Yuji Kikukawa, Kosuke Suzuki, Midori Sugawa
Apr 10, 2012
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Angewandte Chemie
Abstract
Cyanohydrins are a very important class of compounds in chemistry as well as biology, and have been widely utilized as important synthetic intermediates for organic compounds, such as a-hydroxy acids, a-hydroxy aldehydes, and b-amino alcohols. For the synthesis of cyanohydrins, various cyanating reagents have been employed, and trimethylsilyl cyanide (TMSCN) is one of the most useful and safe cyanating reagents for nucleophilic addition to carbonyl compounds to give cyanohydrin trimethylsilyl ether. Hence, the development of efficient catalysts for cyanosilylation of carbonyl compounds with TMSCN is a very important subject in current research, and several efficient catalysts have been developed so far. Lewis acid catalysts can act as electrophilic catalysts to activate carbonyl compounds and have been extensively investigated for cyanosilylation (see Table S1 in the Supporting Information). 3] Several nucleophilic catalysts, such as amines, phosphines, phosphazanes, and alkalineearth metal oxides, can activate TMSCN and promote cyanosilylation (Table S1). 3] Asymmetric cyanosilylations have also been successfully developed by employing customly designed chiral ligands. Polyoxometalates (POMs) are a large family of anionic metal–oxygen clusters that consist of Group V and VI metals in their highest oxidation states, and are thermally and oxidatively stable in comparison with commonly utilized organometallic catalysts and organocatalysts. The chemical properties of POMs, for example, redox potentials, (multi)electron-transfer properties, acidities, and solubilities, and negative charges, can be finely tuned by choosing the constituent anion and countercations, and diverse structures can be synthesized. In addition to the above-mentioned properties, the important feature of POMs is the presence of bare nucleophilic surfaces as a result of external oxygen atoms (M O M and M=O species, M = W or Mo), which might act as nucleophilic sites as well as stabilizers of cationic intermediates. 6] In particular, it is expected that POMs with large negative charges can nucleophilically activate TMSCN, thus resulting in promotion of cyanosilylations, as observed for Lewis base catalyzed cyanosilylations. 3h] Initially, the cyanosilylation of 2-adamantanone (1a) with TMSCN was carried out with the 8-charged POM TBA4H4[g-SiW10O36] (SiW10, TBA = tetra-n-butylammonium) in 1,2-dichloroethane at 30 8C. SiW10 promoted the cyanosilylation, giving the corresponding cyanohydrin trimethylsilyl ether 2a in 15% yield after 20 minutes (Scheme 1). In the case of TBA4H4[a-SiW11O39] (SiW11), 2a was also obtained in 16% yield (Table S2). In the absence of the catalysts, cyanosilylation was not observed under the present conditions.