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)(2)C6H2(6-CMe2CH2))(R) (R = H, Me, Ph) slowly at room temperature (Mes* = 2,4,6-(t-Bu)(3)C6H2). 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)(2)C6H2(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 H-2 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.