Cyclisation of 2-substituted 2-bromomethyl-1,3-dioxacyclohept-5-enes; hydrogen transfer reactions of 1,3-dioxacyclohept-5-enes and 1,3-dithiacyclohept-5-enes

2-Bromomethyl-1,3-dioxacyclohept-5-enes (2-bromomethyl-4,7-dihydro-1,3- dioxepins), containing an additional substituent at the 2-position, cyclise to afford 1-substituted 2,7-dioxabicyclo[3.2.1]-octanes on treatment with tributyltin hydride. The rate constants for cyclisation of the 2-methyl- and 2-phenyl-4,7-dihydro-1,3-dioxepin-2-ylmethyl radicals are 8.4 × 10 ⁵ and 4.9 × 10⁵ s^-1 respectively at 298 K. Hydrogen is readily abstracted from the 4- and 7-positions of 4,7-dihydro-1,3-dioxepins by tert-butoxyl radicals to give 4,7-dihydro-1,3- dioxepin-4-yl radicals which have been characterised by EPR spectroscopy. The Arrhenius parameters for inversion of the 'flap' conformers have been determined from the exchange-broadened spectra of the 2,2-dimethyl and 2,2-diethyl radicals. The analogous radicals, expected on hydrogen abstraction from 1,3-dithiacyclohept-5-enes (4,7-dihydro-1,3-dithiepins). cannot be spectroscopically observed. Instead, the same spectrum, which we attribute to a degradation intermediate, is obtained from a series of 2,2-dialkyl-4,7-dihydro- 1,3-dithiepins.