2- phenyl-l-butene

COOH or NHCOCH3, for example, 2-phenyl-l-butene. Enantioselective reduction of certain alkenes has also been achieved by reducing with baker s yeast. Hydrogenation with Ni2B on borohydride exchange resin (BER) has also been... 

An example of an alcohol that can undergo rapid skeletal rearrangement is 3,3-dimethyl-2-phenyl-2-butanol (Eq. 29). Attempts to reduce this alcohol in dichloromethane solution with l-naphthyl(phenyl)methylsilane yield only a mixture of the rearranged elimination products 3,3-dimethyl-2-phenyl-l-butene and 2,3-dimethy 1-3-phenyl-1 -butene when trifluoroacetic acid or methanesulfonic acid is used. Use of a 1 1 triflic acid/triflic anhydride mixture with a 50 mol% excess of the silane gives good yields of the unrearranged reduction product 3,3-dimethyl-2-phenylbutane, but also causes extensive decomposition of the silane.126 In contrast, introduction of boron trifluoride gas into a dichloromethane solution of the alcohol and a 10 mol% excess of the silane... 

Kagan et al. were the first to report the corresponding enantioselective catalytic hydrogenation using chiral metallocene derivatives [94, 95]. By using menthyl- and neomenthyl-substituted cyclopentadienyl titanium derivatives in the presence of activators (Scheme 6.5) [96], these authors observed low ee-values (7-14.9%) for the catalytic hydrogenation of 2-phenyl-l-butene into 2-phenylbutane. In contrast, no enantiomeric excess was obtained with the corresponding zirconocene derivatives. 

The influence of pco and pHz on the selectivity and optical yield of the reaction were investigated in the case of 2-phenyl-l-propene and 2-phenyl-l-butene 46-47). 

In addition to achiral precatalysts the chiral lanthanide metallocenes (R)-[Me2SiCp" ( — )-menthylCp ]SmCH(SiMe3)2 and (S)-[Me2SiCp" ( - )-men-thyl Cp ]SmCH(SiMe3)2 have been employed [71]. The hydrocarbyl derivatives have been shown to mediate the enantioselective hydrosilylation of 2-phenyl-l-butene by PhSiH3 with exclusive 1,2-addition and with N, 50h 1. In this case enantioselection proceeds with 68% ee ((R) product) and 65% ee ((S) product) for the (R)-Sm and (S)-Sm catalysts (70% enantiopure), respectively. 

Some examples for complexes of this type are shown in Scheme 475. They show catalytic ability to effect the asymmetric hydrogenation of 2-phenyl-l-butene and 2-(a-naphthyl)-l-butene with variable enantioselectivity depending on the characteristics of the particular ligand system.1106... 

Keywords Asymmetric hydrogenation. Simple olefins. Unfunctionalized olefins. Chiral metallocenes, Chiral phosphines, Rhodimn, Ruthenimn, Chiral titanocenes. Chiral zirconocene. Chiral cyclopentadienyUanthanides, Phosphanodihydrooxazole, Iridium, 2-Phenyl-l-butene... 

Marks has developed a class of remarkably effective catalysts for the enanti-oselective hydrogenation of 2-phenyl-1-butene [29,35,36,37]. Incorporation of a menthyl chiral auxiliary on the 3-position of an a sa-bis(cyclopentadienyl) ligand leads to two diastereomeric complexes of samarium 30 [29]. Similarly, the neomenthyl-substituted complexes 31 can be formed and isolated as pure diastereomers. These complexes are very active catalysts for the hydrogenation of 2-phenyl-l-butene under mild conditions (25 °C, 100-1000 1 substratercata-lyst, 760 mm H2,5 min) [29]. As shown in Table 5, as the lanthanide metal in neomenthyl-substituted complexes 31 becomes smaller, the enantioselectivity decreases from 58% ee (La) to 10% ee (Lu) (entries 1-5). Using a 70 30 mixture of the diastereomers 30a and 30b of the menthyl-substituted samarium complex the good enantioselectivity at 25 °C (64% ee) becomes superb at -80 °C (96% ee) (entries 6-10). This same mixture of 30a/30b catalyzed the deuteration of styrene at 25 °C in 72% ee. 

Asymmetric hydrocarboxylation of a-ethylstyrene (2-phenyl-l-butene) giving asymmetric induction in both regioisomers shows that regioselectivity is different for the two enantio-faces12. 

High enantioselectivities in the hydrogenation of 2-phenyl-l-butene have been achieved using chiral samarium complexes such as 55 (96% ee at — 80°C, 64-80% cc at 25°C)129. The reaction was carried out in heptane at 1 bar of H2 using a substrate/catalyst ratio of 200 1, and quantitative conversion and high turnover frequencies were observed under these conditions. The same catalyst gave 72% ee in the deuteration of styrene with D2 at 25 "C. Substantial enantiomeric excesses in the hydrogenation of 1,1-disubstituted olefins have also been obtained with the chiral bis(cyclopentadienyl)titanium complex 5690. 

Chiral zirconocene complexes have also been studied as catalysts for the hydrogenation of nonfunctionalized olefins115. Using homogeneous Ziegler Natta-type catalyst systems derived from [ethylenebis(4,5.6,7-tetrahydro-l-indenyl)]zirconium complexes and methyl aluminoxane [A1(CH3)0] , 2-phenyl-l-butene was hydrogenated in 36% optical yield (20 bar H2, benzene, 25 °C). Under the same conditions, the reaction of styrene with D2 gave optically active 1,2-dideuteroethylbenzene with 65% ee. 

Other catalysts such as rhodium phosphane complexes are much less effective in the hydrogenation of unfunctionalized olefins. The best results have been obtained with 2-phenyl-l-butene using a polymeric phosphinite ligand derived from cellulose113, (S,S)-1,2-bis-(diphenylphosphinoxy)cyclopentane114, or BDPP24 (50-60% ee). 

The reported enantioselectivity for 2-phenyl-l-butene seems too be to high according to a recently published value for the specific rotation of 2-phenylbutanc (ref 129). 

Chloro-2-phenyl-l-butene is dissolved in a three-fold volume of pyridine and heated on a boiling water-bath for 1 h. After cooling, the mixture is stirred into ice-cold ca. 30% sulfuric acid and treated at once with a little hydroquinone. The hydrocarbon is taken up in ether, washed, dried, and distilled in a vacuum it has b.p. 66-67°/13 mm the yield is about 50%. 

Draw the product formed when 2 phenyl-l-butene is treated with a catalytic amount of tosic acid in aqueous ether. 

Chiral lanthanide catalysts, (R)- and (S)-Me2Si(Me4Cp)[(—)-menthylCp]SmCH (SiMe3)2, are effective for the asymmetric hydrosUylation of 2-phenyl-l-butene with HsSiPh, giving (R)- and (S)-2-phenyl-2-silylbutanes with 68 and 65% ee, respectively (Scheme 23) ... 

---