Attempted Synthesis of a 10-P-3 Species: Cyclization Interrupts the Pathway

Abstract

Main group (MG) element-based catalysis provides an enticing solution to the health and environmental problems associated with traditional transition metal catalysis. However, a major obstacle is the large HOMO-LUMO gap present in MG compounds. Confining the active center in a geometrically-distorted rigid chelated structure has shown promise by lowering this barrier and enabling fundamental organometallic-type processes like oxidative addition, insertion, and reductive elimination, leading us to pursue the synthesis of 10-P-3 species 8. To that end, a modified procedure was developed to generate Martin ligand 7-Br, an electron-deficient diol. The ligand was methylated with MeI in the presence of K2CO3 affording diether 9, which was characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. We hypothesized that exposure of 9 to a three-step metal-halogen exchange/phosphination/reduction sequence would generate target 8, a hypervalent 10–P–3 species stabilized within a rigid pincer framework by electron-withdrawing axial O-donors. Instead, after phosphination, the presumed PCl2-substituted intermediate underwent cyclization to monochlorinated 10, which was functionalized to p-tolyl-substituted 11 and characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. This work provides insights into cyclization chemistry in fluorinated OCO pincer compounds that will guide future attempts at generating hypervalent 10-P-3 species.