Alkylthio unit as an alpha-pyrrole protecting group for use in dipyrromethane synthesis

The synthesis of porphyrin precursors requires the successive introduction of substituents at the pyrrole alpha- and alpha'-positions (2- and 5-, respectively). An alpha-pyrrole substituent that serves as a temporary masking agent and is not deactivating would greatly facilitate such syntheses, particularly for beta-(3,4)-unsubstituted pyrroles, but has heretofore not been available. A series of alpha-RS groups (R = Me, Et, n-decyl, Ph) have been investigated in this regard, including the determination of the kinetics of substitution at the pyrrolic 3-, 4-, and 5-positions and the application to dipyrromethane formation. The RS group was readily introduced into the pyrrole alpha-position by the reaction of 2-thiocyanatopyrrole (prepared from pyrrole, ammonium thiocyanate, and iodine) and the corresponding Grignard reagent RMgBr. Each 2-alkylthio group activated the pyrrole ring toward deuteration at the 3- or 5- (vs 4-) position. The dipyrromethane synthesis was carried out using a 2:1 ratio of 2-(RS)pyrrole/benzaldehyde with a catalytic amount of InCl3 at room temperature in the absence of any solvent. The alpha-RS group was removed by hydrodesulfurization using Raney nickel or nickel complexes. This stoichiometric synthesis using the alpha-RS-protected pyrrole is in contrast to the traditional synthesis that employs an aldehyde and 25-100 mol equiv of pyrrole. Six meso-substituted dipyrromethanes were prepared by the reaction of 2-(n-decylthio)pyrrole/aldehyde/InCl3 (2.2:1:0.2 ratio) followed by hydrodesulfurization. Other reactions of the 1,9-bis(RS)dipyrromethane include oxidation to give (i) the 1,9-bis(RS)dipyrrin or (ii) the 1,9-bis(RSO2)dipyrromethane, which underwent subsequent complexation with dibutyltin dichloride. In summary, under mild reaction conditions, the 2-alkylthio group is readily introduced to the pyrrole nucleus, directs electrophilic substitution to the 5-position, and is readily removed as required for elaboration of porphyrinic precursors.