This is the accepted version of the journal article: Belov, Dmitry S.; Fenoll, Didac A.; Chakraborty, Indranil; [et al.]. «Synthesis of Vanadium Oxo Alkylidene Complex and Its Reactivity in Ring-Closing Olefin Metathesis Reactions». Organometallics, Vol. 40, Issue 17 (September 2021), p. 2939-2944. DOI 10.1021/acs.organomet.1c00425 This version is available at https://ddd.uab.cat/record/280631 under the terms of the license Synthesis of Vanadium Oxo Alkylidene Complex and its Reactivity in Ring-Closing Olefin Metathesis Reactions Dmitry S. Belov,1 Didac A. Fenoll,2 Indranil Chakraborty,1 Xavier Solans-Monfort,2 and Konstantin V. Bukhryakov1,* 1 Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, USA 2 Departament de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain O SiMe3 Et3P V Cl PEt3 X X - ethylene up to 59 TON ABSTRACT: V imido alkylidenes have been applied for the ring-opening metathesis polymerization involving cyclic olefins. However, those complexes found limited application in reactions with acyclic terminal olefins due to instability toward ethylene. Experimental and theoretical studies show that the β-hydride elimination from unsubstituted metallacyclobutene is the primary decomposition pathway in those systems. Herein, we report the synthesis of the first catalytically active V oxo alkylidene, VO(CHSiMe3)(PEt3)2Cl, that exhibits the highest reported productivity with various terminal olefins in ring-closing metathesis reactions among known V catalysts. Presented DFT studies indicate that β-hydride elimination is significantly disfavored for V oxo species. Since its discovery, the olefin metathesis (OM) reaction has metal methylidene.17 It has been shown that the main found numerous applications in total synthesis,1 industrial decomposition pathway of V-based catalysts is β-H processes,2 pharmaceutical,3 and material chemistry.4 Ru, Mo, elimination.15, 18, 19 Noteworthy, the structure of V catalysts and W-based homogeneous catalysts are the most prominent resembles the structure of Mo and W Schrock complexes. Thus, due to their high activity and functional group stability.5-7 they are d0, high oxidation state complexes with the second However, the transition to more abundant first-row metals is multiply bound ligand (imido) in addition to the alkylidene. desirable due to the low cost, decreased environmental footprint, and reduced toxicity. In addition, first-row metal alkylidenes can offer a unique reactivity, such as C-H bond activation,8, 9 which has the potential to be coupled with OM. Although first-row metal alkylidenes can perform the critical steps of OM (cycloaddition and cycloreversion), catalytic systems based on those metals remain underdeveloped.10, 11 V alkylidenes are a promising class of compounds that have shown reactivity in OM,12 especially in ring-opening metathesis polymerization (ROMP) of cyclic olefins.13, 14 Recently, our Scheme 1. Syntheses of V imido chloride alkylidenes.14 group reported a method of promoting α-hydrogen abstraction from V trialkyl complexes 4 by using phosphonium Mo and W oxo species are at the heart of many large-scale hydrochlorides to obtain complexes 5a-e in good yields processes involving OM;2 although the precise structure of the (Scheme 1).15 Complexes 5a-e are active catalysts in OM and active species often remains elusive. In contrast, well-defined can tolerate various functional groups. Furthermore, it was the W oxo alkylidenes, known for four decades,20 had limited first report of V-based catalysts capable of performing ring- application in homogenous catalysis due to the higher closing metathesis (RCM) reactions, besides proposed susceptibility toward bimolecular decomposition than imido backbiting reaction during ROMP of cyclopentene,16 which complexes. One strategy to preclude bimolecular resembles some elements of RCM. However, the described decomposition is to generate isolated active sites by grafting on complexes have limited stability toward ethylene that precluded a silica surface.21 Using this approach, direct comparison a high turnover number (TON) in reactions with terminal between well-defined oxo and imido W complexes was olefins, where ethylene is a byproduct. Two major pathways are achieved22, 23 and the data suggest that the oxo complexes known for catalyst deactivation: β-hydride (β-H) elimination of outperform imido counterparts when bimolecular metallacyclobutane (MCB) and bimolecular decomposition of decomposition is avoided. DFT calculations further supports experimental results. Thus it has been shown that oxo Mo and observed by 1H NMR (Scheme 4). Initially, we proposed that a W MCBs are significantly less prone to 𝛽-H elimination mixture of syn and anti-alkylidenes were formed. However, the compared to imido complexes due to electronic reasons.24 ratio of two products varied from one experiment to another. Those studies sparked a new interest in Mo25 and W26 oxo Finally, we were able to crystallize one