Alkylation of substituted phenols in DMF by MeI using TMGN (bis- 1,1,8,8-(tetramethylguanidino)naphtalene) a proton sponge as base: a kinetics study by NMR spectroscopy

Evaluation of a new proton sponge, bis-1,1,8,8-(tetramethylguanidino)naphtalene (TMGN), in substituted phenols O-alkylation by methyl iodide in DMF has been studied. Kinetic measurements were performed in N,N-dimethylformamide-d7 and followed by H NMR using stoichiometric amounts of reagents. Plot of the results shows that the reaction follows an almost perfect second order rate law. However the Hammett plots for substituted phenols are not linear but bell shaped. In order to separate the deprotonation and alkylation contribution to the kinetics, deprotonation of phenols by TMGN has been investigated by quantitative C NMR. By combining these data a linear Hammett plot with a negative slope was obtained for the alkylation step and substituted phenol acidity constants in DMF, not accessible by NMR measurements, were determined which are in agreement with literature data. Introduction The functionalization of natural molecules plays an important role with the aim of obtaining new biologically active molecules. In the continuity of our laboratory’s work, we decided to look for original methods for polyphenol selective O-alkylations. Classical approach generally requires numerous protection and deprotection steps. Recent results from our lab, based on the use of microreactor technology, allowed us to selectively alkylate quercetin, a natural polyphenol, when the base is mixed with the alkylating reagent. Phenol ether are usually synthesized under basic conditions following Williamson synthesis. However Williamson synthesis using mineral bases is hazardous to apply in a microreactor of micrometric capillary size as the low solubility of such bases in organic solvents led to clogging. An alternative method based on more soluble but weaker organic bases is more adapted to micrometric flow reactor; 1,8diazabicyclo [5.4.0] undec-7-ene (DBU) and tetrabutylammonium hydroxide are for example two appropriate organic bases. Phenolates, obtained by deprotonation of the hydroxyl group by a strong base, are good nucleophiles. Considering that deprotonation step is complete, the phenolate alkylation is usually considered as the rate-determining step. For primary halides the O-alkylation step follows a SN2 mechanism. Sodium phenolate alkylation with methyl iodide in dry sulfolane demonstrated faster kinetics Alkylation of substituted phenols in DMF by MeI using TMGN 2 rates for electron donating group than for electron withdrawing groups. The kinetics plot are correlated by Hammett with a negative slope (ρ = -1.1). Phenol alkylation by ethyl iodide in acetonitrile has been kinetically studied by Kondo et al. on pre-formed phenolates and the data also are correlated by a Hammett plot with negative slope (ρ = -1.39). Preparation of the corresponding phenolate anion was achieved by phenols deprotonation from tetramethylammonium hydroxide solution. Strong basicity of hydroxide base gives access to a completely deprotoned phenolate form while ammonium counterion assures a high solubility in organic solvents and a loose ion-pair. However tetrabutylammonium hydroxide is available only in protic solvents like water or methanol and the reaction of phenol with tetrabutylammonium hydroxide produce a molecule of water, which both hampers the reactivity. Recently, a new “proton sponge”, bis-1,1,8,8-(tetramethylguanidino)naphtalene (TMGN), incorporating two hindered guanidine site on a naphthalene backbone, which is now commercially available has been described. TMGN has several advantages over DBU we used previously: TMGN is more basic than DBU and more hindered. This last property is crucial in order to prevent any side alkylation of the base during the reaction while keeping a high deprotonation rate (figure 1). TMGN DBU pKBH (MeCN) = 25.1 pKBH (MeCN) = 24.32 Figure 1. bis-1,1,8,8-(tetramethylguanidino)naphtalene (TMGN) and 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU) structure and their relative basicity in acetonitrile. Preliminary studies on p-nitrophenol O-methylation in the presence of TMGN and methyl iodide as the alkylating reagent produced very promising results, with a full respect of solubility conditions imposed by working on microsystem. Therefore, we decided to follow in the time alkylation of several substituted phenols, deprotonated in-situ by TMGN (scheme 1).