Strongly σ Donating Thiophenoxide in Silica-Supported Tungsten Oxo Catalysts for Improved 1-Alkene Metathesis Efficiency

Abstract

We report the synthesis of tungsten oxo alkylidene complexes bearing bulky thiophenoxide ligands [W(O)(CHtBu)-(SHMT) 2 (PMe 2 Ph)] and [W(O)(CHtBu)(SHMT) 2 ] (SHMT = 2,6-dimesitylthiophenoxide) and their grafting on partially dehydroxylated silica, affording the supported complexes [(SiO)W(O)(CHtBu)-(SHMT)(PMe 2 Ph)] and [(SiO)W(O)(CHtBu)(SHMT)]. While the molecular precursors are not significantly active in the metathesis of alkenes, the grafted analogue without bound phosphine ligands displays activity comparable to that of its aryloxide analogue [(SiO)W(O)( CHtBu)(OHMT)] (OHMT = 2,6-dimesitylphenoxide). It is worth noting that [(SiO)W(O)(CHtBu)(SHMT)] showed unprecedented activity in the metathesis of 1-alkenes, probably because of the lower stability of metallacyclobutane intermediates.A dvances in alkene metathesis over the last 30 years have transformed the way organic molecules are prepared. 1−3 Of d 0 alkene metathesis catalysts, 4−7 supported tungsten oxo alkylidene complexes have recently emerged and been shown to have unprecedented activities and stabilities. 8,9 In particular, silica-supported tungsten oxo alkylidene bearing phenolate ligands catalyze the metathesis of internal olefins at loading as low as 50 ppm. However, they are surprisingly sluggish for 1-alkenes, because of the formation of very stable unsubstituted metallacycles generated by the ethylene byproduct. This reversible deactivation process could be circumvented by using a reactor setup, allowing for the continuous removal of ethylene.DFT calculations have shown that dissymmetry at the metal center (X ≠ Y) for d 0 Schrock-type catalysts of general formula (X)(Y)M(E)(CHR) (M = Mo, W, Re; E = O, NR, CR) not only decreases the energy barrier for coordination of the olefin substrate (the key step in metathesis) but also raises the energy of the metallacyclobutane intermediates. 10−14 In silica-supported phenolate tungsten oxo alkylidene complexes such as [(SiO)W(O)(CHtBu)(OHMT)] and [(SiO)W( O)(CHtBu)(dAdPO)] (OHMT = 2,6-dimesitylphenoxide, dAdPO = 2,6-diadamantyl-4-methylphenoxide), the σ donations of the phenoxide and the surface siloxide ligands are very similar. We thus reasoned that thiolate analogues 15 would be ideal targets, since they are isoelectronic with the corresponding phenolates 16−19 with the advantage of restoring the dissymmetry at the metal center.Herein we report the synthesis, the grafting on partially dehydroxylated silica at 700°C (SiO 2-(700) ), and the catalytic performance of oxo alkylidene complexes with pendant thiolate ligands [W(O)(CHtBu)(SHMT) 2 (PMe 2 Ph)] (1) and [W(O)(CHtBu)(SHMT) 2 ] (2) (SHMT = 2,6-dimesitylthiophenoxide) 15,20 and the corresponding supported systems.The thiolate complex 1 was synthesized by salt metathesis of [W(O)(CHtBu)Cl 2 (PMe 2 Ph) 2 ] 21,22 with 2 equiv of potassium 2,6-dimesitylthiophenolate in 70% yield. A similar synthesis was independently described by Schrock and Hoveyda during the course of our studies. 16 The phosphinefree complex 2 was prepare...

Activating Thiolate‐Based Imidoalkylidene Tungsten(VI) Metathesis Catalysts by Grafting onto Silica

“…This is in sharp contrastw ith the corresponding oxo complex, which failed to yield the corresponding metallacyclobutane. [11] Both heteronuclear correlation (HETCOR)a nd spin diffusion experiments ( Figure S7 and Figure 2) indicate that the peaks at d = 40.5 and 45.6 ppm are consistent with the a and b-carbons of am etallacyclobutane in squarep yramidal (SP) geometry, which is consistent with assigning thiolatea sastrong sdonor. [9] The peak at d = 64.7 ppm only shows an autocorrelation peak and does not show any correlation in spin diffusion experiments.…”

“…[9] Worthy of note is that ar uthenium catalysts bearingb ulky thiolate ligandsi sahighly Z-selective in olefin metathesis catalytic activity, [10] and the activity of silica-supported Wo xoalkylidene complexes towards 1-alkenes is improved compared with the corresponding alkoxide/phenoxide analogues. [11] In contrast, the activity of imidoalkylidene tungsten complexesw ith two thiolate ligandsi sn ot significant, and their MAP derivatives have been recentlys hown to be only slightly active. [11][12] Here, we report that graftinga[W(NAr)(CHCMe 3 )(SAr') 2 ]c omplex (Ar = 2,6-diisopropylphenyl, Ar' = 2,4,6-triisopropylphenyl) onto silica, which has been partially dehydroxylated at 700 8C (SiO 2À(700) ), yields [( SiO)W(NAr)(CHCMe 3 )(SAr')],w hich is ah ighly active, well-defined catalyst.…”

“…[11] In contrast, the activity of imidoalkylidene tungsten complexesw ith two thiolate ligandsi sn ot significant, and their MAP derivatives have been recentlys hown to be only slightly active. [11][12] Here, we report that graftinga[W(NAr)(CHCMe 3 )(SAr') 2 ]c omplex (Ar = 2,6-diisopropylphenyl, Ar' = 2,4,6-triisopropylphenyl) onto silica, which has been partially dehydroxylated at 700 8C (SiO 2À(700) ), yields [( SiO)W(NAr)(CHCMe 3 )(SAr')],w hich is ah ighly active, well-defined catalyst. [13] The bis-thiolate complex [W(NAr)(CHCMe 3 )(SAr') 2 ]( 1)w as obtained as yellow crystals in 68 %y ield by salt metathesis of [W(NAr)(CHCMe 3 )(DME)(OTf) 2 ]w ith two equivalents of Ar'SK at À40 8Ci nt oluene (Scheme 1).…”

Isostructural Molecular and Surface Mimics of the Active Sites of the Industrial WO3/SiO2 Metathesis Catalysts

“…In contrast, using Surface Organometallic Chemistry, [3][4][5][6] a large family of highly active supported alkene metathesis catalysts has been reported based on welldefined metal alkylidenes in the past 15 years. [7][8][9][10][11][12][13][14][15] These numerous complexes have given access to a large number of spectroscopic and catalytic data set that have contributed to a better understanding of the mechanism and reaction intermediates involved in alkene metathesis. 10,12,14 In an attempt to bridge the gap between these and classical illdefined WO 3 /SiO 2 catalysts, 1,2,16 we have recently investigated the synthesis of silica supported oxo alkylidene complexes, which present very high activity ( Figure 1b).…”

Olefin Metathesis by Group VI (Mo, W) Metal Compounds

“…Indeed, the coordination environment of 22 More recently, Schrock et al synthesized new oxo tungsten complexes bearing various ligands and studied their grafting on silica dehydroxylated at 700°C. Ligands such as 2,6-mesitylphenoxy, 2,6-diadamantyl-methylphenoxy, thio-2,6-masitylphenoxy, or tris(tert-butoxy)siloxy were used and the corresponding carbenes were synthesized (Figure 25) [34,[89][90][91]. 2 ], is formed by protonation of the carbene moiety by the silanol group [90].…”