Chemistry Reference
In-Depth Information
reacted with reverse regioselectivity and moderate enantioselectivity (entry 6). (
Z
) -
allylic substrates react with complete retention of the olefi n geometry. Interestingly,
other chiral ligands such as
L43a
are not suitable for W-catalyzed allylations, although
they give excellent results in the Mo-catalyzed process [211].
8B.5. IRON - AND RUTHENIUM - CATALYZED ALLYLIC ALKYLATIONS
8B.5.1. Iron - Catalyzed Allylic Alkylations
Iron is probably the cheapest and least toxic transition metal, which can be used for
cross-coupling reactions. Nevertheless, the Fe-catalyzed allylic alkylation is a rather
underdeveloped fi eld. First examples using stoichiometric amounts of (π - allyl)Fe com-
plexes were carried out by Whitesides and Arhardt [244], Pearson [245], and Nicholas
et al. [246]. Enders et al. investigated the steric course of iron-mediated allylic substitu-
tions proceeding via π - allyl - Fe complexes [247] . The reactions proceed under complete
retention of the confi guration via a double inversion mechanism, comparable to the Pd-
catalyzed reactions.
First catalytic processes were carried out by Roustan et al. [248], Ladoulis and Nicho-
las [249] , and Xu and Zhou [250] using iron - carbonyl - nitrosyl complexes. The reactions
were slow and required relatively large amounts of catalyst (up to 25%). An important
fi nding was that the reaction with the catalyst [Bu
4
N[Fe(CO)
3
NO]], developed by
Roustan, preferentially occurs at the carbon carrying the leaving group with very high
stereospecifi city, similar to Rh-catalyzed allylic substitutions.
Based on the early fi ndings, Plietker recently developed an improved version [31,251].
Plietker observed that the toxic CO atmosphere previously required can be avoided if
PPh
3
is used as additional ligand to stabilize the intermediate Fe complexes. Best results
were obtained with coordinating solvents such as DMF. Under optimized conditions the
reactions proceeded with excellent regioselectivity (rs), while the incoming nucleophile
is replacing the leaving group in a double S
N
2 ′ reaction (Scheme 8B.59 ).
OCO
2
i
-Bu
CH(CO
2
Me)
2
CH
2
(CO
2
Me)
2
[Bu
4
N][Fe(CO)
3
(NO)] (2.5 mol %)
PPh
3
,DMF,80°C,24h
98% rs (81%)
Scheme 8B.59.
Regioselective iron - catalyzed allylic alkylations.
Interestingly, a nearly 1:1 mixture of regioisomers was formed with acetonitrile as
solvent; this indicates a reaction mechanism different from that with DMF as solvent.
While the outcome of the reaction in DMF can best be explained via a (σ - allyl)Fe
complex, the reaction in CH
3
CN probably proceeds via a (π - allyl)Fe complex.
This dichotomy caused further detailed investigations and optimizations. If the PPh
3
is replaced by a
N
-heterocyclic carbene ligand (NHC-ligand), DMF as solvent can be
replaced by methyl
tert
-butyl ether (MTBE), which also allows the use of nonstabilized
nucleophiles such as azlactones [252]. Interestingly, the related NHC ligands
L51
and
L52
(Fig. 8B.29) induced a complete different reaction behavior.
