Reactions of Hexamethyldisilazanes with Derivatives of Cyclic and Acyclic Carboxylic Acids

Abstract

Although alkali metal salts of hexamethyldisilazane (HMDS) are well-known to be hindered bases for organic synthesis, only a few reports concerning synthetic utilities of disilazane itself can be found in the literature. In this paper, we examined the reactions of acid anhydrides (( 1), (6), (12)-(14)) and an acid chloride (5) with HMDS and N-phenylhexamethyldisilazane (PhHMDS). While the reaction of phthalic anhydride (1) with HMDS in refluxed CH2Cl2 gave phthalamic acid ( 3), the similar reaction in acetonitrile or chlorobenzene gave phthalimide (2). With PhHMDS, (1) was converted into N-phenylphthalimide-( 4) in chlorobenzene. Phthaloyl dichloride (5) reacted with HMDS o(r PhHMDS to give 2) or (4), respectively. From the mechanistic study, (2) was postulated to be obtain (ed from trimethylsilyl o-(N-trimethylsilylcarbamoyl)benzoate, which was converted into (3) by hydrolytic desilylation, via an intramolecular elimination of hexamethyldisiloxane. Maleic anhydride (6 ) was transformed into maleamic acid (8), (9), together with only a trace amount of maleimide (10), under the employed conditions. As for the cyclization, an intermediate obtained from (5) was less reactive than the one obtained from (1). The reaction of benzoic anhydride (12)-(14) with HMDS gave benzamides (15), (17), (19) or dibenzamides (16), (18), depending upon the quantity of HMDS and the substituent. Dibenzamide was found to be obtained not via an intermolecular elimination of disiloxane but via a nucleophilic attack of N-silylbenzamide toward benzoic anhydride.