Cyclodimerization versus Polymerization of Methyl Methacrylate Induced by N ‐Heterocyclic Carbenes: A Combined Experimental and Theoretical Study

The activation behavior of two N ‐heterocyclic carbenes (NHCs), namely, 1,3‐bis(isopropyl)imidazol‐2‐ylidene(NHC i Pr) and 1,3‐bis( tert‐ butyl) imidazol‐2‐ylidene (NHC t Bu), as organic nucleophiles in the reaction with methyl methacrylate (MMA) is described. NHC t Bu allows the polymerization of MMA in DMF at room temperature and in toluene at 50 °C, whereas NHC i Pr reacts with two molecules of MMA, forming an unprecedented imidazolium–enolate cyclodimer (NHC i Pr/MMA=1:2). It is proposed that the reaction mechanism occurs by initial 1,4‐nucleophilic addition of NHC i Pr to MMA, generating a zwitterionic enolate 2 , followed by addition of 2 to a second MMA molecule, forming a linear imidazolium–enolate 3 (NHC i Pr/MMA=1:2). Proton transfer, generating intermediate 5 , followed by cyclization and release of methanol yielded the aforementioned zwitterionic cyclodimer 1:2 adduct 7 , the molecular structure of which has been established by NMR spectroscopy, X‐ray diffraction, and mass spectrometry. This unexpected difference between NHC t Bu and NHC i Pr in the reaction with MMA (polymerization and cyclodimerization, respectively) can be rationalized by using DFT calculations. In particular, the nature of the NHC strongly influences the cyclodimerization pathway, the cyclization of 5 and the release of methanol are the discriminating step and limiting step, respectively. In the case of NHC t Bu, both steps are strongly disfavoured compared with that of NHC i Pr (energetic difference of around 14 and 9 kcal mol −1 , respectively), preventing the cyclization mechanism from a kinetic viewpoint. Moreover, addition of a third molecule of MMA in the polymerization pathway results in a lower activation barrier than that of the limiting step in the cyclodimerization pathway (difference of around 14 kcal mol −1 ), in agreement with the formation of polymethyl methacrylate (PMMA) by using NHC t Bu as nucleophile.