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 H2 in toluene solution at
room temperature is described, by which the corresponding H2 addition product 4 was formed. The hydrogen addition
product 4 crystallizes from benzene in the monoclinic
space group P21/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 °C (monoclinic space group P21/c). The isolation of the
pure trans-6 was achieved from toluene
in the presence of 2 equiv of acetonitrile at −30 °C (triclinic
space group P1̅). 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 P21/c).