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

Abstract

The benzylidyne complex [PhC≡W{OSi(OtBu) } ] (1) catalyzed the cross-metathesis between 1,4-bis(trimethylsilyl)-1,3-butadiyne (2) and symmetrical 1,3-diynes (3) efficiently, which gave access to TMS-capped 1,3-diynes RC≡C-C≡CSiMe (4). Diyne cross-metathesis (DYCM) studies with C-labeled diyne PhC≡ C- C≡CPh (3*) revealed that this reaction proceeds through reversible carbon-carbon triple-bond cleavage and formation according to the conventional mechanism of alkyne metathesis. The reaction between 1 and 3* afforded the 3-phenylpropynylidyne complex PhC≡ C- C≡W{OSi(OtBu) } ] (5*), indicating that alkynylalkylidyne complexes are likely to act as catalytically active species. Attempts to isolate 5* from mixtures of 1 and 3* afforded crystals of the ditungsten 2-butyne-1,4-diylidyne complex [(tBuO) SiO} W≡ C- C≡ C- C≡W{OSi(OtBu) } ] (6*), which was additionally characterized by X-ray diffraction analysis. Depolymerization-macrocyclization of a carbazole-butadiyne polymer, obtained from 3,6-diethynyl-9-dodecylcarbazole (7) under copper-catalyzed Hay coupling conditions, was also efficiently catalyzed by 1 and afforded a mixture of mono-, diyne- and triyne-containing tetrameric macrocycles, revealing that diyne disproportionation into monoynes and triynes occurs as a slow side reaction that interferes with a high diyne metathesis selectivity. Potential catalytic pathways were studied by means of quantum-chemical calculations, and kinetic studies were performed to substantiate an α,α-mechanism for the catalytic diyne metathesis reaction, which involves intermediate alkynylalkylidyne and α,α'-dialkynylmetallacyclobutadiene intermediates.

Well‐Defined Alkyne Metathesis Catalysts: Developments and Recent Applications

“…Furthermore, 1,3-diyne cross-metathesis (diyne cross-metathesis, DYCM) can also be adopted to the synthesis of unsymmetrical 1,3-diyne targets,w hen using one substrate in a4 :1 excess (Scheme 26). [182,183] TMS-cappedd iynes (RCCÀC CSiMe 3 )f or instance, which are important synthetic intermediates, were generated from symmetric aromatic or aliphatic 1,3diynesa nd bis(trimethylsilyl)-1,3-butadiyne (Me 3 SiCCÀC CSiMe 3 ). Diyne metathesis can be av aluable method in preparative acetylenic chemistry because diynes are established reversibly under thermodynamic control and unreacted starting materialmay be recovered for subsequentuse.…”

Pentadiynylidyne and Pentacarbido Complexes

“…[4] Very recently,p ropargylidynes have emerged from being simply fundamental laboratory curiosities to now serve as competent mediators for the diyne/alkyne crossmetathesis reaction. [5] Thed evelopment of propargylidyne/ tricarbido chemistry along these various avenues led us to consider the viability of pentadiynylidynes and whether similarly rich chemistry might follow based on 5-carbon chain ligands.W ereport herein the synthesis of the first such pentadiynylidyne complex and demonstrate that it may indeed serve as an entry point into the uncharted chemistry of bi-and polymetallic pentacarbido complexes.…”

“…Theprogressive diminution of the magnitudes of J SiC and J WC couplings along the WC 5 Si chain (see the Supporting Information) allows confident 13 CNMR identification of the resonances for each of the five carbon nuclei of interest (d C = 244.9 C a ,93.3 C b ,56.6 C g ,89.5 C d ,90.3 C e ,T able 1) whilst the localization of WCa nd CCt riple and CÀCs ingle bonds follows from ac rystallographic analysis ( Figure 1, Table 2) that confirms the pentadiynylidyne valence bond description. …”

Pentadiynylidyne and Pentacarbido Complexes

“…Molecular structure of 5 in acrystal of 5·2 CH 2 Cl 2 (ellipsoids set at 50 %p robability,h ydrogen atoms and solvent omitted,p henyl and pyrazolyl groups simplified). [17] Selected bond lengths []and angles [8 8]: W1-C1 1.870(5), Ru1-C5 1.965(5), C1-C2 1.336 (7), C2-C3 1.234 (7), C3-C4 1.345 (7), C4-C5 1.232 (7);W1-C1-C21 75.8(4), C1-C2-C3 175.5 (5), C2-C3-C4 179.2(5), C3-C4-C5 175.1(5), C4-C5-Ru1 174.6-(4), P1-Ru-P2 101.29(4)8 8. . Molecular structure of 7 in acrystal of 7·4 CHCl 3 (ellipsoids set at 50 %p robability,h ydrogensa nd solvent molecules omitted, phenyl and Tp*ligands simplified).…”

“…[7] We have therefore considered an alternative approach involving aS onogashira-type C À C cross-coupling process,r ecently optimized [8] for reactions between the halocarbyne complex [W( CBr)(CO) 2 (Tp*)] (1) [9] and simple 1-alkynes. Theprogressive diminution of the magnitudes of J SiC and J WC couplings along the WC 5 Si chain (see the Supporting Information) allows confident 13 CNMR identification of the resonances for each of the five carbon nuclei of interest (d C = 244.9 C a ,93.3 C b ,56.6 C g ,89.5 C d ,90.3 C e ,T able 1) whilst the localization of WCa nd CCt riple and CÀCs ingle bonds follows from ac rystallographic analysis ( Figure 1, Table 2) that confirms the pentadiynylidyne valence bond description.…”