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Tandem carbonylation reaction
Monsanto Process (Acetic acid Synthesis):
150-200oc, 1-40 atm
Ref: BASF process: cobalt-based high pressure process (200-250oC, 500-700 atm)
Rh cat./ICH3OH +
CO
CH3COOH
o
180 C/ 30 bar
CH 3
CO
I
Rh
CO
I
I
-
CH 3I
CH 3OH
CO
Rh
CO
I
I
HI
H2O
O
CH 3COOH
I
-
O
CH 3
-
I
CH 3
Rh
CO
I
I
O
I
I
CH 3
Rh
OC I CO
-
CO
8.3 Reactions Involving Organonickel Compounds
Allylic halides react with nickel carbonyl, Ni(CO)4 to give p-allyl complexes.
These reactions are believed to involve Ni(I) and Ni(III) intermediates in a
chain 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 preparation
of large rings.
Nickel carbonyl is an extremely toxic compound, and a number of other nickel
reagents 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 which
occurs via a diaryl Ni(III) intermediates and (2) the oxidative addition which
involves 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 cross
coupling is that the nickel reaction can be more readily applied to saturated
alkyl groups because of a reduced tendency for b-elimination.
The synthesis of cyclophane-type structures by use of dihaloarenes and
Grignard reagents from a,w-dihalides.
When secondary Grignard reagents are used, the coupling product sometimes
is derived from the corresponding primary alkyl group. This transformation can
occur by reversible formation.
Styrene serves to stabilize the active catalytic species, and among the styrene
derivatives, m-trifluoromethylstyrene was the best.
The main advantage of nickel is that it reacts more readily with arylchlorides and
methanesulfonates than does the Pd system. These reactants may be more
economical than iodides or triflates in large-scale synthesis.
Vinyl phosphates can be used, and these are in some cases more readily
obtained 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 of
the 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 alkane
by 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 decarbonylation
of aldehyde and acyl chlorides.
The use of cobalt for synthetic purpose is quite limited. Vinyl bromides and
idodides couple with Grignard reagents in good yields, but a good donor
solvent such as NMP or DMPU is required as a cocatalyst.
8.5 Organometallic Compounds with p bonding
Among the classes of organic compounds that serve as p ligands are alkene,
allyl, dienes, cyclopentadiene anion, and aromatic compounds.
The reactivity depends on the following factors
a) The number of electrons that can be accommodated by the metal orbitals
b) the oxidation level of the metal,
c) the electron character of other ligands on the metal
Both thermal and photochemical reactions are used.
P-allyl complexes of nickel can be prepared either by oxidative addition on
Ni(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 p-organometallic compounds is ferrocene.
The molecules behave as an electron-rich aromatic system, and electrophilic
substitution reactions occur readily. Reagents that are relatively strong oxidizing
agents, such as the halogens, effect oxidation at iron and destroy the compound.
Effective Atomic Number: 18
One of the most useful types of p-complexes of aromatic compounds from the
synthetic point of view are chromium complexes obtained by heating benzene
or other aromatics with Cr(CO)6.
The Cr(CO)3 unit is strongly electron-withdrawing and activates the ring to
nucleophilic attack.
oxidize
Existing substituent groups such as CH3, OCH3, and +NMe3 exert a directive
effect, often resulting in a major amount of the meta substitution product.