Carbohydrate-Derived Ligands in Asymmetric Heck Reactions - Carbohydrates: Tools for Stereoselective Synthesis - page 240

11

Carbohydrate-Derived Ligands in Asymmetric Heck Reactions
Montserrat Diéguez and Oscar Pàmies

11.1
Introduction

The asymmetric Mizoroki-Heck reaction, generally described as the palladium-
mediated coupling of aryl or vinyl halides or triflates with alkenes in the presence
of base, has become one of the most versatile methods for C-C bond formation
(Scheme 11.1). This process has found extensive applications in asymmetric syn-
thesis. Shibasaki and Overman have convincingly demonstrated the value of such
a transformation in the synthesis of complex natural molecules [1].

R R

Base

+ R-X +

R 1 R 2 R 1 R 2 R 1 R 2

[M] / L*

R 3 R 3 R 3

Scheme 11.1

Pd-catalyzed Heck reaction; X = halide or triflate.

The Heck reaction has been known to synthetic chemists since the late 1960s.
However, reports of successful examples of the asymmetric Heck reaction were
first published at the end of the 1980s. The bulk of the reported examples involve
intramolecular reactions, which have the advantage of allowing easy control of
alkene regiochemistry and geometry in the product [1g]. In contrast, successful
intermolecular reactions have until very recently been limited to quite reactive
substrates, principally O-,N-heterocycles, which again simplifies the question of
alkene regiochemistry [1]. Nowadays several substrates have been applied in inter-
molecular asymmetric Heck reactions. Most of them are cyclic substrates, such as
enol ethers, dihydropyrroles, dihydrodioxepines, and alkenes (Figure 11.1). Tradi-
tionally, 2,3-dihydrofuran has been the substrate of choice for testing a new ligand.
With regard to the aryl or vinyl source, various triflate compounds have been
applied. However, the most widely used is phenyl triflate. The base is also an