This is the accepted version of the journal article: Belov, Dmitry S.; Acosta, Carlos M.; Garcia-Molina, Miquel; [et al.]. «Syn- thesis and Activity of Vanadium Oxo NHC Alkylidenes. Remarkable Prefer- ence for Degenerate Metathesis and Application for Carbon Isotope Exchange». Organometallics, Vol. 41, Issue 21 (November 2022), p. 2897-2902. DOI 10.1021/acs.organomet.2c00465 This version is available at https://ddd.uab.cat/record/280614 under the terms of the license Synthesis and Activity of Vanadium Oxo NHC Alkylidenes. Remark- able Preference for Degenerate Metathesis and Application for Car- bon Isotope Exchange Dmitry S. Belov,1 Carlos Acosta,1 Miquel Garcia-Molina,2 Kelly L. Rue,1 Xavier Solans-Monfort,2 and Konstantin V. Bukhryakov1,* 1 Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, USA 2 Departamento de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain ABSTRACT: V imido NHC phosphine alkylidenes are the most efficient V catalysts for ring-closing olefin metathesis of various terminal dienes. The presence of imido and phosphine ligands is responsible for catalyst decomposition. Therefore, the development of phosphine-free V oxo NHC alkylidenes is a logical next step to further improve V-based olefin metathesis. Herein we report V oxo NHC chloride and alkoxide alkylidenes and their reactivity in olefin metathesis. V oxo NHC chloride is readily involved in cycloaddition/cycloreversion steps with olefins. However, a remarkable preference for the formation of 1,3-metallacyclobutane (MCB) leads to exclusive methylene groups exchange (degenerate metathesis) utilized for carbon isotope exchange. DFT studies further support the preference for the 1,3-MCB formation. Olefin metathesis (OM) is a crucial reaction to construct C−C with 1 by suppressing both 𝛽-H elimination from MCB and bi- double bonds utilized to produce commodity and fine chemi- molecular decomposition. The main degradation pathway for cals.1-3 The first reported well-defined complex that can pro- catalysts 5 is the exchange of NHC to phosphine that forms dur- mote catalytic OM is Tebbe's reagent (Cp2TiCH2ClAlMe2), ing the initiation step leading to bis phosphine complex analo- which can slowly catalyze the exchange of methylene groups gous to 3, which is unstable toward ethylene.21 (degenerate metathesis)4 between two terminal olefins, as Scheme 1. Maximum Turnover Numbers (TONs) for Previ- shown by Tebbe in 1979.5 The subsequent research led to the ously Synthetized V Catalysts in the Model RCM Reaction. development of highly efficient OM catalysts based on second- and third-row transition metals.6-8 The recent progress, driven cat. by green chemistry and economic factors, shows that the devel- N Ts N Ts + 22 °C opment of OM catalysts based on first-row metals, primarily 1, Ts = p-CH3C6H4SO2 2 ethylene vanadium, is achievable.9-13 Thus, Nomura reported a series of highly active and selective V-based catalysts for ring-opening active catalysts metathesis polymerization of cyclic olefins.14-16 Some of those PMe3 PMe3 − PMe3 complexes have been utilized by Farrell in cross-metathesis Cl V Cl V TON = 6 + PMe3 N (CM) but showed limited activity.17-18 N SiMe3 SiMe3 PMe3 Recently, our group has developed V-catalysts for ring-clos- 3 3a ing metathesis (RCM) of internal and terminal olefins and re- ported the highest V-based OM productivity involving terminal PEt3 − PEt3 PEt3 dienes.19-21 Representative V alkylidene complexes 3-5 and Cl V Cl V TON = 59 + PEt3 their productivities (turnover number, TON) in RCM involving O SiMe3 O SiMe3 the model substrate 1 are shown in Scheme 1. The dissociation PEt3 4 4a of one neutral ligand is required to access four-coordinate 14 electron active catalysts 3a-5a during the reaction. Catalyst 3 Mes N N Mes Mes N N Mes exhibits low RCM productivity with 1 due to limited stability − PMe3 toward ethylene. The main decomposition pathway involves the Cl V O + PMe3 Cl V O TON = 170 N N 𝛽-hydride (𝛽-H) elimination