Question

Account for the regioselectivity and stereoselectivity observed when 1 -methylcyclopentene is treated with each reagent. (a) \(\mathrm{BH}_{3}\) (b) \(\mathrm{Br}_{2}\) in \(\mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{Hg}(\mathrm{OAc})_{2}\) in \(\mathrm{H}_{2} \mathrm{O}\)

Short answer

Question: Explain the regioselectivity and stereoselectivity observed in the reactions of 1-methylcyclopentene with (a) BH3, (b) Br2 in H2O, and (c) Hg(OAc)2 in H2O. Answer: In the reaction of 1-methylcyclopentene with (a) BH3, the regioselectivity follows the anti-Markovnikov rule and the stereoselectivity is syn-addition. In the reaction with (b) Br2 in H2O, the regioselectivity follows Markovnikov's rule and the stereoselectivity is anti-addition. In the reaction with (c) Hg(OAc)2 in H2O, the regioselectivity follows Markovnikov's rule and the stereoselectivity is also anti-addition.

Step-by-step solution

Reaction with BH3

When 1-methylcyclopentene reacts with borane (BH3), it undergoes hydroboration. The mechanism of this reaction is as follows: 1. The \(\pi\) bond of the alkene attacks the boron atom in BH3, forming a new bond between the carbon atom of the alkene and boron. 2. At the same time, a hydrogen atom from BH3 migrates to the other carbon atom in the alkene, breaking the B-H bond. In terms of regioselectivity, the reaction follows the anti-Markovnikov rule, as the hydrogen atom binds to the more substituted alkene carbon. Stereoselectivity is syn-addition, with the boron and hydrogen atoms adding to the same face of the alkene.

Reaction with Br2 in H2O

When 1-methylcyclopentene reacts with bromine in water, it undergoes a halohydrin (Br-OH) formation reaction. The mechanism is as follows: 1. The alkene \(\pi\) bond attacks one of the bromine atoms in Br2, forming a bromonium ion (three-membered ring containing a bromine atom). 2. Water acts as a nucleophile, attacking the more substituted bridgehead carbon of the bromonium ion and opening the ring. 3. The H2O molecule loses a proton, resulting in the formation of a Br-OH moiety on the alkene. The regioselectivity of the bromine addition follows Markovnikov's rule, as the hydroxyl group adds to the more substituted alkene carbon. The reaction is stereoselective as an anti-addition, with Br and OH on opposite faces of the alkene, due to the bromonium ion intermediate.

Reaction with Hg(OAc)2 in H2O

When 1-methylcyclopentene reacts with mercuric acetate (Hg(OAc)2) in water, it undergoes an oxymercuration-demercuration reaction to form an alcohol. The mechanism is as follows: 1. The alkene \(\pi\) bond attacks the mercuric ion, forming a bond with it and creating a mercurinium ion (3-membered ring containing a mercury atom). 2. Water, as a nucleophile, attacks the more substituted bridgehead carbon of the mercurinium ion, opening the ring and attaching an OH group to it. 3. A reduction step with NaBH4 replaces the mercuric group with a hydrogen atom, resulting in the formation of an alcohol. The regioselectivity of the reaction follows Markovnikov's rule, as the hydroxyl group is attached to the more substituted alkene carbon. Stereoselectivity is anti-addition, since the OH group and the hydrogen atom which replaces the mercury group are on opposite faces of the alkene.

Key concepts

Regioselectivity

Regioselectivity is a key concept in organic chemistry that refers to the preference for one direction of chemical bond formation in a molecule over all other possible directions. When we consider the reactivity of alkenes, regioselectivity helps us predict which carbon atom in the double bond will preferentially form a bond with the incoming reagent.

For example, in the hydroboration of 1-methylcyclopentene, the regioselectivity follows the anti-Markovnikov rule, meaning that the boron attaches to the less substituted carbon of the double bond. Contrastingly, in halohydrin formation and oxymercuration-demercuration reactions, Markovnikov's rule applies, leading to nucleophile (water) attack on the more substituted carbon. Markovnikov's rule is generally guided by the stability of the carbocation intermediate, which is more stable when at the more substituted carbon. However, no carbocation is formed in hydroboration, explaining its anti-Markovnikov outcome.

Stereoselectivity

Stereoselectivity involves the preference for the formation of a specific stereoisomer when a chemical reaction can produce multiple isomers. Alkene reactions often exhibit stereoselectivity because the double bond restricts rotation, creating an opportunity for different spatial outcomes of product molecules.

In 1-methylcyclopentene reactions, hydroboration is syn-addition, meaning both the hydrogen and boron atoms add to the same side of the molecule. This gives a specific stereoisomer as a product. Contrastly, halohydrin formation and oxymercuration-demercuration with Br2 in H2O and Hg(OAc)2, respectively, show anti-addition. This leads to bromine and water or mercury and water adding across the double bond on opposite sides, thus resulting in different stereoisomers compared to those obtained from hydroboration.

Hydroboration

Hydroboration is a unique reaction in organic synthesis characterized by the addition of borane (BH3) to alkenes. It is noteworthy for its regioselectivity and stereoselectivity, often deviating from more traditional alkene reactions.

In the provided example, hydroboration of 1-methylcyclopentene includes syn-addition of boron and hydrogen across the double bond, leading to a product with both substituents on the same side of the molecule. The regioselectivity is explained by the anti-Markovnikov rule due to steric and electronic reasons. As hydroboration proceeds without a carbocation intermediate, it doesn't follow the typical Markovnikov's rule seen in many other addition reactions.

Halohydrin Formation

The formation of halohydrins from alkenes involves the addition of halogens and water across the carbon-carbon double bond. This reaction proceeds via a halonium ion intermediate, and its regioselectivity and stereoselectivity are characteristic features of the process.

With 1-methylcyclopentene, reacting with bromine and water, a bromonium ion is formed. Water then opens this cyclic intermediate, leading to the formation of the halohydrin, with the OH group ending up on the more substituted carbon (following Markovnikov's rule) and halogen on the less. The reaction is stereoselective, showing anti-addition because of the way the ring is opened by the attack of water.

Oxymercuration-Demercuration

Oxymercuration-demercuration is a two-step process that converts alkenes into alcohols. The first step, oxymercuration, involves the addition of Hg(OAc)2 across the double bond to create a mercurinium ion, followed by nucleophilic attack by water. The demercuration step typically uses a reducing agent such as NaBH4 to replace the mercury with a hydrogen atom.

This method particularly emphasizes Markovnikov regioselectivity, as the OH group is directed to the more substituted carbon atom. The stereoselectivity is typically anti, with the hydroxyl group and the hydrogen from the reducing step ending up on opposite sides of the original double bond, similar to halohydrin formation.