Question

The cis hydroxylation of alkenes can be effected by (a) reaction with \(\mathrm{OsO}_{4}\) (b) addition of Baeyer's reagent (c) reaction with \(\mathrm{H}_{2} \mathrm{O}_{2}\) and formic acid (d) reaction with an acid in the presence of water.

Short answer

(a) Reaction with \(\mathrm{OsO}_{4}\) (b) Addition of Baeyer's reagent (c) Reaction with \(\mathrm{H}_{2} \mathrm{O}_{2}\) and formic acid (d) Reaction with an acid in the presence of water Answer: (a) Reaction with \(\mathrm{OsO}_{4}\)

Step-by-step solution

Analyze each given reaction

Start by analyzing each of the given reactions to determine if they can lead to cis hydroxylation of alkenes.

Reaction with \(\mathrm{OsO}_{4}\)

(a) A reaction with \(\mathrm{OsO}_{4}\) (osmium tetroxide) is known as the osmium tetroxide-catalyzed dihydroxylation, where an alkene is transformed into a diol molecule by the addition of two hydroxyl groups. The hydroxyl groups are added syn (or cis) to the alkene, leading to cis hydroxylation. So this reaction is a possible candidate for the correct answer.

Addition of Baeyer's reagent

(b) Baeyer's reagent is a mixture of potassium permanganate (\(\mathrm{KMnO}_{4}\)) and water. When this reagent is added to an alkene, it undergoes an oxidative cleavage, leading to the formation of two carbonyl compounds, not a diol. So, this reaction does not result in cis hydroxylation.

Reaction with \(\mathrm{H}_{2} \mathrm{O}_{2}\) and formic acid

(c) Reaction with \(\mathrm{H}_{2} \mathrm{O}_{2}\) (hydrogen peroxide) and formic acid is referred to as the Sharpless asymmetric dihydroxylation, where an alkene is transformed into a diol molecule by the addition of two hydroxyl groups. However, this reaction is used to create chiral diols from prochiral alkenes, and since the question does not specify chiral diols, this reaction is not the most appropriate answer.

Reaction with an acid in the presence of water

(d) A reaction with an acid in the presence of water typically results in hydration of the alkene. However, this reaction involves the formation of an intermediate carbocation, and the hydroxyl group is added non-stereospecifically, leading to both the syn and anti diastereomers. Consequently, this reaction does not involve cis hydroxylation specifically.

Determine the correct answer

Based on the analysis of each reaction, the only reaction that leads to the cis hydroxylation of alkenes is the reaction with \(\mathrm{OsO}_{4}\). Thus, the correct answer is (a).

Key concepts

Osmium tetroxide-catalyzed dihydroxylation

Osmium tetroxide (\textbf{OsO}\(_4\)) is a powerful reagent used for the cis hydroxylation of alkenes, converting them into vicinal diols, where the two hydroxyl (-OH) groups are on the same side of the double bond. The process typically involves the formation of a cyclic osmate ester intermediate, which is then cleaved to form the cis diol. This reaction proceeds without the shift of any substituent groups on the alkene, which helps in preserving the original stereochemistry of the molecule.

Advantages and Applications

This method is highly preferred for its stereospecificity, allowing chemists to synthesize specific compounds without dealing with a mixture of isomers. Because of its selectivity, the osmium tetroxide-catalyzed dihydroxylation is a valuable tool in the synthesis of various natural products and pharmaceuticals that have complex structures and chiral centers.

Baeyer's reagent

Baeyer’s reagent, a solution of potassium permanganate (\textbf{KMnO}\(_4\)) in aqueous alkaline medium, is traditionally used to test for the presence of double bonds in molecules. Alkenes react with Baeyer's reagent to yield glycols; however, the reaction proceeds through syn addition followed by oxidative cleavage. This leads to the formation of carbonyl compounds, such as ketones or aldehydes, rather than the desired cis diols in the case of simple alkenes. Occasionally, when applied to more complex structures, this reagent can result in various oxidation products depending on the substrate.

Limitations

Due to this oxidative nature, Baeyer's reagent is not suitable for the cis hydroxylation of alkenes when the retention of the original carbon framework is required. It is more commonly utilized as a qualitative test to identify alkenes rather than a preparative tool for synthesis.

Sharpless asymmetric dihydroxylation

The Sharpless asymmetric dihydroxylation is an enantioselective chemical reaction that introduces two hydroxyl groups across an alkene to form a chiral diol. It employs \textbf{OsO}\(_4\) catalysis in the presence of a chiral ligand, which imparts enantioselectivity into the reaction. The reagents typically used include hydrogen peroxide (\textbf{H}\(_2\)\textbf{O}\(_2\)) and a secondary alcohol like tert-butanol, which work together with the osmium tetroxide and the chiral ligand to add hydroxyl groups to the less hindered face of the alkene.

Strategic Use in Synthesis

This reaction has revolutionized the field of synthetic organic chemistry and has found widespread applications in the synthesis of complex natural molecules, pharmaceuticals, and other biologically active compounds. The ability to create chiral centers with high enantioselectivity makes the Sharpless asymmetric dihydroxylation a powerful tool in the arsenal of synthetic chemists.

Alkene hydration

Alkene hydration is a chemical reaction in which water is added across the double bond of an alkene, resulting in the formation of an alcohol. This reaction is typically acid-catalyzed and involves the formation of a carbocation intermediate. Because the carbocation can lead to different reaction pathways, the product can be either a Markovnikov or an anti-Markovnikov alcohol, and it is not stereospecific.

Industrial Relevance

Despite the lack of stereospecificity, alkene hydration is of great industrial importance. For example, the hydration of ethene leads to the production of ethanol, a widely used solvent and precursor in the manufacture of various chemicals. However, for the specific transformation of alkenes to cis diols, other more controlled methods, such as osmium tetroxide-catalyzed dihydroxylation, are necessary to achieve the desired selectivity and stereochemistry.