Dual Catalysis Based on N‑Heterocyclic Olefins for the Copolymerization of Lactones: High Performance and Tunable Selectivity

The cooperative interaction of four structurally different N-heterocyclic olefins (NHOs) with a range of simple metal halides as Lewis acidic cocatalysts is employed for the homo- and copolymerization of ε-caprolactone (CL) and δ-valerolactone (VL). While the single components are inactive on their own, their combination provides a powerful and operationally simple platform for the controlled preparation of polyesters from these monomers, whereby molecular weights and end groups can be predicted in a room temperature-based process using low catalyst loadings (0.25–0.50 mol %). A narrow molecular weight distribution is observed (1.05 < <i>Đ</i>_M < 1.15) for multiple combinations of NHOs and Lewis acids. Importantly, the supposed mechanism involves activation of the lactone monomers via coordination to the metal-based Lewis acids. This ensures rapid polymerization and additionally decouples reactivity and activity; a trade-off between fast monomer consumption and the suppression of side reactions (transesterification) can be circumvented this way. Furthermore, this dual catalytic setup can be used to direct preferential monomer incorporation when CL/VL are copolymerized. From the same 1:1 mixture of both monomers, either VL or CL can be consumed more rapidly, or more random incorporation can be achieved, depending on the employed cocatalysts. Well-defined copolymers with moderate gradients result, where the copolymerization selectivity is dictated by choice of the Lewis acid present, which is remarkable in view of the very different NHOs involved (saturated five- and six-membered rings, benzimidazole and imidazole derivatives). While most metal halides (such as MgI₂, ZnI₂, and AlCl₃) entail VL-enriched polyester, YCl₃ favors CL incorporation

Olefin Metathesis and Stereoselective Ring-Opening Metathesis Polymerization with Neutral and Cationic Molybdenum(VI) Imido and Tungsten(VI) Oxo Alkylidene Complexes Containing N‑Chelating N‑Heterocyclic Carbenes

Neutral and cationic molybdenum(VI) imido and tungsten(VI) oxo alkylidene complexes containing an N-chelating N-heterocyclic carbene, [Mo(N-2,6-Me2-C6H3)I(3-(2-(2,6-diisopropylphen-1-ylamido)phen-1-yl)-1-methylimidazol-2-ylidene)(CHCMe2Ph)] (Mo1), [Mo(N-2-tBu-C6H4)I(3-(2-(2,6-diisopropylphen-1-ylamido)phen-1-yl)-1-methylimidazol-2-ylidene)(CHCMe2Ph)] (Mo2), [Mo(N-2,6-Me2-C6H3)(3-(2-(2,6-diisopropylphen-1-ylamido)phen-1-yl)-1-methylimidazol-2-ylidene)(CHCMe2Ph)(L)][B(ArF)4] (Mo3: L = none, Mo4: L = CH3CN), [Mo(N-2-tBu-C6H4)(3-(2-(2,6-diisopropylphen-1-ylamido)phen-1-yl)-1-methylimidazol-2-ylidene)(CHCMe2Ph)][B(ArF)4] (Mo5), [W(O)(B(C6F5)3)Cl(1-methyl-3′-(2-N-(2,6-iPr2-C6H4)-1-C6H4)imidazol-2-ylidene)(CHCMe2Ph)] (W1), and [W(O)(1-methyl-3′-(2-N-(2,6-iPr2-C6H4)-1-C6H4)imidazol-2-ylidene)(CHCMe2Ph)][B(ArF)4] (W2) have been prepared. Catalysts Mo2, Mo4, W1, and W2 were characterized by single-crystal X-ray analysis. Catalysts Mo4 and W2 were benchmarked in homo-, cross-, ring-closing metathesis (RCM) as well as in ring-opening cross-metathesis (ROCM) reactions. In the ring-opening metathesis polymerization (ROMP) of endo,exo-2,3-dicarbomethoxynorborn-5-ene (DCMNBE), methyl-N-(S)-(−)-α-methylbenzyl-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate, exo-N-(R)-(+)-α-methylbenzyl-5-norbornene-2,3-dicarboximide, and 2,3-bis((menthyloxy)carbonyl)norbornadiene Mo4 and W2 offered access to trans-isotactic and cis-syndiotactic polymers, respectively

Olefin Metathesis and Stereoselective Ring-Opening Metathesis Polymerization with Neutral and Cationic Molybdenum(VI) Imido and Tungsten(VI) Oxo Alkylidene Complexes Containing N‑Chelating N‑Heterocyclic Carbenes

