Boron Reagent Activation in Suzuki–Miyaura Coupling - New Trends in Cross-Coupling: Theory and Applications - page 337

N 2

[Pd II ] +

BF 4 -

Ar

Ar-N 2 BF 4

Ar'-BF 3 -

Pd II

Ar F

[Pd 0 ]

Ar' B F

via

F

BF 4 - + BF 3

[Pd II ]

Ar Ar'

Ar'-Ar

Scheme 8.14 Cationic palladium species in SM coupling with arenediazonium salts.

rapid that the benefit in reducing side reactions through ''slow release'' is
reduced. Equally, organotrifluoroborates that are very electron poor, e.g.
3-nitrophenyl, 3,5-trifluoromethylphenyl or alkynyltrifluoroborates, are
so stable that hydrolytic activation often will not occur suciently fast for
the cross-coupling. These species are therefore more likely to undergo direct
transmetallation with palladium(II), which requires higher temperatures
and catalyst loadings and longer reaction times. 63,70 The entire set of orga-
notrifluoroborate reagents were categorized into three classes based on
these observations. Class I reagents have a half-life of less than 1 h, class II
between 1 and 24 h and class II over 24 h. As Class II organotrifluoroborates
hydrolyse on the same time-scale as most SM couplings, the greatest benefits
of the slow-release mode of reactivity are harnessed. The other two classes do
not benefit from slow release; however, there are cases where fast release has
been shown to be beneficial. 71
The use of organotrifluoroborate salts in the SM coupling with arenedia-
zonium salts has been reported to be an ecient process under anhydrous
conditions. 72 This implies that no indirect activation is required and the
trifluoroborate undergoes direct transmetallation. These SM couplings gave
superior yields to the corresponding coupling with boronic acids, which is
intuitive considering that the cationic palladium complex formed reacts
more eciently with the anionic trifluoroborate species than the neutral
boronic acid 73 (Scheme 8.14). During the transmetallation or its precursor,
hydroxide has been shown to bridge B with Pd more proficiently than
fluoride, 65 hence arenediazonium salts in combination with a preformed
trihydroxyborate may be particularly effective partners for SM coupling.

8.4.2 MIDA Boronates
N-Methyliminodiacetic acid (MIDA) can displace water from arylboronic
acids to generate aryl MIDA boronates 74 (Scheme 8.15). Preparation of het-
eroaryl and ethynyl MIDA boronates can be achieved with organometallic
reagents, 75,76 and a transmetallation approach between vinyl TMS and BBr 3
with subsequent trapping by the bis-sodium salt of MIDA gives excellent
yields of the vinyl MIDA boronate. 77