Specific microwave effect

In addition to the above mentioned thermal/kinetic effects, microwave effects that are caused by the unique nature of the microwave dielectric heating mechanisms (see Section 2.2) must also be considered. These effects should be termed specific 

Closely related to the superheating effect under atmospheric pressure are wall effects, more specifically the elimination of wall effects caused by inverted temperature gradients (Fig. 2.6). With microwave heating, the surface of the wall is generally not heated since the energy is dissipated inside the bulk liquid. Therefore, the temperature at the inner surface of the reactor wall is lower than that of the bulk liquid. It can be assumed that while in a conventional oil-bath experiment (hot vessel surface, Fig. 2.6) temperature-sensitive species, for example catalysts, may decompose at the hot reactor surface (wall effects), the elimination of such a hot surface will increase the lifetime of the catalyst and therefore will lead to better conversions in a microwave-heated as compared to a conventionally heated process. 

The same concept of volumetric in situ heating by microwaves was also exploited by Larhed and coworkers in the context of scaling-up a biochemical process such as the polymerase chain reaction (PCR) [25], In PCR technology, strict control of temperature in the heating cycles is essential in order not to deactivate the enzymes involved. With classical heating of a milliliter-scale sample, the time required for heat transfer through the wall of the reaction tube and to obtain an even temperature in the whole sample is still substantial. In practice, the slow distribution of heat 

These results provide clear evidence for the existence of selective heating effects in MAOS involving heterogeneous mixtures. It should be stressed that the standard methods for determining the temperature in microwave-heated reactions, namely with an IR pyrometer from the outside of the reaction vessel, or with a fiber-optic probe on the inside, would only allow measurement of the average bulk temperature of the solvent, not the true reaction temperature on the surface of the solid reagent. 

A selective heating in liquid/liquid systems was exploited by Strauss and coworkers in a Hofmann elimination reaction using a two-phase water/chloroform system (Fig. 2.10) [32]. The temperatures of the aqueous and organic phases under micro-wave irradiation were 110 and 55 °C, respectively, due to the different dielectric properties of the solvents (Table 2.3). This temperature differential prevented decomposition of the final product. Comparable conditions would be difficult to obtain using traditional heating methods. A similar effect has been observed by Hallberg and coworkers in the preparation of /3,/3-diarylated aldehydes by hydrolysis of enol ethers in a two-phase toluene/aqueous hydrochloric acid system [33], 

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The origin of specific microwave effects is twofold - those which are not purely thermal and a special thermal effect connected with possible intervention of hot spots . 

This is essentially true, as is evidenced by the rates of esterification in alcoholic media of propan-l-ol with ethanoic acid [27] or of propan-2-ol with mesitoic acid [28], The absence of a specific microwave effect became apparent from several experiments carefully conducted in alcohols or in DMF under similar conditions but with microwave or classical heating [7]. 

Specific microwave effects can be expected for polar mechanisms, when the polarity is increased during the reaction from the ground state towards the transition state (as more or less implied by Abramovich in the conclusion of his review in 1991 [42]). The outcome is essentially dependent on the medium and the reaction mechanism. 

Such a conclusion is, nevertheless, connected with the synchronous character of the mechanism. If a stepwise process is involved (nonsimultaneous formation of the two new bonds), as for unsymmetric dienes and/or dienophiles or in hetero Diels-Alder reactions, a specific microwave effect could intervene, because charges are developed in the transition state. This could certainly be so for several cycloadditions [47, 48] and particularly for 1,3-dipolar cycloadditions [49]. Such an assumption has... 

It is apparent there is a definite advantage to operating under solvent-free conditions. The specific microwave effect is here of low magnitude, but evident, because after 3 min the yield increases from 64 to 98%. Prolongation of the reaction time with classical heating led to an equivalent result. The microwave effect is rather limited here, because of a near-synchronous mechanism. 

These reactions are among the most propitious for revealing specific microwave effects, because the polarity is evidently increased during the course of the reaction from a neutral ground state to a dipolar transition state. 

A large specific microwave effect was observed in the solvent-free synthesis of N-sulfonylimines, a similar type of reaction [64] (Eq. 12). 

The very important specific microwave effect is consistent with the mechanism which involves the formation of a dipolar TS from neutral molecules (Scheme 3.8). 

In this case, the anion being hard and with a high charge density, the reactions are concerned with tight ion pairs. During the course of the reaction, ionic dissociation is increased and hence polarity is enhanced from the GS towards the TS. Specific microwave effects should be expected. 

Dialkylations were attempted as model reactions before subsequent polymerizations and revealed very important specific microwave effects [88] (Eq. (34) and Tab. 3.14). 

The specific microwave effect can be attributed to two different facts ... 

Finally, the magnitude of a specific microwave effect could be indicative of a polar mechanism or to access the rate-determining step in a procedure involving several steps. For instance, during the study of microwave effect in the solvent-free synthesis... 

Scheme 4.12 Diels-Alder reactions reported to show a specific microwave effect.

Because this change in selectivity is difficult to explain by a classical heating effect, Langa et al. [9] consider that it is one of the most convincing examples of a possible specific microwave effect. 

The synergy between the dry media and microwave irradiation was convincingly demonstrated in this work. For instance, with the allyl compound, the yield is only 16% after 24 h in toluene under reflux, and no reaction occurs after 10 min at 100 °C (classical heating), thus revealing an important specific microwave effect. 

Superheating also can account for acceleration of reactions under microwave conditions70,71. Mingos has estimated that this can lead to 10-50-fold reductions in reaction times in comparison with conventional reflux conditions, but that rate enhancements of 100-1000-fold at atmospheric pressure would be required before specific microwave effects could be invoked72. 

The second group of theories supposes that during microwave irradiation of the reaction mixture there is a specific effect of microwave activation that causes an increase of the reaction rates for which the bulk temperature of the reaction mixture is an inadequate to explain. Such an effect has been accepted to be called the non - thermal microwave effect or the specific microwave effect. Recent critical reviews concerned with both group of theories have been published by Loupy et al. [16], Nuchter et al. [17], and de la Hoz et al. [18], respectively. 

The specific microwave effects were presumably responsible for the observed 40-fold acceleration in microwave-assisted synthesis of 1,3,5-triazines 107 compared to conventional thermal conditions. Thus, microwave heating of benzonitrile and dicyandiamide 106 in an ionic liquid ([bmim]PF6) in the presence of powdered KOH at 130 °C for just 12 min afforded 2,4-diaminotriazine 107 in 87% yield. Under otherwise identical conditions the reaction in a pre-heated oil-bath (130 °C temperature) took 8 hours to afford the target heterocycle 107 in 79% yield [140] (Scheme 57). 

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