P-Alkynyl functionalized benzazaphospholes as transmetalating agents

Abstract

Exposure of 10π-electron benzazaphosphole 1 to HCl, followed by nucleophilic substitution with the Grignard reagent BrMgCCPh afforded alkynyl functionalized 3 featuring an exocyclic –CC–Ph group with an elongated P–C bond (1.7932(19) Å). Stoichiometric experiments revealed that treatment of trans-Pd(PEt₃)₂(Ar)(I) (Ar = p-Me (C) or p-F (D)) with 3 generated trans-Pd(PEt₃)₂(Ar)(CCPh) (Ar = p-Me (E) or p-F (F)), 5, which is the result of ligand exchange between P–I byproduct 4 and C/D, and the reductively eliminated product (Ar–CC–Ph). Cyclic voltammetry studies showed and independent investigations confirmed 4 is also susceptible to redox processes including bimetallic oxidative addition to Pd(0) to give Pd(I) dimer 6-Pd₂-(P(t-Bu)₃)₂ and reduction to diphosphine 7. During catalysis, we hypothesized that this unwanted reactivity could be circumvented by employing a source of fluoride as an additive. This was demonstrated by conducting a Sonogashira-type reaction between 1-iodotoluene and 3 in the presence of 10 mol% Na₂PdCl₄, 20 mol% P(t-Bu)Cy₂, and 5 equiv. of tetramethylammonium fluoride (TMAF), resulting in turnover and the isolation of Ph–CC–(o-Tol) as the major product.