Tuning the Catalytic Alkyne Metathesis Activity of Molybdenum and Tungsten 2,4,6-Trimethylbenzylidyne Complexes with Fluoroalkoxide Ligands OC(CF₃)_nMe_3–n (n = 0–3)

Abstract

The molybdenum and tungsten 2,4,6-trimethylbenzylidyne complexes [MesCM{OC(CF 3 ) n Me 3−n } 3 ] (M = Mo: MoF0, n = 0; MoF3, n = 1; MoF6, n = 2; MoF9, n = 3; M = W: WF3, n = 1; Mes = 2,4,6-trimethylphenyl) were prepared by the reaction of the tribromides [MesCMBr 3 (dme)] (dme = 1,2-dimethoxyethane) with the corresponding potassium alkoxides KOC(CF 3 ) n Me 3−n . The molecular structures of all complexes were established by X-ray diffraction analysis. The catalytic activity of the resulting alkylidyne complexes in the homometathesis and ring-closing alkyne metathesis of internal and terminal alkynes was studied, revealing a strong dependency on the fluorine content of the alkoxide ligand. The different catalytic performances were rationalized by DFT calculations involving the metathesis model reaction of 2-butyne. Because the calculations predict the stabilization of metallacyclobutadiene (MCBD) intermediates by increasing the degree of fluorination, MoF9 was treated with 3-hexyne to afford the MCBD complex [(C 3 Et 3 )Mo{OC(CF 3 ) 3 } 3 ], which was characterized spectroscopically.

Well‐Defined Alkyne Metathesis Catalysts: Developments and Recent Applications

“…However,t he calculations also indicated energetically stabilized MCBD complexes for MoF9, WF6 and WF9 and their [2+ +2] cycloelimination could become the rate-determining step for these catalysts. [64,65] As for actual catalytic performance, the activity increased in the order MoF0 < MoF3 < MoF6,w hichw as in line with ad ecreased activation barrier.C orrelating witha no verly stabilized MCBD, the catalytic activity was diminished for MoF9 andn ot erminal alkyne metathesis could be detected. For the tested tungsten complex WF3,h igh yields even in terminal alkynem etathesis were accomplished.…”

“…[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. With increasing degree of fluorination, the inter-Scheme4.Synthesis of tungsten and molybdenum benzylidyne complexes 11 as precursors for active alkyne metathesis catalysts.…”

Unraveling the Mechanism of 1,3‐Diyne Cross‐Metathesis Catalyzed by Silanolate‐Supported Tungsten Alkylidyne Complexes

“…In line with previous studies, we have computed all relevant intermediates and transition states for the two considered catalytic cycles for a model reaction of the tungsten ethylidyne complex [MeC≡W{OSi(O t Bu) 3 } 3 ] with 2,4‐hexadiyne as the substrate (Scheme , R 1 =R 2 =Me). In addition, alternative reaction pathways for diyne disproportionation were studied computationally.…”

Syntheses of Molybdenum Oxo Alkylidene Complexes through Addition of Water to an Alkylidyne Complex

“…13 More recently, Veige has reported conversion of a tungsten alkylidyne into a tungsten oxo alkylidene 14 and theoretical studies have addressed movement of protons in the primary coordination sphere of high oxidation state complexes. 15 Because molybdenum(VI) alkylidyne complexes can now be prepared from Fisher-type alkylidenes, 16 reactions between a Mo alkylidyne complex and water would seem to be a plausible approach to the synthesis of Mo oxo alkylidene complexes. We have now prepared Mo oxo alkylidene complexes in this manner that are reactive in olefin metathesis reactions.…”

“…The reaction between 1 16a and C 6 H 4 ( o- OMe)C≡CTMS gave 2 in moderate yield (Scheme 1). Addition of one equiv of water to 2 in the presence of one equiv of PPhMe 2 led to 3(PPhMe 2 ) in 34% yield.…”