ABSTRACT:The successful use of dipyrromethanecarbinols in rational routes to porphyrinic macrocycles requires catalysis conditions that enable irreversible condensation, thereby avoiding substituent scrambling and formation of undesired porphyrin products. Previously, successful conditions of trifluoroacetic acid (TFA) (30 mM) in acetonitrile were identified following a lengthy survey of TFA and BF 3 -etherate catalysis in diverse solvents. In this study, focus was placed on the acid catalyst by examining 17 acids in CH 2 Cl 2 , the traditional solvent for two-step, one-flask porphyrin syntheses. In the self-condensation of the carbinol derived from 1-(4-methylbenzoyl)-5-phenyldipyrromethane, porphyrin yields of 9-55% were obtained from the various acids, compared to 20% under TFA catalysis in acetonitrile. A number of catalytic conditions that produce little to no porphyrin in reactions of pyrrole benzaldehyde afforded good yields of porphyrin and the suppression of scrambling in reactions of dipyrromethanecarbinols. The four best acid catalysts (InCl 3 , Sc(OTf) 3 , Yb(OTf) 3 , and Dy(OTf) 3 ) initially identified were then examined with dipyrromethanecarbinols bearing challenging substituents (alkyl, pyridyl, or no substituent). The greatest improvement was obtained with the pyridyl substrates. Selected reactions performed on a preparative scale (115 to 460 mg of isolated porphyrin) verified the results of the analytical-scale experiments and revealed the more facile isolation of the porphyrin from reactions performed in CH 2 Cl 2 rather than acetonitrile. This study provides alternatives to the use of TFA/acetonitrile that offer advantages in terms of yield and isolation of the porphyrin without sacrificing suppression of scrambling. Furthermore, the finding that poor catalysts for the benzaldehyde pyrrole reaction can be excellent catalysts for dipyrromethanecarbinols provides guidance for the identification of other catalysts for use with reactive precursors in porphyrin-forming reactions.

The reaction of dipyrromethanedicarbinols with pyrrole leading to meso-substituted corroles was investigated to determine whether mild acid catalysts [Dy(OTf)(3), Yb(OTf)(3), Sc(OTf)(3), and InCl(3)] known to provide porphyrins from dipyrromethanecarbinol species while suppressing undesired reversibility (resulting in scrambling) are applicable to reactions affording corrole, and to explore the requirements of the oxidation step. We examined a model reaction leading to meso-triphenylcorrole (TPC) to survey the effect of acid catalyst, acid concentration, ratio of pyrrole to dipyrromethanedicarbinol, oxidant, oxidant quantity, and reaction time on the yield of TPC (by UV-vis) in reactions performed at room temperature in CH(2)Cl(2). Key to this survey was a modification of the well-known spectrophotometric method for monitoring reactions leading to porphyrin. The survey revealed that TPC could be prepared via a subset of the mild acid catalysts [Dy(OTf)(3) and Yb(OTf)(3)], and a preparative-scale reaction afforded an isolated yield of TPC of 49%, devoid of porphyrin. Suppression of reversible processes was further demonstrated by the synthesis of three corroles bearing different meso substituents in defined locations in isolated yields ranging from 50% to 80%. The reaction conditions were applicable to a dipyrromethanedicarbinol bearing electron-withdrawing pentafluorophenyl substituents-provided that the reaction time of the condensation step was increased. We identified circumstances under which DDQ can cause severe interference with the detection and isolation of some corroles, we found that the yield and purity of the corrole depend on judicious selection of oxidation conditions, and we assessed the sensitivity toward light of dilute solutions of the corroles prepared in this study.

Synthesis of Meso-Substituted Corroles via Dipyrromethanedicarbinols and Pyrrole. -A model reaction leading to corrole (IVa) is examined to study the effect of acid catalyst, acid concentration, ratio of pyrrole to methanedicarbinol, oxidant, oxidant quantity, and the reaction time on the yield. It is found that mild acid Dy(O-Tf)3 [as well as Yb(O-Tf) 3 and TFA] can be applied to prepare corroles (IV) devoid of porphyrins. A critical role of the choice and quantity of oxidant is identified, including a direct comparison of p-chloranil and DDQ. Dilute corrole solutions are found to be instable toward light and air. -(GEIER*, G. R. I.; CHICK, J. F. B.; CALLINAN, J. B.; REID, C. G.; AUGUSCINSKI, W. P.; J. Org. Chem. 69 (2004) 12, 4159-4169; Dep. Chem., Colgate Univ., Hamilton, NY 13346, USA; Eng.) -Klein 41-121