1372694-85-3Relevant academic research and scientific papers
Cu-catalyzed enantioselective 1,4-additions of aryl-Grignard reagents to cyclohexenone in the presence of TADDOL-derived phosphane-phosphite ligands
Naeemi, Qaseem,Dindaroglu, Mehmet,Kranz, Darius P.,Velder, Janna,Schmalz, Hans-Guenther
experimental part, p. 1179 - 1185 (2012/04/10)
Asymmetric conjugate additions (1,4-additions) of aryl-Grignard reagents to cyclohex-2-enone, currently a more or less unsolved challenge, were investigated. For this purpose, a small library of phenol-derived chiral phosphane-phosphite ligands containing TADDOL- or BINOL-based phosphite moieties was evaluated. These ligands are easily prepared by a short modular scheme previously developed in this laboratory. Two particularly powerful ligands (4a and 4b, both TADDOL-derived and each possessing a bulky tert-butyl substituent ortho to the phosphite group) were identified. Conditions were optimized with use of the addition of (4-methoxyphenyl)magnesium bromide to cyclohexenone as a standard reaction system. Under optimized conditions [CuBr·SMe 2 (4 mol-%), ligand 4a (6 mol-%), 2-methyl-THF, -78 °C, slow addition of Grignard reagent] the 1,4-product was obtained with high enantioselectivity (up to 95 % ee) and good regioselectivity (r.r. = 90:10). The scope of the method was probed with different aryl-Grignard reagents. It was found that reagents with electron-donating substituents in meta- or para-positions performed particularly well, whereas the presence of F or CF 3 substituents led to decreased ee values. Only ortho-substituted aryl-Grignard reagents did not give rise to useful results. A series of phosphane-phosphite ligands were also tested in the Rh-catalyzed 1,4-addition of phenylboronic acid to cyclohexenone, but enantioselectivities did not exceed 70 % ee in this case. The difficult task of employing aryl-Grignard reagents in Cu-catalyzed enantioselective 1,4 addition reactions was achieved with the assistance of readily accessible chiral modular P,P ligands. High enantioselectivities were obtained in a number of synthetically relevant cases. Copyright
