PG-S R PG-S R N N Mes Mes Cl Cl Ph Ru PCy 3 Ru Cl Cl O N N Mes Mes i-Pr PCy 3 PCy 3 Ru Cl Cl Ph 1 2 3 (1) ethylene 5 mol % [Ru] cat. TETRAHEDRON LETTERS Tetrahedron Letters 43 (2002) 209–211 Pergamon Ethylene metathesis of sulfur-containing alkynes Jason A. Smulik, Anthony J. Giessert and Steven T. Diver* Department of Chemistry, University at Buffalo, State University of New York, Buffalo, NY 14260 -3000, USA Received 20 August 2001; revised 21 September 2001; accepted 5 November 2001 Abstract—Enyne metathesis of sulfur-containing alkynes and ethylene has been achieved. High yields were obtained by use of thiol esters in the alkyne partner for the cross metathesis with ethylene. The necessary reactivity and functional group compatibility were achieved through the use of the Grubbs’ second generation benzylidene carbene catalyst. © 2002 Elsevier Science Ltd. All rights reserved. There have been tremendous advances in alkene metathesis due largely to functional group compatibility of the ruthenium carbenes developed by Grubbs. With the supporting dihydroimidazole carbene ligand, the ruthenium carbenes are more active in alkene metathesis 1 and more tolerant of potentially coordinat- ing functional groups like alcohols and ethers. Despite these recent advances and the widespread utilization of metathesis, there has been little application of metathe- sis to unsaturated organosulfur compounds. 2,3 The ear- liest examples 2a,b described ring-closing metathesis (RCM) using both the Grubbs and Schrock catalysts. To the best of our knowledge, there have been no applications using sulfur-containing alkynes. The lim- ited number of examples of metathesis with sulfur-con- taining substrates can be partly explained by the fact that middle to late transition metals used as catalysts may interact favorably with the soft sulfur atom (e.g. Pearson hard–soft acid–base theory). 4 In metathesis, any stabilization of species on the catalytic reaction coordinate could deplete active catalyst and shut down catalysis. Previous work suggested that the Grubbs’ catalyst 1, possessing the strong sigma-donating N -het- erocyclic carbene ligand, could overcome coordination by oxygen in enyne metathesis. 5 In this communica- tion, cross metathesis of sulfur-containing alkynes with ethylene employing ruthenium carbene 1 is reported (Scheme 1). Lack of reactivity or lack of turnover in some alkenes bearing coordinating functionality has been attributed to chelated metal carbenes. 6 Coordination depends on the proximity of functional groups to the intermediate metal carbenes and can result in stabilized chelated structures or result in catalyst decomposition. How catalyst 1 overcomes or averts these problems in enyne metathesis is not well understood, although recent mechanistic studies explaining the high activity of 1 in alkene metathesis 1b,c provide some important clues. Our investigation was prompted by the question whether potentially coordinating sulfur functionality could be tolerated by the new catalyst 1 as it pertains to syn- thetic effort in our group directed toward the synthesis of sulfur-containing natural products. Of more general interest is the question under what circumstances is sulfur permissible in ruthenium-catalyzed alkene– alkyne metathesis. Initial enyne metathesis of propargyl thioethers with ethylene gas 7 gave disappointing results. Using catalyst 1 (5 mol%), the benzyl ether 4A gave only 3% conver- sion (gc) after 6 h, reaction conditions that result in complete conversion of the ether 6 to its diene. 5 This is Scheme 1. * Corresponding author. E-mail: diver@nsm.buffalo.edu 0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII:S0040-4039(01)02098-6