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O
Ac
O
A c
O
Pd(IPr)(OTs)
2
Cu( OTf )
2,
O
2
DMA, 40°C, 18 h
+
B
Ph
()
4
()
4
Ph
O
ee
99%
Scheme 9.142 Retention of configuration of a chiral centre sitting in the path of
probable a double-bond shift.
O
Pd(IPr)(OTs)
2
Cu(OTf)
2,
O
2
DMA, 40°C, 24 h
B
Ar
+
MeO
2
C
O
+
MeO
2
C
Ar
MeO
2
C
Ar
A
B
B:A=0.69-2.92
Scheme 9.143 Regioselectivity of arylation of methyl 3-butenoate by arylboronates
in the presence of a cationic Pd complex with heterocyclic carbene
ligand.
This work clearly showed the importance of a stable ancillary ligand in Pd
complexes with respect to both catalyst stability and regioselectivity. The
complex Pd(MeCN)
2
(OTs)
2
showed comparable activity, but much lower
regioselectivity. Apparently, labile ligands in the coordination shell give rise
to multiple variably ligated Pd species and thus multiple pathways can
be taken.
The suppression of a bond shift, however, is not a general phenomenon -
it is valid only if travel of the double bond along the chain must pass through
unsubstituted CH
2
links or a link bearing donor O or N atoms destabilizing
the emerging double bond. In these cases, the Z
3
-benzylic complex serves as
a thermodynamically favoured anchor for the double-bond migrations. On
the other hand, the arylation of methyl 3-butenoate gave mixtures of two
products in which the product with conserved double-bond position was the
minor isomer formed. The ratio was found to depend on the substituent in
the boronic acid aryl group - the amount of rearranged product was the
highest for phenyl boronate (B : A
¼
2.92), with both electron-donating and
electron-withdrawing substituents resulting in a loss of selectivity
(Scheme 9.143). In this case, both double-bond positions are thermo-
dynamically favoured and the product ratio obtained would depend on the
interplay of equilibria.
On the other hand, in other catalytic systems double-bond shifts take
place which can be employed for the introduction of remote aryl groups into
carbonyl compounds: in the arylation of unsaturated alcohols, the double
bond readily travels from its initial position to a hydroxyl group and converts
it into carbonyl, skipping 1-4 carbon atoms. Sigman and co-workers de-
veloped a regio- and enantioselective version of this process.
404
In the
presence of a chiral oxazoline ligand, the emerging tertiary carbon accepts
the chirality. High optical yields are generally obtained despite wide vari-
ations of both arylboronic acids and unsaturated alcohols (Scheme 9.144).
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