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There are a number of other instances43'45 where the heuristic models presented above can be used to rationalize unusual stereochemical outcomes in hexenyl radical cyclizations including an interesting case of a tandem cyclization (Scheme XIV).46 ConclusionsIn this study of cyclizations of conformationally rigid 2-but-3-enylcyclohexyl radicals, we have conclusively shown that the stereochemical outcome of the reaction is critically influenced by the orientation of the butenyl side chain; an equatorial butenyl group leads predominantly to 1,5-cis cyclization products, whereas an axial butenyl group preferentially gives rise to 1,5-trans products.47 These stereochemical consequences can be satisfactorily accounted for by the cyclohexane "chairlike" transition states originally proposed by Beckwith and co-workers for acyclic hex-5-enyl radical cyclizations. In related but conformationally less rigid systems, the transition states having an equatorial and an axial butenyl side chain may compete. Minor ring-

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Reactivities of eight phosphate diesters each coordinated to a dinuclear Co(III) complex were investigated ([Co2(tacn)2(OH)2{O2P(OR)2}]3+; tacn = 1,4,7-triazacyclononane). Four of the complexes were coordinated by substituted phenyl methyl phosphates (substituent m-F, p-NO2 (1a); p-NO2 (1b); m-NO2 (1c); unsubstituted (1d)) and two by substituted phenyl 2-hydroxypropyl phosphates (substituent p-NO2 (2a); unsubstituted (2b)). Reactivities of dialkyl phosphates coordinated to the dinuclear Co(III) complex (1,2-propylene phosphate (3); dimethyl phosphate (4)) were also investigated. Hydrolysis of the phosphate diesters in 1a to 1d takes place by intramolecular oxide attack on the bridging phosphate while hydrolysis of 3 principally takes place by intermolecular hydroxide attack on the bridging phosphate. The diester in 2a cleaves by intramolecular oxide attack while that in 2b cleaves by intramolecular transesterification. Dimethyl phosphate dissociates from 4 without any observable cleavage of the diester. These results can be understood in terms of the More O'Ferrall−Jencks energy diagram, and they indicate that there will be considerable cooperativity between the oxide activation and leaving group activation.