Neomenthyldiphenylphosphine

The nontrivial synthetic procedures to give the tertiary phosphines chiral at phosphorus (10) led Morrison s group (221, 222) to synthesize neomenthyldiphenylphosphine [(+)-NMDPP] (12) and led Kagan s group (10. 223, 224) to synthesize 2,3-o-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane [(-)-DIOP] (13) from the commercially... 

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

With [Rh2Cl2(CO)4] as catalyst precursor, together with (R)-benzyl(methyl)phenylphosphine or neomenthyldiphenylphosphine, styrene was hydroformylated to 2- and 3-phenylpropanal (equation 65) 2-phenylpropanal was the major product. The optical yields were not given, but... 

With [Pd(DBA)2] (105) in the presence of (+)-neomenthyldiphenylphosphine, the hydroesterification of styrene with CO and methanol gave a best optical yield of 52% for the methyl 2-phenylpropionate formed.578 The reactions were carried out in presence of trifluoroacetic acid. Other acids were of far lesser value. It was believed that the complex [PdH(02CCF3)PPh2 (neomenthyl)] was formed. 

FIG. 8. Asymmetric homogeneous hydrogenation with a neomenthyldiphenylphosphine (NMDPP) catalyst. 

Application of various chiral palladium complexes to the stereoselective hydrosilylation of dienes results in the formation of 3-silylcvcloalkenes irrespective of the isomer of the diene used (in the case of cyclohexadienes). When complexes containing menthyldiphenylphosphine (MDPP) and neomenthyldiphenylphosphine (NMDPP) are used, the chiral products with an excess of the (S)-enantiomer are always formed1011. 

Hydrosilylation of monosubstituted alkenes with palladium catalysts and trichlorosilane follows a course which favors branched products. By using a chiral phosphine ligand, asymmetric reaction is feasible. Initially, menthyldiphenylphosphine (MDPP, 93) and neomenthyldiphenylphosphine (NMDPP, 94) were employed with little success. Later, (/ )-/VA -dimethyl-l-[(S)-2-diphenylphosphinoferroce-nyl]ethylamine [(R)-(S)-PPFA] (95) and its enantiomer were prepared, and these have proved to be the... 

Early studies on the palladium-catalyzed asymmetric hydrosilylation of cyclic conjugated dienes employing menthyl- and neomenthyldiphenylphosphine and ferrocenylamino-phosphine ligands gave low enantiomeric excesses of the corresponding allylsilane [13]. 

Palladium-catalyzed hydroesterification of styrene gives predominantly the branched ester, in which a new chiral center has b n created at the 2-position (equation 34). Carrying out this reaction at low pressure (1-2 bar) in the presence of the bulky chiral phosphine neomenthyldiphenylphosphine and tri-fluoroacetic acid leads to significant asymmetric induction (50% ee) this work seems to represent the only significant advance towards stereocontrol of catalytic hydroesterification reactions. 

S = solvent. Ligands DIOP = 2,3-0-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane47 NMDPP = neomenthyldiphenylphosphine MDPP = menthyldiphenylphosphine48. b Enantiomeric excess, determined after conversion of the alkoxysilane to a trisubstituted silane. 

The asymmetric hydrosilylation of styrene is effected by palladium(II) complexes with menthyldiphenylphosphine (MDPP) and neomenthyldiphenylphosphine (NMDPP) to give optically active a-phenylethyltrichlorosilane68 (equation 23). 

Preparation. This phosphine is prepared in 35% yield by the reaction of sodium diphenylphosphine with (-)-menthyl chloride in THF. This material contains about 5% of neomenthyldiphenylphosphine oxide as a tenacious impurity. 

Asymmetric hydrogenation. Morrison et al. have reported on asymmetric hydrogenations catalyzed by rhodium(I) complexes of the Wilkinson type containing chiral ligands. This type of asymmetric synthesis had been carried out previously with relatively inaccessible phosphine ligands that are asymmetric at phosphorus. Phosphines that are asymmetric at carbon are more readily available and appear to be more efficient. Thus reduction of (E)- 3-methylcinnamic acid with prereduced tris(neomenthyldiphenylphosphine)chlororhodium in the presence of triethylamine leads to 3-phenylbutanoic acid, +34.5°, which contains 61% enantiomeric excess of the S-isomer. Hydrogenations of olefins exhibit a lower degree of asymmetric bias. 

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