Boron Reagent Activation in Suzuki–Miyaura Coupling - New Trends in Cross-Coupling: Theory and Applications - page 333

towards transmetallation with boronic acid. A similar mechanism is
proposed to be responsible for the greatly enhanced the rates of transme-
tallation reported when thallium bases are employed. 56 Addition of TlOH
leads to three orders of magnitude faster turnover than KOH and thus TlX
precipitation may encourage oxo-palladium formation. Alternatively, an
ecient pre-transmetallation to form a Tl-R intermediate may be viable. 8
When Tl 2 CO 3 is employed as base, it was suggested that decarboxylation
generated a cationic palladium centre, 57 which would be particularly reactive
towards a boronate species.

8.4 Boron Reagents Requiring Prior Activation
There are a number of boron reagents used in SM coupling that are known to
require activation, usually hydrolytic, in order to undergo transmetallation.
Ligand substitution has to occur as the parent reagent is stable towards
palladium(II) complexes. Examples of these are organotrifluoroborate salts,
MIDA boronates and 1,8-diaminonaphthylboronamides (DAN).

8.4.1 Organotrifluoroborate Salts
Potassium organotrifluoroborate (R-BF 3 K) salts were discovered in 1960 by
Chambers et al., 58 but the following three decades saw them appearing in
only a handful of publications. Since the mid-1990s, their application has
grown substantially-indeed, they have rapidly become a very widely used
class of organoboron reagent. They are readily prepared from the boronic
acid or ester, by reaction with either KHF 2 , 59 or a much less corrosive process
involving sequential treatment with KF and tartaric acid 60 (Scheme 8.10).
In contrast to boronic acids and esters, organotrifluoroborates are tetra-
hedral in geometry and not Lewis acidic, owing to the additional ligand
bound to the boron centre. This quaternization with exceptionally strong
B-F bonds gives them their favourable physical characteristics of being free-
flowing crystalline salts, which tend to melt and decompose only at very high
temperatures. In addition to being monomeric in nature, they are stable to
air and moisture in the solid state. These factors render them easy reagents
to handle, unlike, for example, small alkylboronic acids that can decompose
in air, 61 or pinacolboronic esters, many of which are liquids or low-melting
solids. In solution, the trifluoroborate moiety is stable under anhydrous
conditions, but when subjected to aqueous or protic media they hydrolyse,
via equilibrium, to form the corresponding boronic acid or ester. 49 Upon
hydrolysis, HF is formally liberated, which can cause etching of glassware

KHF 2
KF / tartaric acid

or

R-BX n R-BF 3 K

BX n = B(OH) 2 or Bpin

Scheme 8.10 Preparation of trifluoroborate salts from boronic acids or esters.