A first P,N-bidentate phosphite with a chiral ketimine fragment. Catalytic properties of its RhI and PdII complexes in comparison with those of phosphine analogs

Abstract

A chiral P,N bidentate aryl phosphite ligand containing peripheral (R) (+) camphor derived ketimine and its rhodium(I) and palladium(II) chelate complexes were synthesized for the first time. These compounds were found to be suitable for asymmetric allylic substitution. The Pd catalyzed sulfonylation of 1,3 diphenylallyl acetate with sodium p toluenesulfinate gave the product in 73% ee; in the alkylation of the same substrate with dimethyl malonate, the ee was 94%. These ee values are higher than the enantioselectivity achieved with the known phosphine analogs.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 Since recently, imino phosphites have been successfully used in enan tioselective catalysis because of their high π acidity, resis tance to oxidation, synthetic accessibility, and low cost (see Refs 3 and 7 and references cited therein and Ref. 8). However, all these ligands contain a peripheral aldimine group; no phosphite with a ketimine fragment has been documented to date. In the present work, a first chiral aryl phosphite containing a ketimine substituent based on (R) (+) camphor was synthesized and successfully used in Pd catalyzed asymmetric allylation. Results and DiscussionA new P,N bidentate chiral aryl phosphite was easily obtained by phosphorylation of camphor N (o hydroxy phenyl)imine (1) with accessible reagent 2 (Scheme 1).It should be noted that imino phosphite 3 is stable when stored in a dry atmosphere. For instance, the 31 P NMR spectrum of compound 3 recorded several months after it has been synthesized shows no signals for decomposition products. In addition, the starting reagents, including (R) (+) camphor, are inexpensive. As regards the complexation character, ligand 3 is a typical chelating agent. In particular, its reactions with [Pd(All)Cl] 2 in the presence of AgBF 4 and with [Rh(CO) 2 Cl] 2 gave cationic and neutral metal chelates 4 and 5 with cis orientation of the P and N atoms (Scheme 2). Scheme 1

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

“…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.…”

“…It is noteworthy that all starting reagents are inexpensive; iminoalcohols 2a, 2b and iminophenol 2c were obtained by one-pot synthesis from 2-aminoethanol, (S)-isoleucinol or 2-aminophenol and (R)-(+)-camphor. In contrast to the use of hydrochloric acid 12 in the synthesis of 1c described previously, 9 we applied anhydrous zinc(II) chloride 13 as a catalyst in the synthesis of ketimines 2a-c. Despite moderate yields, this procedure has the advantage of avoiding a three-step synthesis through the N-nitroimine of (R)camphor.…”