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

Abstract

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.