Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 383

L

L L

Pd

Pd Pd

Cl

Cl Cl

H

Z Z

L

L

P d

Cl

Pd

Cl

Z

Z
sl ow fast

Z Z

normal product "rearranged" product

Scheme 9.36 Reversible addition-elimination of PdH in the rearrangement of an
alkenyl residue, the Curtin-Hammett regime.

NH t Bu

[PdCl 2 (cod)] (5 mo l%)
(t-B u) 3 P·HBF 4, LiCl, Cy 2 NMe
DMF, 10 0°C, 24 h

+

OP(O )(OPh) 2

O

NH t Bu

O

72%

Scheme 9.37 Example of alkenylation with rearrangement.

alkenylpalladium intermediate (Scheme 9.36). The selectivity in such cases is
known to obey Curtin-Hammett control (the other case of Curtin-Hammett
control in Mizoroki-Heck reactions is discussed in Ref. 87).
The result is a spectacular rearrangement applicable to a wide range
of bulky alkenyl

tosylates and phosphonates and various olefins

(Scheme 9.37).
Moreover, Skrydstrup and co-workers 152 showed that the rearrangement
can be controlled by a judicious choice of ligand. This is one of the rarest
cases in Heck reaction chemistry where such tight control over the course of
the reaction is established through a monodentate ligand. The use of the
bulky phosphine XPhos allowed the rearrangement to be blocked. In this
case, XPhos most likely interferes with the alkenyl ligand rotating and oc-
cupying the in-plane conformation in which Pd and H atoms are close to
each other to allow syn elimination (Scheme 9.38).
The result is a complete absence of rearrangement in practically the same
system as shown in Scheme 9.37 with only the ligand being changed to
XPhos (Scheme 9.39).
The application of the ligand-accelerated Heck reaction in enantioselec-
tive arylation/alkenylation was demonstrated by Datta and Larhed 155 using