Chemistry Reference
In-Depth Information
Four possible transmetallation scenarios were considered: (a) the base plays
no role, (b) the base reacts initially with the boronic acid, (c) the base reacts
initially with the palladium(II) or (d) the base reacts with both the boronic
acid and palladium(II) (Figure 8.3).
22
The electrochemical data were interpreted to reveal that the only reaction
that proceeds at a sucient rate to allow catalytic competency under SM
coupling conditions is that between the oxo-palladium species and the
neutral boronic acid. Moreover, in contrast to the high (undetected) barrier
to palladium halide hydrolysis found computationally by Maseras and co-
workers, experimentally the oxo-palladium species was found to be rapidly
generated.
43
Although the trihydroxyborate species was also found to react
with the halide complex, in the presence of an excess of bromide to ensure
the concentration of oxo-palladium species was kept low, the rates were very
low indeed. Overall, it could be concluded that the formation of the aryl
trihydroxyborate is detrimental to the reaction rate. An analogous investi-
gation was performed on the effect of fluoride in the system under an-
hydrous conditions. The study verified that fluoride exhibited similar
activation pathways to hydroxide,
46
which is highly informative for SM
coupling systems that are undertaken using KF as base under nominally dry
conditions.
The rates of stoichiometric transmetallation between the halide complex
[PdXAr(PPh
3
)
2
] and aryl trihydroxyborate (boronate pathway), and also be-
tween the oxo-palladium and boronic acid (oxo-palladium pathway), were
determined by Carrow and Hartwig, by way of
31
P NMR measurements made
at low temperatures (-30 to -55 1C).
47
There was a rate differential of around
four orders of magnitude between transmetallation via the boronate path-
way (trihydroxyborate and the bromide complex) versus that via the oxo-
palladium pathway (neutral boronic acid and the oxo-palladium complex),
the latter being the faster. Crucially, studies of the equilibrium between the
bromide complex and the oxo-palladium complex confirmed that su
cient
populations of the latter are present for the dominant catalytic flux to
proceed via the oxo-palladium pathway.
No base
Boronate
(a)
(b)
L
L
Ar'
Pd
Br
Ar'
Pd
Br
OH
L
L
OH
OH
OH
OH
Ar
B
Ar
B
Oxo-palladium
Oxo-palladium + boronate
(c)
(d)
L
L
H
Ar'
Pd
O
Ar'
Pd
OH
OH
OH
L
L
OH
Ar
B
OH
OH
Ar
B
Figure 8.3 Four transmetallation scenarios considered by Amatore et al.
22
Search WWH ::
Custom Search