Free-radical reactions of halogenated bridged polycyclic compounds. Part VI. The addition of ethanethiol and thiophenol to 1,2,3,4-tetrachloronorborna-2,5-diene, 1,2,3,4,7-syn-pentachloronorborna-2,5-diene, and 1,2,3,4,7-anti-pentachloronorborna-2,5-diene