of them (14) and alkylidene chemistry and led to new highly efficient and perform X-ray studies.37 Surprisingly, the isolated complex selective homogenous well-defined catalysts. contained chloride ligand instead of triflate (Figure 1). The synthesis and evaluation of V oxo alkylidenes is a Complex 13 can be quickly converted to 14 by adding a Cl− logical next step in developing reliable V-based OM. However, source, such as BnNEt3Cl. Formation of complex 14 can be there is only one example of a V oxo alkylidene (complex 7, explained by slow release of Cl− by the reaction of CH2Cl2 and reported by the Mindiola group)8, 27, 28 and it does not show OM PEt3.38 When the reaction was attempted in the presence of activity. In 7, the V oxo alkylidene moiety is stabilized by a BnNEt3Cl, it resulted in decomposition than the formation of pincer ligand (Scheme 2). During the catalytic cycle, active alkylidene. intermediates adopt different geometries. Rigid chelating ligands usually do not allow these conformational changes and are often associated with low catalytic activity.29, 30 Scheme 4. Synthesis of oxo alkylidene complexes. The proposed mechanism of the formation of V oxo alkylidenes includes protonation of one of three alkyl groups with acid (HX) to form 15 (Scheme 5). The second critical step of the alkylidene formation is the α-hydrogen abstraction induced by the coordination of the L-type ligand. The nature of Scheme 2. Reported V oxo alkylidene. the X group plays a crucial role in alkylidene formation. Some imido V complexes (Ar’O-V(NAr)(CH2TMS)2) are isolable Here we report the first catalytically active V oxo and can be converted to alkylidenes upon the addition of alkylidene, its reactivity toward terminal dienes, and functional PMe3.39 In contrast, isolable Ph3SiO-VO(CH2TMS)2 (15a) group tolerance. Furthermore, we provide DFT studies of the complex33 does not react with neutral ligands to form V oxo activation, productive metathesis, decomposition pathways, and alkylidene.40 15b has limited stability in solution. Still, it can comparison with the V imido counterparts. react with neutral ligands to form alkylidenes 16. In the case of The synthesis of V oxo alkylidene from the corresponding 15c, the rate of decomposition is higher than alkylidene trialkyloxovanadium complex seems straightforward in formation. analogy to V imido complexes. However, complex 12 (Scheme 3) is not readily available. Unlike the reaction of 1 with 2 or 3 (Scheme 1), direct alkylation of 10 leads to the reduction and formation of V(IV) complex 11.31, 32 The reported procedures for the synthesis of 12 did not reproduce well in our hands.32-34 Eventually, we adapted a procedure reported for the preparation of (t-BuCH2)3VO.35 Alkylation of 8 with 3 produced 9, which was used for the next step immediately due to the limited Scheme 5. Proposed mechanism of alkylidene formation. stability.36 The latter can be oxidized by several reagents, and styrene oxide was found to give the highest yield of 12. Complex 14 is a mixture of syn and anti-alkylidenes in the ratio 97:3 in the solution by 1H NMR (C6D6). An X-ray structural study showed that syn-14 (Figure 1) has a distorted trigonal bipyramidal geometry with phosphines in axial positions [V−P1 2.4884(8) Å, V−P2 2.4701(8) Å, P1−V−P2 164.89(2)°]. The V1−C1 distance is 1.8403(19) Å and V1−O1 bond is 1.6079(15) Å, that are similar to reported V oxo alkylidene.27 The large V=C−Si angle (140.01(12)°) is indicative of α-hydrogen agostic interactions with V center.15 Scheme 3. Synthesis of V oxo trialkyl complex 12. With the complex 12 in hand, we tried our previously optimized conditions to prepare V alkylidenes.15 Unfortunately, this mostly led to the decomposition of staring material into unidentified paramagnetic compounds. However, we observed a small amount of desired alkylidene complex by 1H NMR (the presence of V=CH signal, 16.07 ppm, C6D6). After considerable optimization, which included searching for appropriate phosphine and anionic ligands, solvent, and a proton source, we found optimal reaction conditions. Treatment of 12 with PEt3⦁TfOH in the presence of five equivalents of Figure 1. Molecular structure of syn-14. The thermal ellipsoids are PEt3 in CH2Cl2 resulted in two V alkylidenes 13 and 14 shown at 50% probability level. We explored the metathesis activity of 14 with diallyl N- d0 complexes.43 The 𝛽-H elimination transition state is 26.8 and tosylamide 17 and compared its reactivity with V imido 18.6 kcal mol-1 above initial reactants for 14 and 5b, which complex 5b (entry 1 and 2, Table 1). agrees with the experimentally observed larger stability toward Catalyst 14 outperforms 5b in reaction with 17. Notably, ethylene of the V oxo complex. The 𝛽-H elimination imposes conversion to 18 is higher in an open vial in both cases (see the allyl fragment almost trans to the oxo or imido ligand table S3), suggesting that the active species are sensitive to (Figure 2), and this