Journal
CHEMICAL SCIENCE
Volume 4, Issue 5, Pages 2028-2035Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c3sc50500f
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Funding
- EPSRC [EP/I003479/1]
- EaStChem
- Engineering and Physical Sciences Research Council [EP/I003479/1] Funding Source: researchfish
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Substituted benzyloxycarbonyloxyl radicals were generated by sensitised photolyses of benzyl oxime carbonates. EPR spectroscopy showed they ring closed exclusively by spiro-cyclisation onto the ipso-C-atoms of the aromatic rings. beta-Scission of the alkoxycarbonyloxyls to CO2 and benzyloxyl radicals increasingly competed and became dominant above 270 K. The first rate parameters for spiro-cyclisations of O-centred radicals onto aromatics were obtained by the steady-state kinetic EPR method. Pentafluoro-substitution of the ring substantially reduced the spiro-cyclisation rate. Activation barriers of the spiro-cyclisations were computed by DFT to be about half those of the alternative ortho-cyclisations. Consideration of the TS structures suggested the preference for spiro-over ortho-cyclisation resulted from better overlap of the oxyl SOMO with the aromatic pi-system during spiro closure.
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