Boron Reagent Activation in Suzuki–Miyaura Coupling - New Trends in Cross-Coupling: Theory and Applications - page 327

8.3 Direct Transmetallation of Boron Reagents
The mechanism of transmetallation, and the preceding activation of the
boron reagent if required, has been studied in less detail than that of
the oxidative addition or reductive elimination events. This is possibly due to
the increased complexity involved in identifying, characterizing and
measuring rates of reaction of the possible palladium(II) intermediates.
Additionally, it is not a straightforward task to identify the active boron
species, as this is often in equilibrium with inactive species and may thus be
present in only low concentration. As a general class of reagent, organo-
boranes and boronic acids both undergo direct transmetallation, sometimes
after the addition of a Lewis base. In other words, no ligands at boron need
to be substituted prior to the delivery of the organic component to the Pd.
This direct activation renders them distinct from other classes of boron
reagent that can require the exchange of ligands on boron to occur in order
to transmetallate effectively.
8.3.1 Organoboranes
As noted above, simple organoboranes were the first boron reagents to be
employed for SM coupling, due in part to their ease in preparation by
hydroboration. 23-29 The 9-BBN and disiamylboranes are the most common
motifs on boron, as the secondary carbons that are appended to boron
provide the differentiation required to achieve the selective delivery of the
primary alkyl or alkenyl group to palladium in the transmetallation step.
Dialkylboranes (HBR 2 ) readily add to unsaturated carbon linkages with syn
selectivity and in an anti-Markovnikov manner to furnish the requisite
organoborane for SM coupling. The reagent is often used immediately in the
SM coupling as triorganoboranes can readily degrade over time.
Early mechanistic studies focused on organoboranes, as these regularly
featured in SM coupling reactions. Dimeric hydroxide-bridged palladium
complexes and monomeric hydroxide-ligated palladium complexes had
been characterized, 30 and it was proposed that hydroxide base provided a
m-bridged hydroxide to link between the boron and palladium in a meta-
thetical-like transition state. 31 However, there has been considerable debate
regarding the sequence of events that precede the assembly of this transition
state, specifically, whether the hydroxide activates the borane (I, Scheme 8.4)
through coordination to give a more nucleophilic boronate species (Boronate
pathway), or activates palladium (II, Scheme 8.4), via exchange of the
halide on the palladium(II) centre to generate an oxo-palladium species (oxo-
palladium pathway). 32
In early work, Suzuki and Miyaura conducted mechanistic studies on the
coupling of bromoalkenes with alkenylboranes employing alkoxide bases.
Palladium tetrakis(triphenylphosphine) catalysed coupling of a tetra-
alkylated borane ''ate'' complex (Li[BR 3 -alkenyl]) with a styrenyl bromide
afforded the cross-coupled product in only 9% yield, and this was interpreted