_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Cyclohexadienyltricarbonyliron salts
Pyrrole forms a few complexes in which it is 1] 5 -bonded as C 4 H 4 N. These include
bis(pyrrolyl)iron, Fe(C 4 H 4 N) 2 , which is isoelectronic with ferrocene, and
pyrrolyltricarbonylmanganese.
o
N
H
9. 5 Cyclohexadienyltricarbonyliron salts
Cyclohexadienyl cations are well known as intermediates in the electrophilic
substitution of benzene. Indeed benzene is protonated by superacids to give
solutions containing the ion C 6 H;, which can be characterized spectroscopically.
These ions are, however. far too reactive tobe isolated as salts, and as they are
generated only in acidic media, their reactions with nucleophiles cannot be
studied. In contrast, cyclohexadienyltricarbonyliron hexafluorophosphate is a
stable yellow solid which can be stored indefinitely at room tempera ture and can
even be recrystallized from water. This is an especially striking example of the
stabilization of a reactive organic species by coordination to a transition element.
Birch reduction of arenes affords 1, 4-cyclohexadienes. On heating these with
Fe(C0) 5 in di-butyl ether, complexes ofthe isomeric 1. 3-dienes are obtained (see
p. 362). Treatment with Ph 1 C+pp; yields the salts. These salts react readily with
a wide range of nucleophiles. Attack on (1J 5 -C 6 H 7 )Fe(CO); is regiospecific,
occurring exclusively at a terminal carbon atom of the dienyl system. It is also
stereospecific, taking place from the exo side of the ligand, away from the metal.
The Fe(C0) 3 group can be removed cleanly by oxidation with trimethylamine
oxide in organic media, releasing a substituted cyclohexadiene.
o
Noi
o
n Fe(CO)
~ 3
QFe(C0) 3
1 Q
1
1
H Nu H Nu
303