Synthesis of trans-Isotactic Poly(norbornene)s through Living Ring-Opening Metathesis Polymerization Initiated by Group VI Imido Alkylidene N-Heterocyclic Carbene Complexes

Abstract

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.

Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalysts

“…Catalysts containing the more basic alkyl‐substituted imido ligands ( 8 , 11 , 17 a ) displayed decreased reactivity compared to complexes with aromatic imido ligands. This was also observed earlier . To demonstrate that this is a reactivity issue, we mixed catalyst 11 with 10 equiv 10‐undecenoic acid and monitored the reaction by 1 H NMR (Figure S122).…”

“…Moreover, it was shown that protonation of Mo( N ‐Ad)(CHCMe 2 Ph)(NHC)(pyrrolide)(aryloxide) selectively eliminates pyrrole . Also, other examples in which pyrrole was eliminated from metal imido alkylidene NHC bispyrrolide complexes or cationic metal imido alkylidene NHC monopyrrolide complexes were reported . All this evidence indicates that X‐type ligands are the first and preferred target for proton transfer.…”

Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalysts

“… Synthesis of cationic monocarboxylate species 11 – 15 from bistriflates 9 a – d via mixed monotriflate/monocarboxylate complex 10 or in situ generation of cationic monotriflate species and subsequent salt metathesis. Synthesis of cationic monocarboxylate complexes 17 a , b from dichloride NHC alkylidenes 16 a , b .…”

“…This is understandable when revisiting the crystal structure of 4 a⋅H 2 O (Figure ), where a weak interaction between one of the hydrogen atoms of water and a triflate ligand shows the way to X‐type ligand protonation. On top, in stark contrast to catalysts bearing weakly basic ligands, [Mo( N ‐2,6‐Me 2 ‐C 6 H 3 )(CHCMe 2 Ph)(NC 4 H 4 )(5‐ i Pr)][B(Ar F ) 4 ] ( 5 c ), with a basic pyrrolide ligand (p K a,pyrrole (H 2 O)=16.5) exhibited no productivity in any of the investigated reactions. Instead, when 5 c was reacted with 5 equiv of 4‐penten‐1‐ol, the formation of pyrrole was immediately observed in the 1 H NMR spectrum, accompanied by the emerging of a new alkylidene signal ( δ =14.67 ppm, t, 3 J HH =5.7 Hz, CD 2 Cl 2 ).…”

Stereoselective Olefin Ring‐Opening Cross Metathesis Catalyzed by Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Complexes

“…[12] Our studies also revealed that the cationic tetracoordinated 14-VE complexes are indeed the active species to which an olefin approaches trans to the NHC followed by metallacyclobutane formation. [13] We were also able to correlate E-selectivity with the buried volume, V bur , of the imido ligand in molybdenum imido alkylidene NHC complexes. [13] We were also able to correlate E-selectivity with the buried volume, V bur , of the imido ligand in molybdenum imido alkylidene NHC complexes.…”