Decomposition of Vanadium(V) Alkylidenes Relevant to Olefin Metathesis

Abstract

Vanadium alkylidenes can be highly active initiators for ring-opening metathesis polymerization of cyclic olefins; however, attempts to expand their use to cross metathesis have been unsuccessful due to catalyst decomposition. Detailed knowledge of the decomposition reaction is imperative to guide future catalyst design. Herein, we demonstrate that β-hydride elimination is the dominant decomposition pathway during cross metathesis. The isolated vanadium decomposition products 11), generated from two separate catalysts, agree with this pathway. Compounds 10 and 11 were shown computationally to form via bimolecular routes after β-hydride elimination from the metallocyclobutane and reductive elimination of propylene, which was itself demonstrated to be exergonic with low thermodynamic barriers to metallocyclobutane formation. Relative conversions in the selfmetathesis of 1-hexene with the series of vanadium alkylidenes of various sterics and electronics ( 6), X = CH(CH 3 ) 2 , Y = F (7)] were determined. It was found that bulkier, yet more electron-donating ligands decreased conversions, indicating that β-hydride elimination is favored over bimolecular decomposition. Analysis of organic reaction products demonstrated that reductive elimination of propylene occurs before insertion of ethylene into the newly formed vanadium−hydrogen bond.

Ring‐Closing Olefin Metathesis Catalyzed by Well‐Defined Vanadium Alkylidene Complexes

“…An X‐ray diffraction study of 1 e (Figure 1) revealed a mixture of two isomers: anti ‐ 1 e (≈91 %) and syn ‐ 1 e (≈9 %) [28] . anti ‐ 1 e has a distorted trigonal bipyramidal geometry with phosphines in axial positions [V−P1 2.4708(4) Å, V−P2 2.4935(4) Å, P1‐V‐P2 170.12(2)°], similar to the structure reported for anti ‐V( N ‐2,6‐( i Pr) 2 C 6 H 3 )(CHSiMe 3 )(OC 6 F 5 )(PMe 3 ) 2 ( anti ‐ A) [V−P1 2.472 Å, V−P2 2.480 Å, P1‐V‐P2 168.92°] [24] . The V−N and V−C bond lengths and Si‐C‐V angles in anti ‐ 1 e (1.6873(14) Å, 1.9153(14) Å, 131.64(7)°) and anti ‐ A (1.691 Å, 1.917 Å, 132.5°) are also similar.…”

“…However, a 15‐fold increase of the catalyst concentration did not lead to a significant decrease of TON (9.0 vs. 5.3, entries 7 and 9, Table 1). We conclude that β‐hydride elimination of metallacyclobutane 6 , and not bimolecular decomposition, is the primary degradation pathway for our system, as is observed for V alkoxide complexes [23, 24] …”

“…Despite the successful application of V complexes for ROMP, examples of V‐catalyzed olefin metathesis of acyclic olefins are surprisingly limited. Although a few examples of cross‐metathesis (CM) have been reported recently utilizing Nomura's catalysts, [23, 24] rapid V alkylidene decomposition precluded complete conversion.…”

Pushing the Bounds of Olefin Metathesis with Vanadium

“…The more electron poor catalyst 13 showed higher conversions than 14, [108] and in a subsequent analysis, catalyst 14 showed higher conversions than the more electron rich and more sterically hindered isopropyl-substituted imido complexes (17), X=Cl (18)]. [111] Together, these results demonstrate that more sterically demanding and electron donating ligands cause lower conversions, which is a sign of an increased rate of decomposition. This strongly indicates that β-hydride transfer is the primary decomposition route, and that bimolecular pathways do not play a significant role.…”

“…[108] Computations demonstrated that this process is exothermic with low barriers. [111] Secondly, the relative conversions before complete decomposition of several catalysts indicate that β-hydride transfer is the primary decomposition route, rather than bimolecular pathways. The more electron poor catalyst 13 showed higher conversions than 14, [108] and in a subsequent analysis, catalyst 14 showed higher conversions than the more electron rich and more sterically hindered isopropyl-substituted imido complexes (17), X=Cl (18)].…”