Carbenerhodium(I) complexes of the half-sandwich-type: reactions with electrophiles

The reaction of the carbenerhodium(I) complexes [(η5-C5H5)Rh(CR2)(L)] (R = aryl) with HX (X = Cl, CF3CO2) led, depending on the size and donor properties of the ligand L, to two different types of products. While compounds 1, 2 with R = Ph and L = CO or PMe3 react with HX to give rhodium(III) alkyls [(η5-C5H5)RhX(CHPh2)(L)] 3, 4a,b, the analogues 5a and 5b with R = Ph, p-Tol and L = PPri3 afford upon treatment with HX (X = Cl, Br, I, CF3CO2) the ring-substituted products [{η5-C5H4(CHR2)}RhHX(PPri3)] 6a–e. In the presence of excess HX, the latter are converted into the dihalo or bis(trifluoroacetato) derivatives [{η5-C5H4(CHR2)}RhX2(PPri3)] 7a–e. A labelling experiment using [(η5-C5D5)Rh(CPh2)(PPri3)] 5a-d5 as a precursor indicates that the migratory insertion of the carbene into a C–H bond of the cyclopentadienyl ring probably occurs via an η4-cyclopentadienerhodium(I) species as an intermediate. The triphenylphosphine complex [{η5-C5H4(CHPh2)}RhCl2(PPh3)] 7f was prepared analogously from 5c and two equiv. of HCl. The reactions of 5a and 5d (R = Ph, L = SbPri3) with either HBF4, [Me3O]BF4 or methyl triflate give via attack of the electrophile on the carbene carbon atom and subsequent σ/π rearrangement cationic η3-benzylrhodium(III) complexes 9 and 10a–c in good to excellent yields. Treatment of 5a and 5d with iodine results in the cleavage of the metal–carbene bond and affords the diiodo compounds [(η5-C5H5)RhI2(L)] 12a,b.