Rhodium-Catalyzed Asymmetric Dehydrocoupling-Cyclophosphination of Supermesitylphosphines: Enantioselective Synthesis of a P‑Stereogenic Benzophospholane via C–H Activation–Functionalization

M(diphos*) catalyst precursors (M = Co or Rh) converted supermesitylphosphines PHR(Mes*) to cyclophosphinated P(2,4-(t-Bu)2C6H2(6-CMe2CH2))(R) (R = H, Me, Ph) slowly at room temperature (Mes* = 2,4,6-(t-Bu)3C6H2). Dehydrocoupling-cyclophosphination of PHPh(Mes*) occurred in up to 85:15 enantiomeric ratio (er) with [Rh((R,R)-Me-DuPhos)(Cl)]2 and NaOSiMe3. Diastereoselective formation of the resting state Rh-hydrides Rh(diphos*)(P(2,4-(t-Bu)2C6H2(6-CMe2CH2))(R))(H) (10-12) suggested that substitution of the phosphine product by substrate was rate-determining, consistent with faster turnover for smaller ligands with the lowest bite angles. We propose that P–H oxidative addition in Rh(diphos*)(PHR(Mes*))(H) (15) gave Rh(diphos*)(PR(Mes*))(H)2 (16), whose reductive elimination of H2 formed Rh(diphos*)(PR(Mes*)) (13), in which C–H oxidative addition of an o-t-Bu methyl group followed by P–C reductive elimination gave the resting state. Density functional theory (DFT) studies suggested that both P–H oxidative addition of PH(Ph)(Mes*) and P–C reductive elimination from Rh-PPh(Mes*) groups proceeded with inversion of configuration at three-coordinate phosphorus, and enantioselection occurred due to rapid interconversion of Rh-phosphido diastereomers 13 (R = Ph) by pyramidal inversion, along with their relative speciation and C–H activation rates. Intrinsic bond orbital (IBO) analyses of the P–H and C–H activation steps are consistent with proton, rather than hydride, transfer to the metal, which may be more widely relevant in such processes.