Reactivity of Phosphaboradibenzofulvene toward Hydrogen, Acetonitrile, Benzophenone, and 2,3-Dimethylbutadiene

The reaction of 9-bromo-9-borafluorene (1) with Li[PtBu2] in toluene gave quantitatively the corresponding di-tert-butyl-phosphaboradibenzofulvene (9-di-tert-butylphosphanyl-9-borafluorene, 2). Degradation of thf occurred by treatment of 2 with thf in toluene at room temperature. In this paper the reaction of 2 with gaseous H-2 in toluene solution at room temperature is described, by which the corresponding H-2 addition product 4 was formed. The hydrogen addition product 4 crystallizes from benzene in the monoclinic space group P2(1)/n. Addition reactions of 2 with acetonitrile, benzophenone, and 2,3-dimethylbutadiene were also investigated. Treatment of 2 with a 20-fold excess of acetonitrile afforded the corresponding adduct, which itself dimerized to a mixture of cis and trans isomers of the corresponding cycloiminoborane 6. Cocrystals of cis-6 and trans-6 (ratio 2:1) were obtained from toluene in the presence of 20 equiv of acetonitrile at 6 degrees C (monoclinic space group P2(1)/c). The isolation of the pure trans-6 was achieved from toluene in the presence of 2 equiv of acetonitrile at -30 degrees C (triclinic space group P (1) over bar). Benzophenone reacted with the phosphaboradibenzofulvene 2, forming the corresponding addition product 7 (orthorhombic space group Pbca). The reaction of 2 with a 6-fold excess of 2,3-dimethylbutadiene gave the related Diels-Alder adduct 8 (monoclinic space group P2(1)/c).