The pentacoordinated, 16-valence electron (VE) Mo imido alkylidene N-heterocyclic carbene (NHC) complexes I1-I5 and the hexacoordinated 18-VE Mo imido alkylidene NHC complexes 1-4, 8, 10 and 12 containing a chelating ligand have been prepared and used as thermally latent catalysts in the ring-opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD). Both 10 and 12 are the first Mo imido alkylidene complexes with a chelating alkylidene featuring a carboxylate group. Complexes I1-I3 and I5 as well as 1-4 proved to be fully thermally latent in the presence of DCPD. With the changes in both the electronic and steric situation at the imido ligand provided by these pre-catalysts, different temperatures of the onset of polymerization (T =65-140 °C) and for the exothermic maximum of the curing curve (T =98-183 °C) of DCPD were achieved. Also, the degree of crosslinking was successfully varied as indicated by swelling experiments in toluene, which revealed degrees of swelling between 0 and 50 %. While the introduction of a chelating alkylidene increases T , the introduction of more electron-donating anionic ligands (tert-butoxide, phenoxide) resulted in a drastic reduction in T , underlining the high flexibility of these systems. The hexacoordinated high-oxidation state molybdenum imido alkylidene NHC complexes 2, 3 and 4 were stable under air for at least twelve hours in the solid state.

The synthesis of isocyanurate-free, linear poly­(oxazolidin-2-one)­s starting from diepoxides and aromatic as well as aliphatic diisocyanates is reported. N-Heterocyclic carbenes (NHCs), liberated in situ from thermally labile CO2 adducts, in combination with Lewis acids of the simplest kind (metal halides such as LiCl and MgCl2) were employed in a cooperative manner to prepare linear polymers with molecular weights (M n) ranging from 6 to 50 kg/mol. Crucially, it is demonstrated that action of either NHC (Lewis base) or metal halide (Lewis acid) alone entails the formation of significant amounts of trimerized isocyanates (isocyanurate) and concomitant gelling of the thus cross-linked material, highlighting the advantages of a cooperative, dual catalytic approach. Reactions were conducted at 200 °C with low NHC loadings (0.5 mol %) to deliver isolated yields of 60–90% within 3–8 h polymerization time. Investigations regarding regioselectivity revealed that exclusively 5-substituted oxazolidin-2-one was formed. Notably, these transformations can be catalyzed by a combination of 1,3-dimethyl­imidazolium-2-carboxylate, a readily accessible and robust NHC-precursor tolerant toward atmospheric conditions, and well-available LiCl. A mechanism is proposed whereby the high molecular weights and the selectivity for oxazolidinone formation over side reactions are attributed to the high nucleophilicity of the NHC, cooperative monomer activation by the metal halide, and specifically chosen reaction conditions.

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We report on a solvent-free approach for the synthesis of low molecular weight, α,ω-dihydroxy telechelic poly(β-butyrolactone). In the presence of Ti(IV) alkoxides, mixtures of β-butyrolactone and diols, like di-or triethylene glycol, were reacted in ratios between 4:1 and 10:1. The oligomerization proceeds at elevated temperatures (80-100 C). Different alkoxide substituents (R = Me, iPr, tBu) of the Ti(IV)(OR) 4 catalyst were investigated. The resulting oligomers were characterized by nuclear magnetic resonance (NMR), infra-red (IR), gel-permeation chromatography (GPC), titration, and matrix-assisted laser desorption-time-of-flight mass spectrometry (MALDI-ToF-MS) analysis. Aside from low molecular weight products, special effort was devoted to achieve high O-acyl cleavage selectivity and to circumvent the formation of unsaturated end-groups in order to form exclusively dihydroxy-telechelic oligomers. Optimized results in terms of selectivity and reaction rates were achieved at 100 C using catalyst loadings of 0.2 mol% with respect to the monomer. The molecular weights determined by GPC were in good accordance with the ratio of monomer to diol used, confirming successful oligomer formation. Polyurethanes prepared from crude macrodiols without any additional catalyst feature molecular weights up to 50,000 g/mol. The reported work serves as concept to utilize β-lactones for tailored polyol synthesis; the resulting products are suitable for polyurethane chemistry.

