Chemistry Reference
In-Depth Information
L*Pd
R
R'
R
R'
R'
R
PdL*
R
R
X
R
[PdL
n
]
Nu
R
R'
∗
or
Rotation
π
-
σ
-
π
Nu
R
R
R
R'
R
R
R'
R'
L*Pd
X
R
PdL*
R
Scheme 8B.20.
Isomerization of geminally disubstituted π - allyl complexes.
early studied in the development of the allylic substitution and will be briefl y discussed
at the end of this chapter.
In a formal sense, regioselectivity can be enhanced by performing the allylic substitu-
tion as an intramolecular reaction, that is, the preferred ring size can direct the nucleo-
philic attack to the sterically more hindered position [105,106]. However, with Pd
catalysts, results of intramolecular alkylations were not satisfactory so far.
8B.2.2.3.1. Allylic Alkylation via Monosubstituted (π - Allyl) Complexes
Monosubsti-
tuted allylic acetates or carbonates (Scheme 8B.21) are particularly suited for applica-
tions in organic synthesis because many linear allylic alcohols, the standard precursors,
are commercially available. A promising solution of the regioselectivity problem
has been found by a team led by Dai and Hou at the Shanghai Institute of Organic
Chemistry.
R'
Pd L*
MeO
2
C
CO
2
Me
R
R
X
Ph
R'CH(CO
2
Me)
2
/
base
Pd
0
/L
*
b
or
X
PdL*
CO
2
Me
R
Ph
R'
CO
2
Me
R
l
Scheme 8B.21.
Asymmetric allylic alkylations via terminally monosubstituted (
π
- allyl)Pd
complexes.
Results are given in Table 8B.11. For comparison, a reaction catalyzed by Pd/PPh
3
is described in entry 1. A ratio PPh
3
/Pd = 2:1 is required for the allylation to proceed
smoothly, yielding preferentially the linear product (cf. Scheme 8B.21). Upon use of a
1:1 ratio, conversion was low (60%). Early efforts for improvement were only partially
successful with substrates R = Ar and failed for substrates with R = alkyl. Pfaltz et al.
argued that a shift from an S
N
2 - type mechanism (
A
) into the direction of an S
N
1 - type
mechanism (
B
) should enhance preference for the branched product (Scheme 8B.22).
Assuming that transition state
B
would be favored by an electron-poor ligand,
