Hydrogenation

Two forms of Hydrogenation :

1. Heterogeneous Catalytic Hydrogenation

2. Homogeneous Catalytic Hydrogenation


1) Heterogeneous Catalytic Hydrogenation

- Catalyst insoluble in reaction medium.

- Reactions take place on catalyst surface.

- Rate of reaction and selectivity dependant on active sites on surface.

- Active sites are the part of the catalyst substrate and hydrogen can adsorb on it.

- By blocking or poisoning active sites the reactivity of a catalyst is reduced and the selectivity increased.

1.1) Hydrogenation of Carbon-Carbon Double Bonds:

Hydrogenation of carbon-carbon double bonds is frequently carried out in the presence of a heterogenous metal catalyst and generally proceeds under mild conditions.

Selective reduction of a double bond in the presence of other unsaturated groups is usually possible, except when the compound contains triple bonds, nitro groups, or an acyl halide.

The mechanism of heterogenous hydrogenation involves :

(a) dissociative chemisorption of H2 on the catalyst,

(b) coordination of the alkene to the surface of the catalyst, and

(c) addition of the two hydrogen atoms to the activated π-bond in a synmanner.

1.1.1) Catalyst Selection

For low-pressure hydrogenations (1-30 atm), Pt, Pd, Rh, and Ru are used.

The reactivity of a given catalyst decreases in the following order:

Pt > Pd > Rh - Ru > Ni.

For high-pressure hydrogenations (100-300 atm), Ni is usually the metal of choice.

Platinum, prepared by reduction of PtO, (Adams catalyst) with H2 is pyrophoric.

Usually 0.1-1% of the catalyst is employed.

A more convenient procedure for the preparation of Pt is by reduction of chloroplatinic acid with NaBH, in ethan.

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Niclcel is used for high-pressure hydrogenations.

Supported-Ni catalysts such as Rnney-Ni and Ni-Boride are employed for hydrogenolysis of the C-S bonds in thioaceta.

Ni,B, which catalyzes the regioselective 1,4-reduction of α,β-unsaturated aldehydes and ketones.

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1.1.2) Substrate Reactivity

The reactivity of α,β-unsaturated substrates decreases in the following order:

RCOCl > RNO, > RC-CR > RCH=CHR > RCHO > RC=N > RCOR > benzene.

Both Pt and Pd catalysts fail to reduce RCOOR', RCOOH, and RCONH, groups.

Pd is usually more selective than Pt.

The ease of reduction of an olefin decreases with increasing substitution of the double bond.

Conjugation of a double bond with a carbonyl group can markedly increase the rate of hydrogenation of the double bond.

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Order of reactivity in Isolated double bond:

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1.1.3) Stereochemistry

In general, hydrogenation takes place by a syn-addition of hydrogen to the less hindered side of the double bond.

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1.1.4) Hydrogenolysis

With Pt and Pd catalysts, hydrogenation of allylic and benzylic alcohols, ethers, esters, amines, and halides is often accompanied by hydrogenolysis of the C-X bond where X = OH, OR, OAc, NR, or halide, respectively.

Rh catalysts are particularly useful for hydrogenations when concomitant hydrogenolysis of an oxygen function is to be avoided.

Divalent sulfur, Hg, and to a lesser degree, amines poison hydrogenation catalysts

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1.1.5) Heteroatom Hydrogenations Carbonyl Moiety

- Can be hydrogenated.

- Platinum reagents preferred as C=O faster than C=C (vide supra) especially when poisoned

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Order of Carbonyl Reduction:

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1.1.6) Hydrogenation of Carbon-Carbon Triple Bonds

Lindlar catalyst (Pd / CaCO3 / PbO): optimum catalyst to prevent over-reduction and gives cis isomer exclusively

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1.1.7) Hydrogenation of Carbon-Nitrogen Triple Bonds (Nitrile)

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1.1.8) Hydrogenation of Nitro Group

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1.1.9) Hydrogenation of Azide Group

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2) Homogeneous Catalytic Hydrogenation

- Soluble in reaction medium.

- Mechanisms much better understood.

Advantages :

- mild conditions (non-polar solvents which dissolve H2 better).

- less catalyst required (each molecule is available for reaction and not just surface).

- improved or complimentary selectivity (far more predictable).

- directed hydrogenations.

- asymmetric hydrogenations

2.1) Hydrogenation of Carbon-Carbon Double Bonds

The complex RhCl(PPh3)3 (also known as Wilkinson’s catalyst) became the first highly active homogeneous hydrogenation catalyst that compared in rates with heterogeneous counterparts.

The Rh complex is readily prepared by heating rhodium chloride with excess triphenylphosphine in ethanol.

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Characteristic features of this Rh-catalyst include

(a) hydrogenation of double bonds via syn-addition of H2

(b) cis-double bonds are hydrogenated faster thantrans-double bonds,

(c) terminal double bonds are hydrogenated more rapidly than more substituted double bonds,

(d) less isomerization of double bonds,

(e) little hydrogenolysis of allylic or benzylic ethers and amines,

(f) R-C≡N, R-NO2, R-Cl, RCOOH, RCOOR', and R,C=O are not reduced.

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2.2) Order of reactivity in Isolated double bond

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Like heterogeneous catalysts there is a strong steric selectivity for the least hindered alkenes

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2.3) Stereochemistry

As indicated in the mechanism reductive elimination is fast so no isomerisation can occur and syn addition results

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Like heterogeneous catalysts, hydrogenation occurs from the least hindered face

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Cis-disubstituted C=C react faster than trans-disubstituted C=C:

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2.4) Functional Group Compatibilty

Compatible with most functional groups wilkinson's catalyst having strong affinity toward's carbon monoxide so only Aldehydes often undergo decarbonylation

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2.5) Directed Hydrogenation

Intramolecular H-bonding or chelation by an adjacent functionality, such as a hydroxyl group, can direct the approach of a metal catalyst to favor hydrogenation of one diastereotopic n-face over another.

The most effective catalysts for directed hydrogenation are the coordinatively unsaturated Crabtree's catalyst" and the 2,5-norbornadiene-Rh(1) catalyst shown below :

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In the case of cyclic unsaturated alcohols, face-selective hydrogenation occurs when the hydroxyl group binds to the Ir during hydrogenation of the double bond

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2.6) Asymmetric Hydrogenation

Many chiral phosphorus-based auxiliary ligands are available for transition metals in asymmetric, catalytic, homogeneous reductions of alkenes.

Particularly noteworthy are the diphosphine ligands DTPAMP, developed by William S, Knowles and BINAP, developed by Ryoji Noyori.

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BINAP is available as either the (+) or (-)-enantiomer and displays broad utility in rhodium- and ruthenium-catalyzed asymmetric hydrogenations of β-keto esters and alkenes.

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