Stereoselectivity of Metathesis of Various Acyclic Olefins with Chromium-, Molybdenum-, and Tungsten-Based Catalysts

The stereoselectivity of metathesis of cis- and trans-RCH=CHCH₃ has been determined by the trans/cis ratios of 2-butenes (C₄) at 0% conversion; the catalytic systems used were M(NO)₂X₂(PPh₃)₂ + EtAlCb (M = Mo, W; X = CI, Br, I) (1); M(CO)₅(PPh₃) + EtAlCl₂ + O₂ (M = Mo, W) (2); (arene)M(C0)₃ + EtAiC₂ (arene = benzene, mesitylene; M = Cr, Mo, W) (3). For 1, M = Mo, X = CI, and R = Et, the trans/cis ratios of C4 are equal to 0.20 and 12.5, respectively, for a cis and a trans olefin; increasing the bulkiness of R above R = Et results in a significant decrease of selectivity (and activity); with a cis olefin, the trans/cis ratio of C₄ increases when the bulkiness of R increases above a certain size (trans/cis ratio of C₄ = 0.44 for R = ;-Bu and 1, M = Mo, X = CI); with a trans olefin, the trans/cis ratio of C₄ decreases when the bulkiness of R increases above a certain size (trans/cis ratio of C₄ = 8.3 for R = r-Bu and 1, M = Mo, X = CI). This effect does not seem to be the result of an electronic effect of the R group since similar stereoselectivities are obtained with 1, M = Mo, R = ;-Bu, phenyl, or benzyl; changing the coordination sphere of the precursor catalyst (1, M = Mo, W; X = CI, Br) does not result in a meaningful variation of stereoselectivity. For cis-and trans-2-pentene, a comparison was made between the trans/cis ratios of C4 and the trans/cis ratios of Ce: for 1, M = Mo, W; X = CI, Br, the trans/cis ratio of Ce is always higher than the trans/cis ratio of C4. This result, which seems to be related to 1-2 a-e e-a repulsive interaction in the metallocyclobutane transition state, is corroborated by the stereoselectivities of the metathesis of a olefins propylene, 1-butene, and 1-pentene: for 1, M = Mo, X = CI, the trans/cis ratio of the products increases in the order 2-butene (1.1) 〼3-hexene (1.42) < 4-octene (2.0). The stereoselectivity of metathesis of acyclic olefins is explained on the basis of many competitive interactions occurring during olefin coordination to the metallocarbene and in the metallocyclobutane transition state: 1-3 diaxial interaction, 1 -2 equatorial-axial interaction, and olefin-metal (and/or its ligands) interaction M₄-C₁-C₂-C₃. Comparison of the stereoselectivities between Cr, Mo, and W was carried out with R = Et and 1 (M = Mo, W; X = CI), 2 (M = Mo, W), and 3 (M = Cr, Mo, W; arene = mesitylene). The variations of stereoselectivity observed (Cr < Mo ≫ W) may be explained on the following basis: the Mo-C distance is smaller than the W-C distance in the metallocyclobutane; this would result in an increase of the 1 -3-diaxial interaction.