Chemistry Reference
In-Depth Information
O
X
CO
CO
O
R
O
O
N
CO
N
CO
R
Ph
Y
Mo
Mo
NH
NHCOPh
X
N
Y
N
Ph
Ph
Ar
O
O
Ph
Ph
R
X
B
C
Azlactone: X = O, Y = N
4-Oxazolones: X = N, Y = O
Y
Ph
Scheme 8B.57.
Mechanistic proposal for the allylation of oxazolones.
Me
3
SiO
O
Me
3
SiO
O
OSiMe
3
OSiMe
3
OSiMe
3
R
1
R
2
MeO
OMe
OMe
NR
2
OMe
R
R
R
O
R
1
OR
R
O
O
Figure 8B.28.
Nucleophiles used in Mo(II) - catalyzed allylations.
react as nucleophiles as well. But with these nucleophiles, the Mo(II) catalyst probably
acts as a Lewis acid catalyst, giving a Friedel-Crafts-type alkylation rather than a metal
template - controlled process [212c] .
Very recently, Trost and Zhang described the fi rst asymmetric allylation of 3-alkyl-
ated oxindoles (Table 8B.40). Optimization of the reaction parameters revealed that the
counterion of the base had the largest effect on the enantioselectivity. Lithium bases
gave the best results with unsubstituted allyl carbonates, and the best conversion was
observed in the presence of 2 equiv of LiO
t -
Bu or a mixture of LiO
t -
Bu and LHMDS.
In nearly all cases, excellent yields and enantioselectivities were obtained. A wide range
of substituents on the indol ring are tolerated, and increasing the sterical size of the
3-substituent results in higher ees (entries 3-5) [237]. Cinnamyl carbonates and related
aryl-substituted substrates give good yields and selectivities in the presence of NaO
t -
Bu
(entries 6 - 9) [238] .
8B.4.2. Tungsten - Catalyzed Allylic Alkylations
Based on the great similarity of the group VI metals, Mo and W complexes were inves-
tigated together in most cases
.
[211,212b,e] .
8B.4.2.1. Fundamentals of W-Catalyzed Allylic Substitutions
Like Mo complexes,
the W complexes also give preferential rise to the branched products [239]. According
to Trost and Hung, W complexes are less reactive than the corresponding Mo analogues
[240]. No reaction was observed between dimethyl sodiomalonate and allylacetate in the
presence of W(CO)
6
, while (MeCN)
3
W(CO)
3
led to slow conversion (31% in 16 h).
