Rhodium-Catalyzed Isomerization of a Bis(secondary phosphine) to an Unsymmetrical Diphosphine via P-C Cleavage and P-P and C-H Bond Formation

To develop Rh-catalyzed dehydrocoupling for stereocontrolled synthesis of P-P bonds, we prepared potential intermediates, chiral rhodium phosphine-phosphido complexes, and investigated their stoichiometric and catalytic transformations. Treatment of [Rh(cliphos*)(COD)][X] with the bis(secondary phosphine) IsHPCH(2)PHIs (1, Is = 2,4,6-(i-Pr)(3)C6H2) gave the cations [Rh(diphos*)(IsHPCH(2)PHIs)] [X] (diphos* = (R,R)Me-DuPhos (2), (R,R)-i-Pr-DuPhos (3), or (R,R)-Me-BPE (4); X = BF4 or OTf) with high diastereoselectivity as single C-2-symmetric diastereomers. Deprotonation of 2-4 with NaN(SiMe3)(2) yielded phosphine-phosphido chelates Rh(diphos*)(IsHPCH(2)PIs) 5-7 with high diastereoselectivity. Thermolysis of 5-7 in toluene at 90 degrees C caused isomerization to give Rh(diphos*)(IsMeP-PIs) (8-10) as mixtures of diastereomers. Under catalytic conditions, Rh-DuPhos precursors slowly converted 1 to a mixture of diastereomers of its isomer, the unsymmetrical diphosphine IsHP-PMeIs (12). We propose a possible mechanism for the isomerization and discuss its relationship to dehydrocoupling catalysis.