Amido functionalized POSS

The simplest method of preparing amides is by reacting the amines with carboxylic acids (for example in the presence of DCC as a coupling agent), acid anhydrides, esters or acyl halides. In the work by Janeta et all (Chemistry - A European Journal 2014, 20, 15966-15974), the reactions were carried out with the use of acid chlorides, which were selected because these reactions do not require an additional coupling agent, the reaction is rapid even at reduced temperature, and the resulting by-product (ammonium salt) is easy to remove.
The reaction of amines with acid chlorides, the products of which are amides (Schotten-Baumann reaction), is usually carried out in a two-phase system. The alkaline aqueous phase containing sodium hydroxide neutralizes the acid ammonium salt and the product remains in the organic phase. However, as under these conditions the silsesquioxane core decomposes (a strong base breaks the Si–O–Si bonds), the solution was to use triethylamine as a neutralizing agent for hydrogen chloride. Et₃N as a weak base does not decompose the silicon core, and at the same time is able to deprotonate the ammonium salt of POSS, which enabled the amidation reaction to proceed. Triethylamine is a slightly stronger base than propylamine. The pKa of its protonated form in the aqueous environment is 10.75, while for propylamine this value is 10.71, therefore it was used to deprotonate the POSS salt. Reactions were carried out in anhydrous DMF at 0 °C. An excess of triethylamine was used to obtain the 3-aminopropyl moiety in situ, which was then reacted with the appropriate acyl chloride, and to neutralize the hydrogen chloride produced. Quench the reaction by adding a 1 M aqueous hydrochloric acid solution. The resulting precipitate was filtered off and washed with deionized water to remove the resulting triethylamine hydrochloride. The next step in product isolation was washing the precipitate with sodium bicarbonate in order to remove the carboxylic acid by-product formed in the reaction. During this process, no opening of the siloxane cage was observed, which proves that the amide bond stabilizes the final product. The resulting compounds are crystalline solids, stable in air. The reactions were performed several times, which confirmed the repeatability of the method. As shown in scheme below, the reaction for all derivatives was very good i.e. with yields ranging from 58% to 91% when using the chloride salt as substrate and from 82% to 97% when using the trifluoromethanesulfone salt. According to this method, a series of amide derivatives of POSS were prepared. Mainly aromatic derivatives substituted in 4 position  and alkyl derivatives were selected for the research.

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