Chiral phosphine catalysts

The review of Morrison et al. (10) traces the development of the use of rhodium-chiral phosphine catalysts to about the end of 1974. This field was initiated by the suggested incorporation (216) of chiral phosphines, instead of triphenylphosphine, into the so-called Wilkinson catalyst, RhCl(PPh3)3 (Section II,A), or into closely related systems. Horner s group (217, 218) used such catalysts, formed in situ in benzene... 

Besides the major thrust using chiral phosphine catalysts, other chiral ligands have been used with rhodium and other metals for asymmetric hydrogenation. 

Hydrosilylation of imine compounds was also an efficient method to prepare amines. The hydrosilylation product TV-silylamines can readily be desilylated upon methanol or water treatment, yielding the corresponding amines. The amines can be converted to their corresponding amides by subsequent acyl anhydride treatment. The first attempt to hydrogenate prochiral imines with Rh(I) chiral phosphine catalysts was made by Kagan102 and others. These catalysts exhibited low catalytic activity, and only moderate ee was obtained. 

To a much smaller extent non-enzymic processes have also been used to catalyse the stereoselective acylation of alcohols. For example, a simple tripeptide has been used, in conjunction with acetic anhydride, to convert rram-2-acctylaminocyclohexanol into the (K),(R)-Qster and recovered (S),(S)-alcohol[17]. In another, related, example a chiral amine, in the presence of molecular sieve and the appropriate acylating agent, has been used as a catalyst in the conversion of cyclohexane-1(S), 2(/ )-diol into 2(S)-benzoyloxy-cyclohexan-1 f / j-ol1 IS]. Such alternative methods have not been extensively explored, though reports by Fu, Miller, Vedejs and co-workers on enantioselective esterifications, for example of 1-phenylethanol and other substrates using /. vo-propyl anhydride and a chiral phosphine catalyst will undoubtedly attract more attention to this area1191. 

The same authors compared catalysts prepared from these precursors and [Ru(BINAP)Cl2]2 adsorbed on MCM-41 (with 26 and 37 A pores) and an amorphous mesoporous silica (with 68 A pores) all treated with combinations of SiPh2Cl2 and Si(CH2)3X (X = NH2, CO2H). Catalysts were also prepared in which the organometallic precursors were immobilized by entrapment into silica (using sol-gel techniques). This is one of the few studies in which the performance of chiral phosphine catalysts immobilized by covalent and noncovalent procedures are compared directly. The materials were examined as catalysts for the hydrogenation of sodium a-acetamidocinnamate and of a-acetamidocinnamic acid under similar conditions to those used for the catalysts on unmodified MCM-41. The catalysts... 

The SHB concept was expanded to chiral phosphine catalysts by de Rege et al., who reacted the trifluoromethanesulfonate (triflate) counter anion of the cationic complex [Rh(COD)((R,Rj-MeDuPhos)] with the surface hydroxyl groups of the silaceous mesoporous material MCM-41 [122]. The complex was loaded to a level of 1.03 wt% Rh. A decrease in support surface area and pore volume is consistent with the complex being located within the support pores. The counterion is very important in this process if the anion of the homogeneous catalyst precursor is altered to BArp no adsorption of the catalyst is observed. It is postulated that the mechanism of triflate binding is hydrogen bonding with the support, and that the... 

Other hydrogenation methods are less chemoselective. Use of Raney nickel provides hydroxylamines in low yield °. Hydrogenation of 1-acetonaphthone oxime over rhodium-chiral phosphine catalysts was found to proceed under harsh conditions and provided low... 

Makino, K., Hiroki, Y. and Hamada, Y. Dynamic Kinetic Resolution Catalyzed by Ir Axially Chiral Phosphine Catalyst Asymmetric S3mthesis of anti-Aromatic -Hydroxy-ot-amino Acid Esters. J. Am. Chem. Soc. 2005, 127, 5784—5785. 

An efficient asymmetric hydrogenation of a-acetamidocin-namic acids has been achieved using a rhodium-chiral phosphine catalyst. This paper describes the preparation of the catalyst and the hydrogenation procedure as well as studies on structure vs. activity. 

Scheme 3. Chiral phosphine catalyst for kinetic resolution of secondary alcohols...

For an account of enantioselective acyl transfer reactions using chiral phosphine catalysts see E. Vedejs,... 

II. Homogeneous Rhodium-Chiral Phosphine Catalyst Systems. 85... 

Another example of how catalysis plays a key role in enabling our lives is in the synthesis of pharmaceuticals. Knowles s development, at Monsanto in the early 1970s, of the enantioselective hydrogenation of the enamide precursor to L-DOPA (used to treat Parkinson s disease), using a Rh-chiral phosphine catalyst (Section 3.5), led to a share in the Nobel prize. His colaureates, Noyori and Sharpless, have done much to inspire new methods in chiral synthesis based on metal catalysis. Indeed, the dramatic rise in the demand for chiral pharmaceutical products also fuelled an intense interest in alternative methodologies, which led to a new one-pot, enzymatic route to L-DOPA, using a tyrosine phenol lyase, that has been commercialized by Ajinomoto. 

