Boron Reagent Activation in Suzuki–Miyaura Coupling - New Trends in Cross-Coupling: Theory and Applications - page 343

Ar-H
AlCl 3 , NR 3
B(cat)Cl

"Electrophilic
borylation"

[M] cat + HB(OR) 2
or
HB(cyclohex) 2 + HB(OR) 2

[Pd] + KOAc + B 2 pin 2
or
[Ir] + B 2 pin 2

R

OR

R

or B

R OR

"Hydroboration" "Miyaura Borylation" R

R

t -BuONO
B 2 pin 2

"Sandmeyer
borylation"

Ar-NH 2

Scheme 8.23 Selected methods for the preparation of boronic esters.

enantioselectively, 103-105 with high regio-control for the unusual
Markovnikov addition 106 and to give products from trans-hydroboration
rather than the usual cis route. 107,108 The second major method used is a
palladium-catalysed borylation of aryl halides 109,110 or alkenyl halides, 111 i.e.
the Miyaura borylation. Iridium complexes can catalyse the borylation of
arenes 112 and alkanes 113,114 and rhodium can also catalyse the borylation of
arenes. 115 These reactions are conducted under remarkably mild conditions
that made it very functional group tolerant. Boronic esters can also be pre-
pared from arylamines using a recently developed method based on the
Sandmeyer reaction 116,117 or via an electrophilic arene borylation protocol
that employs a very strong Lewis base (Scheme 8.23). 118,119
It is unclear at present what the precise active transmetallating species is
during the SM coupling of boronic esters. Boronic esters have the capacity to
behave analogously to boronic acids, whereby they transmetallate directly
with an oxo-palladium species or become activated by hydroxide to undergo
transmetallation via the boronate pathway. Alternatively, boronic esters
could undergo complete or partial hydrolysis to form a more reactive species
that can also transmetallate via the oxo-palladium or boronate pathways
(Scheme 8.24). A complete mechanistic investigation to elucidate this
aspect has not yet been reported. Carrow and Hartwig, however, demon-
strated that catechol-, neopentyl- and pinacolboronic esters react rapidly,
even at -55 1C, with a stoichiometric quantity of oxo-palladium species,
generated in situ from the bridged hydroxy-palladium dimer and PPh 3 . 47
Kinetic data showed that the catechol- and neopentylboronic esters reacted
at a similar rate to the boronic acid but the bulky pinacolboronic ester was
the slowest. However, all three reacted faster than the trihydroxyborate and a
palladium halide complex, for which no reaction was detected over the same
period even at -30 1C. It was therefore concluded that these boronic esters
transmetallate via the oxo-palladium pathway. Measuring rates of hydrolysis
and rates of transmetallation for the boronate species of a wide range of
boronic esters would be a valuable addition in this area.
Pinacolboronic esters can be coupled under nominally anhydrous con-
ditions. 120 However, the presence of trace amounts of adventitious water, or
indeed small proportions of deliberately added water, 89,121 a common