Amines and Catalytic C–C Bond Formation: Structure, Properties, and Reactions

Chapter 23: Chemistry of Amines

Classification and Structure of Amines

Amines are organic compounds derived from ammonia (NH3) by replacement of one or more hydrogen atoms with alkyl or aryl groups. They are classified based on the number of organic substituents attached to the nitrogen atom.

• Primary (1°) amine: One organic group attached to nitrogen. Example: methylamine (CH3NH2).

• Secondary (2°) amine: Two organic groups attached. Example: dimethylamine ((CH3)2NH).

• Tertiary (3°) amine: Three organic groups attached. Example: trimethylamine ((CH3)3N).

• Aromatic amines: Nitrogen attached to an aromatic ring. Example: aniline (C6H5NH2).

Common cyclic amines include pyrrolidine, piperidine, pyrrole, and pyridine.

Nomenclature of Amines

Amines are named by replacing the '-e' ending of the parent alkane with '-amine'. For diamines, the suffix '-diamine' is used. Substituents on nitrogen are indicated with the prefix 'N-'.

• Examples:

• 2-propanamine

• 1,6-hexanediamine

• N-methylamine

• 2-aminoethanol

• 4-aminobenzoic acid

The largest group is the parent; other groups on nitrogen are given the 'N-' prefix. The NH2 group has the lowest precedence in nomenclature.

Salts of Amines

Amines react with acids to form ammonium salts, which are generally water-soluble and used in drug formulations.

• Example: Triethylamine (Et3N) forms triethylammonium chloride (Et3NH+ Cl-).

Chirality of Amines

Amines with three different substituents on nitrogen are technically chiral, but rapid pyramidal inversion prevents resolution of enantiomers.

• Pyramidal inversion: Nitrogen in amines rapidly inverts between sp3 and sp2 hybridization, making isolation of chiral amines difficult.

Properties of Amines

Basicity

Amines are bases due to the lone pair of electrons on nitrogen, which can accept a proton.

• General reaction: $\text{MeNH}_2 + \text{H}_2\text{O} \rightleftharpoons \text{MeNH}_3^+ + \text{OH}^-$

• pKa of amine: Refers to the conjugate acid ($\text{MeNH}_3^+$). Lower pKa means more acidic.

• Example: $\text{MeNH}_3^+$ pKa = 10.6; $\text{PhNH}_3^+$ pKa = 4.6

• Aromatic amines (anilines) are less basic than alkyl amines due to delocalization of the nitrogen lone pair into the aromatic ring.

Substituent Effects on Aniline Basicity

Aniline's basicity is sensitive to substituents on the aromatic ring. Electron-withdrawing groups decrease basicity.

Compound	pKa
Aniline	4.63
p-Nitroaniline	2.47
o-Nitroaniline	1.0

Para-substituted anilines have lower pKa than meta due to resonance effects.

Heterocyclic Amines

Heterocycles containing nitrogen exhibit varied basicity and aromaticity depending on the position and involvement of the nitrogen lone pair.

Compound	pKa
Pyrrolidine	10.8
Piperidine	5.3
Imidazole	7.0

• In pyrrole, the nitrogen lone pair is part of the aromatic π system, making it not basic.

• In pyridine, the lone pair is in the plane of the ring and available for protonation, making it basic.

Guanidine

Guanidine is a very strong base due to resonance stabilization of its conjugate acid.

• Reaction: $\text{H}_2\text{N}-\text{C}(=\text{NH})-\text{NH}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{N}-\text{C}(=\text{NH}_2^+)-\text{NH}_2 + \text{OH}^-$

• pKa: 13.6

Reactions of Amines

Formation of Amine Salts

Amines react with acids to form water-soluble salts, important in pharmaceutical formulations.

• Example: R-norepinephrine + HCl → R-norepinephrine hydrochloride (water soluble)

Preparation of Amines

Amines can be prepared via nucleophilic substitution (SN2) reactions, but selectivity is a challenge.

• General reaction: $\text{MeBr} + \text{NH}_3 \rightarrow \text{MeNH}_2 + \text{Me}_2\text{NH} + \text{Me}_3\text{N}$

• Reaction cannot be stopped at primary amine; higher amines are also formed.

• Selective synthesis of primary amines can be achieved using azides:

• $\text{ArCl} + \text{NaN}_3 \rightarrow \text{ArN}_3 \xrightarrow{LiAlH_4} \text{ArNH}_2$

Nitrosylation of Amines

Secondary amines react with nitrous acid to form nitrosamines, which are carcinogenic.

