Ru(II)-Catalyzed Cycloadditions of 1,6-Heptadiynes with Alkenes: New Synthetic Potential of Ruthenacyclopentatrienes as Biscarbenoids in Tandem Cyclopropanation of Bicycloalkenes and Heteroatom-Assisted Cyclocotrimerization of 1,6-Heptadiynes with Heterocyclic Alkenes

The ruthenium(II)-catalyzed tandem cycloaddition of 1,6-heptadiynes with bicyclic alkenes, such
as bicyclo[3.2.1]heptenones and norbornene derivatives, furnishes the 1:2 adducts between the diynes and two
molecules of the bicycloalkenes together with common [2 + 2 + 2] cyclocotrimerization products. The structure
of a representative tandem 1:2 adduct between dimethyl dipropargylmalonate and 2,4-dimethylbicyclo[3.2.1]oct-6-en-3-one was unequivocally determined by X-ray analysis and was concluded to involve an unusual
1,2-dicyclopropylcyclopentene skeleton. On the basis of the spectroscopic analogy, the previously communicated
structures of the tandem cycloadducts between the diynes and norbornene derivatives were corrected. The
formation of the tandem double-cyclopropanation products from the diynes is chemical evidence of a bis-carbenoid hybrid structure, 1,3,5-metallacyclopentatriene, of the corresponding 2,4-metallacyclopentadiene
intermediates. The selectivity for the formation of the tandem cyclopropanation adducts was increased in the
order of (η5-C9H7)Ru(PPh3)2Cl > CpRu(cod)Cl > Cp*Ru(cod)Cl, indicative of the η5 → η3 ring slippage of
the cyclopentadienyl type ligands playing a key role in the tandem cyclopropanation. On the other hand, the
normal [2 + 2 + 2] cyclocotrimerization between 1,6-heptadiynes and alkenes was selectively catalyzed by
Cp*Ru(cod)Cl, in the case of cyclic or linear alkenes possessing heteroatoms at the allylic position. The latter
heteroatom-assisted cyclocotrimerization was also catalyzed by a paramagnetic dinuclear ruthenium(III) complex,
[Cp*RuCl2]2, at lower temperature.