O-aryl imidates, isoureas and thiocarbamates
Tate, Joseph Andrew
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Phenols are some of the most readily available and easily elaborated aromatic compounds, but the strength of the CAr-O bond hampers their conversion to highly sought CAr-N, CAr-S and CAr-C analogues. Attempts have therefore been made to establish new protocols for achieving such transformations by derivatising phenols with suitable CAr-O bond activating groups. In particular, investigations have focussed on the development of reactions with the potential to enable phenols to be conveniently converted to anilines. Towards this goal, the synthesis of O-aryl trihaloacetimidates was investigated with a view to probing their ability to rearrange to N-aryl trihaloacetamides via transition metal catalysis (Scheme 1). It was found that O-aryl trichloroacetimidates could be obtained from the base-catalysed reactions of phenols with trichloroacetonitrile, but only when electron-rich phenols were applied. In contrast, N-(4-methylphenyl)-O-aryl trifluoroacetimidates were generated in good yields from electron-rich and electron-poor phenols by their condensation with N-(4-methylphenyl)trifluoroacetimidoyl chloride. With these substrates in hand, a number of transition metal catalysts were screened for activity in the proposed rearrangement reactions, but the desired transformations were not achieved. As part of this screen, a novel mono-NHC palladium(II) precatalyst with the potential to be thermally activated was developed. Scheme 1 The proposed strategy for converting phenols to anilines. The hydroxide-catalysed rearrangement of O-aryl-N,N’-diisopropyl isoureas to N-aryl- N,N’-diisopropyl ureas was reported in 1983, but there have been no reported applications of this reaction to date. The reaction was therefore revisited with the intention of realising its unexplored synthetic potential. The reported hydroxide-catalysed rearrangement of O-phenyl-N,N’-diisopropyl isourea to N-phenyl-N,N’- diisopropyl urea was, however, discredited on the basis of 1H NMR and UV spectrometric analyses (Scheme 2). This isourea was instead, found to be converted to phenoxide and diisopropyl urea under the reported conditions. A detailed kinetic study revealed that the isourea was not directly hydrolysed, but underwent hydroxide-mediated elimination to produce phenoxide and diisopropyl carbodiimide. The hydrolysis of diisopropyl carbodiimide to diisopropyl urea then occurred in a slower, second step which was catalysed by hydroxide. Attempts to identify and synthesise N-heterocylcic isourea structures which were more disposed towards rearrangement were unsuccessful. Scheme 2 The reported and observed reactivity of O-phenyl-N,N’-diisopropyl isourea in aqueous base. Early attempts to use O-aryl-N,N’-dimethyl thiocarbamates as phenol-derived pseudohalides in palladium(0)-catalysed, CAr-C bond-forming cross-coupling reactions showed little promise due to the onset of their base-induced decomposition. However, the formation of a diaryl thioether side product was observed during these studies, leading to a preliminary investigation into the use of aryl thiocarbamates as hydrogen sulfide surrogates and thiophenol precursors in palladium(0)-catalysed C-S coupling reactions (Scheme 3). The promise of this approach was demonstrated by the synthesis of both symmetrical and unsymmetrical diaryl thioethers in the palladium(0)-catalysed couplings of O- and S-(4-trifluoromethyl)-N,N-dimethyl thiocarbamate with 1-bromo-4- fluorobenzene. Scheme 3 The preparation of diaryl thioethers from O-aryl thiocarbamates and aryl bromides via palladium(0) catalysis.