SummaryThe novel dicationic metathesis catalyst [(RuCl2(H2ITapMe2)(=CH–2-(2-PrO)-C6H4))2+ (OTf−)2] (Ru-2, H2ITapMe2 = 1,3-bis(2’,6’-dimethyl-4’-trimethylammoniumphenyl)-4,5-dihydroimidazol-2-ylidene, OTf− = CF3SO3 −) based on a dicationic N-heterocyclic carbene (NHC) ligand was prepared. The reactivity was tested in ring opening metathesis polymerization (ROMP) under biphasic conditions using a nonpolar organic solvent (toluene) and the ionic liquid (IL) 1-butyl-2,3-dimethylimidazolium tetrafluoroborate [BDMIM+][BF4 −]. The structure of Ru-2 was confirmed by single crystal X-ray analysis.

The first reversible N‐ heterocyclic carbene (NHC) induced α‐H abstraction in tungsten(VI) imido‐dialkyl dialkoxide complexes is reported. Treatment of W( N Ar)(CH 2 Ph) 2 (O t Bu) 2 (Ar=2,6‐dichlorophenyl, 2,6‐dimethylphenyl, 2,6‐diisopropylphenyl) with different NHCs leads to the formation of complexes of the type W( N Ar)(CHPh)(NHC)(CH 2 Ph)(O t Bu) in excellent isolated yields of up to 96 %. The highly unusual release of the tert‐ butoxide ligand as t BuOH in the course of the reaction was observed. The formed alkylidene complexes and t BuOH are in an equilibrium with the NHC and the dialkyl complexes. Reaction kinetics were monitored by 1 H NMR spectroscopy. A correlation between the steric and electronic properties of the NHC and the reaction rates was observed. Kinetics of a deuterium‐labeled complex in comparison to its non‐deuterated counterpart revealed the presence of a strong primary kinetic isotope effect (KIE) of 4.2, indicating that α‐H abstraction is the rate‐determining step (RDS) of the reaction.

So far, the earlier reported strong correlation between basicity of an N-heterocyclic carbene (NHC) and its reactivity in poly(ε-caprolactam) (PA6) synthesis resulted in a substantial limitation of applicable carbenes. Here, to overcome this issue, 1,3-dimethylimidazolium-2-carboxylate, an easily accessible, air and moisture-stable NHC, was applied as thermally latent initiator. In order to compensate for its low basicity, reactivity was enhanced by the addition of both a Lewis acid and an activator to ease the initial polymerization step. The resulting mixtures of ε-caprolactam, the CO 2-protected NHC, a Lewis acid and N-acylazepan-2-one constitute homogeneous, thermally fully latent "single-component" blends for the anionic polymerization-based synthesis of PA6. They can be stored both in the liquid and solid state for days and months, respectively, without any loss in activity. The role of the Lewis acid as well as technical implications of the prolonged pot-times are discussed.

Abstract1,3‐Dicyclcohexyl‐6,9‐dimethyl‐1,3,6,9‐tetraazaspiro[4.4]non‐7‐ene‐2,4‐dione, a spirocyclic parabanic acid derivative of N,N‐dimethylimidazole, is used as thermally latent, protected N‐heterocyclic carbene (NHC) in polymerizing anhydride‐cured epoxide resins, and azepan‐2‐one, respectively. The protected carbene is synthesized from 1,3‐dimethylimidazolium‐2‐carboxylate in the presence of two equivalents of cyclohexyl isocyanate. In the synthesis of epoxide resin thermosets, this class of latent NHC allows the production of fast and fully cured materials with high crosslinking content. Fast and complete conversion is found in the anionic ring opening polymerization (AROP) of azepan‐2‐one (ε‐caprolactam, CLA) with and without additional activators.