Tandem carbonylation reaction

+ CORh cat./I-

180oC/ 30 barCH3COOHCH3OH

CH3OH CH3I

HI H2OI

RhCO

I CO

IRh

CO

I COI

CH3

-

-

IRh

I COI

-O

CH3

IRh

OC COI

-O

CH3

II

O

CH3CH3COOHCO

Monsanto Process (Acetic acid Synthesis): 150-200oc, 1-40 atmRef: BASF process: cobalt-based high pressure process (200-250oC, 500-700 atm)

8.3 Reactions Involving Organonickel Compounds

Allylic halides react with nickel carbonyl, Ni(CO)4 to give -allyl complexes.

These reactions are believed to involve Ni(I) and Ni(III) intermediates in achain process which is initiated by formation of a small amount of a Ni(I) sepcies.

This couplig reaction has been used intramolecularly to bring about cyclization of bis-allylic halides and was found useful in the preparationof large rings.

Nickel carbonyl is an extremely toxic compound, and a number of other nickelreagents with generally similar reactivity can be used in its place.

Mediun sized ring can be formed in intramolecular reaction.

The key aspects of the mechanism are (1) the reductive elimination whichoccurs via a diaryl Ni(III) intermediates and (2) the oxidative addition whichinvolves a Ni(I) species.

A soluble bis-phosphine complexes, Ni(dppe)2Cl2, is a particularly effective catalyst. The main distinction between this reaction and Pd-catalyzed crosscoupling is that the nickel reaction can be more readily applied to saturatedalkyl groups because of a reduced tendency for -elimination.

The synthesis of cyclophane-type structures by use of dihaloarenes andGrignard reagents from -dihalides.

When secondary Grignard reagents are used, the coupling product sometimesis derived from the corresponding primary alkyl group. This transformation canoccur by reversible formation.

Styrene serves to stabilize the active catalytic species, and among the styrenederivatives, m-trifluoromethylstyrene was the best.

The main advantage of nickel is that it reacts more readily with arylchlorides andmethanesulfonates than does the Pd system. These reactants may be moreeconomical than iodides or triflates in large-scale synthesis.

Vinyl phosphates can be used, and these are in some cases more readilyobtained and handled than vinyl triflates.

8.4 Reactions Involving Rhodium and Cobalt

Hydroformylation

The key steps in reaction are addition of hydridorhodium to the double bond ofthe alkene and migration of the alkyl group to the complexed carbon monoxide.

The acylrhodium intermediate is trapped by internal nucleophiles.

Fischer-Tropsch Process: reductive conversion of carbon monoxide to alkaneby reacting with hydrogen gas. Synthetic hydrocarbon fuels.(1923-1925)In1944, 600,000 ton/yr was produced. Since 1957 South Africa use this method, Sasol Process.

Under appropriate conditions, rhodium catalyst can be used for the decarbonylationof aldehyde and acyl chlorides.

The use of cobalt for synthetic purpose is quite limited. Vinyl bromides andidodides couple with Grignard reagents in good yields, but a good donorsolvent such as NMP or DMPU is required as a cocatalyst.

8.5 Organometallic Compounds with bonding

Among the classes of organic compounds that serve as ligands are alkene,allyl, dienes, cyclopentadiene anion, and aromatic compounds.

a) The number of electrons that can be accommodated by the metal orbitalsb) the oxidation level of the metal, c) the electron character of other ligands on the metal

The reactivity depends on the following factors

Both thermal and photochemical reactions are used.

-allyl complexes of nickel can be prepared either by oxidative addition onNi(0) or by transmetallation of a Ni(II) salt.

Oxidative decomposition

Trapping experiments

In 1956, Longuet and Orgel propose the complex compound.In 1959, Criegee isolated the complex.

One of the best known of the -organometallic compounds is ferrocene.

The molecules behave as an electron-rich aromatic system, and electrophilicsubstitution reactions occur readily. Reagents that are relatively strong oxidizingagents, such as the halogens, effect oxidation at iron and destroy the compound.

Effective Atomic Number: 18

One of the most useful types of -complexes of aromatic compounds from thesynthetic point of view are chromium complexes obtained by heating benzeneor other aromatics with Cr(CO)6.

The Cr(CO)3 unit is strongly electron-withdrawing and activates the ring tonucleophilic attack.

oxidize

Existing substituent groups such as CH3, OCH3, and +NMe3 exert a directiveeffect, often resulting in a major amount of the meta substitution product.