Electroorganic synthesis: inter- and intra-molecular anion radical cycloadditions, and electrogenerated base promoted coupling reactions

Date
2005
Authors
Felton, Greg Andrew Nicholas
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Abstract

The electrochemical promotion of a variety of organic synthetic procedures has been investigated. These procedures are seen to fall into two categories, those promoted by anion radicals, and those promoted by electrogenerated base. The intramolecular anion radical cyclization of bis(enones) leads to the formation of both cyclobutane and Diels-Alder cyclic products. The formation of these products is shown to occur via a distonic anion radical intermediate. The majority of reactions are seen to be mildly electrocatalytic, indicating an electron transfer chain. The primary use of tetraethylammonium tetrafluoroborate as the electrolyte leads to high pericyclic yields, and is therefore presented as an effective synthetic route. Synthetic and mechanistic considerations are widely explored, particularly via systematic substrate alteration. Additionally, variation of the perchlorate electrolyte cation is seen to lead to dramatic diastereoselectivity, indicative of electrolyte chelation with the distonic anion radical. Indeed this work is the first to synthetically utilize Barium perchlorate electrolyte. Intermolecular anion radical cross-cyclobutanation reactions are shown to yield novel compounds, albeit in moderate yield. The cross reactions are generally based upon the previously known anion radical cyclobutanating reactivity of phenyl vinyl sulfone. The reactivity of ketones, with sulfones, is only the third example of intermolecular anion radical cyclobutanation. The Electrogenerated Base (EGB) promoted additions of allyl phenyl sulfone to electron deficient alkenes was studied. This carbanion based chemistry leads to Michael additions, primarily yielding two classes of product, that of single addition and of double addition. Addition to vinyl sulfones approached quantitative yields, giving exclusive 1:2 (double addition) product formation, and electrocatalytic factors of greater than 10. Addition to a variety of other electron deficient alkenes gave a mixture of 1:2 and 1:1 adducts and in some cases, secondary reaction products. Although formed in moderate yield, the formation of an eight member ring from the ,-addition of the allyl phenyl sulfone carbanion to divinyl sulfone is quite remarkable. Further EGB research is briefly explored. Firstly, use of a pro-base (in some cases the in situ substrate) to give reactive cyanomethyl anions. Leading to near quantitative yields of acyclic cyano sulfones, produced with electrocatalytic factors greater than 30. Secondly, use of a pro-base, namely diphenyl sulfone, to increase EGB reaction efficiency and promote carbanion mechanism addition reactions of allyl/propenyl species. Lastly, the use of EGB’s to promote electro-isomerization is observed.

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