at unsubstituted metallacyclobu- PMe3 tane (MCB) formed during the catalysis.19 Substitution of an 5 5a imido group to oxo significantly disfavors 𝛽-H elimination The next logical step for the catalyst optimization is the syn- leading to a higher RCM productivity with catalyst 4. However, thesis of V oxo NHC alkylidenes to preserve remarkable stabil- the bimolecular decomposition of V oxo phosphine alkylidenes ity toward 𝛽-H elimination of V oxo complexes and disfavor becomes the primary reason for catalyst degradation.20 Ex- bimolecular decomposition by introducing a large NHC ligand. change of phosphine in imido complex 3 to N-heterocyclic car- In addition, we hypothesized that the shift to phosphine-free V bene (NHC) ligand increases TON from 6 to 170 in reaction oxo alkylidenes would improve catalysts' performance since phosphines can participate in side reactions and deactivation Scheme 3. Synthesis of complex 7. pathways, such as reduction of high-oxidation state V complexes22 and reaction with alkylidenes.23-24 Here, we present F3C CF the synthesis of phosphine-free V oxo NHC complexes and 3 Mes N N Mes their reactivity in OM. We also report the unusual preference of 6 OK O V SiMe3 the degenerate metathesis employed for carbon isotope Et2O, 24 h F3C O exchange. − IMes, − KCl F3C Mes 7, 89% Complex 6 has been synthesized from 4 and 2 equiv. of IMes utilizing ligand exchange reaction in 78% yield (Scheme 2). Proton NMR spectrum of complex 7 in C6D6 revealed a Scheme 2. Synthesis of complex 6. singlet resonance for alkylidene proton at 17.66 ppm (1JCH = 112 Hz). X-ray studies showed that 7 is a four-coordinate com- plex (syn-isomer) with distorted tetrahedral geometry (Figure N N IMes Mes N N Mes 2). Four-coordinate V alkylidenes are rare.26-29 To our Cl knowledge, complex 7 is the first crystallographically charac- 4 n-hexane, 30 min O V terized four-coordinate V alkylidene that contains oxo or alkox- SiMe3 − PMe3 ide ligands. The relevant bond distances are V1−C1 = 1.853(10) Mes N N Mes Å, V1−C17 = 2.140(7) Å, V1−O1 = 1.588(5) Å, and V1−O2 = 6, 78% 1.845(5) Å. The V=C−Si angle is 130.5(5)°, and V1−O2−C is A 1H NMR spectrum of 6 in C6D6 showed that the alkylidene 145.9(5)°. proton resonance is a singlet at 14.45 ppm (1JCH = 114 Hz). The X-ray structural study revealed that 6 is a syn-isomer (Figure 1) and has a distorted trigonal bipyramidal geometry with NHCs in axial positions [V1−C1 2.173(2) Å, V1−C22 2.191(2) Å, C11−V1−C22 156.52(10)°], which is similar to imido NHC complex 5 [V−C(NHC) is 2.1828(11) Å and P−V1−C(NHC) 154.52(3)°]. The V1−C43 distance is 1.889(3) Å, and the V1−O1 bond is 1.586(2) Å, which are similar to 4.20 Notewor- thy, the V=C−Si angle [133.01(15)°] is lower than the V=C−Si angle in 4 [140.03(9)°]. It can be explained by less extent of α- hydrogen agostic interaction with the metal center20 due to the lower electrophilicity of V in 6 resulting from the high σ-donat- ing abilities of two NHC ligands.25 Figure 2. Perspective view of the crystal structure of complex 7 with thermal ellipsoids shown at 30% probability. An important feature of an oxo ligand is the ability to coordi- nate Lewis acids. It has been shown that the coordination of B(C5F6)3 to W oxo alkylidene can improve the activity and se- lectivity of the OM catalysts.30 However, adding B(C6F5)3 to 6 in C6D6 led to the rapid decomposition of 6. The reaction be- tween B(C6F5)3 and 7 resulted in the formation of new alkyli- dene by 1H NMR (singlet, 14.63 ppm, C6D6), presumably due to coordination of B(C6F5)3 to oxo ligand followed by slow deg- radation of 7⦁B(C6F5)3 adduct. Complex 7 does not react with 1, 1,7-cyclooctadiene, 1- hexene, styrene, or ethylene at 22 °C. However, 7 slowly catalyze RCM reactions of 1 and 1,7-cyclooctadiene; and CM reaction of 1-hexene at 100 °C (1-4 TON after 3-4 days). Heat- Figure 1. Perspective view of the crystal structure of complex 6 ing of 7 in the presence of ethylene led to slow