First chiral phosphite with a quaternary ammonium fragment: Synthesis and use in Rh-catalyzed asymmetric hydrogenation

Abstract

The first representative of chiral P monodentate phosphite ligands containing quaternary ammonium substituents and its rhodium complex [Rh(COD)(L) 2 ]BF 4 (COD is 1,5 cyclo octadiene) were obtained. The use of this ligand in Rh catalyzed asymmetric hydrogenation of prochiral methyl esters of unsaturated acids allowed one to achieve optical yields up to 99%.Hydrogenation of prochiral compounds catalyzed by transition metal complexes belongs to the most signifi cant asymmetric transformations because it involves in expensive molecular hydrogen and produces no side reac tions. A further development of this branch necessitates perfection of the ligand pool. First of all, this refers to phosphite type compounds, which differ from conven tional phosphines in synthetic accessibility, oxidation re sistance, and high ligating power. 1 It is also important that chiral phosphites are inexpensive. For instance, the price for phosphite derivatives of BINOL (BINOL is 2,2´ dihydroxy 1,1´ binaphthyl) was only 2% of the price of the well recognized diphosphine ligand BINAP. 2 In recent years, perfect results have been reached by using P monodentate phosphites and phosphoramidites in enantioselective Rh catalyzed hydrogenation, 1-5 Cu catalyzed conjugated addition, 6 Ir and Pd catalyzed allylation, Pd catalyzed hydrosilylation-oxidation, and Ru catalyzed hydrogenation of ketones. 7-10The synthesis and catalytic properties of achiral ionic phosphites are well studied (see Ref. 11 and references therein), while data on the use of phosphites as chiral ionic ligands are virtually lacking. The sole example we know is phosphite derivatives of (1R,2R) trans diamino cyclohexane containing carbene imidazolium fragments, which have been employed in Pd catalyzed allylic substi tution and Ir catalyzed hydrogenation. 12 This is an inter esting fact since, e.g., optically active cationic phosphinites have been successfully used in asymmetric hydrogenation and allylic substitution. 13,14Here we describe the synthesis of chiral P mono dentate cationic phosphite from an accessible quaternized amino alcohol and successful use of this ligand in Rh catalyzed asymmetric hydrogenation. Results and DiscussionThe starting synthon 1 containing an alkylammonium fragment was prepared by quaternization of 2 (dimethyl amino)ethanol with heptyl bromide followed by replace ment of the counteranion in intermediate product 2 (Scheme 1). Its direct phosphorylation in CH 2 Cl 2 gave novel cationic P monodentate chiral phosphite 3.Ligand 3 is well soluble in CH 2 Cl 2 , CHCl 3 , acetone, and acetonitrile and is fairly resistant to hydrolysis. In particular, it is acceptable to separate compound 3 from its by product triethylamine hydrochloride by extraction with water from the reaction mixture in CH 2 Cl 2 .A reaction of P monodentate phosphite 3 (2 equiv.) with [Rh(COD) 2 ]BF 4 afforded the corresponding cationic rhodium complex 4 (see Scheme 1) identified from 31 P NMR, ESI MS, and elemental analysis data (see Ex perimental).Phosphite 3 and its complex 4 were tes...