Know More

Weblinks

 https://en.wikipedia.org/wiki/Reductions_with_metal_alkoxyaluminium_hydrides  https://en.wikipedia.org/wiki/Sodium_borohydride  https://en.wikipedia.org/wiki/Diisobutylaluminium_hydride

CHEMISTRY Paper 9: ORGANIC CHEMISTRY-III (Reaction Mechanism-2) Module17: Reduction by Metal hydrides – Part-II

Suggested readings

 Organic Chemistry by Clayden, Greeves, Warren and Wothers

 March’s Advanced Organic Chemistry, Reaction, Mechanism and structure by Michael B. Smith and Jerry March

 A Guidebook to Mechanism in Organic Chemistry, Sixth edition by Peter Sykes

CHEMISTRY Paper 9: ORGANIC CHEMISTRY-III (Reaction Mechanism-2) Module17: Reduction by Metal hydrides – Part-II

Glossary

C

Conjugate addition: Conjugate addition is the vinylogous counterpart of direct nucleophilic addition. A reacts with a α,β-unsaturated carbonyl compound in the β position. The negative charge carried by the nucleophile is now delocalized in the alkoxide anion and the α carbanion by resonance.

E

Electron donating group: In organic chemistry, an electron donating group (EDG) or electron releasing group (ERG) is an atom or functional group that donates some of its electron density into a conjugated π system via resonance or inductive electron withdrawal, thus making the π system more nucleophilic.

Electrophile: In chemistry, an electrophile (literally electron lover) is a reagent attracted to electrons. Electrophiles are positively charged or neutral species having vacant orbitals that are attracted to an electron rich centre.

L

Lactone: An organic compound containing an ester group —OCO— as part of a ring.

N CHEMISTRY Paper 9: ORGANIC CHEMISTRY-III (Reaction Mechanism-2) Module17: Reduction by Metal hydrides – Part-II

Nucleophile: A nucleophile is a chemical species that donates an electron pair to an electrophile to form a chemical bond in relation to a reaction. All molecules or with a free pair of electrons or at least one pi bond can act as . Because nucleophiles donate electrons, they are by definition Lewis bases.

Nucleophilic addition reaction: In organic chemistry, a nucleophilic addition reaction is an addition reaction where a with an electron-deficient or electrophilic double or triple bond, a π bond, reacts with electron-rich reactant, termed a nucleophile, with disappearance of the double bond and creation of two new single, or σ, bonds.

O

Oxidizing agent: An oxidizing agent is a chemical species that removes an electron from another species.

R

Reducing agent: A reducing agent (also called a reductant or reducer) is an element or compound that loses (or "donates") an electron to another chemical species in a chemical reaction. Since the reducing agent is losing electrons, it is said to have been oxidized.

S

Stereocenter: A stereocenter or stereogenic center is an atom bearing groups such that an interchanging of any two groups leads to a stereoisomer. The most common stereocenters are chiral centers (such as asymmetric carbon atoms) and the double-bonded carbon atoms in cis-trans .

History of metal hydrides

The first reported metal hydrido complex was H2Fe(CO)4, obtained by the low temperature protonation of an carbonyl anion. The next reported hydride complex was (C5H5)2ReH. In 1957, Chatt and Shaw described trans-PtHCl(PEt3)2 the first non-organometallic hydride (i.e., lacking a metal-carbon bond).

Iron tetracarbonyl hydride is the organometallic compound with the formula H2Fe(CO)4. Also known as tetracarbonyldihydridoiron, tetracarbonyldihydroiron, or iron tetracarbonyl CHEMISTRY Paper 9: ORGANIC CHEMISTRY-III (Reaction Mechanism-2) Module17: Reduction by Metal hydrides – Part-II

dihydride, this compound was the first metal hydride discovered. The complex is only stable at low temperatures and decomposes rapidly at temperatures above –20 °C

Iron tetracarbonyl hydride Iron tetracarbonyl hydride was originally produced by Hieber and Leutert, who developed a two-step process starting from iron pentacarbonyl:

− − 3− Fe(CO)5 + 2OH → HFe(CO)4 + HCO − + HFe(CO)4 + H → H2Fe(CO)4 Current procedures consist of treatment of iron pentacarbonyl with potassium and barium hydroxide to yield an orange solution. From this point in the reaction, ideal conditions consist of a cold dark environment, thus dubbing the method the "polar night 2− synthesis". This dark, cold environment stabilizes the dianion species Fe(CO)4 , which is light and temperature sensitive. The orange solution is then treated with sulfuric acid to protonate the anionic intermediate, giving the neutral product.

Structure and properties

In iron tetracarbonyl hydride the Fe(CO)4 group has C2v with a geometry intermediate between octahedral and tetrahedral. Viewed as an octahedral complex, the hydride are cis. Viewed as a tetrahedral Fe(CO)4 complex, the hydrides occupy adjacent faces of the tetrahedron. Although the structure of tetracarbonyliron with the hydrogen atoms bound as a single H2 has been proposed as an intermediate in some rearrangement reactions, the stable state for the compound has the two atoms as independent ligands.

Chemistry

Iron tetracarbonyl hydride undergoes rapid ligand substitutions. Upon warming, the complex liberates H2, giving trans tetracarbonyliron.

H2Fe(CO)4 + PPh3 → H2Fe(CO)3PPh3 CHEMISTRY Paper 9: ORGANIC CHEMISTRY-III (Reaction Mechanism-2) Module17: Reduction by Metal hydrides – Part-II

H2Fe(CO)3PPh3 → trans-Fe(CO)3PPh3 + H2

The hydrides in tetracarbonyldihydroiron have a pK1 of 6.8 and pK2 of 15. The monoanion itself has been reviewed extensively. The monoanion is an important intermediate in the water-gas shift reaction (WGSR). The slow step in the iron carbonyl-catalyzed WGSR is the proton transfer from water to the iron hydride anion.

− HFe(CO)4− + H2O → H2Fe(CO)4 + OH Another way the complex has been used is for cooperative bimetallic activation of CO2.

− Cp2MoCO2 + H2Fe(CO)4 → Cp2MoHCO+ HFe3(CO)11 + H2O

CHEMISTRY Paper 9: ORGANIC CHEMISTRY-III (Reaction Mechanism-2) Module17: Reduction by Metal hydrides – Part-II