The Lewis Pair Polymerization of Lactones Using Metal Halides and N-Heterocyclic Olefins: Theoretical Insights

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.

Homo‐ and random copolymerizations of ω‐pentadecalactone with ε‐caprolactone using isothiourea‐based dual catalysis

“…A better understanding of the proposed mechanism of the copolymerization of PDL and CL (Scheme ) is assumed to work via the coordination of lactones to magnesium‐center, thereby increasing the electron deficiency on the carbonyl carbon, which facilitates the ring opening through nucleophilic attack of activated alcohol, according to the previous works . The mode of alcohol activation is often described as the deprotonation/activation of an initiating alcohol species by THTP in the presence of MgI 2 .…”

“…However, only a limited number of N‐heterocyclic organic catalysts, strong acids, and phosphazene superbases are able to polymerize the PDL (Scheme ). From this perspective, cooperative catalytic systems appear attractive due to their capacity for providing a powerful alternative to enhance the polymerization activity and selectivity . Crucially, Buchmeiser, Naumann, and co‐workers reported that the inactive N‐heterocyclic carbenes in combination with Lewis acids (MgCl 2 , ZnCl 2 , SnCl 2 ) are capable of polymerizing CL.…”

Two zinc(ii) coordination polymers based on flexible co-ligands featuring assembly imparted sensing abilities for Cr2O72− and o-NP

“…This phenomenon is a typical example of aggregation-induced emission (AIE) and may be explained with that the coordination to metal ions in a complexes restricted the deformation of the ligand and the induced nonradiative relaxation. 41,42 Sensing of Cr 2 O 7…”

Synthesis, Characterization of Pyridyl Heterocyclic Olefins (PHOs) and Activation of Heterocumulenes

“…[16][17][18] In addition, NHOs were utilized in CO 2 functionalization, [19] transesterificiation, [20] silylation, [21] hydroborylation [22] , alkylation [23] as well as polymerization. [24][25][26][27][28] With suitable structural design, NHOs can serve as a new class of Brønsted/Lewis base organocatalysts for a wide range of chemical reactions. Up to now, all of the NHOs published were based on imidazole, [1,29] benzylimidazole, [12] triazole, [35] imidazoline [30] and six-membered enediamines [32] (Scheme 2).…”