Applications of the on water method

Since the publication of our first report describing the on water phenomenon, a number of other research groups have used this method to their advantage. The reported examples encompass a broader range of reaction classes than before, and provide evidence for the broad utility and versatility of the on water method. 

An elegant experimental and theoretical study on the influence of water on transition states of Huisgen 1,3-dipolar cycloadditions was reported by Butler and coworkers. The authors concluded that the polarity of the transition state in cycloadditions of phthalazinium ylides such as 52 (Fig. 11.17) with various olefins remains nearly the same in acetonitrile and 9 1 water-acetonitrile. A few representative preparative reactions were also outlined such as the example shown in Fig. 11.17. 

Bergdahl and coworkers have reported the successful use of on water conditions for Wittig reactions of stabilized ylides. Although the protic stabUity of stabilized ylides is well known, these types of Wittig reactions are generally carried out in aprotic solvents. The authors describe a number of examples where rate acceleration is apparent in reactions carried out in water. For instance, the olefination of aldehyde 55 was reported to be unsuccessful in organic solvents (Fig. 11.18) on the other hand, the reaction in water afforded a 67% yield of a,-unsaturated ester 56. 

Two interesting reports describing the advantages of water in radical reactions have recently appeared. In the first, Chatgilialoglu and coworkers demonstrate that radical 

Radical brominations also take place readily in aqueous media, without the requirement of any cosolvents. In a recent paper, Iskra and coworkers report a number of cases of efficient benzylic bromination with NBS, using visible light (from simhght or an incandescent lamp) as the initiator (Fig. 11.20). ° However, the authors also report that in certain activated cases such as mesitylene (61), electrophilic bromination of the benzene ring predominates. 

---