decomposition with thermal ellipsoids shown at 30% probability. of 7. Corresponding methylidene and MCBs have not been ob- Surprisingly, complex 6 does not exhibit RCM activity with served by NMR. We believe bulky Mes(CF3)2CO alkoxide pre- 1. We initially hypothesized that one NHC-ligand does not dis- vents the coordination of olefin to the metal center, suppressing sociate readily to form a four-coordinate 14 electron active cat- the catalytic activity. alyst. To overcome this challenge, we decided to synthesize V Complex 6 does not catalyze RCM of 1 or 1,7-cyclooctadiene oxo NHC complexes bearing bulky alkoxide ligands to facili- and CM of 1-hexene or allylbenzene. However, 6 reacts with tate the dissociation of one NHC. Screening various alkoxides styrene to form V benzylidene complex 6-Ph (Scheme 4A). The (see the complete list in SI) led to the synthesis and isolation of previously reported V imido alkylidene reacts with styrene to complex 7 in 89% yield (Scheme 3). produce metallacyclopropane.31 6-Ph does not catalyze RCM of 1 and CM of 1-hexene or styrene. Thus, heating of 6-Ph in the presence of 20 equiv. of styrene at 80 °C for seven days does not lead to thermodynamically favorable E-stilbene (Scheme 4B). Noteworthy, 6-Ph is relatively thermally stable in the so- lution. Thus, we observed ~50% decomposition of 6-Ph at 80 °C after seven days. The addition of 4-MeO-styrene to 6-Ph results in an equilib- cases the feasibility of the two processes. This transition state is rium between 6-Ph and 6-OMe (Scheme 4C), Keq is 0.8 in C6D6 lower for the degenerate metathesis by 8.0 kcal mol-1 and this at 22 °C. Noteworthy, benzylidene 6-OMe can be formed di- corresponds to a 10-6 ratio between the corresponding kinetic rectly from 6 and 4-MeO-styrene 8. The addition of styrene to constants (k1,2/k1,3). The reaction of 6-Me-1 with propene pre- 6-OMe leads to the same equilibrium between 6-Ph and 6- sents lower energy barriers in general. The most favorable CM OMe. The presence of electron-donating groups in the phenyl pathways involve 1,2-MCB-1 and 1,2-MCB-2, but they are still ring destabilizes the negative charge on the 𝛼-C atom, explain- 4.1 and 4.3 kcal mol-1 higher in energy than the degenerate me- ing the preference for 6-Ph. tathesis (10-3 k1,2/k1,3 ratio). For comparison, reported data on Scheme 4. Reactivity of complex 6. Ru NHC alkylidenes shows that 1,3-MCB is ~2 kcal/mol more favorable than 1,2-MCB.37 We believe 𝛼-C atoms in V MCBs A. Alkylidene exchange have a higher negative charge than those in Ru MCBs due to IMes IMes the lower V electronegativity.38 The higher polarization of the Cl Ph Cl V=C bond favors that the substituted moiety of the reacting ole- V V + Me3Si O SiMe3 C6D6, 22 °C, 16 h O Ph fin interacts with the positively charged metal and, thus, overall IMes IMes 1,3-MCB. 6 6-Ph A. Cross-metathesis B. Cross-metathesis is unfavorable Ph Ph Ph O O Ph not observed IMes V V IMes 5 mol% 6-Ph Ph no CM Ph Cl Ph + Cl C 6D 6, Ph 80 °C, 7 days O TSadd-1 1,2-MCB-1 TSrev-1 6-m ΔGE-stilbene = − 1.1 kcal/mol IMes V Ph 17.7 16.3 26.3 9.4 C. Evidence of degenerate metathesis Cl Ph Ph O IMes syn-6-Ph-1 O Ph IMes Ph IMes V V IMes Cl 0.0 Keq = 0.8 V Cl Cl O Cl Ph V + + Ph O Ph C6D6, 22 °C TSadd-2 1,2-MCB-2 TSrev-2 6-m IMes IMes 16.7 13.8 21.9 4.8 O O 8 Ph Ph O 6-Ph 6-OMe O Ph Ph V IMes V oxo NHC complexes 6, 6-Ph, and 6-OMe are readily in- Ph IMes V Cl volved in cycloaddition/cycloreversion steps with styrenes. O Cl TSadd-3 1,2-MCB-3 TSrev-3 6-m However, the absence of CM products indicates that degenerate IMes V 19.4 15.8 24.8 4.8 metathesis is remarkably favorable for V oxo NHC species. Cl Ph Ph Ph We hypothesize that the formation of 1,3-MCB is signifi- anti-6-Ph-1 O Ph O Ph IMes V V IMes cantly more favorable than 1,2-MCB for studied systems. To 2.8 Cl Ph Cl support our hypothesis, we performed DFT (B3LYP-D3)32-34 studies on the cross (via 1,2-MCB) and degenerate (via 1,3- TSadd-4 1,2-MCB-4 TSrev-4 6-m 23.1 