The formation of diarylamines by reacting anilines with aromatic electrophiles is, usually, a reaction demanding long reaction times, high temperatures and strong bases, in order to obtain acceptable yield, due to the low nucleophilicity of anilines.1 The Buchwald-Hartwig cross-coupling reaction is a very useable and elegant solution to this problem.2,3,4 Anticipating the classical metal-free SNAr-amination reaction, without using Pd-catalysts and strong bases, with only higher temperature and pressure as tool as depicted in the Scheme below,1 gives no product at all at 180 ºC and very low conversion (9 %) at 220 ºC for 15 minutes when heating by microwaves.5 Remainder was unreacted starting material.
For high efficiencyin parallel synthesisand method developement short reaction timesare essential. We have examinedthe possibilityof maintaining reported selectivitiesin rapid, high temperaturepalladium-catalyzed asymmetric allylic alkylations. Reactions were promotedby useof singlemode microwave heating(vide infra),which oppositeto conventional oilbath heating(wallheat transfer) rapidlyand almost homogeneouslyheat largeparts of the sample(internalheat transfer).
Trends on chemical synthesis have changed over recent years, with a more targeted approach to molecular design becoming more prevalent. As a result, the speed with which a new compound can be synthesized is key to an efficient laboratory.
In order to obtain a fast synthesis of an N-unprotected tripeptide, microwave (mw) assisted deprotection, coupling and partial deprotection were applied starting with a purified protected dipeptide [Fig. 1]. The three consecutive mw assisted steps were performed without any intermediate purifications and were accomplished within some 30 minutes. The resulting crude mixture containing the Boc deprotected tripeptide ester was shown to be quite complex [Fig. 2]. The crude product was subjected to automated reversed phase flash chromatography, followed by automated fraction pooling and “thin film evaporation”. The total time for the entire procedure was 1 h 20 min. The purity of the product was > 99%.
Project I •Metal-mediated [2 + 3] cycloaddition to coordinated nitriles -- synthesis of new transition metal complexes -- synthesis of new heterocycles -- enantioselective synthesis of heterocycles role of microwaves? Project II • Platinum-catalysed cycloisomerisation of alkynes -- Synthesis of Taxol-like compounds -- highly efficient and atom economic chemistry role of microwaves?