chemistry

Hydrogenation of carbon–nitrogen double bonds

Hydrogenation of carbon–nitrogen double bonds is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Hydrogenation of carbon–nitrogen double bonds rather than just read about it. In short: In chemistry, the hydrogenation of carbon–nitrogen double bonds is the addition of the elements of dihydrogen (H2) across a carbon–nitrogen double bond, forming amines or amine derivatives. Although a variety of general methods have been developed for the enantioselective hydrogenation of ketones, methods for the hydrogenation of carbon–nitrogen double bonds are less general.

Key takeaways

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Reference excerpt

In chemistry, the hydrogenation of carbon–nitrogen double bonds is the addition of the elements of dihydrogen (H2) across a carbon–nitrogen double bond, forming amines or amine derivatives. Although a variety of general methods have been developed for the enantioselective hydrogenation of ketones, methods for the hydrogenation of carbon–nitrogen double bonds are less general. Hydrogenation of imines is complicated by both syn/anti isomerization and tautomerization to enamines, which may be hydrogenated with low enantioselectivity in the presence of a chiral catalyst. Additionally, the substituent attached to nitrogen affects both the reactivity and spatial properties of the imine, complicating the development of a general catalyst system for imine hydrogenation. Despite these challenges, methods have been developed that address particular substrate classes, such as N-aryl, N-alkyl, and endocyclic imines. If the complex is chiral and non-racemic and the substrate is prochiral, an excess of a single enantiomer of a chiral product can result.

Mechanism and stereochemistry Hydrogen for the reduction of C=N double bond can either be provided by hydrogen gas (H2) or transferred from sources of H2, such as alcohols and formic acid. The process is usually catalyzed by transition metal complexes. For metal catalyzed reactions, the transfer of H2 to the imine can proceed by either inner sphere or outer sphere mechanisms.

Inner sphere mechanisms

Relevant to the inner sphere mechanism are the two modes by which imines can coordinate, as a π or as a σ-donor ligand. The pi-imines are also susceptible to conversion to iminium ligands upon N-protonation. The binding mode for the imine is unclear, both η1 (σ-type) and η2 (π-type). The final step in the mechanism is release of the amine. In some iridium-catalyzed hydrogenations, the mechanism is believed to proceed via a monohydride species. The oxidation state of iridium is always +3. Examples:

Outer sphere mechanisms

Ruthenium(II) complexes of amine ligands are known for engaging in the outer-sphere mechanism, during which the imine/iminium substrate does not bind to the metal center directly. Instead, substrate receives the elements of H2 by interaction with Ru-H and N-H sites. This process is utilized by the Shvo catalyst and many ruthenium amine complexes. One such complex is Baratta's catalyst RuCl2(PPh3)2(ampy) (ampy = 2-picolylamine) for transfer hydrogenation.

Metal-free hydrogenations Because the substituents attached to the imine nitrogen exert a profound influence on reactivity, few general catalyst systems exist for the enantioselective hydrogenation of imines and imine derivatives. However, catalyst systems have been developed that catalyze hydrogenation of particular classes of imines with high enantioselectivity and yield. This section describes some of these systems and is organized by the substitution pattern of the imine. α-Carboxy imines are attractive precursors for α-amino acids. Organocatalytic reduction of these substrates is possible using a Hantzsch ester and a chiral phosphoric acid catalyst.

Applications Imine hydrogenation provides a practical route to chiral amines. Metolachlor is the active ingredient in the widely used herbicide Dual Magnum. A key step in its industrial production involves the enantioselective reduction of an N-aryl imine. This reduction is achieved with extremely high turnover number (albeit moderate enantioselectivity) through the use of a specialized catalyst system consisting of [Ir(COD)Cl]2, modified Josiphos ligand 3, and acid and iodide additives.

Comparison with other methods Imines may be reduced enantioselectively using stoichiometric amounts of chiral metal hydrides. Such methods have the advantage that they are easy to implement. Reduction with hydrosilanes is a second alternative to transition-metal catalyzed hydrogenation.

References

Illustrations

Hydrogenation of carbon–nitrogen double bonds illustration

Worked examples

Example 1 — a first encounter with Hydrogenation of carbon–nitrogen double bonds

Start with the simplest possible case. Write down what Hydrogenation of carbon–nitrogen double bonds claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Hydrogenation of carbon–nitrogen double bonds before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Hydrogenation of carbon–nitrogen double bonds ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Hydrogenation of carbon–nitrogen double bonds

In research: Hydrogenation of carbon–nitrogen double bonds appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Hydrogenation of carbon–nitrogen double bonds in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Hydrogenation of carbon–nitrogen double bonds is common in secondary-school and first-year university syllabi. It links to neighbouring topics Hydrogenation, Organic reactions, so understanding it makes those chapters shorter.
In everyday life: Look for Hydrogenation of carbon–nitrogen double bonds outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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Frequently asked questions

What is Hydrogenation of carbon–nitrogen double bonds in simple terms?

Why does Hydrogenation of carbon–nitrogen double bonds matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Hydrogenation of carbon–nitrogen double bonds?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Hydrogenation of carbon–nitrogen double bonds.