B(C₆F₅)₃ Activation of Oxo Tungsten Complexes That Are Relevant to Olefin Metathesis

Abstract

We have found that coordination of B(C 6 F 5 ) 3 to an oxo ligand in tungsten oxo alkylidene bis(aryloxide) complexes, where the aryloxide is O-2,6-(mesityl) 2 C 6 H 3 (HMTO) or 2,6-diadamantyl-4-methylphenoxide (dAdPO), accelerates the formation of metallacyclobutane complexes from alkylidenes as well as the rearrangement of metallacyclobutane complexes. In contrast, a tungstacyclopentane complex, W(O)(C 4 H 8 )(OHMT) 2 , is relatively stable toward rearrangement in the presence of B(C 6 F 5 ) 3 . A careful balance of steric factors allows a single isomer of W(O)(trans-4,4-dimethylpent-2-ene)(dAdPO) 2 to be formed from W(O)(CH-t-Bu)(dAdPO) 2 in the presence of both ethylene and B(C 6 F 5 ) 3 .

Interaction of Metal Oxido Compounds with B(C6F5)3

“…It was shown that upon using sterically demanding alkoxido, amido, and other ligands, bimolecular deactivationm ay be prevented but such compounds tend to be lessr eactive in metathesis reactions. [83][84][85][86][87][88] The investigation of Lewis acid-base interaction in olefin metathesis dates back to Osborn's description of ab ridge between catalyst and cocatalyst as described above (section 2), however, B(C 6 F 5 ) 3 was at that time not considered. [29] Significant reactivity enhancemento ft he oxido alkylidene catalysts used in metathesis reactions was achieved by addition of the borane Lewis acid.…”

“…Although the coordinationo fb orane to the oxido group is proposed,f urthere vidence remains to be found. [84] Also, molybdenumo xido alkylidene compounds were reacted with B(C 6 F 5 ) 3 and used as catalyst precursors in metathesis reactions. The adduct [Mo{O-B(C 6 F 5 ) 3 }(CHSiMe 3 )(N = PtBu 3 ) 2 ] (50)w as easily accessible by reactiono ft he parent complex with the borane.…”

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

“…It is thus tentatively attributed to a p-olefin W IV complex,b ased on the chemical shift of similarmolecular complexes. [19] To confirm that hypothesis, as ample of catalyst, which had previously been exposed to 13 Cd ilabelede thylene and subsequently dried under high vacuum (10 À5 mbar) for 2h at RT,w as suspended in C 6 D 6 and treated with Br 2 . 13 CNMR spectroscopy of the solution revealed the releaseo f1 ,2-dibromoethane (FigureS8) as the main 13 Ce nriched product, which is consistentw ith the presence of the proposed p-ethylene complex ( Figure S13).…”

Metathesis by Molybdenum and Tungsten Catalysts

“…29 Several examples of natural products synthesized in the last few years that involve one or more olefin metathesis steps are Epothilone C, 30 metathesis catalysts best achieves each of the metathesis reactions is determined through screening procedures, and trends are beginning to emerge. Newer catalysts, e.g., Lewis acid activated 33 or high oxidation state complexes that contain an NHC ligand, 34 have not yet been examined to any significant degree as catalysts for various types of metathesis reactions. It is becoming increasingly clear that small changes in a catalyst can have significant consequences in terms of yields, turnover, and selectivities, and that access to a large variety of catalysts therefore is required for continued advances toward solutions to the problems that remain.…”