Cyclodimerization versus Polymerization of Methyl Methacrylate Induced by N‐Heterocyclic Carbenes: A Combined Experimental and Theoretical Study. Issue 14 (6th March 2014)

• Record Type: Journal Article
• Authors: Nzahou Ottou, Winnie; Bourichon, Damien; Vignolle, Joan; Wirotius, Anne‐Laure; Robert, Fredéric; Landais, Yannick; Sotiropoulos, Jean‐Marc; Miqueu, Karinne; Taton, Daniel
• Is Part Of: Chemistry. Volume 20:Issue 14(2014)
• Page Start: 3989
• Page End: 3997
• Publication Date: 2014-03-06
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201304492
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>The activation behavior of two <italic>N</italic>‐heterocyclic carbenes (NHCs), namely, 1, 3‐bis(isopropyl)imidazol‐2‐ylidene(NHC<italic>i</italic>Pr) and 1, 3‐bis(<italic>tert‐</italic>butyl) imidazol‐2‐ylidene (NHC<italic>t</italic>Bu), as organic nucleophiles in the reaction with methyl methacrylate (MMA) is described. NHC<italic>t</italic>Bu allows the polymerization of MMA in DMF at room temperature and in toluene at 50 °C, whereas NHC<italic>i</italic>Pr reacts with two molecules of MMA, forming an unprecedented imidazolium–enolate cyclodimer (NHC<italic>i</italic>Pr/MMA=1:2). It is proposed that the reaction mechanism occurs by initial 1, 4‐nucleophilic addition of NHC<italic>i</italic>Pr to MMA, generating a zwitterionic enolate <bold>2</bold>, followed by addition of <bold>2</bold> to a second MMA molecule, forming a linear imidazolium–enolate <bold>3</bold> (NHC<italic>i</italic>Pr/MMA=1:2). Proton transfer, generating intermediate <bold>5</bold>, followed by cyclization and release of methanol yielded the aforementioned zwitterionic cyclodimer 1:2 adduct <bold>7</bold>, the molecular structure of which has been established by NMR spectroscopy, X‐ray diffraction, and mass spectrometry. This unexpected difference between NHC<italic>t</italic>Bu and NHC<italic>i</italic>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 <bold>5</bold> and the release of methanol are the discriminating step and limiting step, respectively. In the case of NHC<italic>t</italic>Bu, both steps are strongly disfavoured compared with that of NHC<italic>i</italic>Pr (energetic difference of around 14 and 9 kcal mol<sup>−1</sup>, 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<sup>−1</sup>), in agreement with the formation of polymethyl methacrylate (PMMA) by using NHC<italic>t</italic>Bu as nucleophile.</p> </abstract>