The efficiency of microwave flash heating in accelerating organic transformations (reaction times reduced from days and hours to minutes and seconds) has recently been proven in several different fields of organic chemistry. This specific account mainly summarizes our own experiences in developing rapid, robust, and selective microwave-assisted transition metal-catalyzed homogeneous reactions. Applications include selective Heck couplings, cross-couplings, and asymmetric substitutions. The science of green chemistry was developed to meet the increasing demand for environmentally benign chemical processes. We believe the combination of metal catalysis and microwave heating will be of importance in the search for green laboratory-scale synthesis.
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.
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Dual activation by a chiral Lewis acid and an achiral or chiral Lewis base enabled cyanation of both aromatic and aliphatic aldehydes with acetyl cyanide and ethyl cyanoformate to provide direct access to O-acetylated and O-alkoxycarbonylated cyanohydrins, respectively, under mild conditions. With a combination of a Ti-salen catalyst and Et3N, benzaldehyde was converted to the O-acetylated cyanohydrin with 94% ee within 10 h at -40 degrees C in 89% isolated yield.
Microwave heating allows regio‐ and enantioselective allylic alkylation to be carried out in air in a few minutes in a convenient one‐pot procedure [Eq. (1)]. The catalytically active molybdenum(0) species was formed in situ from inexpensive [Mo(CO)6] and a chiral ligand.
A minor enantiomer recycling one-pot procedure employing two reinforcing chiral catalysts has been developed. Continuous regeneration of the achiral starting material is effected via selective enzyme-catalyzed hydrolysis of the minor product enantiomer from Lewis acid-Lewis base catalyzed addition of acyl cyanides to prochiral aldehydes in a two-phase solvent system. The process provides O-acylated cyanohydrins in close to perfect enantioselectivities, higher than those obtained in the direct process, and in high yields. A combination of a (S,S)-salen Ti Lewis acid and Candida antarctica lipase B provides the products with R absolute configuration, whereas the opposite enantiomer is obtained from the (R,R)-salen Ti complex and Candida rugosa lipase.
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