Intramolecular cyclization of ruthenium vinylidene complexes with a tethering vinyl group: facile cleavage and reconstruction of the C-C double bond.

Protonation of ruthenium acetylide complexes [M]-CCCPh2CH2CH=CH2 (2a, [M] = (eta5-C5H5)(P(OPh)3)(PPh3)Ru; 2a', [M] = (eta5-C5H5)(dppp)Ru; *C = 13C-labeled carbon atom) with HBF4 in ether produces [[M]=C=CHCH2CPh2CH=CH2][BF4] (4, 4') exclusively via a metathesis process of the terminal vinyl group with the *C=*C of the resulting vinylidene group. For 4 in methanol, bond reconstruction of the two labeled *C atoms readily takes place via a retro-metathesis process followed by a cyclization of the resulting vinylidene ligand giving the cyclic carbene complex 5, which is fully characterized by single-crystal X-ray diffraction analysis. The protonation of 2a in MeOH is followed by a cyclization, also giving 5. Deuterium-labeling study indicates that the C-C bond formation of this cyclization proceeds simultaneously with the formation of 4 consistent with facile cleavage and reconstruction of C=C bonds. For comparison, complex 4 in alcohol yields, besides 5, the corresponding alkoxycyclohexene 6. Formation of 6 from 4 also involves a skeletal rearrangement with reconstruction of the C=C bond. Interestingly, [[Ru']=C=C(Me)CH2CPh2CH=CH2][BF4] (8') originally from a complex with two connected labeled carbon atoms also undergoes reestablishment of the *C=*C bond yielding the cyclic allenyl complex 9'. 13C-labeling studies clearly reveal the reestablishment of two C=C double bonds in the transformation of both 4 to 5 and 8' to 9'. The proposed mechanism implicates a cyclobutylidene intermediate formed either via a regiospecific [2+2] cycloaddition of two double bonds in the ruthenium vinylidene 4 or via a cyclization of 4 giving a nonclassical ion intermediate followed by a 1,2-alkyl shift.