Reactions of Ruthenium Complexes with Terminal Aryl Alknyes and 2-Ethynylphenyl Vinyl Ethers

Herein, we report that η5-pentadienyl ruthenium complex (η5-C5H7)(PPh3)2RuCl (1) is an efficient catalyst for the synthesis of 1,4-disubstituted butenynes in THF from the dimerization of aryl alkynes with a catalyst loading as low as 1-2 mol % under mild conditions, furnishing the (Z)-butenyne as the major product in the moderate yield along with a small amount of the E isomer. When aryl group which contains the substituent in the ortho position, the catalytic procedure results seem to indicate that the steric effect affects the yields of the desired enyne product more than the electronic effect. Dimerization reactions in different solvents showed that oxygen-containing solvents such as THF, diethyl ether, and acetone are better solvents than nitrogen-containing solvents. O-containing solvents are known to have relatively weaker coordinating power than the N-containing solvents. These results possibly indicate that the process could be assisted by the weak coordination between a solvent molecule and the metal center to stabilize the active intermediate. Treatment of a phenyl alkyne with 1 and KPF6, both in stoichiometric quantities, gives a phenyl phosphonium salt 6, whose structure has been confirmed. A series of synthesis of polyaryl phosphonium salts (APS) by cyclotrimerization of various aryl alkynes are also induced by a stoichiometric amount of the ruthenium η5-pentadienyl complex (η5-C5H7)(PPh3)2RuCl (1). A mechanism consisting of continuous insertion reactions of aryl alkynes into Ru-C bonds is proposed for the formations of enynes and APSs. In second study, catalytic cyclization reactions of three 2-ethynylphenyl vinyl ether 7a, 7b-Z and 7d in various alcohol are efficiently catalyzed by [Ru]Cl to give the benzoxepine derivatives, respectively. Surprisingly, in CH2Cl2, 7a and 7b containing no methyl and one methyl substituted vinyl groups undergo different skeletal rearrangement processes involving cleavage of C≡C and C=C bonds, respectively, in their reactions with [Ru]Cl. For both 7a and 7b, the terminal triple bonds are converted to the vinylidene ligands, then different [2+2] cycloadditions of the vinyl group with either the Cα=Cβ or the Ru=Cα bonds of the vinylidene ligand are proposed to take place. In the reaction of 7a, with no methyl group, the [2+2] cycloaddition process involves the vinylidene Cα=Cβ bond leading to formation of a bicyclic carbene intermediate followed by cleavage of the original C≡C triple bond, to give the cationic γ,δ-unsaturated vinylidene complex 9a which contains the trans-form double bond detected by coupling constant in NMR spectrum. The skeletal rearrangement of 7b-Z and -E, with one terminal methyl group on the vinyl unit, is believed to proceeds via a different [2+2] cycloaddition process involving the Ru=Cα bond, followed by cleavage of the C=C double bond of the original vinyl group to afford the carbene complex 14b. The structure of 14b is determined by single crystal X-ray diffraction analysis. Thus in the reaction of [Ru]Cl with a similar aryl vinyl ether 7c containing two terminal methyl groups on the vinyl unit, only the vinylidene complex 16c is isolated with no C-C bond formation. In addition, this catalytic cyclization is inhibited by the presence of a terminal methyl group on the vinyl unit trans to the phenolic oxygen atom as in the case of 7b-E. We also found that hydrolysis of i-propenyl phenyl ether to phenol probably occurs prior to cyclization to lead the formation of carbine complex 18d. Labeling experiments using 13C and 2D isotopes lead to better discernment of the skeletal rearrangement of the enyne system.