Supported reagents activities

Gupta et al. reported that the Vilsmeier-Haack cyclisation of acetanilides 20 using supported reagents and microwave-irradiation in solvent-free conditions is rapid and efficient. Reaction yields are good, although only a few activated derivatives have been investigated. 

Two polymer-supported reagents have been developed for the oxidation of sulphoxides to sulphones these involve peracid groups150, and bound hypervalent metals activated by t-butyl hydroperoxide151,152. 

Reaction of glycosyl thioimidate 79 with aa -dibromo-m-xylene in the presence of NaH as a base and 15-crown-5 as a supporting reagent allowed the intermediate 80 (Scheme 4.88). Treatment of the diol 81 with dibutyltin oxide in dry toluene and then reaction with 80 in the presence of tetrabutylammonium iodide afforded the desired O-linked intermediate 82. Activation of this compound with NIS-TMSOTf afforded 83 in a good yield. Hydrogenolysis followed by acetylation gave the desired disac-... 

For the 9-fluorenylmethoxycarbonyl (Fmoc) protection of amino acids, Chinchilla et al.36,37 prepared a similar ROMP-polymer that supports an activated. V-hydroxysuccinimide Fmoc-carbonate (Table VII, entry 31). Various Fmoc-amino acids are prepared in pure form after removal of the polymer reagent by filtration and aqueous phase separation. 

Interestingly, the same reaction can be performed with the polymer-supported reagent 18 in water, because the reagent 18 was found to be considerably stable in this solvent. A mechanistic rationale of the catalytic activation of iodosobenzene 5 by potassium bromide is given in Scheme 14. This reaction provides a very facile and environmentally benign method for the alcohol oxidation. 

Solid-supported reagents which have found utility include Nafion-scandium Lewis acid catalyst (allyl additions to aldehydes) [62], HOBt (medium-ring lactamization) [63], EDC (preparation of active esters) [64], and thiazolium hydrotribromide (brominations) [65], A review has also appeared describing the use of supported reagents in separation science, primarily for the selective sequestration of metal ions [66],... 

A further example used the supported amino alcohols 45,46 and 47 (Scheme 4.76), where the reagents were pumped up from the bottom of the polymer using a pair of long needles connected to peristaltic pumps. The product was collected from the top using another pump and quenched in a solution of dilute hydrochloric acid. For the first run with catalyst 46, the yields and ee were excellent (94% yield in 97% ee), but when 46 was recovered and reused, the yield dropped to 75% and the ee to 50%. This was ascribed to degradation of both the chiral and backbone sites of the polymer by diethyl zinc, again demonstrating that not only do the solid supports need to be mechanically sound but both the backbone support and active site must be also chemically resistant to the reaction conditions [171]. 

This development was certainly apparent at the 4th International Symposium on Supported Reagents and Catalysts in Chemistry held at the University of St Andrews, 2-6 July 2000. Six Keynote and nine Invited Lectures were presented, along with 22 Oral Contributions. In addition 40 posters were presented. Keeping up the tradition of this meeting, therefore, a large proportion of the participants were active in presenting their work in one format or another. 

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