Phenyl transfer reactions enantioselective

Dosa and Fu reported the first catalytic enantioselective phenyl transfer reaction to ketones (equation 25)100. In the presence of 1.5 equivalents of MeOH, the chiral tertiary alcohol was produced in good yield and with high enantioselectivity. Walsh and workers recently reported the Ti(OE -i Vchiral dihydroxybis(sulfonamide) catalyst 34 system, whereby enones have been converted to enantioenriched allyl alcohols101. 

Scheme 7.46 An aminoalcohol ligand for the Zn-catalyzed enantioselective phenyl transfer reaction to aromatic aldehydes, as described by Pericas and coworkers [75].

Chiral Lewis acid promoted atom transfer reaction (Kharasch reaction) of a-halo oxazolidinone imide 90 and 1-octene 92 has been reported by Porter et al. (Scheme 23) [78]. The enantioselective atom transfer utilizing Zn(OTf)2 and phenyl bisoxazoline ligand 93 as a chiral Lewis acid. The yields of the products, however, were quite low ranging from 5-15% and only moderate enantioselectivities were achieved (up to 40%). 

A crystalline inclusion complex of 10-(4-f-butylphenyl)-3-(2-ethyl-phenyl)-pyrimido[4,5-fc]quinoline-2,4(3H,10H)-dione/urea/EtOH obtained. X-ray analysis showed that the urea is doubly H-bonded to the pyrimidinone (96TL8905). The proximity of the chiral axis might give interesting applications in chiral recognition. The enantiomers were involved in an enantioselective hydride transfer reaction (Figure 25). 

Although similar efforts have been devoted to related polymer systems (Overberger and Cho, 1968 Overberger and Dixon, 1977 Okamoto, 1978), large enantioselectivity has not been observed. Goldberg et al. (1978) conducted borohydride reduction of phenyl ketones in micelles of the chiral surfactant [44]. The result was disappointing, since the maximal enantioselectivity was only 1.66% for phenyl propyl ketone. A much better optical yield was reported when this reaction was carried out under phase-transfer conditions (Masse and Parayre, 1976). The cholic acid micelle and bovine serum albumin exhibited the relatively high enantioselectivity in the reduction of trifluoroacetophenone (Baba ef al., 1978). 

Table 17) with two substituents in position C3 the oxygen transfer by the chiral hydroperoxides occurred from the same enantioface of the double bond, while epoxidation of the (ii)-phenyl-substituted substrates 142c,g,i resulted in the formation of the opposite epoxide enantiomer in excess. In 2000 Hamann and coworkers reported a new saturated protected carbohydrate hydroperoxide 69b , which showed high asymmetric induction in the vanadium-catalyzed epoxidation reaction of 3-methyl-2-buten-l-ol. The ee of 90% obtained was a milestone in the field of stereoselective oxygen transfer with optically active hydroperoxides. Unfortunately, the tertiary allylic alcohol 2-methyl-3-buten-2-ol was epoxidized with low enantioselectivity (ee 18%) with the same catalytic system . 

There are only a few reports on chiral phase transfer mediated alkylations". This approach, which seems to offer excellent opportunities for simple asymmetric procedures, has been demonstrated in the catalytic, enantioselective alkylation of racemic 6,7-dichloro-5-methoxy-2-phenyl-l-indanone (1) to form ( + )-indacrinone (4)100. /V-[4-(tnfluoromethyl)phenylmethyl]cinchoninium bromide (2) is one of the most effective catalysts for this reaction. The choice of reaction variables is very important and reaction conditions have been selected which afford very high asymmetric induction (92% cc). A transition state model 3 based on ion pairing between the indanone anion and the benzylcinchoninium cation has been proposed 10°. 

It is important to mention that the catalysis with this modified arylzinc reagent not only leads to improved enantioselectivity (at high product yield), but also that in this process swfestoichiometric quantities of diphenylzinc could be applied. This also meant that now both of the phenyl groups could be activated and transferred to the aldehydes. A reaction profile obtained by FT-IR studies revealed that the modification of the zinc reagent had a significant effect on its reactivity [41]. 

---