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Scope and Mechanism of a Novel Base Induced Cyclization of Benzyl 1 Alkynyl Sulfones

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Title: Scope and Mechanism of a Novel Base Induced Cyclization of Benzyl 1 Alkynyl Sulfones
Author: Hossain, Mohammad Selim
Department: Department of Chemistry
Program: Chemistry
Advisor: Schwan, Adrian L
Abstract: Recent work has shown that sulfones are unique synthetic tools capable of undergoing numerous transformations and reactions. The sulfone group is a strong electron withdrawing group and has the ability to stabilize an α-sulfonyl carbanion. As such, unsaturated sulfones can undergo a variety of reactions, i.e. conjugate additions, alkylation etc. Beside this, α sulfonyl carbanion can be generated with strong base and α sulfonyl carbanion can also be used in various reactions. This report is an investigation of a novel base induced cyclization of various alkynyl sulfones. The results revealed that the reaction does not significantly depend on the electronic effects of substituents in the aromatic ring. Additional versatility in this process was demonstrated with respect to a diverse array of functionality at various positions i.e. benzylic position, aromatic ring and terminal position of alkyne. These alkynyl sulfones, bearing a larger group at ortho position and any group at benzylic position underwent intramolecular cyclization when treated with base affording benzothiopyran S,S-dioxide. The results demonstrated that the cyclization efficiency was significantly influenced by the steric effects of substituents of aromatic ring at ortho positions and blocking the terminal position with phenyl since the cyclization reaction gave low or no yields with aromatic rings having a substituent at para, meta or to position than those having no substituent. It was also found that 2-thiophenylmethyl sulfones cyclize more efficiently than benzyl and furfuryl sulfones. The evidence from mechanistic studies points to a mechanism for the cyclization that involves disruption of aromaticity. To disrupt aromaticity, a deprotonation at benzylic position is a requirement for this cyclization reaction. Results suggest that the cyclization also requires an available hydrogen at the point where the potential ring would be fused. Several intermediates were also observed spectroscopically and identified by ReactIR®.
Date: 2012-11
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