Olefins 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,... 

This reaction is very facile, and the use of 1 mole of acid provides a convenient method for the synthesis of vinyl complexes. The proposed mechanism (287) is shown in Fig. 6 which suggests an oxidative addition of HX followed by insertion into the Pt—H bond to form the intermediate vinyl compound. If excess acid is present, a second oxidative addition of HX occurs to give a hydrovinyl platinum(IV) intermediate. If a second insertion reaction occurs, the olefin is formed along with PtX2(PPh3)2. 

Spectacular enantioselection has been observed in hydrogenation (cf. Section 2.2) [3] and hydrometallation of unsaturated compounds (cf. Section 2.6) [4], olefin epoxidation (cf Section 2.4.3) [5] and dihydroxylation (cf Section 3.3.2) [6], hydrovinylation (cf Section 3.3.3) [7], hydroformylation (cf Section 2.1.1) [4a, 8], carbene reactions [9] (cf Section 3.1.10), olefin isomerization (cf Section 3.2.14) [10], olefin oligomerization (cf Section 2.3.1.1) [11], organometallic addition to aldehydes [12], allylic alkylation [13], Grignard coupling reactions [14], aldol-type reactions [15], Diels-Alder reactions [12a, 16], and ene reactions [17], among others. This chapter presents several selected examples of practical significance. 

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],... 

The regiochemistry of the hydrovinylation product (3-phenyl-1-butene) requires the exclusive addition of the Ni atom to the phenyl-substituted olefinic C atom (Ni C2) and of the hydrogen atom to the terminal C atom. The pathway for this addition, which is presumably accompanied by an anticlockwise rotation of the styrene molecule about the Ni-alkene axis in 22, is not clear but has precedence in the preferred Ni—>C2 addition which is observed in the initial step... 

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... 

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... 

Palladium-catalysed hydroarylations and hydrovinylations of olefins and acetylenes have been reviewed. Vinyl iodide, for instance, reacts with internal acetylenes in the presence of (PPh3)2Pd(OAc)2, triethylamine and formic acid to yield the alkenes 204... 

The co-dimerization of ethylene with olefins has also been studied extensively and has been called hydrovinylation. A seminal example, discovered by Bogdanovic and Wilke, involved the co-dimerization of ethylene and norbornene catalyzed by a (TT-aUyl)nickel catalyst (Equation 22.34). This chemistry and more modem versions of these additions of one olefin C-H bond across another were presented in Chapter 16 on the hydrofunctionalization of olefins. 

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. 

---