Observation by flow 1H NMR and dimerization kinetics and products of reactive ortho-quinodimethanes and benzocyclobutadiene

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1990
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Fischer, David
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The reactive o-quinodimethanes, 1,2-dimethylene-1,2-dihydronaphthalene (9) and o-xylylene (1) were observed by flow [superscript]1H NMR spectroscopy at room temperature. The [superscript]1H NMR spectrum of 9, generated in acetonitrile-d[subscript]3 (CD[subscript]3CN) by fluoride ion induced 1,4-elimination from (1-(trimethylsilylmethyl)) trimethyl(2-naphthylmethyl)ammonium iodide (10), was obtained in the absence of precursor and dimers. However, the [superscript]1H NMR spectrum of the more reactive 1, generated in a similar manner from (o-((trimethylsilyl)methyl)benzyl) trimethylammonium iodide (5), could be obtained only in the presence of its stable (4 + 2) and (4 + 4) dimers;The dimerization kinetics of 3-methyl-(5[superscript]'), 3,6-dimethyl-(11), 3-isopropyl-(12), and 3,6-diisopropyl-1,2-xylylene (13) in acetonitrile (CH[subscript]3CN) were studied by stopped-flow UV-visible spectroscopy. These reactive o-quinodimethanes were generated by fluoride ion induced elimination from the appropriate amine salt precursors. The dimerization rates (25°C) for each of these is somewhat slower than that of o-xylylene (1) itself (k[subscript] 5[superscript]'/k[subscript]1 = 0.92, k[subscript]11/k[subscript]1 = 0.42, k[subscript]12/k[subscript]1 = 0.44, and k[subscript]13/k[subscript]1 = 0.02). The dimerization of the unsymmetrical 5[superscript]' and 12 resulted in formation of all four possible (4 + 2) dimers for each. Dimerization of the symmetrical 11 and 13 gave only (4 + 2) dimer for each. In all cases, little or none of the respective (4 + 4) dimer was formed. The [superscript]1H NMR spectra for 5[superscript]' and 11-13 were also observed in CD[subscript]3CN by flow [superscript]1H NMR spectroscopy;Fluoride ion induced 1,2-elimination from 2-trimethylsilylbenzocyclobutenyl-1 mesylate (26) was used to generate the reactive molecule benzocyclobutadiene (1[superscript]') in CD[subscript]3CN, which was observed by flow [superscript]1H NMR spectroscopy at room temperature. The signal for the 4-membered ring protons of 1[superscript]' is at 6.36 ppm, while the multiplets for the 6-membered ring protons are at 6.26 and 5.78 ppm. Based on the positions of the [superscript]1H NMR signals, 1[superscript]' appears to be nonaromatic and not aromatic or antiaromatic;The [superscript]1H NMR spectrum (in CD[subscript]3CN) of 1,2-dimethylene-1,2-dihydrothiophene (1[superscript]''), obtained by fluoride ion induced 1,4-elimination from 3-(trimethylammonium-methyl)-2-(trimethylsilylmethyl)thiophene iodide (11[superscript]'), was observed by flow [superscript]1H NMR spectroscopy at room temperature. The dimerization rate of 1[superscript]'' in CH[subscript]3CN, generated in the same manner, was measured by UV-visible spectroscopy (k[subscript] 1[superscript]'' = 10.8 M[superscript]-1 s[superscript]-1, at 25°C). The reactivity of 1[superscript]'' is thus intermediate between o-xylylene (1) and 1,2-dimethylene-1,2-dihydrofuran (9).

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Chemistry, Organic chemistry
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