Neutral and cationic molybdenum(VI) imido and tungsten(VI) oxo alkylidene complexes containing an N-chelating N-heterocyclic carbene, [Mo(N-2,6-Me2-C6H3)I(3-(2-(2,6-diisopropylphen-1-ylamido)phen-1-yl)-1-methylimidazol-2-ylidene)(CHCMe2Ph)] (Mo1), [Mo(N-2-tBu-C6H4)I(3-(2-(2,6-diisopropylphen-1-ylamido)phen-1-yl)-1-methylimidazol-2-ylidene)(CHCMe2Ph)] (Mo2), [Mo(N-2,6-Me2-C6H3)(3-(2-(2,6-diisopropylphen-1-ylamido)phen-1-yl)-1-methylimidazol-2-ylidene)(CHCMe2Ph)(L)][B(ArF)4] (Mo3: L = none, Mo4: L = CH3CN), [Mo(N-2-tBu-C6H4)(3-(2-(2,6-diisopropylphen-1-ylamido)phen-1-yl)-1-methylimidazol-2-ylidene)(CHCMe2Ph)][B(ArF)4] (Mo5), [W(O)(B(C6F5)3)Cl(1-methyl-3′-(2-N-(2,6-iPr2-C6H4)-1-C6H4)imidazol-2-ylidene)(CHCMe2Ph)] (W1), and [W(O)(1-methyl-3′-(2-N-(2,6-iPr2-C6H4)-1-C6H4)imidazol-2-ylidene)(CHCMe2Ph)][B(ArF)4] (W2) have been prepared. Catalysts Mo2, Mo4, W1, and W2 were characterized by single-crystal X-ray analysis. Catalysts Mo4 and W2 were benchmarked in homo-, cross-, ring-closing metathesis (RCM) as well as in ring-opening cross-metathesis (ROCM) reactions. In the ring-opening metathesis polymerization (ROMP) of endo,exo-2,3-dicarbomethoxynorborn-5-ene (DCMNBE), methyl-N-(S)-(−)-α-methylbenzyl-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate, exo-N-(R)-(+)-α-methylbenzyl-5-norbornene-2,3-dicarboximide, and 2,3-bis((menthyloxy)carbonyl)norbornadiene Mo4 and W2 offered access to trans-isotactic and cis-syndiotactic polymers, respectively

Interconversion Rates and Reactivity of <i>Syn</i>- and <i>Anti</i>-rotamers of Neutral and Cationic Molybdenum and Tungsten Imido Alkylidene <i>N</i>‑Heterocyclic Carbene Complexes

The interconversion rates of the syn- and anti-isomers of the neutral molybdenum and tungsten imido alkylidene N-heterocyclic carbene (NHC) complexes of the general formula [M(NR)(CHCMe2R′)(NHC)(OR″)2] (M = Mo, W; R = adamantyl, 2,6-Me2-C6H3, 2,6-iPr2–C6H3, 2,6-Cl2–C6H3, 2-tBu-C6H4; NHC = 1,3-diisopropylimidazol-2-ylidene (IiPr), 1,3-dimethylimidazol-2-ylidene (IMe), 1,3-dicyclohexylimidazol-2-ylidene (ICy), and 1,3-dimesitylimidazol-2-ylidene (IMes); R′ = Me, Ph; R″ = CMe(CF3)2, CMe2(CF3), C(CF3)3, C6F5, SO2CF3) and of the cationic complex [Mo(N-2,6-Me2-C6H3)(CHCMe2Ph)(IMes)(SO3CF3)(CD3CN)+ B(3,5-(CF3)2C6H3)4–] were determined by irradiating solutions of the catalysts with 366 nm UV light followed by recording the back-isomerization of the anti- to the syn-isomer. Both the rate of anti to syn and syn to anti interconversion, ka/s and ks/a were found to be orders of magnitude lower than in tetracoordinated, neutral Schrock catalysts of the general formula [Mo(NR)(CHCMe2R′)(OR″)2]. Accordingly, the values for the Gibbs free energy of the transition state, ΔG‡, are significantly higher for both neutral and cationic molybdenum and tungsten imido alkylidene NHC complexes than for Schrock catalysts. NMR investigations strongly suggest that these higher ΔG‡ values are attributable to dissociation of an anionic ligand from a neutral pentacoordinated catalyst that precedes interconversion. As with Schrock catalysts, the anti-isomer proved to be the more reactive isomer, in both ring-opening metathesis polymerization and ring-closing metathesis (RCM) reactions, allowing for higher productivities, expressed as turnover numbers, in RCM