Anti addition of hydrogen

Reduction of an alkyne with sodium metal in liquid ammonia gives an alkene formed by anti addition of two hydrogen atoms. The reaction occurs by a very different mechanism than the catalytic hydrogenation reaction. 

This reaction occurs in four steps two are electron transfer reactions and two are acid-base reactions. An electron of the alkali metal moves to ammonia to give a solvated electron, which is the active reducing agent. The formation of this solvated electron causes a deep blue color, cerulean blue, when the metal is placed in ammonia. 

In the first step, an electron adds to a 7t orbital of the alkyne to form a radical anion. The two non-bonded orbitals are shown trans to each other. However, the stereochemistry is not critical at this point. Vinyl carbanions are configurationally stable, but vinyl radicals are less so. 

The second step is an acid-base reaction. Because C—H bonds of sp -hybridized carbon atoms are less acidic than N—H bonds, the equilibrium lies to the right. At this point, even if the vinyl radical is not configurationally stable, the two allq l groups are more stable trans to each other because steric hindrance would result if they were cis. 

In the third step, a second electron transfer occurs to form a vinyl anion in which the alkyl groups are trans. Finally, in the fourth step, an acid—base reaction occurs to protonate the vinyl anion. 

---

An anti addition of hydrogen atoms to the triple bond occurs when alkynes are reduced with lithium or sodium metal in ammonia or ethylamine at low temperatures. 

The best way of ensuring anti addition of hydrogen across any triple bond is to treat the alkyne with sodium in liquid ammonia. 

Most of the electron attacks eventually lead to anti addition of hydrogen to the triple bond. Therefore, it has often been assumed that a irons dianion is a necessary intermediate. Because both free dianions and dimetal adducts may be isolated or detected, either or both may be important in different systems. 

B Anti Addition of Hydrogen Synthesis of frans-Alkenes... 

Synthesis of frans-2-butene can be accomplished by treating 2-butyne with lithium in liquid ammonia. The anti addition of hydrogen by this reaction gives us the trans product that we need. 

Hydrobromination of the same substrate in the a-cyclodextrin complex gave the monobromide with the opposite configuration at 46 % e.e.. This clearly shows that ethyl trans-cinnamate forms complexes with a- and 3-cyclodextrins such that the anti-addition of hydrogen bromide occurs with high but different enantioselectivities in the two cases to yield monobromide derivatives of opposite chiralities. A detailed mechanism, however, could not be described at the present time for the observed asymmetric induction in the hydrobromination and bromination of the a-cyclodextrin complex, because no crystalline or molecular structures were determined for the ester included in a-cyclodextrin. 

Under some experimental conditions, particularly with tetrasubstituted double bonds, some percentage of the product may appear to be formed by anti addition of hydrogen. Catalytic reduction of 1,2-dimethylcyclohexene, for example, yields predominantly ds-l,2-dimethylcyclohexane. Along with the cis isomer are formed lesser amounts of frflns-l,2-dimethylcyclohexane as a racemic mbcture. 

Reduction Using Na or Li Metal in NHjC/) (Section 7.8C) Alkali metal reduction is stereoselective anti addition of hydrogens to an internal alkyne gives a trans-aikene. The mechanism involves a radical mechanism with two sequential single-electron transfers from the Na or Li metal, each followed by deprotonation of the amine solvent. 

---