Modified Organotin Reagent for Easy Separation

The diphenyltin hydride derived from 4-vinylpyridine has the advantage that the resulting tin halide sticks to the baseline during column chromatography on silica Rf of ca. 0) eluted with 1 3 ethyl acetate hexane, allowing easy separation from non-polar products. Residual reagent has an R/ of ca. 0.2 compared to an of ca. 0.9 for Iriphenyltin hydride. 

Water-soluble polyether tin hydride and the bis(2-cardoxyethyl)tin hydroxide allowed reductions and cyclizations to be carried out in water with easy separation from organic soluble products. The latter tin reagent is generated in situ and has not been characterized, but may be a multiplicity of tin oxides and hydroxides. It is soluble in dilute alkali and catalyzes the reduction of alkyl and aryl bromides in the presence of NaBH4 and the water-soluble initiator 4,4 -azobis(4-cyanovaleric acid) (AVCA). 

The pyrene-tethered dimethyltin hydride is made in six steps and 29% overall yield from commercially available pyrene carboxyaldehyde, but can be used catalytically with a stoichiometric source of hydride. Workup with activated carbon provides product in yields comparable to tri-n-butyltin hydride and with organotin contamination below 2 mol% and undetectable by NMR spectroscopy. Unfortunately, reagent recovery from the activated carbon is low-yielding. Allyl and phenyl pyrene-tethered tin reagents have also proved effective for use in allylation and Stille coupling respectively and, again, tin contamination of the product are undetectable by and NMR spectroscopy. 

Some degradation of the flnorons tin hydride does appear to occnr with multiple recycling. The first four reductions of bromoadamantane were complete in 4 honrs while the last two required 5 hours. Nevertheless, the ability to use 1-10% tin catalyst with almost complete recovery substantially improves the green credentials of tin hydride redactions. Most importantly, tin contamination of the prodnct of fluorous tin hydride-mediated reactions cannot be detected to an estimated detection limit of 1%. 

A fluorous surfactant covalently tethered to silica provides a thin film of perfluorinated solvent for reactions and/or extractions. This material was used for the small-scale hydrocyclization of 6-bromo-1-hexene with NaBH and a catalytic amount of a fluorous tin bromide in 1-butanol. The yield of methylcyclopentane was modest, however, and this technology is a long way from being viable on an industrial scale. The same idea has been more successfully employed with fluorous silica-supported tin Lewis acid catalysts for Baeyer-Villiger oxidations.  

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