Designing the "search pathway" in the development of a new class of highly efficient stereoselective hydrosilylation catalysts.
The direct coupling of oxazolines and N-heterocyclic carbenes leads to chelating C,N ancillary ligands for asymmetric catalysis that combine both an "anchor" unit and a stereodirecting element. Reacting various N-substituted imidazoles with 2-bromo-4(S)-tert-butyl- and 2-bromo-4(S)-isopropyloxazoline gave the imidazolium precursors of the stereodirecting ancillary ligands. A library of ten different ligand precursors was obtained by using this simple procedure (65-97 % yield). These protioligands were metalated in a subsequent step by reaction with [{Rh(mu-OtBu)(nbd)}2] (nbd=norbornadiene), generated in situ from KOtBu and [{RhCl(nbd)}2] giving the corresponding N-heterocyclic carbene complexes [RhBr(nbd)(oxazolinyl-carbene)] 4 a-j in good yields. X-ray diffraction studies of two of the rhodium complexes, 4 d and 4 j, established a distorted square-pyramidal coordination geometry with the bromo ligand occupying the apical position. The rhodium-carbene bond length was found to be 2.070(4) A (4 d) and 2.012(3) A (4 j). Complexes 4 a-j were treated with AgBF4 in dichloromethane, giving the active cationic square-planar catalysts for the hydrosilylation of ketones. As a reference reaction for the catalyst optimisation, the hydrosilylation of acetophenone with diphenylsilane was studied and the system optimised with respect to the counterion (BF(4) (-)), solvent (THF) and the silane reducing agent (diphenylsilane). The reaction product (1-phenylethanol) was obtained with the highest enantiomeric excess (ee) by carrying out the reaction at -60 degrees C, whilst the enantioselectivity drops upon going both to lower and higher temperatures. The observation that the temperature dependence of the ee values goes through a maximum indicated a change in the rate-determining step as the temperature is varied. The determination of the initial reaction rate in the hydrosilylation of acetophenone upon varying the catalyst (4 d) and substrate concentrations at -55 degrees C established a rate law for the initial conversion which is first-order in both substrates as well as the catalyst (Vi = k[4][PhCOMe][Ph2SiH2]). The catalytic system derived from complex 4 d was found to afford high yields and good enantioselectivities in the reduction of various aryl alkyl ketones (acetophenone: 92 % isolated yield and 90 % ee, 2-naphtyl methyl ketone: 99 % yield, 91 % ee). The selectivity for the reduction of prochiral dialkyl ketones is comparable or even superior to the best previously reported for prochiral nonaromatic ketones; whereas cyclopropyl methyl ketone is hydrosilylated with an enantioselectivity of 81 % ee, the increase of the steric demand of one of the alkyl groups leads to improved ee's, reaching 95 % ee in the case of tert-butyl methyl ketone. Linear chain n-alkyl methyl ketones, which are particularly challenging substrates, are reduced in good asymmetric induction, such as 2-octanone (79 % ee) and even 2-butanone (65 % ee).
César
Vincent
V
Laboratoire de Chimie Organométallique et de Catalyse (UMR 7513), Institut Le Bel, Université Louis Pasteur, 4 rue Blaise Pascal, 67070 Strasbourg, France.
Bellemin-Laponnaz
Stéphane
S
Wadepohl
Hubert
H
Gade
Lutz H
LH