Trimethylsilanolate-Promoted Activation of Alkynyl Trimethylsilanes: Hiyama-Type Sonogashira Cross-Coupling for the Synthesis of Arylene-Ethynylene-Linked Porphyrin Arrays.

Abstract

To synthesize meso-ethynylene-conjugated porphyrin arrays, the Sonogashira cross-coupling reaction is straightforward to construct the C(sp)-C(sp2) bonds, but the reaction is often accompanied by side reactions such as the Glaser homocoupling. The rate-determining oxidative addition step results in the unexpected kinetic competition with the Glaser homocoupling, which is desired to be circumvented. We here propose two sets of improved strategies for the synthesis of arylene-ethynylene-linked porphyrin arrays from the meso-brominated porphyrin and alkynes. First, we explored the solvent-modulated approach employing dichloromethane as the reaction solvent to minimize the formation of the copper acetylide dimer as the intermediate for the Glaser homocoupling, while the scope of the approach is limited. Subsequently, we have developed the trimethylsilanolate-promoted Hiyama-type approach that activates alkynyl trimethylsilanes (TMSs) by use of potassium trimethylsilanolate under amine-free conditions. The latter approach is advantageous not only in skipping the preprotodesilylation of the TMS group but also in achieving an excellent isolated yield.