Stereoselectivity alkene hydrogenation

The preferred stereochemistry of addition to cyclic alkenes is anti The additions are not as highly stereoselective as hydrogen bromide addition, however. 

More recently homogeneous hydrogenation catalysts, such as RhCl(Ph3P)3, have been developed which are soluble in the reaction medium. These are believed to transfer H to an alkene via a metal hydride intermediate they, too, lead to a considerable degree of SYN stereoselectivity in hydrogen addition. 

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. 

Alkene synthesis. A regio- and stereoselective alkene synthesis is formulated in equations (I)—(III). The first step involves formation of the alkyliron complex 1. Treatment of 1 with trityl tetrafluoroborate abstracts a -hydrogen to give a cationic iron complex 2. Liberation of the free alkene is effected by Nal in acetone. This sequence is useful because 1- and 2-alkyliron complexes (1) are converted into 1-alkcncs exclusively 3-alkyliron complexes are converted exclusively into the less stable (Z)-2-alkenes. The paper includes some 20 examples of alkenes prepared in this way. 

As was discussed in Sections 11.1.5 and 11.2.3, the stereoselective partial hydrogenation of alkynes to either cis or trans alkenes is of key importance. Chemical reductions can also be applied to achieve both selective transformations. 

Sophorolipid lactone (34) was obtained from an open-chain bisalkyne precursor (concomitant formation of 2-butyne) in a macrocyclization yield of 78% [29], The epothilones have been obtained through non-stereoselective alkene metathesis [26d], making them worthwhile targets for the combination alkyne metathesis/Lindlar hydrogenation to assemble the C12-C13 Z double bond. Furthermore, epothilone C (35) shows the most complex array of functional groups among the substrates of alkyne metathesis. The macrocyclization yield, starting from the expected OTBS-protected precursor (not shown) was 81% [23, 30]. Neither... 

Reactions with Alkenes,—Further examples have been reported of the cycloaddition of 1,3-dienes to chlorophosphines. - Certain of these reactions show a high overall stereoselectivity when hydrogen sulphide is used to quench the initial complex. Thus the sulphide (11) is formed as one isomer only. 15-Phospha-steroids and 17-phospha-steroids have been prepared by this type of reaction. In general, both possible isomers (at phosphorus) are formed, as with the oxides (12). ... 

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... 

In this volume, innovative aspects of ruthenium applications in their contribution to green chemistry have been included, notably formation of hydrogen, hydrogenation and hydration of polar multiple bonds, stereoselective alkene metathesis, alkyne transformations via various activation modes, sp C-H and sp C-H bond activation and functionalization, photoredox catalysis and nanoparticles in catalysis. 

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