Selective head-to-tail dimerization of phenylacetylene catalyzed by a diruthenium μ-methylene complex

The mu-methylene complex [Ru-2(mu-CH2)(CO)(4)(mu-dppm)(2)] (1) in toluene solution at 75 degreesC is the first known catalyst for the selective dimerization of PhC=CH or of PhC=CD to give the head-to-tail dimer PhC=CC(Ph)=CH2 or PhC=CC(Ph)-CD2 respectively, probably by a mechanism involving alkenyl-alkynyl group coupling at the diruthenium center. After many turnovers, a new diruthenium complex was detected and identified as [Ru-2{mu-eta(2)-PhCCH}(CO)(4)(mu-dppm)(2)] (2) which was not an active catalyst for the dimerization reaction, and the organic product PhCCMe was also detected at this stage. Complex 2 was prepared more readily by reaction of PhCCH with [Ru-2(mu-CO)(CO)(4)(mu-dppm)(2)] (3) in toluene at 75 degreesC. The reaction of 1 with a large excess of PhCCH gave further oligomerization and the major products were identified as an octamer and hexamer of PhCCH. Complex 1 was a catalyst for the dimerization of 1-hexyne to a mixture of dimerization products E-BuC=C-CH=CHBu and BuC=C-C(Flu) CH2 in a ratio of about 2: 1, and also for polymerization of HCCH, but it failed to react with the alkynes PhCCMe, PhCCPh or 3-hexyne at 75 degreesC in toluene. (C) 2002 Elsevier Science B.V. All rights reserved.