• General reaction: $\text{R}_2\text{NH} + \text{HNO}_2 \rightarrow \text{R}_2\text{N-NO}$

• Mechanism involves formation of nitrosyl cation and nucleophilic attack by the amine.

Diazotization of Amines

Primary aromatic amines react with nitrous acid to form diazonium salts, which are reactive intermediates in organic synthesis.

• General reaction: $\text{R-NH}_2 + \text{NO}^+ \rightarrow \text{R-N}_2^+ \xrightarrow{-N_2} \text{R}^+$

• Diazonium salts are explosive and must be handled with care.

Reactions of Diazonium Salts

Diazonium salts can be converted to a variety of functional groups via substitution reactions.

Reagent	Product
H2O	Phenol
HBF4	Fluorobenzene
CuCl	Chlorobenzene
CuBr	Bromobenzene
CuCN	Benzonitrile
KI	Iodobenzene
H3PO2	Benzene

Electrophilic Substitution on Aromatic Amines

Nitration, reduction, diazotization, and substitution can be used to install groups on aromatic rings. The NH2 group is an ortho/para-directing activator, while Cl is ortho/para-directing but deactivating.

Hofmann and Cope Eliminations

Hofmann elimination of amines produces less substituted alkenes via an E2 mechanism. Cope elimination is a syn stereoselective elimination of amine oxides.

• Hofmann elimination: $\text{R-NMe}_3^+ \xrightarrow{Ag_2O, \Delta} \text{Alkene} + \text{NMe}_3 + \text{H}_2\text{O}$

• Cope elimination: $\text{Amine oxide} \xrightarrow{\Delta} \text{Alkene} + \text{N-oxide}$

Cope elimination is syn stereoselective, meaning the eliminated groups leave from the same side of the molecule.

Chapter 24: Catalytic C–C Bond Formation

General Principles

Catalytic formation of carbon–carbon bonds is a cornerstone of modern organic synthesis, often using transition metal catalysts.

• Key steps: Oxidative addition, migratory insertion, and reductive elimination.

• General reaction: $\text{ML}_n + \text{X}_2 \rightarrow \text{ML}_n\text{X}_2$ (oxidative addition)

Heck Reaction

The Heck reaction couples aryl or vinyl halides with alkenes using a palladium catalyst and a base.

• General reaction: $\text{R-X} + \text{R}'-CH=CH_2 \xrightarrow{Pd, \text{base}} \text{R-CH=CH-R}'$

• Mechanism involves oxidative addition, migratory insertion, and reductive elimination.

Intramolecular Heck Reaction

Intramolecular Heck reactions can create new chiral centers and quaternary carbons, often with high enantiomeric excess.

Catalytic Allylic Alkylation

Allylic alkylation uses a metal catalyst to couple nucleophiles with allylic substrates, forming new C–C bonds.

Cross Coupling Reactions

Cross coupling reactions allow formation of C–C bonds between two different organic groups under mild conditions, revolutionizing pharmaceutical synthesis.

Reaction Name	Organometallic Partner
Suzuki	B(OR)2
Stille	SnR3
Negishi	ZnX

• General reaction: $\text{R-M} + \text{R}'-X \xrightarrow{Pd(0)} \text{R-R}'$

• Mechanism involves oxidative addition, transmetallation, and reductive elimination.

Alkene Metathesis

Alkene metathesis is a catalytic process that exchanges alkene fragments between molecules, often using Ru or Mo catalysts.

• General reaction: $\text{R}_1\text{CH}=\text{CH}_2 + \text{R}_2\text{CH}=\text{CH}_2 \xrightarrow{\text{catalyst}} \text{R}_1\text{CH}=\text{CH}\text{R}_2 + \text{CH}_2=\text{CH}_2$

• Ring-closing metathesis forms cyclic alkenes.

Mechanism involves [2+2] cycloaddition and cycloreversion steps.

Summary Table: Key Reactions and Properties

Reaction/Property	Description
Amines	Classified as 1°, 2°, 3°, aromatic; basicity depends on structure
Diazotization	Primary aromatic amines form diazonium salts
Hofmann/Cope Elimination	Produce less substituted alkenes; syn stereoselective (Cope)
Heck Reaction	Pd-catalyzed coupling of aryl halides and alkenes
Cross Coupling	Suzuki, Stille, Negishi reactions for C–C bond formation
Alkene Metathesis	Exchange of alkene fragments; ring-closing metathesis

Additional info: These notes cover advanced topics in amine chemistry and organometallic C–C bond formation, including mechanisms, selectivity, and synthetic applications relevant for college-level organic chemistry.