Stereoselective reactions catalytic hydrogenation

A second stereochemical aspect of alkene hydrogenation concerns its stereoselectivity. A reaction in which a single starting material can give two or more stereoisomeric products but yields one of them in greater amounts than the other (or even to the exclusion of the other) is said to be stereoselective. The catalytic hydrogenation of a-pinene (a constituent of turpentine) is an example of a stereoselective reaction. Syn addition of... 

What is more interesting is that no one to date has utilized a stereoselective imine catalytic hydrogenation strategy to make (5)-dapoxetine directly according to a four-step reaction sequence such as the one proposed in the following. 

Two stereochemical aspects—stereospecificity and stereoselectivity—attend catalytic hydrogenation. Stereospecificity will be considered more fully in the next chapter, but we can apply one of its principles—that the stereochemical outcome of a particular reaction depends on the stereochemistry of the reactants—to catalytic hydrogenation. According to Mechanism 6.1, even though the two hydrogen atoms are not transferred from the catalyst surface to the double bond simultaneously, both do add to the same face of the double bond. This is called syn addition and is one of several terms applied to stereospecificity. Its counterpart—anti addition—signifies addition to opposite faces of a double bond. 

The Birch reductions of C C double bonds with alkali metals in liquid ammonia or amines obey other rules than do the catalytic hydrogenations (D. Caine, 1976). In these reactions regio- and stereoselectivities are mainly determined by the stabilities of the intermediate carbanions. If one reduces, for example, the a, -unsaturated decalone below with lithium, a dianion is formed, whereof three different conformations (A), (B), and (C) are conceivable. Conformation (A) is the most stable, because repulsion disfavors the cis-decalin system (B) and in (C) the conjugation of the dianion is interrupted. Thus, protonation yields the trans-decalone system (G. Stork, 1964B). 

The stereoselectivity obtained in hydrogenation of an enone can be due to the formation of an intermediate in which alkene, hydrogen, and alcohol groups bind simultaneously to the metal (Equation (5)). This kind of stereoselectivity is typical in catalytic reactions where a polar group resides near to a C=C bond. 

Stereoselective reduction of some triazolodiazines (derivatives of ring systems 33 and 37) bearing chiral terpene residues has been elaborated by Groselj el al. <2006TA79>. With catalytic hydrogenation, partial saturation of the six-membered ring was experienced, while reaction with borane-methyl sulfide resulted in formation of triazole-boron complexes. 

Oximes undergo hydrogenation to hydroxylamines and/or amines depending on reaction conditions. Platinum oxide is the most frequently used catalyst for selective hydrogenation of oximes to hydroxylamines. Reduction of chiral oxime 96 over palladium catalyst (equation 66) proceeds in high yield and stereoselectivity. High stereoselectivity was observed in catalytic hydrogenation of a-alkoxyoximes . 

The synthesis of the C20—C26 fragment started with a 4-alkylation of methyl aceto-acetate The first stereocentre was introduced by enantioselecuve catalytic hydrogenation with Noyort s (S)-binap rhodium complex (cf p 102f.) Stereoselective Frater-Seebach alkylation with allyl bromide introduced the second stereocentre in 90% yield (cf p 27) Stereospecifid introduction of the stereocentres C24 and C2 was achieved by a chelation controlled addition of an allylstannane to an aldehyde (see p 66f) After some experimentation with Lewis acid catalysts and reaction conditions a single diastereomer of the desired configuration was ob-... 

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