Selectivity in MW-assisted Reactions

In summary, observations of increased rates of MW-assisted homogeneous reactions, compared with classically heated reactions at the same apparent temperature, may be explained by one or more of the following problems or effects  

Because observed rate enhancements are usually small, or zero, nonthermal effects do not seem to be important in MW heated reactions in homogeneous media, except possibly in some reactions of polymers and reactions in nonpolar solvents. Relatively few studies have been conducted on MW-assisted reactions of polar reactants in nonpolar solvents. Also, since there is some disagreement as to whether or not these reactions are accelerated significantly by MW, in comparison with conventionally heated reactions at the same temperature, more research on the effect of MW irradiation on the rates of these reactions is required. Nonthermal effects may, however, explain the more substantial MW rate enhancements in solvent-free reactions on solid supports [44] (see Chapt. 5) and solid state reactions [68, 69]. 

Another interesting question is whether MW heating can result in changes in selectivity, leading to different product compositions than those obtained using conventional heating. The effect of MW irradiation on selectivity was discussed in a review by Langa et al. in 1997 [9] and more recently by Loupy et al. [44, 70] 

It was observed that there were only slight differences in composition between the products of the MW and conventionally heated reactions. The greatest change in composition was observed in the reaction of 1-bromooctane with fert-butoxide ion where the percent of the elimination product (the alkene) increased from 5%, under conventional reflux to 14% under MW heating. On the other hand there 

The endo isomer was the major product, but the proportion of the endo isomer decreased with temperature, from approximately 90% at 0 °C to 85 % at 60 °C [69]. Ge-dye et al. [71] performed the reaction in methanol solution under MW heating in a closed Teflon container and found that the product contained 79% of the endo isomer at an estimated temperature of 110 °C. A plot of temperature versus percent endo isomer is effectively linear between 0 °C and 60 °C and assuming it remains linear to 110°C, the product should contain 80% endo isomer at this temperature. Thus it was concluded that the change in product composition was due to the change in temperature rather than to some special effect of MW. At higher temperatures there is an increase in the proportion of the exo isomer, which is thermodynamically more stable than the endo isomer. 

---