031 e quiv of benzyltributylammonium chloride (Bu(3)BzlNCl) (based on [benza ldehyde]) gave similar to 50% yield compared with similar to 25% in th e reaction without salt. The pyrrole-aldehyde condensation is much fas ter in the presence of salt, as measured by the rate of disappearance of benzaldehyde and the rate of formation of the porphyrin. Yield incr eases in the presence of salt were observed with catalysis by BF3 . O( Et)(2), BF3 . 2H(2)O, and CF3CO2H. Significant salt effects also were observed with BF3 . O(Et)(2) or CF3CO2H in the solvent diethyl ether, but the maximum yield was 15%. A survey of nine aldehydes showed yield improvements of up to 2-fold in six cases in the presence of salt. Du ring the pyrrole-aldehyde reaction in CH2Cl2 either in the presence or absence of soluble salts, the medium becomes heterogeneous (measured by nephelometry). The addition of water to BF3 . O(Et)(2) in CH2Cl2 al so yields a heterogeneous medium; in the presence of salt this medium affords twice the yield of porphyrin as that formed in the absence of salt. B-11 NMR and F-19 NMR experiments failed to unveil any new speci es formed by interaction of chloride-containing salts with BF3 . O(Et) (2). The complexity of the reaction medium, as well as insufficient in formation about the nature of the pyrrole-aldehyde condensation, precl ude an assignment of mechanisms underlying the salt effects. However, a rank ordering of salts in the porphyrin reaction does not correlate with their desiccative power, and the generality of the salt effects i s at odds with the selective anion templating of tetrapyrrolic macrocy cles. Irreversible features of the pyrrole-aldehyde condensation have been identified via exchange experiments during the course of the reac tion and C-13 NMR labeling experiments. The improved reaction conditio ns can be used for preparative-scale syntheses, as 720 mg tetraphenylp orphyrin (47% yield) was obtained from a 100 mL-scale reaction with 0. 1 M reactants at room temperature. (C) 1997 Elsevier Science Ltd.