Date of Award

1980

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Abstract

The synthesis of several "fulvenoid dipoles", molecules incorporating both fulvene and 1,3-dipolar moieties, were accomplished. Both aromatic and anti-aromatic resonance structures can be written for all fulvenoid and heptafulvenoid dipoles, but the extent of their contribution to the ground state of these molecules, and therefore, the stability of these systems is not immediately obvious by this theory. However, a qualitative application of an orbital interaction model leads to the prediction that fulvenoid dipoles, formally consisting of diene and 1,3-dipole units, will be stabilized when the 1,3-dipolar unit is a good donor (low ionization potential), while heptafulvenoid dipoles will be stabilized when the 1,3-dipole unit is a good acceptor (high electron affinity). According to this model, the fulvenoid nitrone, N-cyclopentadienylidenemethylamine oxide, is expected to be a reactive system (like cyclopentadienone), while the seven-membered analog, N-cycloheptatrienylidenemethylamine oxide, a relatively stable molecule (like tropone). Only substituted analogs of the fulvenoid nitrone, N-2,5-dimethyl-3,4-diphenylcyclopentadienylidenemethylamine oxide and N-3-t-butylcyclopentadienylidenemethylamine oxide, could be synthesized in this work; the parent molecule is too reactive to permit isolation or trapping in a number of attempted syntheses. The three-membered analog, N-2,3-diphenylcyclopropenylidenemethylamine oxide, also resisted attempts at synthesis, in spite of the prediction of electronic stabilization for this molecule. Reactions of N-2,5-dimethyl-3,4-diphenylcyclopentadienylidenemethylamine oxide with ethyl (or methyl) propiolate and methyl acrylate were regioselective, giving spiro compounds with substituents at C-4 of the isoxazoline or isoxazolidine ring; the isoxazoline adduct from ethyl (or methyl) propiolate reaction rearranged readily to corresponding stable fulvenoid azomethine ylide which was unreactive toward dipolarophiles; reactions of this fulvenoid nitrone with methyl vinyl sulfone and styrene gave mixtures of the 4- and 5-substituted regioisomers, preferentially the 4-substituted adduct. The periselectivity and regioselectivity of these reactions are shown to be compatible with frontier orbital theory. Attempts to react this nitrone with very electron-rich dipolarophiles failed. N-3-t-butylcyclopentadienylidenemethylamine oxide had a very low reactivity toward electron-deficient or electron-rich dipolarophiles; it only reacted with methyl propiolate cleanly to give the spiro-type, 4-substituted isoxazoline, which rearranged to the corresponding fulvenoid azomethine ylide upon heating. Frontier orbital theory predicts that electron-deficient alkenes and alkynes should show a loss or reversal of regioselectivity in reactions with electron-rich nitrones, with 4-substituted adducts favored with very electron-rich nitrones. To confirm this prediction, reactions of two electron-rich nitrones (C-cyclopropyl-N-methylnitrone and C,C-dicyclopropyl-N-methylnitrone) with four electron-deficient dipolarophiles (methyl propiolate, methyl acrylate, acrylonitrile, and phenyl vinyl sulfone) have been investigated. Reaction of C-cyclopropyl-N-methylnitrone and these dipolarophiles gave mixtures of 4- and 5-substituted regioisomers (phenyl vinyl sulfone and ethyl propiolate gave predominantly the 4-substituted adducts). Reaction of C,C-dicyclopropyl-N-methylnitrone and methyl acrylate and acrylonitrile gave predominantly the 4-substituted isoxazolidines, while the reactions of this nitrone with phenyl vinyl sulfone and methyl propiolate were regioselective for the formation of 4-substituted adducts. Both electron-rich nitrones gave 5-substituted isoxazolidines in reactions with styrene. These type of products indicate that steric effects do not control regioselectivity. These results are uniquely compatible with a frontier orbital theory interpretation of regioselectivity.

Pages

216

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