New Dinuclear Catalysts Rh₂(N−O)₂[(C₆H₄)P(C₆H₅)₂]₂ with Imidate Ligands: Synthesis and Isomerization from Head-to-Tail to Head-to-Head Configuration of the Imidate Ligands

Two new dirhodium(II) catalysts of general formula Rh2(N−O)2[(C6H4)P(C6H5)2]2 (N−O = C4H4NO2) are prepared, starting from Rh2(O2CCH3)2(PC)2L2 [PC = (C6H4)P(C6H5)2 (head-to-tail arrangement); L = HO2CCH3]. The thermal reaction of Rh2(O2CCH3)2(PC)2·L2 with the neutral succinimide stereoselectively gives one compound that according to the X-ray structure determination has the formula Rh2(C4H4NO2)2[(C6H4)P(C6H5)2]2 (1). It corresponds to the polar isomer with two bridging imidate ligands in a head-to-head configuration. However, stepwise reaction of Rh2(O2CCH3)2(PC)2·L2 with (CH3)3SiCl and potassium succinimidate yields a mixture of 1 and one of the two possible isomers (structure B) with a head-to-tail configuration of the imidate ligands, Rh2(C4H4NO2)2[(C6H4)P(C6H5)2]2 (2), also characterized by X-ray methods. In solution, compound 2 undergoes slow isomerization to 1; the rate of this process is enhanced by the presence of acetonitrile. Compounds 1 and 2 are obtained as pure enantiomers starting from (M)- and (P)-Rh2(O2CCH3)2(PC)2·L2 rather than from the racemic mixture. Their enantioselectivities in cyclopropanation of 1-diazo-5-penten-2-one are similar to those reported for the dirhodium amidate catalysts.