Connecting the Superelectrophilic and Diels-Alder Reactivities of Nitrobenzofuroxans
|François Terrier , Sami Lakhdar , Taoufik Boubaker , Regis Goumont|
University of Versailles (SIRCOB), Institut Lavoisier,45,Avenue des Etats-Unis, Versailles 78035, France
Nitrosubstituted 2,1,3-benzoxadiazoles and related 1-oxides, commonly referred to as nitrobenzofurazans and nitrobenzofuroxans, respectively, represent a class of neutral heteroaromatic structures which exhibit an extremely high electrophilic character in nucleophilic addition and substitution processes . As an illustration of this behaviour, the assessment of both carbon and nitrogen nucleophilicities of 2-aminothiazoles through coupling of these ambident derivatives with 4,6-dinitrobenzofuroxane (DNBF) will be discussed .
Recently, it has been shown that the carbocyclic ring of a number of nitrobenzofuroxans and related heterocycles, e.g. nitrotetrazolopyridines, can also be involved in a variety of Diels-Alder reactions, contributing both to normal and inverse processes which often proceed with high stereoselectivity . In this regard, we will highlight our finding that there is a close relationship between superlectrophilicity and pericyclic reactivity of our substrates . Compounds which are the most prone to undergo s-complexation, as measured by the pKa values for water addition (eq. 1), are also those which exhibit the greatest pericyclic reactivity. For reactions involving such common dienes as 2,3-dimethylbutadiene, cyclopentadiene or cyclohexadiene (eq. 2), a value of 8-8.5 defines the frontier demarcating those electrophiles which can exhibit dual pericyclic and electrophilic behaviour from those which do not . Importantly, this frontier can also be defined with reference to the ranking of nitrobenzofuroxans on the comprehensive electrophilicity scale of Mayr et al (Acc. Chem. Res. 2003, 36, 66) as well as the theoretical electrophilicity scale of Parr (J. Am. Chem. Soc. 1999, 121, 1922).
Last, the dual electrophilic and pericyclic reactivity of 4-nitrobenzodifuroxan and the demonstration that this compound behaves as a superelectrophilic nitroolefin will be presented.
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Presentation: oral at 18th Conference on Physical Organic Chemistry, Symposium 1, by François Terrier
See On-line Journal of 18th Conference on Physical Organic Chemistry
Submitted: 2006-05-29 16:19 Revised: 2006-07-31 09:17
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