The selectivity of several molybdenum and tungsten imido alkylidene N-heterocyclic carbene (NHC) complexes in the ring opening metathesis polymerization (ROMP) of enantiomerically pure endo,exo-2,3-dicarbomethoxynorborn-5-ene (DCMNBE) was examined by 1H- and 13C NMR spectroscopy. With one exception, all complexes showed a strong bias toward the formation of trans-isotactic polymers, some yielding polymers based on >98% trans-isotactic repeat units. This high selectivity was successfully extended to the ROMP of other monomers such as endo and exo-N-(R)-(+)-α-methylbenzyl-5-norbornene-2,3-dicarboximide, 2,3-bis[(menthyloxy)carbonyl]norbornadiene, and methyl-N-(S)-(−)-α-methylbenzyl-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate. The cationic initiators [Mo(NAr)(5 i Pr)(CHCMe2Ph)(X)][B(ArF)4] (Ar = 2- t BuC6H4, 2,6-Me2C6H3, 2,6- i Pr2C6H3; 5 i Pr = 1,3-diisopropylimidazol-2-ylidene; X = pyrrolide, O-2,6-(2,4,6-Me3C6H2)2C6H3; B(ArF)4 = B(3,5-(CF3)2C6H3)4) were found to be the most reactive ones, while also maintaining very high isoselectivity. Finally, a large imido ligand improved stereospecificity, while the choice of the NHC ligand had only a minor influence. Polymerizations can be terminated with 2-methoxystyrene, as evidenced by matrix-assisted laser-desorption time-of flight mass spectrometry.
Three novel molybdenum imido alkylidene N‐heterocyclic carbene (NHC) pre‐catalysts, that is, Mo(N‐t‐Bu)(1‐(2,6‐diisopropylphenyl)‐3‐isopropyl‐4‐phenyl‐1H‐1,2,3‐triazol‐5‐ylidene)(CHCMe2Ph)(OTf)2 (I1, OTf = CF3SO3), Mo(N‐t‐Bu)(1‐(2,6‐diisopropylphenyl)‐3‐isopropyl‐4‐phenyl‐1H‐1,2,3‐triazol‐5‐ylidene)(CHCMe2Ph)(OTf)(t‐BuO) (I2) and Mo(N‐2,6‐Me2‐C6H3)(1,3,4‐triphenyl‐4,5‐dihydro‐1H‐1,2,4‐triazol‐5‐ylidene)(CHCMe2Ph)(OTf)2 (I3) are presented. Compared to complexes based on imidazol‐2‐ylidenes or imidazolin‐2‐ylidenes, (1‐(2,6‐diisopropylphenyl)‐3‐isopropyl‐4‐phenyl‐1H‐1,2,3‐triazol‐5‐ylidene) used in precatalysts I1 and I2 exerts a comparably strong trans effect to the triflate groups trans to the NHC, while (1,3,4‐triphenyl‐4,5‐dihydro‐1H‐1,2,4‐triazol‐5‐ylidene) used in I3 has a weaker trans effect on the triflate. In combination with a suitable second anionic ligand at molybdenum, that is, OTf, t‐BuO, compounds I1–I3 require higher temperatures to become active and can thus be used as truly room temperature latent pre‐catalysts, even for a highly reactive monomer such as dicyclopentadiene (DCPD). When used as latent precatalysts, I1–I3 offer access to poly‐DCPD with different degrees of cross‐linking and glass‐transition temperatures (Tg). © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3028–3033
Molybdenum- and tungsten-based olefin metathesis catalysts have demonstrated excellent results in the control of cis (Z-) selectivity as well as enantioselectivity. However, their air and moisture sensitivity, which requires the use of a glovebox, has prevented their more widespread use by organic chemists. Now we report on developed, preweighed Mo catalysts formulated in paraffin tablets. The significantly improved air stability, high homogeneity, and uniformity of the pellets allow researchers to carry out reactions on the bench avoiding the need of a glovebox. The two different Mo-based complexes which were packed into tablets are XiMoPac-Mo001 (1) that can be used to achieve endo-selective enyne ring-closing metathesis (RCM) reactions, inter alia; and XiMoPac-Mo003 (2) which was reported among the best catalysts to promote Z-selective cross-metathesis. For the evaluation of the wax-protected catalysts commonly used, highly reproducible robust model reactions were chosen: homo cross-metathesis (HCM) of functionalized (e.g., methyl 9-decenoate) and unfunctionalized (allylbenzene) terminal olefins, and ring closing metathesis (RCM) of diethyl diallylmalonate. The yields and conversions were comparable with those which can be achieved in glovebox with nonformulated catalysts. Exposure to air did not cause any significant reduction in conversion while the product selectivity (targeted product vs homologues derived from double bond isomerization) remained high. In contrast, exposure to air caused a measurable drop in the conversion with the nonprotected catalyst. Furthermore, the formulated catalysts remained unaffected even after 4 h of exposure to air, showing its enhanced air stability. In conclusion, these commercially available air-stable Mo-catalyst tablets allow the reactions to be accomplished using ordinary Schlenk techniques, and hence simplify catalyst handling in pilot laboratories and plants.
The influence of the structure of cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) catalysts, i. e. of [Mo(N-2-tert-butyl-C 6 H 4 ) (CHCMe 2 Ph)(NHC)X + B(Ar F ) 4 À ] (NHC = 1,3-di(2-Pr) imidazol-2-ylidene (iPr), 1,3-dimesitylimidazol-2-ylidene (IMes); X = pyrrolide, OCH(CF 3 ) 2 , B(Ar F ) 4 À = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) and of [Mo(N-3,5-Me 2 -C 6 H 3 )(CHCMe 2 Ph)(NHC)(CH 3 CN)X + B(Ar F ) 4 À ] (NHC = 1,3-dimesitylimidazol-2-ylidene, 1,3-dimesitylimidazolin-2-ylidene (IMesH 2 ); X = CF 3 SO 3 , OCPh(CF 3 ) 2 ) on E/Z-selectivity in the ring-opening cross-metathesis (ROCM) of endo, endo-2,3dicarbomethoxynorborn-5-ene (endo, endo-DCMNBE), exo, exo-2,3-dicarbomethoxynorborn-5-ene (exo, exo-DCMNBE), endo, exo-2,3-dicarbomethoxynorborn-5-ene ((+) DCMNBE) and 2,3-exo,exo-bis(acetoxymethyl)-7-oxabicyclo[2.2.l]hept-5-ene (7-oxa-NBE) with 1-pentene, styrene, allyltrimethylsilane, allyl benzyl ether, allyl phenyl ether and allyl ethyl ether has been studied. With the exception of the ROCM reaction of endo, endo-DCMNBE with styrene, all other ROCM reactions of endo, endo-DCMNBE proceeded under thermodynamic control without any post-metathesis isomerization reactions with full retention of the configuration of the newly formed 1,2-disubstituted double bond as confirmed by kinetic studies. Similar accounts for selected homometathesis reactions. Catalyst structure-selectivity correlations based on the buried volume, V bur , of the N-imido ligand are presented.ROCM, [2] this has been accomplished by the use of chiral catalysts having a stereogenic metal center, while in the case of ruthenium alkylidene-, [3] i. e. Grubbs-and Grubbs-Hoveyda-type catalyst-triggered ROCM, this was realized by the use of chiral Nheterocyclic carbene (NHC) ligands [3] or, again, with the aid of a stereogenic metal center. [4] In several reports [3a,5] the use of 7-oxa-or 7-azanorbornenes compounds has been described, to make use of the heteroatom to increase stereoselectivity. There are also reports, which exclusively use aryl-substituted alkenes i. e. styryl-alkenes or enol ethers or enoates as a cross-
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