New dinuclear catalysts Rh-2(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 Rh-2(N-O)(2)[(C6H4)P(C6H 5)(2)](2) (N-O = C4H4NO2) are prepared, starting from Rh-2(O2CCH3)(2)(PC)(2 )L-2 [PC = (C6H4)P(C6H5)(2) (head-to-tail arrangement); L = HO2CCH3]. The t hermal reaction of Rh-2(O2CCH3)(PC)(2).L-2 with the neutral succinimide ste reoselectively gives one compound that according to the X-ray structure det ermination has the formula Rh-2(C4H4NO2)(2)[(C6H4)P(C6H5)(2)](2) (1). It co rresponds to the polar isomer with two bridging imidate ligands in. a head- to-head configuration. However, stepwise reaction of Rh-2(O2CCH3)(2)(PC)(2) .L-2 with (CH3)(3)SiCl and potassium succinimidate yields a mixture of 1 an d one of the two possible isomers (structure B) with a head-to-tail configu ration of the imidate ligands, Rh-2(C4H4NO2)(2)[(C6H4)P(C6H5)(2)](2) (2), a lso 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)-Rh-2(O2CCH3)(2)(PC)(2).L-2 rather than from the racemic m ixture. Their enantio selectivities in cyclopropanation of 1-diazo-5-penten -2-one are similar to those reported for the dirhodium amidate catalysts.