Chemistry Reference
In-Depth Information
[Pd], DMF,
80
o
C, 14 hr
BX
n
Br
Ph
2
N
+
Br
Ph
2
N
Ph
2
N
BX
n
= B(OH)
2
- no additive = trace
- K
2
CO
3
= 33%
- CuCl = trace
BX
n
=
O
O
- no additive = 77%
- K
2
CO
3
= 84%
- CuCl = 88%
B
-
O
Scheme 8.31
SM coupling of both cyclic triolates and boronic acids with
dibromonaphthalene.
congested aryl systems. In contrast to the couplings outlined above
(Scheme 8.30), the addition of base (K
2
CO
3
, 2 equiv.) was found to be
beneficial, suggesting that prior hydrolytic activation may be necessary.
Interestingly, the triol unit can be tailored to provide the requisite physical
or chemical properties. For example, when the bridging carbon bears a
methyl group, the reagents are more soluble in organic solvents than orga-
notrifluoroborate salts, and when it bears a polar sulfonate group, the
solubility in aqueous solutions becomes appreciable.
132
8.6 Conclusion
Overall, it is concluded that the boron reagents used in SM coupling can be
categorized into three distinct groups (Figure 8.5). It must be emphasized
that such a categorization is subject to future mechanistic studies that will
undoubtedly reveal further information necessary to refine it.
For organoboranes, such as 9-BBN-borane and boronic acids, it is clear
that direct transmetallation can occur, either through reaction of the borane
or boronic acid with an oxo-palladium intermediate or by simple association
of hydroxide to form a boronate species that reacts with a palladium(II)
halide complex or a cationic palladium(II) complex. With their high Lewis
acidity and low steric hindrance around the boron, boronic acids are the
most reactive boron reagent for transmetallation in SM coupling.
The next class of reagents includes organotrifluoroborate salts, MIDA
boronates and DAN boronamides, where it is now established that in the
majority of cases prior activation must occur for transmetallation to proceed.
The reagents themselves are generally very unreactive towards palladium(II),
hence ligand exchange has to occur to prime them for ecient transme-
tallation by either the oxo-palladium or the boronate pathway. For the par-
ticular reagents detailed in this chapter, the activation is hydrolytic.
Organotrifluoroborate hydrolysis is catalysed by acid, but also proceeds to
completion under basic aqueous conditions due to the equilibrium being
driven toward the boronic acid by consumption of the liberated HF. Elec-
tron-poor aryltrifluoroborates and alkynyltrifluoroborates hydrolyse more
slowly than electron-rich aryltrifluoroborates and alkyltrifluoroborates. In
fact, there are instances where their hydrolysis is so slow that direct trans-
metallation occurs and therefore should be placed in the direct
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