causes a strong trans influence between the ethylene.41, 42 two groups. Since the oxo acts as a stronger 𝜎-donating ligand than the imido,24 the destabilization of the 𝛽-H transition state is more significant for 14, hindering its deactivation. The Table 1. RCM catalyzed by 14 and 5b. computed energy barriers are consistent with the deactivation taking place in a few hours for 5b and a longer time for 14, suggesting that other deactivation processes, such as bimolecular decomposition, are important for 14. # cat. solvent cat, mol% conv., %a TON b 1 5b benzene 5 63 (42) 12.6 b 2 14 benzene 5 87 (67) 17.4 3 14 chloroform 1 59 (56)b 59.0 b 4 14 chloroform 2 83 (66) 41.5 5 14 chloroform 3 91 (69)b 30.3 b 6 14 chloroform 4 94 (73) 23.5 b 7 14 chloroform 5 96 (76) 19.2 8 14 chloroform 6 97 (76)b 16.2 a 1 b by H NMR. closed vial. The catalytic experiments were carried out in a glovebox. To investigate the mechanism of catalyst deactivation, we conducted a reaction of 14 with ethylene. Initially, we observed the formation of a small amount of new alkylidene signal (m, 13.8 ppm, C6D6, presumably V methylidene) and vinyl-TMS. After several hours at room temperature, we observed the complete decomposition of alkylidenes. The reaction of 14 with Scheme 6. Gibbs energies (kcal mol-1) of the key intermediates ethylene produces only traces of propylene by 1H NMR, which and the highest transition states relative to syn-14 (blue) and is in contrast to the analogous reaction of 5b, where propylene syn-5b (red). a the difference between the two values is the RCM is the primary decomposition product.15 To examine the Gibbs reaction energy. b triplet ground state. contribution of bimolecular decomposition in the deactivation of the catalysts, we tested the reaction of 14 with 17 at different catalysts loadings (entries 3-8, Table 1). Important to mention, we performed reactions in chloroform since it gives the highest conversion among tested solvents (see table S1). The decrease of the catalyst loading led to an increase of TON, suggesting that bimolecular decomposition plays a role in the catalyst deactivation. Noteworthy, the TON of 59 is the highest reported TON for V-based OM involving terminal olefins.15, 18, 19 To explain the difference in stability of 14 and 5b toward ethylene, we performed DFT calculations on the full catalytic cycle, which includes precursor activation, productive RCM cycle, and mononuclear catalyst deactivation (See SI for structures and detailed energetics). Scheme 6 summarizes the Gibbs energies of key intermediates relative to syn-14 (blue) Figure 2. Optimized structures (left: V imido; right: V oxo) for the and syn-5b (red) and the highest transition states of each part 𝛽-H elimination transition states. of the global process (see SI for complete processes). Computed Noteworthy, we found a difference in reactivity between 14 Gibbs energy barriers for the precursor activation and RCM and 5b. Thus, cycloaddition reaction between V imido catalytic cycle with syn-5b and syn-14 are consistently low alkylidenes and olefin occurs trans to the strongest 𝜎-donating (below 19 kcal mol-1). The difference between the energy span ancillary ligand (PEt3) to form a trigonal bipyramidal (TBP) of syn-14 and that of syn-5b is only 2.2 kcal mol-1 (16.7 vs. 18.9 MCB intermediate similar to other d0 OM catalysts.24, 43-45 TBP kcal mol-1). Therefore, calculations suggest that RCM occurs isomer can undergo cycloreversion step (productive OM) or readily at reaction conditions, and the two precursors behave rearrange to square pyramidal (SP) MCB, which is off-cycle similarly in the absence of catalyst deactivation. Mononuclear intermediate.46 Mononuclear d0 catalyst deactivation implies a catalyst deactivation occurs through 𝛽-H elimination from 𝛽-H elimination trans to the weakest 𝜎-donating ligand (Cl, MCB. This step has the highest energy barrier similar to other Figure 2) from the SP isomer.47 The geometry optimizations for Konstantin Bukhryakov - Department of Chemistry and V oxo MCB evolved to a distorted TBP, where oxo is not Biochemistry, Florida International University, Miami, FL 33199, strictly in axial position. Consequently, one single MCB cannot USA. kbukhrya@fiu.edu be involved in the cycloaddition/cycloreversion steps, and Notes MCB rearrangement is necessary (see SI for details). However, The authors declare no competing financial interests. the MCB interconversion implies low energy barriers that have a small impact on the process. ACKNOWLEDGMENT We explored the scope of RCM, and the results are We are grateful to the Florida International University and the New summarized in Scheme 7. Faculty Development Grant provided by U.S. Nuclear Regulatory Commission for financial support. 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Am. Chem. Soc. 2008, 130 (19), 6288-6297. Dalton Trans. 1972, (4), 533-542. Insert Table of Contents artwork here O SiMe3 Et3P V Cl PEt3 X X - ethylene up to 59 TON 6