Alkenes syn addition

Alkene hydrogenation occurs on the surface of metal particles which act as a catalyst for the reaction. This usually means that both hydrogens are added to the same face of the alkene syn addition). 

Figure 7.9 The mechanism for the hydrogenation of an alkene as catalyzed by finely divided platinum metal (a) hydrogen adsorption (b) adsorption of the alkene (c) and (d), stepwise transfer of both hydrogen atoms to the same face of the alkene (syn addition).

The simple diastereoselectivity is dependent on the structure of the alkene syn addition is normally observed with acyclic (Tfj-alkenes, whereas anti addition occurs with acyclic (Z)-alkenes and cyclic alkenes (Table 5). The isomerization of (Z)- to (Ej-alkenes prior to reaction can explain the prevalent or exclusive formation of the syn //-nitro acetate from either isomeric alkene. 

Have you noticed yet For symmetric alkenes and symmetric reagents (addition of two identical X groups) c/5 -alkene + syn addition meso c/5 -alkene + anti addition racemic trans-3 ktnt + syn addition racemic... 

If addition of hydrogen to an alkene forms a product with two asymmetric carbons, only two of the four possible stereoisomers are obtained because only syn addition can occur. (The other two stereoisomers would have to come from anti addition.) One stereoisomer results from addition of both hydrogens from above the plane of the double bond, and the other stereoisomer results from addition of both hydrogens from below the plane. The particular pair of stereoisomers that is formed depends on whether the reactant is a c/5-alkene or a fran -alkene. Syn addition of H2 to a cis-alkene forms only the erythro enantiomers. (In Section 5.8, we saw that the erythro enantiomers are the ones with identical groups on the same side of the carbon chain in the eclisped conformers.)... 

This process does not produce any tram alkene. The stereochemical outcome of alkyne hydrogenation can be rationahzed in the same way that we rationalized the outcome of alkene hydrogenation (Section 9.7). Both hydrogen atoms are added to the same face of the alkene (syn addition) to give the cis alkene as the major product. 

The particular pair of stereoisomers that is formed depends on whether the reactant is a cis alkene or a trans alkene. Syn addition of H2 to a cis alkene forms only the erythro enantiomers. (In Section 4.11 we saw that the erythro enantiomers are the ones with the hydrogens on the same side of the carbon chain in the eclipsed conformers.)... 

It should be noted that formation of trans-product can be achieved in an anti-addition reaction through the outer-sphere mechanism. Theoretical studies have demonstrated that syn-addition and anti-addition reactions may start from the same 7i-complex, and direction of the multiple bond activation depends on the polarity of solvent [17, 18]. Relative reactivity in the inner-sphere and outer-sphere mechanisms contributes to the overall -/Z- selectivity of the addition reaction to alkynes (stereoselectivity issue). In some cases it is possible to switch the direction of C-Het bond formation by finding a suitable ligand [19]. In case of alkenes syn-addition and a f -addition processes do not necessarily result in different stereochemistry (unrestricted rotation around the single C-C bond in the product). Occurrence of these mechanisms for the N [20, 21], P [22, 23], O [24-26], S, Se [27, 28] heteroatom groups and application of different metal catalysts are discussed in detail in the other chapters of this book. Stereochemical pathways of nucleometallation and development of enantioselective catalytic procedures were reviewed [29]. In this chapter we focus our attention on the mechanism of irmer-sphere insertion reaction involving double and triple carbon-carbon bonds. 

Catalytic hydrogenation is mostly used to convert C—C triple bonds into C C double bonds and alkenes into alkanes or to replace allylic or benzylic hetero atoms by hydrogen (H. Kropf, 1980). Simple theory postulates cis- or syn-addition of hydrogen to the C—C triple or double bond with heterogeneous (R. L. Augustine, 1965, 1968, 1976 P. N. Rylander, 1979) and homogeneous (A. J. Birch, 1976) catalysts. Sulfur functions can be removed with reducing metals, e. g. with Raney nickel (G. R. Pettit, 1962 A). Heteroaromatic systems may be reduced with the aid of ruthenium on carbon. 

