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O
O
N
N
A rB(OH)
2
N
N
N
N
Pd
Pd
Ar
O
O
monomer, from spontaneous
dissociation of the dimer
(HO)
2
B
recycle of
catalyst
O
N
N
N
products
P
d
high
ee
Ar
O
B( OH)
2
Scheme 9.148 Ligand design for enantioselective deborylative arylation - improving
control over the coordination shell.
ArB (OH)
2
Pd(OCOCF
3
)
2
(5 mol% )
BQ, dioxane, 50°C, 2 h
NMe
2
Ar
NMe
2
O
Ar
O
ArPd
+
M
e
2
N
Ar B(OH)
2
Pd
Ar
O
Scheme 9.149 Diarylation via tandem carbopalladation-cross-coupling.
matters for the ligand-restricted (here, enantioselective) pathway to operate.
In this case, both the original ligand and all intermediates are neutral,
bearing two s-bonded anionic ligands. The most serious restriction of this
outstanding system is the low material yield of the arylation product, in
other words, a low TON value, which is likely to be associated with problems
in recycling the reactive form of the catalyst, as the O-sidearm may not get
rid of borate and thus the third coordination location loses the ability to
recoil.
The carbopalladation intermediate can undergo cross-coupling with ex-
cess of organometallic reagent. Such a tandem process takes place readily
with more reactive organotin compounds (see below), but with organoboron
compounds no instances were known until recently. Cross-coupling with
boronic acids usually requires the addition of a hard Lewis base to activate
the C-B bond towards transmetallation and PdH elimination is therefore
faster than transmetallation under the usual conditions of oxidative Heck
reactions. Larhed and co-workers found an interesting class of olefins
bearing a pendant moiety with a basic NMe
2
group, which undergo diary-
lation (Scheme 9.149).
408
The role of the pendant moiety is probably to
stabilize the carbopalladation intermediate by chelation, increasing its life-
time to match it with the rate of transmetallation. The other probable
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