Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 370

ArCOCl

ArX

Pd(0)

Pd (0)

B

A

B

Ar

C

ArPdX ArPdX

CO

Pd X

O

Z Z

A

B

Ar

ArPdX

Pd X

O

Z

Z

Z Ar Z

Ar

O

Scheme 9.24 Three pathways associated with carbonylative Heck reaction:
(A) decarbonylative Heck reaction (Blaser reaction); (B) carbonylative
Heck reaction; (C) regular Mizoroki-Heck reaction.

of CO. These equilibria are highly dynamic and their shift in one or the other
direction is controlled by concentration of CO in the reaction medium and
ancillary ligands. The competitive regular Mizoroki-Heck pathway is ir-
reversible and therefore usually wins the competition, making the real-
ization of the carbonylative Heck reaction a challenge with regard to the
formulation of the catalytic system and choice of proper stable ancillary
ligands.
The problem was recently solved by Beller and co-workers, who developed
the first general protocols for the carbonylative Heck reaction. The synthesis
of chalcones from aryl triflates and styrenes 107 is critically dependent on the
choice of ligand and conditions. The polar pathway suits this specific task
best - aryl triflates or nonaflates as substrates and the bidentate phosphine
ligand dppp secure a free coordination place needed for consecutive binding
of CO and olefin (Scheme 9.25). A slightly increased pressure of CO (2-10
bar) is used to promote the carbonylative process.
The protocol is applicable without modifications to a wide range of tri-
flates, including electron-rich and electron-deficient aryl derivatives and
enol esters (Scheme 9.26).
The method was subsequently extended to aryl iodides and bromides
through the use of Beller and co-workers' ligands, bulky and electron-rich
monophosphines (Scheme 9.27). 108-110 Bulky monophosphines of such a type
are believed to ensure that the coordination shell contains only a single stable
ancillary ligand (phosphine itself), thus effectively pursuing in the non-polar
pathway the same goal as the diphosphine dppp does in the polar pathway -
keeping two coordination sites available for catalytic transformation.
For the reactions involving bromoarenes and styrenes, Beller and co-
workers showed that the classical Heck catalytic system using PPh 3 and NEt 3
unexpectedly outperformed advanced phosphines. 111