Unsymmetrical alkene generally gives Markovnikov orientation of addition, as shown by the oxymercuration of propene. The mercurinium ion has a considerable amount of positive charge on both of its carbon atoms, but there is more of a positive charge on the more substituted carbon atom, where it is more stable. Attack by water occurs on this more electrophilic carbon, giving Markovnikov orientation. The electrophile remains bonded to the less substituted end of the double bond. When mercuration takes place in an alcohol solvent, the alcohol serves as a nucleophile to attack the mercurinium ion. The resulting product contains an alkoxy (-OR) group. For example, oxymercuration-demercuration of 1-methylcyclopentene gives 1-methoxy-1-methylcyclopentane where methanol has added across the double bond. In the first step mercuric acetate adds to the alkene to give mercurium ion which has a partial positive charge on the more substituted tertiary carbon. In the second step, methanol attacks this carbon from the opposite side, leading to anti -addition. Demercuration of this anti -addition product by sodium borohydride gives 1-methoxy-1-methylcyclopentane (Scheme 10).
Scheme 10
4.6.5 Hydroboration–Oxidation
Borane, a neutral molecule, is an electrophile because boron has only six shared electrons in its valence shell. Boron, therefore, readily accepts a pair of electrons in order to complete its octet. Thus, alkenes undergo electrophilic addition reactions with borane (serving as the electrophile). The addition of borane to an alkene, followed by reaction with hydroxide ion and hydrogen peroxide, is called Hydroboration-Oxidation (Scheme 11).
Scheme 11
Diborane (B2H6) is a dimer composed of two molecules of borane (BH3). Diborane has three-centered two-electron (banana-shaped) bonds with protons in the middle of them. Diborane is in equilibrium with a small amount of borane (BH3), a strong Lewis acid with only six valence electrons (Scheme 12).
Scheme 12
In the first step, the addition of a boron atom and hydrogen atom takes place to the double bond. In the second step, the oxidation followed by hydrolysis gives the target alcohol and boric acid. Hydroboration can be accomplished with diborane (B2H6) or more conveniently with a reagent prepared by dissolving diborane in THF. When diborane is introduced to THF, it reacts to form a Lewis acid-base complex of borane and THF (represented as BH3·THF) (Scheme 13).
Scheme 13
Mechanism
As an electron-deficient compound, BH3 is a strong electrophile, capable of adding to a double bond. This hydroboration of the double bond occurs in one step, with the boron atom adding to the less substituted end of the double bond. In the transition state, the electrophilic boron atom withdraws electrons from the π-bond, and the carbon at the other end of the double bond acquires a partial positive charge. This partial charge is more stable on the more substituted carbon atom. The product shows boron bonded to the less substituted end of the double bond and hydrogen bonded to the more substituted end. Also, steric hindrance favors boron adding to the less hindered end of the double bond (Scheme 14).
Scheme 14