Question

Human white cells produce an enzyme called myeloperoxidase. This enzyme catalyzes the reaction between hydrogen peroxide and chloride ion to produce hypochlorous acid, \(\mathrm{HOCl}\), which reacts as if it were \(\mathrm{Cl}^{+} \mathrm{OH}^{-}\). When attacked by white cells, cholesterol gives a chlorohydrin as the major product. Cholesterol Cholesterol chlorohydrin (a) Propose a mechanism for this reaction. Account for both its regioselectivity and stereoselectivity. (b) On standing or (much more rapidly) on treatment with base, the chlorohydrin is converted to an epoxide. Show the structure of the epoxide and a mechanism for its formation. This epoxide is believed to be involved in induction of certain cancers.

Short answer

Question: Propose a mechanism for the reaction of cholesterol with hypochlorous acid to form cholesterol chlorohydrin, accounting for its regioselectivity and stereoselectivity. Explain how the chlorohydrin can be converted into an epoxide and provide a mechanism for its formation. Answer: The formation of cholesterol chlorohydrin involves the reaction of the double bond of cholesterol with hypochlorous acid, forming a chloronium ion intermediate. The regioselectivity occurs due to the chlorine atom forming a bond with the more substituted carbon atom, while the stereoselectivity results in the trans isomer as the major product. The chlorohydrin is then converted into an epoxide through an intramolecular nucleophilic substitution reaction involving neighboring group participation. In the presence of a base, the hydroxyl group is deprotonated, and the negatively charged oxygen atom attacks the adjacent carbon atom bearing the chlorine atom, resulting in the formation of the epoxide ring and elimination of a chloride ion.

Step-by-step solution

Formation of chlorohydrin

The reaction begins when the myeloperoxidase enzyme catalyzes the reaction between hydrogen peroxide and chloride ion to produce hypochlorous acid (HOCl). The HOCl then acts as an electrophile, reacting with cholesterol at the alkene position. The double bond of the cholesterol attacks the chlorine atom of the \(\mathrm{HOCl}\). This results in the formation of a chloronium ion intermediate and the release of a water molecule.

Regioselectivity and stereoselectivity

The regioselectivity of the reaction can be explained by the fact that the chlorine atom, being a more electronegative atom, is more likely to form a bond with the more substituted carbon atom. This leads to the formation of a more substituted chlorohydrin. The stereoselectivity is due to the reaction occurring at the most available position, producing the trans isomer as the major product.

Formation of cholesterol chlorohydrin

The intermediate chloronium ion is then attacked by a water molecule, leading to the formation of a chlorohydrin as the major product. This involves the opening of the three-membered ring and formation of a new bond with the oxygen of the water molecule. (b) Formation of epoxide from cholesterol chlorohydrin

Epoxide structure

When the chlorohydrin is left standing or treated with a base, it is converted to an epoxide. The epoxide structure is formed by an intramolecular nucleophilic substitution reaction involving a neighboring group participation mechanism.

Mechanism for epoxide formation

In the presence of a base, the hydroxyl group of the chlorohydrin is deprotonated, generating a negatively charged oxygen atom. This nucleophile then attacks the adjacent carbon atom bearing the chlorine atom, forming the three-membered epoxide ring and leading to the elimination of a chloride ion.

Key concepts

Enzyme Catalysis

Enzyme catalysis is an essential biological process where enzymes accelerate chemical reactions within cells. In our textbook exercise, myeloperoxidase, an enzyme found in human white cells, acts as a catalyst in the formation of hypochlorous acid from hydrogen peroxide and a chloride ion. This reaction demonstrates the potent ability of enzymes to facilitate complex biochemical processes.

Enzymes work by lowering the activation energy of a reaction, thus allowing it to proceed more quickly than it would without the aid of an enzyme. The myeloperoxidase enzyme accomplishes this by providing an active site that brings substrates together in an optimal orientation for the reaction to occur. The enzyme uniquely binds to the substrates, forming an enzyme-substrate complex, which then leads to the product formation—a key concept in enzyme catalysis is the specificity of enzymes, meaning that each enzyme typically catalyzes only one kind of reaction.

Regioselectivity and Stereoselectivity

The themes of regioselectivity and stereoselectivity are paramount when understanding how enzymes can direct the outcome of biochemical reactions. Regioselectivity refers to the preference of a chemical reaction to occur on specific directions or positions of a molecule, while stereoselectivity involves the preference for the formation of one stereoisomer over another when multiple stereoisomers are possible.

In our example, the reaction exhibits regioselectivity as hypochlorous acid preferentially interacts with the more substituted carbon of the cholesterol's double bond. This predilection for more substituted carbon is due to its higher degree of stabilization, which influences the formation of the chloronium ion intermediate. As for stereoselectivity, the process creates the trans isomer of the chlorohydrin as the major product. This selectivity may be attributed to the steric hindrance on the molecule that affects how groups are oriented in the three-dimensional space during the reaction, a key concept in understanding the detailed outcome of protein interaction with specific substrates.

Epoxide Formation Mechanism

Epoxides are three-membered cyclic ethers, and their formation mechanism is an important reaction in organic chemistry, particularly in the process described in our textbook exercise. When a chlorohydrin is treated with a base or left to stand, it undergoes a transformation to become an epoxide through an intramolecular reaction.

During this process, the base deprotonates the hydroxyl group in chlorohydrin to generate an alkoxide ion. This ion then acts as a nucleophile, attacking the adjacent carbon that bears the chlorine atom. This nucleophilic substitution yields the epoxide ring and concurrently releases a chloride ion. This mechanism illustrates the interplay between a substrate's molecular structure and the possibility of intramolecular reactions leading to the formation of cyclic compounds. Understanding this mechanism gives insight into the reactivity and chemical behavior of heteroatoms within organic compounds.