1812 Organometallics 2006, 25, 1812-1819 Theoretical Investigation of Alkyne Metathesis Catalyzed by W/Mo Alkylidyne Complexes Jun Zhu, Guochen Jia,* and Zhenyang Lin* Department of Chemistry and Open Laboratory of Chirotechnology of the Institute of Molecular Technology for Drug DiscoVery and Synthesis, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ReceiVed February 5, 2006 In this paper, the mechanism of alkyne metathesis catalyzed by W/Mo alkylidyne complexes has been theoretically investigated with the aid of density functional theory calculations. Calculations on various model alkylidyne complexes M(tCMe)(OR)3 (M ) W, Mo; R ) Me, CH2F), W(tCMe)(NMe2)3, and W(tCMe)(Cl)3 allow us to examine the factors that influence the reaction barriers. In the reaction mechanism, metallacyclobutadienes are initially formed from a ring-closing step between alkynes and alkylidyne complexes. A ring-opening step then gives the metathesis products. The factors that determine the metathesis reaction barriers have been examined. The reaction paths leading to the formation of Cp complexes, a possible path deactivating catalytic activity, were also studied. Introduction several other groups,2b,6 consists of structurally not yet well- defined species formed in situ from Mo(CO)6 and a phenolic Catalytic alkene metathesis has become one of the primary additive (e.g., 4-chlorophenol). The simplicity and user-friendly tools in both organic synthesis and polymer chemistry, due to nature of this catalyst system are offset somewhat by its rather their extraordinary generality, chemoselectivity, and functional limited tolerance of polar functional groups and the elevated group tolerance.1 In comparison, the analogous alkyne meta- temperature (ca. 140-150 °C) required to initiate and maintain thesis is relatively less developed. The situation appears to be the catalytic activity. A major breakthrough in rational catalyst changing, given the recent advances in the development of new design for alkyne metathesis came with the development of the alkyne metathesis catalysts.2,3 well-defined and now widely used tungsten alkylidyne com- 7 Several catalytic systems have been used so far for alkyne plexes (RO)3WtCR′ by the Schrock group. Recently, the metathesis. The first effective catalytic system described in the Fu¨rstner group introduced a highly active monochloromolyb- literature consists of a heterogeneous mixture of tungsten oxide denum species which is conveniently prepared in situ by and silica that operates only at a very high temperature (ca. activation of the triamido complex Mo[N(t-Bu)Ar]3 (Ar ) 3,5- 4 8 200-450 °C). The second active catalytic system, which was C6H3Me2) with CH2Cl2 as a chlorine source. 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Soc. 2004, 126, 12796. 103, 3932. (n) Fu¨rstner, A.; Davies, P. W. Chem. Commun. 2005, 2307. (o) Zhang, (8) (a) Fu¨rstner, A.; Mathes, C.; Lehmann, C. W. J. Am. Chem. Soc. W.; Moore, J. S. J. Am. Chem. Soc. 2005, 127, 11863. (p) Nicolaou, K. C.; 1999, 121, 9453. (b) Fu¨rstner, A.; Mathes, C.; Lehmann, C. W. Chem. Eur. Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed. 2005, 44, 4490. J. 2001, 7, 5299. 10.1021/om060116p CCC: $33.50 © 2006 American Chemical Society Publication on Web 03/04/2006 Alkyne Metathesis Catalyzed by W/Mo Complexes Organometallics, Vol. 25, No. 7, 2006 1813 duced more recently by Cummins and Moore.9,10 All these studies show that metal centers and ligands can have drastic effects on the catalytic performance. For example, W(tCtBu)- t (O Bu)3 is astoundingly active for the metathesis of internal alkynes, while the corresponding Mo alkylidyne complexes Mo- t t (tC Bu)(O Bu)3 and the analogous W alkylidyne complexes t containing amide ligands W(tC Bu)(NR2)3 are almost meta- 11,12 t t thesis inactive. In addition, while Mo(tC Bu)(O Bu)3 does not react with internal alkynes and is catalytically inactive for alkyne metathesis, the analogous Mo alkylidyne complexes containing fluoroalkoxide (OC(Me)(CF3)2, OC(CF3)3) or phen- oxide ligands do react with internal alkynes to give isolable metallacyclobutadiene complexes and are excellent catalysts for metathesis of alkynes.11 For the monochloromolybdenum cata- lyst that was first developed by the Fu¨rstner group and refined by the Moore group, R,R,R-trifluoro-o-cresol or perfluoro-tert- butyl alcohol or p-nitrophenol was sometimes added for alcoholysis to produce active species for alkyne metathesis.10 All these observations show that the alkoxide ligands containing electron-withdrawing groups are likely to improve the activity of the catalysts for alkyne metathesis. The following questions t t deserve our attention. Why can W(tC Bu)(O Bu)3 catalyze the Figure 1. Energy profiles for the alkyne metathesis reactions t t t t catalyzed by W(tCMe)R (R ) OMe (a), NMe (b)). The free alkyne metathesis, but W(tC Bu)(N Bu2)3 and Mo(tC Bu)(O - 3 2 t t energies and relative energies (in parentheses) are given in kcal/ Bu)3 cannot? Why is Mo(tC Bu)(O Bu)3 almost metathesis t mol. inactive, whereas Mo(tC Bu)(OR)3 [OR ) OC(Me)(CF3)2, OC- (CF ) , or OAr ] are excellent catalysts for alkyne metathesis? 3 3 Computational Details The main purpose of the project is to answer these questions by computational chemistry through studying the detailed Molecular geometries of all the model complexes in this work mechanisms of alkyne metathesis.