Enantiopure dendritic polyoxometalates: chirality transfer from dendritic wedges to a POM cluster for asymmetric sulfide oxidation.

The quest for new catalytic and highly enantioselective processes is currently one of the most intensively studied areas in chemical synthesis. In this context, chiral polyoxometalates (POMs) have become a topic of recent interest owing to their potential application in medicine and asymmetric catalysis. Among numerous applications of POMs, catalysis is by far the most studied, owing to the enormous versatility that POMs offer in the clean synthesis of fine chemicals. However, few chiral POM compounds for asymmetric catalysis are known. Of those reported, none are based on dendritic structures, although dendritic counter cations are known to play a critical role in determining the properties of anionic POMs. Two main approaches have been developed for the synthesis of chiral POM-based frameworks. The first involves the use of a chiral species (organic moieties or metal complexes) as a chirality transfer agent. The second strategy is based on spontaneous resolution upon crystallization in the absence of any chiral auxiliary, which yields conglomerates. For example, enantiopure hafnium-substituted POMs, which are obtained by spontaneous resolution upon crystallization in the absence of any chiral source have been reported. Recently, two enantiomerically pure 3D chiral POM-based architectures were developed from achiral moieties without any chiral auxiliaACHTUNGTRENNUNGry. However, despite their potential applications in catalysis and separation, enantiopure POM frameworks are still under represented. The search for suitable enantiomerically pure POM-based hybrid materials remains a challenging issue in synthetic chemistry and materials science. To date, chiral dendritic POM systems, as well as the study of chiroptical activity of enantiopure POM-based frameworks in asymmetric catalysis have, to the best of our knowledge, never been reported. Herein, we report the synthesis and characterization of four enantiopure polyoxometalate-cored dendrimers, which contain n-propyl [(R)-(+)-6 and (S)-( )-6] and epoxy [(R)(+)-7 and (S)-( )-7] groups, based on electrostatic interactions between enantiopure dendritic quaternary ammonium ions and an achiral trianionic POM. To the best of our knowledge, these compounds represent the first examples of optically active dendritic POM systems. Importantly, the POM cluster displays chiroptical effects, indicating chirality transfer from the enantiopure dendritic ammonium ions. The optical and chiroptical properties of the n-propyl-terminated POM-cored dendrimers (R)-(+)-6 and (S)-( )-6 have been demonstrated in solution by UV/Vis spectroscopy, circular dichroism (CD) and vibrational circular dichroism (VCD) spectroscopy, as well as fluorimetry. In addition, the use of 6 as a catalyst in the asymmetric oxidation of thioanisole with aqueous H2O2 provides the corresponding optically active sulfoxide with up to 14% enantiomeric excess (ee) as a result of a chirality transfer process from the dendritic wedge to the catalytically active POM unit. Despite the modest enantioselectivity, this proof-of-principle catalytic experiment demonstrates and confirms the transfer of chiroptical properties from organic moieties to a catalytically active POM unit. Interestingly, the catalyst was recovered and reused in three successive cycles without discernable [a] C. Jahier, Dr. M. Cantuel, Dr. N. D. McClenaghan, Dr. T. Buffeteau, Dr. D. Cavagnat, Dr. S. Nlate ISM, UMR CNRS No. 5255, Universit Bordeaux I 351 Cours de la Lib ration, 33405 Talence Cedex (France) Fax: (+33)5-4000-6994 E-mail : [email protected] [b] Dr. F. Agbossou UCCS UMR CNRS N8 8181, Universit de Lille 59652 Villeneuve d’Asq Cedex (France) [c] Dr. M. Carraro, Prof. M. Bonchio Department of Chemical Sciences and ITM-CNR University of Padova, Via Marzolo, 1 35131 Padova (Italy) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200901512.