Abstract
Lewis pair polymerization employing N-Heterocyclic olefins (NHOs) and simple metal halides as co-catalysts has emerged as a useful tool to polymerize diverse lactones. To elucidate some of the mechanistic aspects that remain unclear to date and to better understand the impact of the metal species, computational methods have been applied. Several key aspects have been considered: (1) the formation of NHO-metal halide adducts has been evaluated for eight different NHOs and three different Lewis acids, (2) the coordination of four lactones to MgCl2 was studied and (3) the deprotonation of an initiator (butanol) was investigated in the presence and absence of metal halide for one specific Lewis pair. It was found that the propensity for adduct formation can be influenced, perhaps even designed, by varying both organic and metallic components. Apart from the NHO backbone, the substituents on the exocyclic, olefinic carbon have emerged as interesting tuning site. The tendency to form adducts is ZnCl2 > MgCl2 > LiCl. If lactones coordinate to MgCl2, the most likely binding mode is via the carbonyl oxygen. A chelating coordination cannot be ruled out and seems to gain importance upon increasing ring-size of the lactone. For a representative NHO, it is demonstrated that in a metal-free setting an initiating alcohol cannot be deprotonated, while in the presence of MgCl2 the same process is exothermic with a low barrier.
Highlights
Among the many approaches towards polymerization-active Lewis pairs (LPs) [1,2,3,4,5,6,7,8,9], the combination of commercially available metal halides (MXn ) and readily accessible organobase catalysts (OBCs) provides very simple, yet surprisingly powerful access to Lewishalide pair (LP) catalysts for lactone polymerization [10,11,12]
The increased electron density on the exocyclic carbon should render N-heterocyclic olefins (NHOs) 3 most prone for coordination to a Lewis acid, a conclusion that is supported by the data listed in Tables 1 and 2; the ZnCl2 (3)(THF) adduct accounts for the most exothermic reaction energy for all investigated NHO/metal halide combinations (−53.4 kJ/mol)
The results described indicate that adduct formation for NHO/metal halides Lewis pairs can be readily influenced—perhaps even tailored—by suitable selection of both components
Summary
Among the many approaches towards polymerization-active Lewis pairs (LPs) [1,2,3,4,5,6,7,8,9], the combination of commercially available metal halides (MXn ) and readily accessible organobase catalysts (OBCs) provides very simple, yet surprisingly powerful access to LP catalysts for lactone polymerization [10,11,12]. Coordinates to the monomer, accepting electron density and increases the polarization of the carbonyl group (Scheme 1) This facilitates attack by an (organic) nucleophile (i.e., an alcohol initiator activated by the applied OBC) and subsequent ring-opening. Behavior was attributed to the ability of the activating Lewis acid to differentiate between the the inherent of this setup (both the components be exchanged independently) polymer chain and theadaptability cyclic monomer. Lewis significant amounts of GBL was possible [11,12] This effect was proposed to originate from a acid.
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