Intramolecular cyclization of arylalkynes with C-O bond formation on ruthenium complexes

Cyclization of terminal arylalkynes containing aldehyde (1a), ketone (1b-c) or hydroxyl (6a-c) functionality on the aryl ring induced by ruthenium complex is investigated. For the reaction of Cp(PPh₃)₂RuCl with o-ethynyl benzaldehyde 1a, a vinylidene intermediate was formed which is followed by a carbonyl attack to give 2a. However, for 1b-c, with a ketone replacing the aldehyde group, isobenzofuryl carbene complexes (4b-c) were formed as the major products along with the isochromene carbene complexes (2b-c) as the minor products. For three o-enynyl phenol compounds 6a-c with different substituents on the aryl ring, the reactions take place at the triple bond first giving vinylidene intermediates which undergo facile intramolecular cyclization to yield the cyclic oxocarbene complexes 7a-c in excellent yields. Modification of various substituents at Cβ of the carbene ligand of 7 by the use of different alkyl halides can be readily achieved under mild condition. For the chromane ring derivatives 11, the two allyl groups underwent ring closing metathesis reaction in the presence of Grubb's catalyst and yielded the tricyclic product 12. These chemical reactions and their mechanisms are corroborated by structure determinations of two ruthenium complexes using single crystal X-ray diffraction analysis.