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

Reaction of a variety of C⋮CH 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, CH2CHCH2).
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 RuCHSiMe3 bond with the enyne C⋮CH
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(HC⋮CH)(CH2CH2)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.