Disubstituted alkenes, hydrogenation

Hydration of alkenes by this method however is limited to monosubstituted alkenes and disubstituted alkenes of the type RCH=CHR Disubstituted alkenes of the type R2C=CH2 along with trisubstituted and tetrasubstituted alkenes do not form alkyl hydrogen sulfates under these conditions but instead react m a more complicated way with concentrated sulfuric acid (to be discussed m Section 6 21)... 

The product i n this case is a cis-disubstituted alkene, so the fi rst question is, " What is an immediate precursor of a cis-disubstituted alkene " We know that an alkene can be prepared from an alkyne by reduction and that the right choice of experimental conditions will allow us to prepare either a trans-disubstituted alkene (using lithium in liquid ammonia) ora cis-disubstituted alkene (using catalytic hydrogenation over the Lindlar catalyst). Thus, reduction of 2-hexyne by catalytic hydrogenation using the Lindlar catalyst should yield cis-2-hexene. 

In principle, carbometallation of an alkene (RCH=CH2) with a coordinatively unsaturated organotransition metal compound (R1 M I. ) can produce a monomeric carbometallation product 1 (Scheme 6). This reaction may not, however, stop at this stage. It can be accompanied by other processes of which (i) hydrogen-transfer hydrometallation to produce a potentially thermodynamically more favorable mixture of a 1,1-disubstituted alkene and a hydrometallation product 2 and (ii) polymerization to produce polyalkenes 3 are representative. The extents to which these side-reactions occur are functions of relative rates of various competing processes. For example, accumulation of the monomeric carbometallation product 1 can be favored in cases where the starting R1 MTL is more reactive toward alkenes than 1. The organometal/alkene ratio is also an important parameter, since neither of the two side-reactions can proceed after all of the starting alkene has reacted. 

Enantioselective hydrogenation of 1,6-enynes using chirally modified cationic rhodium precatalysts enables enantioselective reductive cyclization to afford alky-lidene-substituted carbocycles and heterocycles [27 b, 41, 42]. Good to excellent yields and exceptional levels of asymmetric induction are observed across a structurally diverse set of substrates. For systems that embody 1,2-disubstituted alkenes, competitive /9-hydride elimination en route to products of cycloisomerization is observed. However, related enone-containing substrates cannot engage in /9-hydride elimination, and undergo reductive cyclization in good yield (Table 22.12). 

With most rhodium and ruthenium catalysts 4 and 5 (Fig. 30.2), only low en-antioselectivities were obtained (Table 30.1, entries 1-6) [2-6]. However, good results were reported by Noyori and coworkers, who used DuPHOS with potassium tert-buloxide activation to hydrogenate substrate 1 in 86% ee (Table 30.1, entry 6) [6], as well as hydrogenating a range of other 1,1-disubstituted alkenes (see Section 30.2.2). 

In addition to P,N ligands, the carbenoid-oxazoline catalysts 47 (Fig. 30.10) were used to hydrogenate test substrates 36-39, as well as substrates 48 and 49, which were hydrogenated in 93% ee and 84% ee, respectively [21]. These catalysts were also used to hydrogenate 1,1-disubstituted alkenes (see Section 30.2.1). 

Diphenylacetylene and 1-phenyl-1-propyne were hydrogenated to the corresponding 1,2-disubstituted alkenes in aqueous organic biphasic media using [ RuCl2(wtppms)2 2] and an excess of wtppms (80 °C, 1 bar H2, TOFs up to 25 h-1). The stereoselectivity of the reaction depended heavily on the pH of the catalyst-containing aqueous phase (Fig. 38.1) and, under acidic conditions, Z-al-kenes could be obtained with a selectivity close to 100% [71]. 

While monosubstituted alkenes usually react with high regioselectivity, it is not true for disubstituted alkenes. Formation of mixtures of type 163 and 164 (equation 105) has been observed in most cases when unsymmetrical alkenes bearing two different substituents possess similar stereoelectronic properties. In general, regioselectivity is controlled by a combination of HOMO-LUMO interactions, steric effects and hydrogen bonding between suitable substituents in both alkene and nitrone molecules . ... 

Asymmetric hydrogenation has been reported to occur in excellent yield and ee when a trisubstituted alkene is hydrogenated with a chiral titanocene catalyst (equation 84)334. A similar reaction, but with variable enantioselectivity, may also be obtained with chiral Rh, Ru and Co catalysts335-338. Disubstituted alkenes (mainly 1,1-disubstituted) may also be... 

E. J. Allain, L. P. Hager, L. Deng, and E. N. Jacobsen, Highly enantioselective epoxidation of disubstituted alkenes with hydrogen peroxide catalyzed by chloro-peroxidase, J. Am. Chem. Soc. 1993, 115, 4415-4416. 

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