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-1-ORGANOMETALLICS IN SYNTHESIS:
CHROMIUM, IRON & COBALT REAGENTS
Introduction to Metal-Carbon Bonding
Organometallic chemistry involves the interaction of an organic compound with a transition metal
unit to form an intermediate complex. These may or may not be stable, but often have properties
and undergo reactions that are quite different to the parent organic compound.
Before we consider the properties of such complexes, think about the bonding involved between
a metal and an organic ligand.
The 18 electron rule:
• Derives from a desire to achieve a stable configuration – all bonding orbitals full.
• In order to assess the number of electrons contained within a complex, count electrons
on the free metal, add the electrons from the ligands, and consider the charges (see
Inorganic Organometallics Notes for details).
Details of M-C Bonding
CO Bonding
Chatt-Dewar-Duncanson Model: Carbon donates a lone pair of electrons to an empty orbital on
the metal, while the metal donates electrons from a filled orbital into antibonding CO orbitals. See
Inorganic Organometallics Notes for pictures. The result of this is that CO is very electron
withdrawing.
Bonding to π Ligands
A similar situation to the above occurs with these, e.g. alkenes (again, Inorganic
Organometallics Notes for pictures).
Arene-Chromium Tricarbonyl Chemistry
Formation of ArCr(CO)3 Complexes
Heating an aromatic complex with chromium hexacarbonyl in dibutyl ether results in the formation
of a complex between the aromatic ring and the transition metal. Electron-rich arenes complex
the best and strong electron-withdrawing groups (e.g. nitro, keto) are incompatible. Schlenk lines
should be used.
Moderately air
stable
Decomplexation occurs with an oxidant (O2 or CAN – (NH4)2Ce(NO3)6.
The key features of these complexes are:
1) Inductive – e-withdrawing.
2) Steric – size of tricarbonyl groups.
3) Stereochemistry – can be chiral – can influence stereoselectivity.
4) Neighbouring Group Participation – stability of benzylic cations.
These Notes are copyright Alex Moss 2003. They may be reproduced without need for permission.
www.alchemyst.f2o.org
, -2-
This diagram sums up all chromium-arene chemistry, and is brought about by the electron-
withdrawing nature of the Cr(CO)3 group.
Reactivity
Nucleophilic Aromatic Substitution of Aryl Halides
Although simple aryl halides are relatively inert to nucleophilic aromatic substitution, when
complexed to chromium, this reaction is greatly facilitated.
These substitution reactions all proceed by a two step process, involving nucleophilic attack of
the arene ring from the face opposite the metal, forming an anionic η5-cyclohexadienyl complex,
followed by rate-limiting loss of the halide from the endo side of the ring.
The displacement of halide by carbanions is limited to stabilised carbanions capable of adding
reversibly to the complexed arene. More reactive carbanions, such as 2-lithio-1,3-dithiane, attack
These Notes are copyright Alex Moss 2003. They may be reproduced without need for permission.
www.alchemyst.f2o.org
, -3-
the complexed arene ortho and meta to the halide, producing η5-cyclohexadienyl complexes
which cannot directly lose chloride, and furthermore, cannot rearrange to the η5-cyclohexadienyl
complex (which can). Stabilised carbanions also initially attack ortho and meta to chloride, but the
addition is reversible such that eventually attack at the halide-bearing position occurs, followed by
loss of halide (substitution).
Addition of Carbon Nucleophiles to Arenechromium Complexes
A range of stabilised carbanions attack from the face opposite the metal, producing anionic η5-
cyclohexadienyl complexes.
With substituted aromatics, under conditions of kinetic control, the regioselectivity is that expected
for nucleophilic aromatic substitution (meta to e-donating groups, para to acceptor groups) and
predicted by the assumption that the frontier MOs are arene-centred. Ortho addition occurs with
groups capable of coordinating the incoming nucleophile (e.g. imines). In contrast to benzene-
Cr(CO)3 complexes, for which MeLi and n-BuLi react by deprotonation, these reagents add to
phenylmethaneimine or similar. Note also that replacement of an electron-withdrawing CO ligand
with a ligand such as phosphine (e-donating) decreases the reactivity of arene complexes
towards nucleophilic attack.
Very reactive carbanions, such as dithiane lithium or phenyllithium, add irreversibly to the
complexed arene. The resulting anionic η5-cyclohexadienyl complexes are relatively stable and
can sometimes be isolated. Oxidation of these complexes generates the alkylated arene,
protonolysis generates mixtures of isomeric cyclohexadienes, and reaction with carbon
electrophiles produces trans disubstituted species. In contrast, stabilised carbanions add to
complexed arenes reversibly, and are always in equilibrium with the starting compounds.
Reaction with carbon electrophiles results in regeneration of the starting arene complex and
alkylation of the electrophile by the stabilised carbanion, as shown in the equation above.
Both the activating and the directing effects of complexation to chromium are synthetically useful.
For example, allows nucleophilic substitution at indoles, and permits reaction to occur at the
usually inaccessible C4 (or indeed C7 with a bulky substituent at C3).
Protolytic cleavage of the η5-cyclohexadienyl complexes resulting from alkylation of η6-
arenechromium tricarbonyl complexes produces cyclohexadienes rather than arenes.
These Notes are copyright Alex Moss 2003. They may be reproduced without need for permission.
www.alchemyst.f2o.org
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