The enantioselective hydroaminations of allenes with chiral phosphine catalysts was accomplished with substrates that had a terminal symmetric substitution and with the amines protected as carbamates or sulfonamides. The same symmetric substituents were necessary for the enantioselective transformation nsing chiral counterions. However, very recently, high enantiomeric excesses were reached with trisubstituted asymmetric allenes by a dynamic kinetic enantioselective hydroamination of allenyl carbamates (eqnation 110), even thongh the E/Z ratio of the prodncts was not optimal. 

Azlactone(3) + H2 + Ru(lll)[chiral phosphine] (catalyst) - L-NH2CHRC02H after hydrolysis... 

Table 13.7 Screening of polyether dendrimer supported chiral phosphine catalysts.

Jessop and co-workers have pointed out that homogeneous catalysis in supercritical fluids can offer high rates, improved selectivity, and elimination of mass-transfer problems.169 They have used a ruthenium phosphine catalyst to reduce supercritical carbon dioxide to formic acid using hydrogen.170 The reaction might be used to recycle waste carbon dioxide from combustion. It also avoids the use of poisonous carbon monoxide to make formic acid and its derivatives. There is no need for the usual solvent for such a reaction, because the excess carbon dioxide is the solvent. If the reaction is run in the presence of dimethy-lamine, dimethylformamide is obtained with 100% selectivity at 92-94% conversion.171 In this example, the ruthenium phosphine catalyst was supported on silica. Asymmetric catalytic hydrogenation of dehydroaminoacid derivatives (8.16) can be performed in carbon dioxide using ruthenium chiral phosphine catalysts.172... 

Many communications have concentrated on specific amino phosphonic acids or derivative types. Thus, esters of phosphonoaminoacetic add were obtained by the reactions between trialkyl (ethyl) phosphite and (218) and which are thought to proceed via the phosphorane (219). A sequence has been presented for the preparation of the mono- and di-benzyl esters of N-chz protected (a-aminoben-zyl)phosphonic acid. A synthesis of (aminomethylene)bisphosphonic acid from dibenzylamine, dibenzyl hydrogenphosphonate and triethyl orthoformate has been noted and the asymmetric hydrogenation of (220) in the presence of chiral phosphine catalysts yields samples of (221) with e.e.s of 63-96%. The pyrrolidine-based compound (222) has been prepared from methyl S)-N-methoxycarbonyl-4-oxo-2-pyrrolidinecarboxylate and iV-coupled 4-amino-butanal diethyl acetals were the starting materials in syntheses of the pyrrolidine-2-phosphonic add derivatives (223) in which Z represents the iV-protected amino add or peptide moiety. ... 

Liu and co-workers [105] recently designed and synthesized a chiral phosphine catalyst S3 and applied it as catalyst to the aza-BH reactions between A -tosylimines and MVK (Scheme 9.28). It was found that the addition of achiral Brpnsted acid as co-catalyst could remarkably improve the reactivity and enantioselectivity. Subsequently, new catalyst was developed and applied in the asymmetric aza-BH reaction by the same group by replacing e phenol in 53 with //-tosyl aniline. 

Organophosphine-catalyzed allylic amination of MBH adducts has also been applied for the synthesis of a-alkylidene-(3-amino carbonyl compounds. The first catalytic asymmetric example of this reaction was reported by Krische and coworkers [122] in 2004 (Scheme 11.41), who showed the addition of phthalimide to MBH acetate in the presence of 20mol% of commercially available chiral phosphine catalyst (7 )-Cl-MeO-BlPHEP 54. The reaction was carried out at 50°C however, a long reaction time (62 h) was required. The substitution product was produced in 80% isolated chemical yield and with only 56% of... 

In 2004, Krische and colleagues demonstrated that exposure of Morita-Baylis-Hillman acetates to tertiary phosphine catalysts in the presence of 4,5-dichlorophthalimide enabled regiospecific allylic substitution through a tandem Sn2 -Sn2 mechanism. Through the use of the chiral phosphine catalyst, (i )-Cl-MeO-BIPHEP, the racemic Morita-Baylis-Hillman acetate depicted in Scheme 2.108 was converted into the corresponding enantiomerically enriched allylic amination product, thus establishing the feasibility of DKR. 

Ketene [2 + 2] cycloaddition with disubstituted ketones or aldehydes with phosphine catalysis gives P-lactone formation with a preference for the traws-diastereomer (Scheme 4.32). The formation ofphosphonium eno-late intermediates was monitored using NMR, and based on the NMR observation of the intermediates a mechanism was proposed in which the phosphonium enolate 158 reacted with a second molecule of the ketene forming intermediate 159, which then was acylated by the aldehyde followed by loss of 158 and formation of the product p-lactone. The use of the chiral phosphine catalyst BINAPHANE gave enantioselective product formation. 

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