P,N-Bidentate Phosphites with a Chiral Ketimine Fragment, Their Appli­cation in Enantioselective Allylic Substitution and Comparison with Phosphine­ Analogues

Abstract

Novel chiral P,N-bidentate arylphosphites have been prepared by a one-step phosphorylation of appropriate hydroxy ketimines based on (R)-(+)-camphor. Metal chelate complexes [Rh(CO)(PN)Cl] and [Pd(PN)(allyl)]BF 4 with the ligands were obtained and the new compounds were fully characterized by 1 H, 13 C and 31 P NMR, IR, MS (EI, FAB and ESI techniques) and X-ray crystal structure analysis. Using these ligands, up to 73% ee has been achieved in the asymmetric palladium-catalyzed sulfonylation of 1,3-diphenyl-2-propenyl acetate with sodium p-toluenesulfinate.In the enantioselective alkylation of the same substrate with dimethyl malonate, up to 94% enantioselectivity has been achieved.Imino phosphines constitute an important group of chiral P,N-bidentate ligands for asymmetric catalysis by metal complexes. 1-3 The vast majority of such compounds contain an aldimine fragment, whereas systems with ketimine units are fairly uncommon. 4-6 Imino phosphites have been successfully used in enantioselective catalysis because of their high p-acidity, resistance to undesired oxidation, synthetic accessibility, and low cost. 3,7,8 However, all these ligands contain a peripheral aldimine group; phosphites with a ketimine fragment were not known until recently. In 2004 we reported the synthesis of the first chiral aryl phosphite 2c, containing a ketimine substituent, based on (R)-(+)-camphor and the results of its preliminary testing in palladium-catalyzed asymmetric allylation. 9 Developing this topic further, in the present work we discuss the synthesis of a new series of such ligands and their application in palladuim-catalyzed asymmetric allylation. Moreover, we will make a direct comparison of the new chiral iminophosphites with their phosphine analogues.Novel P,N-bidentate chiral aryl phosphites were easily obtained by phosphorylation of appropriate iminoalcohols 2a and 2b or the iminophenol 2c with an easily accessible bis(2,6-dimethylphenyl)chlorophosphite (1; Scheme 1). Usually, this chlorophosphite is synthesized from 2,6-dimethylphenol and PCl 3 in the presence of triethylamine in benzene. 10,11 We have improved the procedure and developed a novel, versatile, solvent-free method for the synthesis of phosphorochloridite (1) through the use of a catalytic amount of 1-methylpyrrolidin-2-one. The reaction time is three-fold shorter and no solvent is required. Hence, this simple, economical and time-saving procedure represents a handy method for the synthesis of convenient and cheap phosphorylating reagents, based on substituted phenols.

β-Pinene and camphor based, pyrazole-tethered triarylphosphines as chiral P,N ligands for palladium

“…It was considered worthwhile, therefore, to attach a chiral backbone to the pyrazole moiety so that enantiopure pyrazolyl ligands can be developed. There are a number of precedents where β-pinene [17][18][19][20][21] or camphor [22][23][24][25][26][27][28][29] moiety has been used as a chiral backbone to obtain optically pure ligands.…”