Asymmetric reactions microwave-accelerated

A variety of other reaction variables, such as the solvent, base and various additives, can have profound effects on asymmetric Mizoroki-Heck cyclizations. These factors have been discussed in recent reviews [3]. The reaction temperature can also play an important role. At temperatures above 100 °C, catalyst decompositiou can contribute to deterioration of enantioselectivity [23, 24], Nonetheless, there are numerous examples of successful asymmetric Mizoroki-Heck cyclizations at higher temperature. Carrying out palladium-catalysed reactions in microwave reactors is now common [25], with several examples of microwave-accelerated asymmetric Mizoroki-Heck reactions being reported [26, 27]. Under microwave heating conditions, reaction times can be reduced and rigorous exclusion of oxygen is often not required. This technology was used to promote an asymmetric Mizoroki-Heck cyclization in the recent total synthesis of minliensine (see Section 16.4.4). 

Many variations on this reaction have been studied over the last several years including use of Co2(CO)s on a solid support to accelerate reaction rates, the use of microwave radiation, and the use of ionic liquids as solvents. Asymmetric synthesis has also been given close attention. ... 

Microwave irradiation has been used to accelerate many reactions through flash heating [28—30]. The potential of this approach was recently shown in the asymmetric allylic alkylation in Figure 13.15 [31]. BINAP and the other ligand used in this work were chosen because they afford relatively slow conversions under conditions without microwave irradiation. 

A series of highly selective reactions have been reported by Guijarro et al., where a chiral sulfinyl group directs the ATH of an imine, using either an asymmetric catalyst or a racemic one (Fig. 49). Using cis-aminoindanol, very selective syntheses of amines are possible [162-164], and imine formation can be accelerated with microwave irradiation. 

In conclusion, microwave irradiation can be a useful tool for accelerating asymmetric organocatalytic reactions, especially with a higher-temperature tolerance. However, for several reactions presented in this section similar or almost identical results in terms of yield and enantioselectivity were obtained with conventional heating at the same temperature. There are also examples where under microwave conditions an increase in yield and/or enantioselectivity were observed. Unfortunately, because of relatively Uttle exploration of this area, it is still hard to speak about specific or non-thermal microwave effects. Therefore, in-depth mechanistic studies are required. 

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