Tungsten and Molybdenum 2,4,6‐Trimethylbenzylidyne Complexes as Robust Pre‐Catalysts for Alkyne Metathesis

Abstract

A series of 2,4,6-trimethylbenzylidyne tungsten and molybdenum complexes was prepared from the tribromides mer-Successive reaction of complexes 9 with lithium or potassium hexafluoro-tert-butoxide and lithium 1,3di-tert-butylimidazolin-2-imide, (Im t-Bu N)Li, afforded the imidazolin-2-iminato complexes [MesC M{OC-} 3 ] (7b) was also prepared, and its molecular structure together with those of 10a, 10b, 11b, 12a and 12b were established by X-ray diffraction analysis. Complexes 10 and 11 were employed as pre-catalysts for the alkyne metathesis of the test substrate 3-pentynyl benzyl ether (13) at low catalyst loadings (1 mol%) in the presence of molecular sieve (5 ). Comparative studies of these 2,4,6trimethylbenzylidyne species (MesC M) with their benzylidyne analogues (PhC M) revealed that the increased steric bulk renders the former more stable and manageable in air in solid form for shorter periods of time, but at the expense of a slower initiation, which requires higher temperatures or longer reaction times.

Well‐Defined Alkyne Metathesis Catalysts: Developments and Recent Applications

“…[62] In-depths tudies on the influence of the alkylidyne ligand revealed ane nhanced air-stability for sterically more demanding 2,4,6-trimethylbenzylidyne complexes. [63] Complexes with a2 ,4,6-trimethylbenzylidyne moiety could be exposed to air for al imited time span while maintaining sufficientc atalytic activity.H owever,u nlimited stability in air could not be achieved by this approach. [63] For an extended study on the structure-activity relationship, as eries of fluoroalkoxy-supported benzylidyne complexes of the type [MesCM{OC(CF 3 ) n Me 3-n } 3 ]( Mes = mesityl, n = 0, 1, 2, 3) was synthesized and ac ombinedt heoretical and experimental investigation on their catalytic activity was performed (Scheme 5).…”

“…[63] Complexes with a2 ,4,6-trimethylbenzylidyne moiety could be exposed to air for al imited time span while maintaining sufficientc atalytic activity.H owever,u nlimited stability in air could not be achieved by this approach. [63] For an extended study on the structure-activity relationship, as eries of fluoroalkoxy-supported benzylidyne complexes of the type [MesCM{OC(CF 3 ) n Me 3-n } 3 ]( Mes = mesityl, n = 0, 1, 2, 3) was synthesized and ac ombinedt heoretical and experimental investigation on their catalytic activity was performed (Scheme 5). [64,65] As for tungsten, only WF3 was synthesized and analyzed because WF6 already showedn oc atalytic activity.D FT calculations on the rate-determining step of alkyne metathesis as af unctiono ft he fluorination degree suggested ac lear trend.…”

“…Inspiredb yt he work of C. CopØret and others, [76] tris(tert-butoxy)silanolate-supported benzylidyne complexes 25 were prepared from precursor 11 as homogeneous analogues to silica-supported catalysts (Scheme 10). [63,77] Catalytic reactions with complexes 25 were carried out under molecular sieves promoted conditions and afforded excellent yields in the self-metathesis and RCAM of internal alkynes. Under conventionalv acuum driven conditions yields were also satisfactory but generally lower,a nd reaction temperatures of 80 8Cw ere partly required.…”

The chemistry of the carbon-transition metal double and triple bond: Annual survey covering the year 2014

“…. 1550, 1551) were prepared and tested in the metathesis of 3-pentynyl benzyl ether [1321]. The catalysts were more stable in air than their benzylcarbyne analogs however were slower initiators of alkyne metathesis.…”

In SituGeneration of Molybdenum-Based Catalyst for Alkyne Metathesis: Further Developments and Mechanistic Insights

“…Since more than four decades, tremendous efforts have been devoted to the search of new catalysts for metathesis reactions in general, in particular via the synthesis and use of well-defined, active metathesis catalysts, essentially based on ruthenium, tungsten and molybdenum. In that context, a strong interest has been devoted within the last decade to the synthesis of molecular initiators for alkyne metathesis, as revealed by recent publications in the field [4][5][6][7]. However, the use of in situ catalysts still remains of interest both from academic and industrial point of views, as they are generally prepared using procedures that are less sensitive to oxygen and water, whereas the synthesis and uses of well-defined and active carbenes or carbynes would require their prior synthesis and working under more specific experimental conditions.…”