This Review compiles the evolution,
mechanistic understanding,
and more recent advances in enantioselective Pd-catalyzed allylic
substitution and decarboxylative and oxidative allylic substitutions.
For each reaction, the catalytic data, as well as examples of their
application to the synthesis of more complex molecules, are collected.
Sections in which we discuss key mechanistic aspects for high selectivity
and a comparison with other metals (with advantages and disadvantages)
are also included. For Pd-catalyzed asymmetric allylic substitution,
the catalytic data are grouped according to the type of nucleophile
employed. Because of the prominent position of the use of stabilized
carbon nucleophiles and heteronucleophiles, many chiral ligands have
been developed. To better compare the results, they are presented
grouped by ligand types. Pd-catalyzed asymmetric decarboxylative reactions
are mainly promoted by PHOX or Trost ligands, which justifies organizing
this section in chronological order. For asymmetric oxidative allylic
substitution the results are grouped according to the type of nucleophile
used.
The asymmetric inter- and intramolecular Heck and related reactions are comprehensively reviewed, from their original development to recent advances in terms of substrate scope, reactivity, regio- and enantioselectivity. Their reaction mechanisms/catalytic cycles are discussed in order to understand the concepts underpinning the significant recent developments of these processes. The design and application of new chiral ligands has been pivotal to reaction enhancements and, for each Heck and related process, we classify ligands by the nature of the denticity, chirality and donor atoms involved. In this manner, the continued development of ligand architectural design and application can be more easily monitored for each process. Significant improvements in reaction times, a disadvantage in many Heck reactions, have been addressed through a combination the use of microwave-assisted protocols and ligand design. The asymmetric Fujiwara-Moritani and oxidative boron Heck-type reactions, recent additions to Heck type processes, will also be discussed in this critical review (149 references).
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