Cycloaddition-ortho benzonitrile

A dipolarophile bearing an ionic group and an associated counterion provides enhanced selectivity as has been recently demonstrated by Raposo and Wilcox [14]. Cycloaddition of benzonitrile 4 and the uncharged amine 5 a (a chiral phenylmaleimide derivative) in THE or chloroform provides a mixture of cycloadducts 6-9a in 1 4 4 4 diastereomeric ratio (i.e., 8 5 in favor of the methyl face approach of the dipolarophile. The ortho-substituents of the... 

The first examples of ortho cycloaddition can be found in a U.S. patent of Ayer and Buchi [1], Benzonitrile and 2-methylbut-2-ene are reported to yield 7,8,8-trimethylbicyclo[4.2.0]octa-2,4-diene-l-carbonitrile upon irradiation under nitrogen with a mercury resonance arc. Similar reactions, all leading to derivatives of bicyclo[4.2.0]octa-2,4-diene-l-carbonitrile occurred when benzonitrile was irradiated in the presence of 2,4,4-trimethylpent-l-ene, ethyl vinyl ether, vinyl acetate, methyl vinyl ketone, and methyl acrylate. The addend pairs para-tolunitrile/oct-l-ene, ort/m-dicyanobenzene/2-methylbut-2-ene, para-dicyanobenzene/but-l-ene, 2,3-dimethylbenzonitrile/propene, and 3,4,5-trimethylbenzonitrile/ethene likewise produced ortho photocycloadducts. 

A correlation between free enthalpy of electron transfer and mode of the photoreaction was also constructed for addition of alkenes to benzonitrile. Four areas could be differentiated Full electron transfer, leading to substitution, is only observed if AG < 0 eV cycloaddition to the cyano group occurs if 0 < AG < 0.4 eV. All olefins for which AG > 0.4 eV preferentially undergo cycloaddition to the aromatic ring, ortho cycloaddition if AG < 1.7 eV and meta cycloaddition if AG > 1.7 eV. 

The ortho cycloaddition is thermally forbidden in a suprafacial-suprafacial manner and the photochemical reaction is forbidden with S benzene and ground-state alkene. On the basis of these considerations, it could be understood that the ortho addition had only been observed with systems where the alkene is the lowest excited singlet species (as with maleimides [37,74,75] or where either the alkene or the arene has marked acceptor properties (the only examples known at that time were benzene-acrylonitrile [127] and benzonitrile + a mono-olefin [1,73], Benzene-acrylonitrile and benzonitrile-olefin systems do not display charge-transfer absorption, but charge transfer could well follow excitation. Bryce-Smith further stated that irradiation of benzene in the presence of simple mono-olefins normally provides B2u (Si) benzene as the lowest excited singlet species, which leads to meta rather than ortho addition, but the latter process might, in principle, be able to occur under conditions where a Biu (S2) state of benzene is populated. 

Fig. 14 Isoxazolines synthesis through 1,3-dipolar [3+2] cycloaddition reactions between ortho-quinone monoketals and benzonitrile oxides...

Another aspect of the versatile reactivity of ortho-qaiaont monoketals was exploited by Chittimalla s and Suzuki s groups [51, 52], who reported examples of [3+2] cycloaddition reactions involving non-dimerizing MOBs of type 10 and benzonitrile oxides 39. The reactions are site- and regioselective, primarily affording isoxazolines 40, as a result of the 1,3-dipolar cycloaddition of the nitrile oxides to the MOBs 2,3-C-C double bond (Fig. 14). 

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