Selective Ruthenium-Catalyzed Transformations of Enynes with Diazoalkanes into Alkenylbicyclo[3.1.0]hexanes

Reaction of a variety of CCH bond-containing 1,6-enynes with N₂CHSiMe₃ in the presence of RuCl(COD)Cp* as catalyst precursor leads, at room temperature, to the general formation of alkenylbicyclo[3.1.0]hexanes with high Z-stereoselectivity of the alkenyl group and cis arrangement of the alkenyl group and an initial double-bond substituent, for an E-configuration of this double bond. The stereochemistry is established by determining the X-ray structures of three bicyclic products. The same reaction with 1,6-enynes bearing an R substituent on the C₁ carbon of the triple bond results in either cyclopropanation of the double bond with bulky R groups (SiMe₃, Ph) or formation of alkylidene−alkenyl five-membered heterocycles, resulting from a β elimination process, with less bulky R groups (R = Me, CH₂CHCH₂). The reaction can be applied to in situ desilylation in methanol and direct formation of vinylbicyclo[3.1.0]hexanes and to the formation of some alkenylbicyclo[4.1.0]heptanes from 1,7-enynes. The catalytic formation of alkenylbicyclo[3.1.0]hexanes also takes place with enynes and N₂CHCO₂Et or N₂CHPh. The reaction can be understood to proceed by an initial [2+2] addition of the RuCHSiMe₃ bond with the enyne CCH bond, successively leading to an alkenylruthenium−carbene and a key alkenyl bicyclic ruthenacyclobutane, which promotes the cyclopropanation, rather than metathesis, into bicyclo[3.1.0]hexanes. Density functional theory calculations performed starting from the model system Ru(HCCH)(CH₂CH₂)Cl(C₅H₅) show that the transformation into a ruthenacyclobutane intermediate occurs with a temporary η³-coordination of the cyclopentadienyl ligand. This step is followed by coordination of the alkenyl group, which leads to a mixed alkyl−allyl ligand. Because of the non-equivalence of the terminal allylic carbon atoms, their coupling favors cyclopropanation rather than the expected metathesis process. A direct comparison of the energy profiles with respect to those involving the Grubbs catalyst is presented, showing that cyclopropanation is favored with respect to enyne metathesis.