22.1 30.1 9.4 MCB) metathesis of styrene and propene. Dissociation of one neutral ligand in 6-Ph is required to form an active four-coordi- B. Degenerate metathesis nate complex 6-Ph-1 (Figure 3). Cycloaddition between 6-Ph- Ph O 1 and styrene can lead to 8 possible pathways depending on the O Ph Ph Ph V IMes relative position of the substituents. Similarly, the reactivity of Ph IMes V Cl the four coordinated ethylidene 6-Me-1 with propene follows Cl O TSadd-5 1,3-MCB-1 TSrev-5 syn-6-Ph-1 equivalent pathways (see SI). We computationally explored the Ph 13.9 7.5 13.9 0.0 IMes V 8 different pathways assuming the accepted OM reaction mech- Cl Ph anism with d0 alkylidenes.35-36 The productive MCB has a trig- syn-6-Ph-1 O O Ph Ph onal bipyramid structure with apical oxo and chlorine ligands. 0.0 IMes V V IMes Cl We analyzed the accuracy of our computational methodology Cl Ph Ph by reproducing the equilibrium constant between 6-Ph and 6- TSadd-6 1,3-MCB-2 TSrev-6 anti-6-Ph-1 OMe. Calculations predict the thermodynamically favorable 17.1 11.0 17.1 2.8 formation of 6-Ph and 6-OMe from 6 (see SI). Moreover, the O Ph Ph O Ph Ph reaction of 6-Ph with 8 to form 6-OMe is computed to be en- Ph IMes V V IMes Cl dergonic by 0.3 kcal mol-1. The computed equilibrium constant O Cl 1,3-MCB-3 syn-6-Ph-1 is thus 0.6, in excellent agreement with the experimentally de- IMes V TSadd-7 TSrev-7 17.1 11.0 17.1 0.0 termined value. Cl Ph Figure 3 reports the Gibbs energies of the 8 pathways in- anti-6-Ph-1 O Ph O Ph Ph volved in the reaction of 6-Ph-1 with styrene (Figure S6 for 6- 2.8 IMes V V IMes Me-1 with propene reaction). Syn-6-Ph-1 + styrene is used as Cl Ph Ph Cl origin of energies. For the reaction of 6-Ph-1 with styrene, cal- TSadd-8 1,3-MCB-4 TSrev-8 anti-6-Ph-1 culations show that the preferred CM pathway is 1,2-MCB-2 21.3 15.9 21.3 2.8 which implies syn-6-Ph-1 and trans-stilbene as final product Figure 3. Calculated relative Gibbs free energies under benzene (Figure 3A). The preferred degenerate pathway implies the solvation (B3LYP-D3) in kcal/mol with respect to syn-6-Ph-1. Red phenyls pointing toward the oxo ligand (1,3-MCB-1, Figure values show the most favorable paths via 1,2- and 1,3- MCBs. 3B). The transition state for cycloreversion determines in both To further support the high preference for degenerate metath- Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or by e-mail: esis, we utilized 13C-labeled 4-MeO-styrene 8-13C for carbon deposit@ccdc.cam.ac.uk. isotope exchange (CIE)39 with styrene in the presence of 3 mol% of 6 (Scheme 5). CIE is slow at room temperature. Heat- AUTHOR INFORMATION ing the reaction mixture at 80 °C for 16 hours led to 50% isotope Corresponding Author exchange as expected at equilibrium conditions (assuming no isotope effect). Konstantin Bukhryakov - Department of Chemistry and Biochem- istry, Florida International University, Miami, FL 33199, USA. Scheme 5. Carbon isotope exchange (CIE) catalyzed by 6. kbukhrya@fiu.edu Notes 13CH 2 The authors declare no competing financial interests. 13CH 3 mol% 6 + + 2 ACKNOWLEDGMENT R C6D6, 80 °C R 16 h We are grateful to the Florida International University and the New O O R = Ph: 50% exchange Faculty Development Grant provided by U.S. Nuclear Regulatory 8-13C 8 R = n-Bu: 12% exchange Commission for financial support. Financial support from MICINN 13CH [PID2020-112715GB-I00] and the Generalitat de Catalunya 2 3 mol% 6 13CH 2 [2017SGR1323] are also gratefully acknowledged. K.L.R. was + + supported by U.S. Nuclear Regulatory Commission (NRC) fellow- n-Bu C6D6, 22 °C n-Bu Ph 72 h Ph ship grant No. 31310018M0012 awarded to FIU. 9-13C 9 20% exchange REFERENCES CIE involving alkyl olefins is less efficient. Thus, the reac- 1. Mol, J. C., Industrial applications of olefin metathesis. J. tion between 8-13C and 1-hexene led to 12% exchange after 16 Mol. Catal. A: Chem. 2004, 213 (1), 39-45. hours at 80 °C. 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