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

The yttrium alkynide (C5Me5)2Y(CCPh)(THF), 1, and the related trienediyl [(C5Me5)2Y]2(μ-η2:η2-PhCCCCPh), 2, can be isolated from the reaction of (C5Me5)2Y(η3-CH2CHCH2)(THF) with phenylacetylene in THF and hexane, respectively. Complex 1 reacts with tBuNCNtBu to afford the conventional amidinate insertion product (C5Me5)2Y[tBuNC(CCPh)NtBu-κ2N,N′], 3. However, the analogous reaction with iso-propyl and cyclohexyl carbodiimides involves C–H activation and forms the iminovinyl complexes (C5Me5)2Y[C(CHPh)C(NCMe2)NiPr-κ2C,N], 4, and (C5Me5)2Y{C(CHPh)C[NC(CH2)5]NCH(CH2)5-κ2C,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)2Y[μ-κ2-(iPrN)2C–C(Ph)CCC(Ph)–C(NiPr)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)2Y{μ-N(H)C(CH2Ph)C[C(Ph)CHPh]CN}]2, 7.