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