C-H Activation via Carbodiimide Insertion into Yttrium-Carbon Alkynide Bonds: An Organometallic Alder-ene Reaction

The yttrium alkynide (C5Me5)(2)Y(C CPh)(THF), 1, and the related trienediyl [(C5Me5)(2)Y](2)(mu-eta(2):eta(2)-PhC=C=C=CPh), 2, can be isolated from the reaction of (C5Me5)(2)Y(eta(3)-CH2CHCH2)(THF) with phenylacetylene in THF and hexane, respectively. Complex 1 reacts with (BuN)-Bu-t=C=(NBu)-Bu-t to afford the conventional amidinate insertion product (C5Me5)(2)Y[(BuNC)-Bu-t(C CPh)(NBu)-Bu-t-kappa N-2,N'], 3. However, the analogous reaction with iso-propyl and cyclohexyl carbodiimides involves C-H activation and forms the iminovinyl complexes (C5Me5)(2)Y[C(=CHPh)C(N=CMe2)=(NPr)-Pr-t-kappa C-2,N], 4, and (C5Me5)(2)Y{C(=CHPh)C[N=C(CH2)(5)]=NCH(CH2)(5)-kappa C-2,N}, 5, respectively. The reaction is formally a variation of the Alder-ene reaction in which a C-H bond of the carbodiimide (ene) is activated and transferred to the alkynide ligand (enophile) bound to yttrium. The trienediyl 2 reacts with iso-propyl carbodiimide via conventional insertion to form a bis(amidinate) product with a trienediyl linker, namely, (C5Me5)(2)Y-[mu-kappa(2)-((PrN)-Pr-i)(2)C-C(Ph)=C=C=C(Ph)-C((NPr)-Pr-i)(2)]Y(C5Me5)(2), 6. The alkynide ligand in 1 is also modified in the reaction with benzylcyanide that forms an unusual insertion product, the amidonitrile complex [(C5Me5)(2)Y{mu-N(H)C(CH2Ph)=C[C(Ph)=CHPh]C N}](2), 7.