Vinyl Cations As Cyclopentenone Precursors Via C‒h Insertion Or Alkene Addition Reactions
We report that Lewis acid mediated reactions of β-hydroxy-α-diazo carbonyls provide facile access to vinyl cation intermediates, which can form mono- and bicyclic cyclopentenone ring systems. Vinyl cations are sp hybridized cations that can be exploited as carbene surrogates or as electrophiles. The methodology reported in this dissertation is important because cyclopentenone rings are common motifs in natural products or can be used as synthetic intermediates.
The first portion of this dissertation will discuss vinyl cations we’ve used as carbene surrogates in metal free C‒H insertion reactions. Treating β-hydroxy-α-diazo carbonyls with a Lewis acid provided vinyl cation intermediates after loss of hydroxide and nitrogen gas. A 1,2-methylene shift then provided a vinyl cation which can insert into an inert C‒H bond to give mono- and bicyclic cyclopentenone rings. Modification at the point of insertion to a more substituted center provides a competitive elimination type reaction. This result sparked a deeper investigation into the concertedness of the C‒H insertion. Further expansion of this work included migratory aptitude studies in the 1,2-shift step for non-symmetric aliphatic and tetralone systems.
The second portion of this dissertation forms vinyl cations in a similar fashion, but uses them as electrophiles that are susceptible to a nucleophilic attack by a pendant alkene. This reaction, which we hypothesize proceeds through a unique acylium intermediate, forms α-alkylidene cyclopentenones in good yields. The efficiency of this reaction was influenced by the group adjacent to the vinyl cation. Aryl groups which were electron rich or were sterically hindered formed α-alkylidene cyclopentenones in higher yields than electron poor aryl rings or alkyl groups. Other nucleophiles were investigated but were not productive compared the alkene nucleophile.