Achiral hydrogenation

In general, the selectivity of hydrogenation is dependent on a variety of parameters, such as the pressure of H2, temperature, concentration of substrate and catalyst, ratio of catalyst to ligand, and solution pH [57c,d, 60, 61]. For instance, J06 and coworkers reported that the selective hydrogenation of tranx-cinnamaldehyde at the C=C bond was achievable at a low pH of 5 with 1 bar H2 but the selectivity favored the C=0 bond when the pH was increased to pH 7. On the other hand. 

Hydrogenation of unsaturated polymers in water is also possible. For example, Rh/TPPTS catalyzed the hydrogenation of polybutadiene-l,4-l)-poly(ethylene 

Supported nanoparticles have been exploited for hydrogenation in the aqueous phase [4, 66]. The support can be a water-soluble polymer or a solid. 

Pd nanoparticles stabilized by sodium carboxymethyl cellulose in water was shown to be a very efficient and stable catalyst for the selective hydrogenation of acetylene under mild reaction conditions [72], 

Transfer hydrogenation in water has gained increasing interest recently. Pioneering work by Joo and coworkers [73] showed that benzaldehydes, cinnamaldehyde. 

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Table 38.1 Aqueous-organic two-phase achiral hydrogenation of alkenes. 

Hydrosilylation can be applied to alkenes, alkynes, and aldehydes or ketones. A wide range of metal compounds can be used as a catalyst. The most common and active ones for alkenes and alkynes are undoubtedly based on platinum. Hydrosilylation of C-0 double bonds gives silyl ethers, which are subsequently hydrolysed to their alcohols. The reaction is of interest in its enantioselective version in organic synthesis for making chiral alcohols, as the achiral hydrogenation of aldehydes or ketones does not justify the use of expensive silanes as a reagent. 

Note An intramolecular hydrogen bond can be stereodirecting in an otherwise achiral hydrogenation reaction. 

However, when this approach was applied to dehydro-pentagastrin (Boc-/3Ala-A -Trp-Met-Asp-Phe-NH2) it became evident that the structure of the catalyst was not the only factor influencing the stereoselectivity of the reaction. Achiral hydrogenation conditions (PdO) led to a 52% diastereomeric excess in favor of the (R)-tryptophan derivative. Asymmetric induction was also very low in the presence of chiral rhodium catalysts 44% in favor of the (/f)-diastereomer with [Rh-(S,S)-DIOP](DIOP = 2,3-C>-isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane) and only 26% in favor of the (5)-diastereo-mer with [Rh-(/ ,i )-DIPAMP] + BF4. ... 

In a second example addition of hydrogen bromide converts 2 butene which is achiral to 2 bromobutane which is chiral But as before the product is racemic because... 

When a reactant is chiral but optically inactive because it is racemic any products derived from its reactions with optically inactive reagents will be optically inactive For example 2 butanol is chiral and may be converted with hydrogen bromide to 2 bromo butane which is also chiral If racemic 2 butanol is used each enantiomer will react at the same rate with the achiral reagent Whatever happens to (/ ) (—) 2 butanol is mir rored m a corresponding reaction of (5) (+) 2 butanol and a racemic optically inactive product results... 

Tbe purpose of tbe bydroxyl group is to acbieve some hydrogen bonding with the nearby carbonyl group and therefore hinder the motion of the chiral center. Another way to achieve the chiral smectic Cphase is to add a chiral dopant to a smectic Chquid crystal. In order to achieve a material with fast switching times, a chiral compound with high spontaneous polarization is sometimes added to a mixture of low viscosity achiral smectic C compounds. These dopants sometimes possess Hquid crystal phases in pure form and sometimes do not. 

In a second example, addition of hydrogen broimide converts 2-butene, which is achiral, to 2-brormobutane, which is chiral. But, as before, the product is racermic because... 

Each act of proton abstraction from the a carbon converts a chiral molecule to an achiral enol or enolate ion. The 5/) -hybridized carbon that is the chirality center in the starting ketone becomes 5/) -hybridized in the enol or enolate. Careful kinetic studies have established that the rate of loss of optical activity of 5cc-butyl phenyl ketone is equal to its rate of hydrogen-deuterium exchange, its rate of bromination, and its rate of iodina-tion. In each case, the rate-detennining step is conversion of the starting ketone to the enol or enolate anion. 

Closely related to the concept of chirality, and particularly important in biological chemistry, is the notion of prochirality. A molecule is said to be prochiral if can be converted from achiral to chiral in a single chemical step. For instance, an unsymmetrical ketone like 2-butanone is prochiral because it can be converted to the chiral alcohol 2-butanol by addition of hydrogen, as we ll see in Section 17.4. 

Simple diastereoselection has also been reported for Strecker syntheses using pentanedial or butanedial with primary amines or hydrogen cyanide which affords 2,5-dicyanopiperidine or 2,5-dicyanopyrrolidine derivatives, respectively35. In both cases, the achiral svn-diastereomers are slightly preferred. At 70 °C, the pure rA-compounds can be partially transformed to the /rfflM-isomcrs. 

Cyclodextrin mediated oxidation ofprochiral sulphides by achiral oxidation reagents leads also to optically active sulphoxides (e.e. up to 30%). When oxidation was carried out in pyridine the highest optical purities were obtained294 with hydrogen peroxide, whereas in water the best results were observed with m-chloroperbenzoic acid295.. 

Diastereoselective addition has been carried out with achiral reagents and chiral substrates, similar to the reduction shown on page. 1201, but because the attacking atom in this case is carbon, not hydrogen, it is also possible to get diastereoselective addition with an achiral substrate and an optically active reagent. Use of suitable reactants creates, in the most general case, two new chiral centers, so the product can exist as two pairs of enantiomers ... 

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