Synthesis of a phosphapyracene via metal-mediated cyclization: structural and reactivity effects of acenaphthene precursors

Abstract

Metal-mediated synthesis of a new heterocycle, 1-phenyl-phosphapyracene (Ph-4, Ph-PyraPhos), by tandem phosphination/cyclization of peri-substituted 5-bromo-6-chloromethylacenaphthene (3) was investigated for comparison to Pt-catalyzed formation of 1-phosphaacenaphthenes (2, AcePhos) from the analogous naphthalene precursor (1). Reaction of PH₂Ph with 3, NaOSiMe₃ and a Cu catalyst gave Ph-4; a Pt catalyst yielded PHPh(CH₂Ar) (Ph-11, Ar = 5-Br-acenaphthyl). Deprotonation of a complex of this secondary phosphine, [Pt((R,R)-Me-DuPhos)(Ph)(PHPh(CH₂Ar))][PF₆] (17), generated the phosphido intermediate Pt((R,R)-Me-DuPhos)(Ph)(PPhCH₂Ar) (Ph-8), which cyclized to give [Pt((R,R)-Me-DuPhos)(Ph)(Ph-PyraPhos)][PF₆] (18). Treatment of Ph-8 with silver triflate gave 18 and the cyclometalated phosphine complex [Pt((R,R)-Me-DuPhos)(κ²-(P,C)-5-Ph₂PCH₂-6-C₁₂H₈)][PF₆] (21), which might form via Pt(IV) intermediates. The effects of the added “ace” bridge on structure and reactivity are discussed.