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

The reaction of the carbenerhodium(I) complexes [(eta (5)-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 [(eta (5)-C5H5)RhX(CHPh2)(L)] 3, 4a,b, the analogues 5a and 5b with R = Ph, p-Tol and L = PPr3i afford upon treatment with HX (X = Cl, Br, I, CF3CO2) the ring-substituted products [{eta (5)-C5H4(CHR2)}RhHX(PPr3i)] 6a-e. In the presence of excess HX, the latter are converted into the dihalo or bis(trifluoroacetato) derivatives [{eta (5)-C5H4(CHR2)}RhX2(PPr3i)] 7a-e. A labelling experiment using [(eta (5)-C5D5)Rh(=CPh2)(PPr3i)] 5a-d(5) as a precursor indicates that the migratory insertion of the carbene into a C-H bond of the cyclopentadienyl ring probably occurs via an eta (4)-cyclopentadienerhodium(I) species as an intermediate. The triphenylphosphine complex [{eta (5)-C5H4(CHPh2)}RhCl2(PPh3)] 7f was prepared analogously from 5c and two equiv. of HCl. The reactions of 5a and 5d (R = Ph, L = SbPr3i) with either HBF4, [Me3O]BF4 or methyl triflate give via attack of the electrophile on the carbene carbon atom and subsequent sigma/pi rearrangement cationic eta (3)-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 [(eta (5)-C5H5)RhI2(L)] 12a,b.