Mechanism of the Regio- and Stereoselective Cyclopolymerization of 1,6-Hepta- and 1,7-Octadiynes by High Oxidation State Molybdenum–Imidoalkylidene N-Heterocyclic Carbene Initiators

Abstract

The regio-and stereoselective cyclopolymerization of a series of 1,6-heptadiynes and 1,7-octadiynes, i.(MAP3) is described. With initiators I1−I4, high regioselectivity >96% was observed in the polymerization of M1−M4, allowing for virtually selective α-insertion. These initiators displayed also high stereoselectivity, allowing for all-trans polyenes with up to 96% syndioselectivity. By contrast, the MAP initiators MAP1−MAP3 showed poor regioselectivity (26−62% α-selectivity). A polymerization mechanism is proposed that explains for the high stereo-and regioselectivity of the Mo− imidoalkylidene NHC complexes. ■ INTRODUCTIONThe cyclopolymerization of 1,6-hepta-and 1,7-octadiynes has been widely used for the syntheses of soluble, conjugated polymers possessing a rigid backbone. 1−29 Equally important, it can also be used as a probe to determine both the regio-and stereoselectivity of insertion and to come up with a polymerization mechanism for a specific initiator system. Two different pathways for the cyclopolymerization of α,ω-diynes with Schrock-type molybdenum carbenes exist (Scheme 1). 3,4 Thus, the monomer can undergo either α-or β-addition. The first addition step of the monomer to the metal alkylidene is decisive for the structure of the resulting repeat unit. In the absence of any backbiting, the polymer structure is a concise record of all previous insertion steps. For first-generation Schrock initiators, the concept of large and small alkoxides was developed by the Schrock group and allowed for the selective β-addition of monomer. 1,2 Our group was capable of extending this concept to selective α-addition by using first-generation Schrock initiators bearing small alkoxides, an additional coordinating base, and low temperatures. 18,[20][21][22][23][24]30,31 In that context not only the alkoxide but also the imido ligands played a predominant role in the control of the regioselectivity. 32,33 Recently, molybdenum−imidoalkylidene NHC bis(triflate) 34 and cationic tungsten−oxoalkylidene NHC complexes 35 have been developed in our group. Particularly molybdenum− imidoalkylidene NHC bistriflates display high reactivity for various norborn-2-enes, 7-oxanorborn-2-enes, and diynes and are also tolerant toward nitrile, amino, hydroxyl, and carboxylic acid groups. 34 We were interested in the polymerization mechanism that applies in the polymerization of α,ω-diynes with these novel initiators. Equally important, we wished to find out about the stereo-and regioselectivity of these novel complexes and therefore addressed the issues of tacticity and cis/trans selectivity in cyclopolymerization. Since the cationic species derived from Mo−imidoalkylidene NHC bis(triflate) complexes resemble to some extent molybdenum−imidoalkylidene monoalkoxide monopyrrolide (MAP) catalysts developed by the Schrock group, 36,37 and since these systems have

Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Complexes: Structure–Productivity Correlations and Mechanistic Insights

“…In the search for ionic group 6 metal alkylidenes, we recently reported a new class of neutral or cationic N‐heterocyclic carbene (NHC)‐based Mo–imido alkylidene NHC and W–oxo alkylidene NHC‐based olefin metathesis catalysts. In particular, Mo–imido alkylidene NHC bistriflate catalysts display functional group tolerance for hydroxyls, aldehydes, carboxylic acids, and secondary amines, at least in ring‐opening metathesis polymerization (ROMP) and in the cyclopolymerization of α,ω‐diynes, exceeding in this regard high‐oxidation‐state molybdenum‐based Schrock catalysts. It is also worth mentioning that, in olefin cross‐metathesis reactions of non‐functionalized olefins, the first cationic Mo–imido alkylidene NHC complex, [Mo( N ‐2,6‐Me 2 ‐C 6 H 3 )(CHCMe 2 Ph)(CH 3 CN)OTf + B(Ar F ) 4 − ] allowed for turnover numbers (TONs) up to 210 000 .…”

First Neutral and Cationic Tungsten Imido Alkylidene N‐Heterocyclic Carbene Complexes

“…The synthesis of W(NAr')(NHC)(=CHR)(2,5-Me 2 pyr) 2 (1;A r ':2 ,6-iPr 2 C 6 H 3 ;N HC:1 ,3-diisopropylimidazol-2-ylidene;2 ,5-Me 2 pyr: 2,5-dimethylpyrrolide;R :C Me 2 Ph), W(NAr')(NHC)(=CHR)(2,5-Me 2 pyr)(OC 6 (6), W(NAr')(NHC)(=CHR)(OtBu) 2 (7), [W(NAr')(NHC)(=CHR)(Ot-Bu) + ][B(Ar F ) 4 À ] ( 8), and W(NAr')(NHC)(=CHR)(OCMe(CF 3 ) 2 ) 2 (9)i s described, and the reactivity of the complexes in olefin metathesis and cyclopolymerization is reported. The cationic complexes 4, 5,a nd 6 showed high productivity and activity in olefin metathesis reactions, with turnover numberso fu pt o 40 000 and turnover frequencies (TOF 5min )o fu pt o3 1s À1 ,a nd also substantial functional group tolerance towarde sters, nitriles, alcohols, and sulfides, particularly in the cyclopolymerization of a,w-diynes.…”

“…The product crystallized out of the reaction mixture. The yellow solution was separated from the crystals, the solvent was partially removed in vacuo, and the remaining dissolved product was crystallized at À30 8C; yield 143.9 mg (65 %); 1 HNMR (400 MHz, C 6 9, 53.3, 52.8, 52.2, 50.5, 36.7, 33.6, 33.0, 32.3, 31.2, 27.2, 26.6, 25.3, 24.8, 24.2, 23.2, 23.1, 23.1, 22.6, 15.5 ( 8):A tÀ30 8C, as olution of N,N-dimethy-lanilinium·B(Ar F ) 4 À (95.3 mg, 0.09 mmol) in diethyl ether (2 mL) was added to as olution of 3 (71.0 mg, 0.09 mmol) in diethyl ether (2 mL). The reaction mixture was stirred for 1h at room temperature.…”

Molybdenum imido alkylidene and tungsten oxo alkylidene N-heterocyclic carbene complexes for olefin metathesis

“…2] Cycloaddition of the monomer to the alkylidene then starts ringopening metathesis polymerization (ROMP). Notably, this activation pathway has so far been confirmed for both ROMP and cyclopolymerization [40]. While the origin of functional group tolerance is still not fully clarified, the high stability of these complexes is a result of the resonance stabilization of the nucleophilic NHC attached to Mo.…”