Phosphine selected syntheses

One of the most selective hydroformylation catalysts was obtained when cobalt acetate was irradiated in the presence of an excess of a phosphine, with synthesis gas at 80 atm, in methanol as the solvent. Propylene was hydroformylated with this catalyst to give butyraldehyde with an n/i ratio of more than 99/1 /10/. In the absence of phosphine, the cobalt acetate forms a more active catalyst which is, however, less selective for straight chain products /23/. 

Amiens, C. et al., Selective synthesis, characterization, and spectroscopic studies on a novel class of reduced platinum and palladium particles stabilized by carbonyl and phosphine ligands, J. Am. Chem. Soc., 115, 11638, 1993. 

Mukaiyama, T., Kobashi, Y. Highly a-selective synthesis of disaccharide using glycosyl bromide by the promotion of phosphine oxide. 

Phosphine-mediated selective synthesis of tetra-substituted furans was developed directly from 1,4-enediones in anhydrous 1,2-dichloroethane (14T6733). 

Similar attempts to induce /Z-selectivity by altering the steric properties of ruthenium-based catalytic systems have also been reported [69]. In 2010, Chen etal. [70] demonstrated that Z-selectivity (up to 51%) in ROMP could be achieved using O-chelated Ru-phosphine catalysts. In this case, the /Z-selectivity was promoted by a steric interaction between the sulfonate anion and cyclic alkene substituents. However, the selectivity of these Ru-based catalysts was not found to be satisfactory for the selective synthesis of Z olefins. 

SCHEME 4.209 Selective synthesis of unsymmetrical phosphine sulfides [347]. 

Allylic phosphonium ylides can also be generated in situ by the nucleophilic attack of phosphines on aziridines (or epoxides) followed by a set of proton transfers. On the basis of these reaction pathways, Hou et al. have realized a slightly E selective synthesis of conjugated dienes from aldehydes (or ketones) and aziridines (or epoxides) in the presence of tributylphosphine (Scheme 27) [122]. 

Homogeneous rhodium-catalyzed hydroformylation (135,136) of propene to -butyraldehyde (qv) was commercialized in 1976. -Butyraldehyde is a key intermediate in the synthesis of 2-ethyIhexanol, an important plasticizer alcohol. Hydroformylation is carried out at <2 MPa (<290 psi) at 100°C. A large excess of triphenyl phosphine contributes to catalyst life and high selectivity for -butyraldehyde (>10 1) yielding few side products (137). Normally, product separation from the catalyst [Rh(P(C2H2)3)3(CO)H] [17185-29-4] is achieved by distillation. 

A Belgian patent (178) claims improved ethanol selectivity of over 62%, starting with methanol and synthesis gas and using a cobalt catalyst with a hahde promoter and a tertiary phosphine. At 195°C, and initial carbon monoxide pressure of 7.1 MPa (70 atm) and hydrogen pressure of 7.1 MPa, methanol conversions of 30% were indicated, but the selectivity for acetic acid and methyl acetate, usehil by-products from this reaction, was only 7%. Ruthenium and osmium catalysts (179,180) have also been employed for this reaction. The addition of a bicycHc trialkyl phosphine is claimed to increase methanol conversion from 24% to 89% (181). 

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). 

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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