Stereochemical features in the oxidative addition and the elimination of /3-hydrogen of cyclic and acyclic alkenes are different. The insertion (palladation) is syn addition. The syn addition (carbopalladation) of R—Pd—X to an acyclic alkene is followed by the syn elimination of 3-hydrogen to give the trans-a ksne 6, because free rotation of 5 is possible with the acyclic alkene. On the other hand, no rotation of the intermediate 7 is possible with a cyclic alkene and the syn elimination of /3-hydrogen gives the allylic compound 8 rather than a substituted alkene. 

Epoxidation of alkenes with peroxy acids is a syn addition to the double bond Substituents that are cis to each other in the alkene remain cis in the epoxide substituents that are trans in the alkene remain trans m the epoxide... 

Epoxidation (Section 6 18) Peroxy acids transfer oxygen to the double bond of alkenes to yield epoxides The reaction IS a stereospecific syn addition... 

Epoxidation of alkenes is a stereospecific syn addition Which stereoisomer of 2 butene reacts with peroxyacetic acid to give meso 2 3 epoxybu tane Which one gives a racemic mixture of (2/ 3/ ) and (25 35) 2 3 epoxybutane ... 

Like the hydrogenation of alkenes hydrogenation of alkynes is a syn addition CIS alkenes are intermediates in the hydrogenation of alkynes to alkanes... 

The reaction of dihalocarbenes with alkenes is stereospecific and syn addition is observed... 

Methylene transfer from lodo methylzinc iodide converts alkenes to cyclopropanes The reaction is a stereo specific syn addition of a CH2 group to the double bond... 

Overall the reaction leads to addition of two hydroxyl groups to the double bond and IS referred to as hydroxylation Both oxygens of the diol come from osmium tetraox ide via the cyclic osmate ester The reaction of OSO4 with the alkene is a syn addition and the conversion of the cyclic osmate to the diol involves cleavage of the bonds between oxygen and osmium Thus both hydroxyl groups of the diol become attached to the same face of the double bond syn hydroxylation of the alkene is observed... 

Entries 1 and 2 in Scheme 2.9 are typical of concerted syn addition to alkene double bonds. On treatment with peroxyacetic acid, the Z-alkene affords the cis-oxirane, whereas the -alkene affords only the iraws-oxirane. Similarly, addition of dibromocarbene to Z-2-butene yields exclusively l,l-dibromo-cw-2,3-dimethylcyclopropane, whereas only 1,1-dibromo-/ra 5-2,3-dimethylcyclopropane is formed from -2-butene. There are also numerous stereospecific anti additions. Entiy 3 shows the anti stereochemistry typical of bromination of simple alkenes. 

A significant modification in the stereochemistry is observed when the double bond is conjugated with a group that can stabilize a carbocation intermediate. Most of the specific cases involve an aryl substituent. Examples of alkenes that give primarily syn addition are Z- and -l-phenylpropene, Z- and - -<-butylstyrene, l-phenyl-4-/-butylcyclohex-ene, and indene. The mechanism proposed for these additions features an ion pair as the key intermediate. Because of the greater stability of the carbocations in these molecules, concerted attack by halide ion is not required for complete carbon-hydrogen bond formation. If the ion pair formed by alkene protonation collapses to product faster than reorientation takes place, the result will be syn addition, since the proton and halide ion are initially on the same side of the molecule. 

Peroxy acid oxidation of alkenes (Sections 6.18 and 16.9) Peroxy acids transfer oxygen to alkenes to yield epoxides. Stereospecific syn addition is observed. 

Draw the other stereoisomer that might have been obtained from syn addition of hydrogen to each alkene. Is the observed product for each addition also the thermodynamic product Compare energies for alkene A+ H2 observed and not observed and alkene B+H2 observed and not observed. What structural factors seem to be responsible for the relative stabilities of the two products of each hydrogenation reaction ... 

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