Problem 20 - Advanced Organic Chemistry. Part A. Structure and Mechanisms

Question

Explain the outcome of the following reactions by a mechanism showing how the product could be formed. a. 2,6-Di- \((t\)-butyl)phenoxide reacts with \(o\)-nitroaryl halides in \(\mathrm{NaOH} / \mathrm{DMSO}\) at \(80^{\circ} \mathrm{C}\) to give 2,6 -di- \((t\)-butyl)-4-(2-nitrophenyl)phenol in \(60-90 \%\) yield. Under similar conditions, 1,4-dinitrobenzene gives 2,6-di- \((t\)-butyl)-4(4-nitrophenyl)phenol. b. 2 -(3-Chlorophenyl)-4,4-dimethyloxazoline reacts with alkyllithium reagents to give 2-(2-alkylphenyl)-4,4-dimethyloxazolines. c. Nitrobenzene reacts with cyanomethyl phenyl sulfide in \(\mathrm{NaOH} / \mathrm{DMSO}\) to give a mixture of 2- and 4-nitrophenylacetonitrile. d. The following transformation occurs: e. Reaction of benzene with 3,3,3-trifluoropropene in the presence of \(\mathrm{BF}_{3}\) gives 3,3,3-trifluoropropylbenzene. f. 3-Chloronitrobenzene reacts with 4-amino-1,2,4-triazole in \(\mathrm{K}^{+} .{ }^{-} \mathrm{O}-t-\mathrm{Bu} /\) DMSO to give 2-chloro-4-nitroaniline. g. Good yields of tetralone can be obtained from 4-phenylbutanoic acid or the corresponding acyl chloride in the presence of the strongly acidic resin Nafion-H. With 3-phenylpropanonic acid, only the acyl chloride gives a cyclization product.

Step-by-Step Solution

Answer

The reactions involve deprotonation, SNAr involving phenoxide, and product rearrangement.

Step 1: Deprotonation of Phenol

In the presence of NaOH, the phenol group on 2,6-di-(t-butyl)phenoxide is deprotonated, generating a phenoxide ion. This ion is a strong nucleophile which can readily attack electrophilic centers, such as an aryl halide.

Step 2: Nucleophilic Aromatic Substitution (SNAr)

The phenoxide ion attacks the ortho position of the nitroaryl halide. The presence of the nitro group enhances the electrophilicity of the aromatic system, facilitating a nucleophilic aromatic substitution (SNAr). This results in the release of the halide ion.

Step 3: Rearrangement

In the case of 1,4-dinitrobenzene, after forming the initial substitution product, a rearrangement occurs leading to the major product 2,6-di-(t-butyl)-4-(4-nitrophenyl)phenol. The electron-withdrawing effect of the nitro groups facilitates migration of the intermediate.

Key Concepts

Nucleophilic Aromatic Substitution

Nucleophilic Aromatic Substitution, often abbreviated as SNAr, is a reaction where a nucleophile, such as a phenoxide ion, replaces a leaving group in an aromatic ring. In aromatic systems, the presence of electron-withdrawing groups like nitro groups increases the susceptibility of electrophilic centers to nucleophilic attack. This makes the aromatic ring more accepting of electron-rich nucleophiles.
This process typically involves the initial attack by the nucleophile on the carbon atom bearing the leaving group, often a halide. The bond between the carbon and the halide is then broken, leading to the departure of the halide ion. The result is a new carbon-nucleophile bond, replacing the one with the leaving group.

• The nucleophile must be strong enough; a powerful ion like the phenoxide is ideal.
• The reaction is significantly affected by the nature of substituents present on the aromatic ring.
• Nitro groups or other electron-withdrawing groups enhance the electrophilicity of the carbon center.

This reaction platform is crucial in organic synthesis, providing pathways for creating complex aromatic compounds.

Phenoxide Ion Deprotonation

Phenoxide ion deprotonation is a key step in many nucleophilic aromatic substitution reactions. When phenol reacts with a strong base like NaOH, it loses a hydrogen ion (H⁺) from the hydroxyl group, forming the phenoxide ion. This ion is a potent nucleophile, meaning it has a high electron density that can attack electrophilic centers. This increased reactivity stems from its negative charge, which enhances its ability to donate electrons.

• In this scenario, phenol acts as an acid, donating a proton.
• The phenoxide ion formed is more stable than the starting phenol in basic conditions.
• It plays a crucial role in subsequent nucleophilic substitutions.

Because of its reactivity, the phenoxide ion is ideally suited to participate in further chemical transformations, making it invaluable in synthetic chemistry.

Electrophilic Centers

Electrophilic centers in organic molecules are targets for nucleophilic attack because they possess a partial positive charge or are electron-deficient. In aromatic systems, electrophilic centers can be accentuated by the addition of electron-withdrawing groups, such as nitro groups, which increase the ring's susceptibility to attack by nucleophiles. This makes regions of the aryl halides prime targets for reactions like SNAr.

• Electron-withdrawing groups pull electron density away from the carbon, enhancing its electrophilicity.
• The stronger the electron-withdrawing ability, the more reactive the electrophilic center.
• These centers play a pivotal role in determining the mechanism and rate of reactions.

The interplay of nucleophilic and electrophilic centers drives many synthetic transformations, allowing chemists to manipulate and design complex organic molecules.

Rearrangement Mechanisms

Rearrangement mechanisms are transformations where the molecular structure of a compound is converted into a different isomer. In some reactions, like the one discussed in the exercise involving 1,4-dinitrobenzene, rearrangement occurs after the initial nucleophilic attack. In such cases, the arrangement of atoms is altered to give a more stable intermediate or product.

Rearrangements often follow the formation of unstable intermediates that facilitate a shift of groups within the molecule. These shifts can be driven by electronic factors, such as the distribution of electron density or the stabilization conferred by electron-withdrawing groups.

• Rearrangements can lead to unexpected products, differing from simple substitution.
• They often proceed through a sequence of bond-breaking and bond-forming steps.
• The outcome is highly dependent on the stability of the intermediates formed.

Understanding these mechanisms is essential in predicting the course and outcome of intricate synthetic pathways, offering vital insights into the adaptability and resilience of chemical structures.