Olefins asymmetric hydrovinylation

Stability, activity and chemo- and enantioselectivity increased with increasing steric demand of the ortho substituent R. Introduction of the trimethylsilyl group at this position (ligand 38) therefore resulted in an excellent enantioselective system which belongs among the best Pd catalysts described so far for asymmetric hydrovinylation. Almost 70% conversion was observed within 15 min. The product was obtained in 78.5% ee and only a small amount of the isomerization products was detected in the reaction mixture. However, at higher conversions, isomerization of the product to the internal achiral olefin took place. Therefore,... 

Scheme 7 displays a possibility of the synthesis of chiral 2-arylpropionic acids via the oxidative tranformation of (7 )-3-aryl-l-butenes. The requisite chiral olefins may be obtained by transition metal-catalyzed asymmetric coupling between a benzylic Grignard reagent and vinyl bromide (93 % optical yield) [28] or, more attractively, asymmetric hydrovinylation of an aromatic olefin with ethylene. The asymmetric combination of styrene and ethylene, giving the adduct 25 in 95 % ee, has been performed on a 10-kg scale with a dinuclear Ni catalyst formed from ( -allyl)NiCl2 and a unique chiral dimeric aminophosphine obtainable from (/ )-myrtenal and (5)-l-phenylethylamine [7a],... 

A detailed mechanism of asymmetric hydrovinylation is discussed in order to explain the pathways of the asymmetric induction4- 51314. The 7t-allylnickel complex, as the catalyst precursor, is activated by phosphanes and ethylaluminum chloride and reacts with an olefin to give a catalytically active nickel hydride-olefin complex. The olefin then inserts into the metal hydride bond and after coordination and insertion of ethene a new alkylnickel compound is... 

Asymmetric hydrovinylations using chiral nickel catalysts constitute a powerful method for achieving enantioselective functionalization of olefins [25, 26], Pioneering results in this area were reported by Wilke [25, 84] who in 1987 disclosed the impressively enantioselective catalytic system depicted in Equation 21 [85], The nickel complex of the chiral dimeric azaphospho-lene ligand 153 thus effected the efficient hydrovinylation of styrene (152) with ethene to give the corresponding product 154 in 97 % yield and 95 % ee. 

A continuous-flow method for asymmetric catalysis in an SCF/IL system was reported by Leitner s group (144), with the hydrovinylation of styrene [Eq. (31)] as the test reaction. The SCCO2 solution of styrene and ethylene was continuously bubbled up through a column of ionic liquid containing the catalyst. The enantio selectivity was found to be high (in one of the ILs) and catalyst stability was enhanced due to the fact that there was a constant concentration of substrate in the system the catalyst was unstable in ILs in the absence of the olefins. 

Asymmetric modifications of hydrovinylation are one of the earliest examples of successful asymmetric transition metal catalysis. After optimization of various dimerization and codimerization reactions using phosphane modified nickel catalysts, the first examples of asymmetric olefin codimerization were reported with n-allylnickel halides activated by organoaluminum chloride and modified by chiral phosphanes7. Thus, codimerization of 2-butene with propene using n-allylnickel chloride/A]X, (X = Cl, Br) in the presence of tris(myrtanyl)phosphane gives low yields of (—)-( )-4-methy 1-2-hexene (I) with 3% ee7,7 . 

Similarly, asymmetric metal-catalyzed hydrovinylation of various other olefins has been investigated, usually catalyzed by nickel systems prepared from Ni(IJ) salts or / -allylnickel halides, activated by Lewis acids such as ethylaluminum dichloride or diethylaluminum chloride, and modified by chiral phosphanes. Surveys of these results are found in more general reviews11-12 and reviews dealing preferentially with this subject -5-7-13 - A... 

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