2-Indolylphosphines: Synthesis and reactivity

Abstract

Organophosphorus compounds bearing indolyl substituents has been synthesized. Using an animal protecting group to direct lithiation and electrophilic trapping with a chlorophosphine, the key P--C bond was formed at the 2-position of indole. Subsequent removal of the aminal protecting groups with NaBH 4 led to the formation of phosphines where the indolyl nitrogen centre may now serve as a secondary site of reactivity with a variety of functional groups to generate a diverse range of phosphines of varying electron-withdrawing and electron-donating character. The stepwise, controlled coordination of the 2-indolylphosphines on Ru 3(CO)12 was examined. Reactions of HL (HL = 2-indolylphosphine) with Ru3(CO)12 led to the formation of Ru3(CO) 11(HL) where HL was mono-coordinated through the phosphorus atom. Gentle heating of Ru3(CO)11(HL) resulted in the formation of Ru3(CO)9(mu-H)(mu3,eta2-L) in which the NH proton has migrated to the triruthenium core to form a bridging hydride ligand. Through its deprotonated nitrogen atom, the indolyl moiety bridged over the face of the triruthenium core, bonding to the two ruthenium centres to which the phosphorus atom is not bound. 2-Indolylphosphines behaved as anionic 6-electron P, N-donors in this mode of coordination. Monodentate 2-indolylphosphines, in the absence of base, served as 2-electron P-donors in reaction with Pd(COD)Cl2 to form [Pd(L)Cl(mu-Cl)] 2 dimers. In the presence of base, one of the dimers aggregated into a 58-electron butterfly cluster Pd4Cl4[P(C6H 5)2(C9H8N)]2[mu3,eta2-P(C6H5) 2(C9H7N)]2 where the ligand served in two positions as a neutral 2-electron P-donor and in one position as an anionic 4-electron N-donor. Lastly, a new generation of 2-indolylphosphines has been prepared with an emphasis on increasing electron richness of the phosphine and increasing overall steric bulk. These phosphines featured non-phenyl phosphorus substituents and phenyl groups on the indolyl nitrogen centre. Preliminary results showed that 2-indolylphosphines could serve as effective ligands for the Heck-Mizoroki and Buchwald-Hartwig animation reactions involving aryl bromides, but their performance in reactions with aryl chlorides was poor. 2-Indolylphosphines served as scaffolds for the synthesis of new unsymmetric P, P and P, N-bidentate ligands which feature a rigid indole backbone. X-ray studies of the Pd(II) square planar complexes provided insights into the nature of the bonding geometry of these P, P and P, N-bidentate ligands.