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

Allyl and diene complexes
Fe(C0} 5
-
Fe2 (C0}9
-
Fe 3 (COl 12
Fe(C0}
Fe(C0}4
-
3Fe(C0)4
4 + co
+ Fe(C0) 5
Fe(C0)4+ ~ - r-Fe(C0}4~ ~
n ot isoloted
Fe
(C0) 3
The mildest thermal conditions are achieved with Fe 2 (C0) 9 , which reacts on
refluxing in diethyl ether. In this case, however, only half ofthe iron is converted
into the complex. A very gentle method is to photolyse Fe(C0) 5 at - 78oC with
Z-cyclooctene in pentane, yielding the labile intermediate Fe(1] 2 -C 8 H 14 ) 2 (C0) 3 •
On treatment with a diene at room temperature in a hydrocarbon solvent,
cyclooctene is displaced, giving the 1]4-diene Fe(C0) 3 in high yield. Even styrenes
afford 1]4-derivatives under these conditions in which one double bond of the
aromatic system becomes coordinated.
Me
oA
Me
~
(C0} 3
Prolonged irradiation of (1]4-C4H6)Fe(C0) 3 with butadiene leads to (1]4-
C4H6)2Fe(CO).
The complexes of cyclic dienes, such as 1, 3-cyclohexadienes, are precursors to
1] 5 -cyclohexadienyltricarbonyliron salts, which have been applied in origanic
synthesis (p. 303). 1,4-Cyclohexadienes, available from the Birch reduction of
arenes, are converted into the 1, 3-diene complexes. The isomerization occurs
through a transient 1] 3 -allyliron hydride (p. 363).
6
OMe
Li/EINH
2 /
Bu 10H
(Birch reductionl
OMe
6
Fe(C0)
5
Bu~O/heat
OMe
6-F•(COI3 +
(j
OMe
f•(COI 3
1]4-Derivatives of 1, 3-dienes which possess functional groups such as -CH 2 0H,
-CHO, -COR and -COOR are prepared by similar methods.
(a) ATTACK OF ELECTROPHILES. (Diene)tricarbonyliron complexes are protonated
by strong acids. Addition of HCl gives [Fe(ry 3 -MeC 3 H4) (C0) 3 Cl]. With acids
containing weakly coordinating anions (e.g. CF 3 C02H, HS0 3 F or HBF4) the
situation is more complicated. 1 H n.m.r. studies indicate that protonation occurs
stereospecifically endo, that is from the same side of the diene as the metal,
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