Phosphines enantioselectivity

Moncarz JR, Brunker TJ, Jewett JC, Orchowski M, Glueck DS, Sommer RD, Lam K-C, Incarvito CD, Concolino TE, Ceccarelli C, Zakharov LN, Rheingold AL (2003) Palladium-catalyzed asymmetric phosphination. enantioselective synthesis of PAMP-BH3, ligand effects on catalysis, and direct observation of the stereochemistry of transmetalation and reductive elimination. Organometallics 22 3205-3221... 

Scriban C, Glueck DS (2006) Platinum-catalyzed asymmetric alkylation of secondary phosphines enantioselective synthesis of P-stereogenic phosphines. J Am Chem Soc 128 2788-... 

Moncarz JR, Laritcheva NF, Glueck DS (2002) Palladium-catalyzed asymmetric phosphination enantioselective synthesis of a P-chirogenic phosphine. J Am Chem Soc 124 13356-13357... 

High enantioselectivities and regioselectivities have been obtained using both mono- and 1,2-disubstituted prochinal olefins employing chiral phosphine phosphite (33,34) modified rhodium catalysts. For example, i7j -2-butene ia the presence of rhodium and (12) (33) gave (3)-2-meth5ibutanal ia an optical yield of 82% at a turnover number of 9.84. ... 

Much effort has been placed in the synthesis of compounds possessing a chiral center at the phosphoms atom, particularly three- and four-coordinate compounds such as tertiary phosphines, phosphine oxides, phosphonates, phosphinates, and phosphate esters (11). Some enantiomers are known to exhibit a variety of biological activities and are therefore of interest Oas agricultural chemicals, pharmaceuticals (qv), etc. Homochiral bisphosphines are commonly used in catalytic asymmetric syntheses providing good enantioselectivities (see also Nucleic acids). Excellent reviews of low coordinate (coordination numbers 1 and 2) phosphoms compounds are available (12). 

Scheme 2.12. Enantioselective Reduction of 2-Acetamidoacrylic Acids by Chiral Phosphine Complexes of Rhodium...

On the basis of the conclusions accumulated over the years, novel highly efficient P-chirogenic phosphines were designed, synthesized, and tested in a variety of enantioselective reactions. These issues will be stressed in the following paragraphs. 

Bischirogenic unsymmetric (S,S)-BisP 6b (cf. (S,S)-BisP -borane 81b) were synthesized from the coupling reaction between synthon 79 and the lithiated (S)-alkylmethylphosphine-boranes 87 in reasonable to quantitative yields with enantioselectivity over 97% (Scheme 15) [94]. These phosphines constitute the unsymmetric version of BisP in that they bear different groups on both phosphorus atoms, breaking the C2-symmetry character of BisP [32,94]. 

As mentioned in Sect. 2.2, phosphine oxides are air-stable compounds, making their use in the field of asymmetric catalysis convenient. Moreover, they present electronic properties very different from the corresponding free phosphines and thus may be employed in different types of enantioselective reactions, m-Chloroperbenzoic acid (m-CPBA) has been showed to be a powerful reagent for the stereospecific oxidation of enantiomerically pure P-chirogenic phos-phine-boranes [98], affording R,R)-97 from Ad-BisP 6 (Scheme 18) [99]. The synthesis of R,R)-98 and (S,S)-99, which possess a f-Bu substituent, differs from the precedent in that deboranation precedes oxidation with hydrogen peroxide to yield the corresponding enantiomerically pure diphosphine oxides (Scheme 18) [99]. 

The enantioselective 1,4-addition addition of organometaUic reagents to a,p-unsaturated carbonyl compounds, the so-called Michael reaction, provides a powerful method for the synthesis of optically active compounds by carbon-carbon bond formation [129]. Therefore, symmetrical and unsymmetrical MiniPHOS phosphines were used for in situ preparation of copper-catalysts, and employed in an optimization study on Cu(I)-catalyzed Michael reactions of di-ethylzinc to a, -unsaturated ketones (Scheme 31) [29,30]. In most cases, complete conversion and good enantioselectivity were obtained and no 1,2-addition product was detected, showing complete regioselectivity. Of interest, the enantioselectivity observed using Cu(I) directly in place of Cu(II) allowed enhanced enantioselectivity, implying that the chiral environment of the Cu(I) complex produced by in situ reduction of Cu(II) may be less selective than the one with preformed Cu(I). 

Finally, the participation of the Ar,P-bidentated phosphinous amides 61-65 (Scheme 45) as chiral ligands in the catalytic enantioselective hydrosilylation of acetophenone with diphenylsilane is worth mentioning, despite low ee (2-20%) [52]. 

Both types of manipulation were applied to achieve the required enantiomer and the highest enantioselectivity in the hydrolysis of the phosphinic acid diester... 

The Rh-NHC complexes, with or without phosphine co-ligands, have been stndied as hydrogenation catalysts of alkenes with molecular hydrogen, with the aim to develop more active, selective (and/or enantioselective) and thermally stable catalysts. 

Table 3.12 surveys current industrial applications of enantioselective homogeneous catalysis in fine chemicals production. Most chiral catalyst in Table 3.12 have chiral phosphine ligands (see Fig. 3.54). The DIP AMP ligand, which is used in the production of L-Dopa, one of the first chiral syntheses, possesses phosphorus chirality, (see also Section 4.5.8.1) A number of commercial processes use the BINAP ligand, which has axial chirality. The PNNP ligand, on the other hand, has its chirality centred on the a-phenethyl groups two atoms removed from the phosphorus atoms, which bind to the rhodium ion. Nevertheless, good enantio.selectivity is obtained with this catalyst in the synthesis of L-phenylalanine. 

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