The Reaction of Quadricyclane with p-Anils

The reaction of quadricyclane (Q) with p-anils has been generalized in order to define its scope.Tetrafluoro-, tetrachloro-, and tetrabromo-p-benzoquinones in dichloromethane or chloroform, at or below room temperature give predominant or exclusive formation of the adduct to the C=O bond of the quinone as confirmed by the X-ray structure of the adduct to chloranil.In the case of DDQ (dicyanodichlorop-benzoquinone), the major product is the standard [4+2]-cycloadduct, while the adduct to the C=O bond is formed as minor product and adds to a second molecule of quadricyclane to form bis-adduct (4).The structure of the latter has also been defined by X-ray analysis.In the course of an investigation aimed to the synthesis of C2-symmetric chiral ketones2 we wanted to prepare the double adduct of quadricyclane (Q) to p-anils (A) (i.e.tetrafluoro, tetrachloro, and tetrabromo) as well as with DDQ (dicyanodichloro-p-benzoquinone), in similarity to the double adduct of the parent p-benzoquinone.2.3In the literature it has been reported that p-chloranil forms with quadricyclane a charge transfer complex4and that converts it photochemically to norbornadiene.5 Except for a recent report by Hirao et a1.,6J no indication on the formation of adducts has ever been reported.Indeed, virtually no reaction was observed when quadricyclane and p-chloranil were mixed at room temperature in acetonitrile under the conditions of the photochemical isomerisation, except for a slow conversion of Q into norbornadiene (N).Differently, however, a clean reaction occurs when the two reagents are mixed in dichloromethane or chloroform with ,formation of a highly predominant product plus two other minor compounds as observed in the 1H nmr spectrum of the crude reaction mixture.The predominant product presents analytical data in accordance with a 1 :I adduct to which was assigned, in similarity to the work of Hirao,' structure (1) on the basis of the 1H nmr spectrum, decoupling experiments, considerations on the size of the coupling constants and NOE experiments.An X-ray analysis on the tetrachloro adduct (Figure 1) allowed to confirm the structure assignment.