Benzoin condensation solvent effects

There have been very few examples of PTV derivatives substituted at the vinylene position. One example poly(2,5-thienylene-1,2-dimethoxy-ethenylene) 102 has been documented by Geise and co-workers and its synthesis is outlined in Scheme 1-32 [133]. Thiophene-2,5-dicarboxaldehyde 99 is polymerized using a benzoin condensation the polyacyloin precursor 100 was treated with base to obtain polydianion 101. Subsequent treatment with dimethyl sulfate affords 102, which is soluble in solvents such as chloroform, methanol, and DMF. The molar mass of the polymer obtained is rather low (M = 1010) and its band gap ( ,.=2.13 eV) is substantially blue-shifted relative to PTV itself. Despite the low effective conjugation, the material is reasonably conductive when doped with l2 (cr=0.4 S cm 1). 

Currently, other catalytic systems have been developed, including N, M -disubstituted o-phenylenediamines, thiazolium cyclophane, ° and enzymes. The benzoin cndensa-tion is normally carried out in aqueous solution, and the addition of salt (LiCl, KCl) can increase the reaction rate however, the condensation can also be performed in organic solvent, such as anhydrous petroleum ether. In contrast to the salt effect in aqueous solution, the addition of salt (LiCl, LiClOa) into organic solvents (ethylene glycol, formamide, and DMSO) results in the decreasing of reaction rate. However, a few aldehydes were found not to form benzoin under the benzoin condensation condition instead ethylenediol and ethanediol were formed. This condensation when catalyzed by cyanide ion, is assumed... 

The reaction of two molecules of benzaldehyde to form benzoin is generally referred to as the benzoin condensation. It is normally catalyzed by cyanide ion, although thiazolium ions will also catalyze it, as we have discussed above and shown in Fig. 1.2. The normal solvent for the benzoin condensation is ethanol, to dissolve all the components of the reaction. However, it seemed to us likely that there would be overlap of the phenyl rings in the transition state for the benzoin condensation, and thus that reaction in water could lead to hydrophobic accelerations. This proved to be the case. We saw that the rate of the cyanide-catalyzed benzoin condensation was 200-fold faster in water than in ethanol. Also, we saw that added LiCl increased the reaction rate, while added lithium perchlorate decreased it. Such salt effects are diagnostic of the presence of some acceleration by hydrophobic packing in the transition state for the reaction. 

We also detected a hydrophobic effect in the benzoin condensation [14]. In this case, in contrast to the Diels-Alder reaction, it is not formally required that the two hydrophobic phenyl groups come together in the transition state, but our studies indicated that they do. Again there was a large increase in rate when water was the solvent, but in an ionic reaction of this sort such solvent effects could well be related only to the effect on the ions of the polar character of the medium. However, we saw that the reaction rate was increased with LiCl, but decreased when LiC104 was added. In this system LiC104 is a salting... 

Very recently [126] the above described catalytic activity of [C2CiIm][OAc] in the benzoin condensation has been reported to provide a novel route for biomass processing. 5-Hydroxymethylfurfural - that can be produced effectively from plant biomass resources such as glucose or cellulose [127] - can undergo a facile benzoin condensation (see Fig. 12) in [C2Cilm][OAc], catalyzed by the solvent itself under very mild conditions (60-80°C) with excellent yields (up to 98% within 1 h). The product of the reaction, 5,5 -di(hydroxymethyl)furoin has been suggested for use as a possible C12 kerosene/jet fuel [126]. Later, also a redox esterification reactirai has been reported in this ionic liquid [174]. 

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