Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 439

An interesting protocol using a ferrocenylimine C,N-palladacycle as pre-
catalyst for the oxidative arylation of standard olefins (styrenes, acrylates,
but also methacrylate, a single example) by arylboronic acids was reported by
Wu and co-workers. 398 The protocol is remarkable for a combination of
unusual features: (i) the extreme simplicity of the catalytic system, involving
only Pd precatalyst with a low loading (0.5 mol%); (ii) the use of oxygen as
the only oxidant; and (iii) the mildness of the conditions (50 1C, DMF)
(Scheme 9.134). The use of a palladacycle for the oxidative Heck reaction is
particularly notable.
The reaction was realized by Larhed and co-workers in a continuous-flow
reactor, which allows for very fast mixing and heating of the reagents, re-
sulting in a dramatic shortening of the reaction time, and provides protocols
that are easily scalable and reproducible, with both standard (Scheme 9.135)
and electron-rich olefins (Scheme 9.136). 66
Depending on the nature of the palladium precatalyst, deborylative Heck
reactions can follow both polar and neutral pathways. In many cases,
deborylative reactions are subject to the same modes of regioselectivity

Ph

(33)

(0.5 mol%)
O 2, DMF, 50°C, 3 h

B(OH) 2 Ph

95%

N

Pd

Cl

Fe

2

(24)

Scheme 9.134 Deborylative Heck reaction in the presence of a palladacyclic pre-
catalyst (24).

CO 2 Bu

B(OH) 2 CO 2 Bu

Pd(OAc) 2 (2 mol%)
dppp, B Q, DMF
continuous flow
130 °C, 5 min

Br Br 89%

Scheme 9.135 Example of deborylative arylation of acrylate in a continuous flow
reactor.

OBu

B(O H) 2

Pd (OAc) 2 (2 mol%)
dppp, BQ , DMF
continuous flow
130°C, 5 min

O

CbzHN CbzHN

82%

Scheme 9.136 Example of deborylative arylation of electron-rich olefins in a con-
tinuous flow reactor.