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(μ‐O t Bu)(nbd)} 2 ] (nbd=norbornadiene), generated in situ from KO t Bu 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) Å ( 4 d ) and 2.012(3) Å ( 4 j ). Complexes 4 a – j were treated with AgBF 4 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 °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 °C established a rate law for the initial conversion which is first‐order in both substrates as well as the catalyst ( V i = k [ 4 ][PhCOMe][Ph 2 SiH 2 ]). 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 ).