Synthesis, structure, and reactivity of (tropon-2-ylimino)arsorane and in situ generation of its stiborane and -bismuthorane analogues: reactions with heterocumulenes and an activated acetylene giving heteroazulenes

Abstract

(Tropon-2-ylimino)pnictoranes of the general structure RNMPh₃ (R = tropon-2-yl; M = As, Sb, and Bi) 4–6 have been prepared for the first time by the reaction of 2-aminotropone with Ph₃MX₂ (M = As, Sb, and Bi) in the presence of a base. The arsorane derivative (M = As) 4 is isolated as a stable crystalline compound, while the stiborane (M = Sb) and the bismuthorane (M = Bi) derivatives 5 and 6 are not isolated and are prepared in situ due to their moisture sensitivity. The X-ray crystal analysis revealed that compound 4 exhibits two different conformations in the solid state, and that the As–O bond distances (2.33 Å) lie below the sum of the van der Waals radii (3.37 Å), and thus, there is appreciable bonding interaction between the arsine and the oxygen atoms. With a view to constructing a series of cyclohepta-annulated heterocycles and in order to gain a better understanding of a series of iminopnictoranes, compounds 4–6 were allowed to react with heterocumulenes such as carbon disulfide, phenyl isothiocyanate, phenyl isocyanate, and diphenylcarbodiimide, in an aza-Wittig/electrocyclization or a formal [8 + 2] type cycloaddition eliminating triphenylpnictorane oxide to give 2H-cycloheptaoxazol-2-one, its thione, and imine derivatives. On the other hand, the reaction of compounds 4 and 5 with dimethyl acetylenedicarboxylate (DMAD) gives postulated dimethyl cyclohepta[b]pyrrole-2,3-dicarboxylate, which subsequently reacts with DMAD to result in the formation of tetramethyl 2H-cyclohepta[gh]pyrrolizine-1,2,4,5-tetracarboxylate, while the reaction of 6 gives only intractable tarry materials. The reactivity of the compounds 4–6, which contain a formal NM (M = As, Sb, and Bi) double bond, has been clarified to be in the order of 6 (M = Bi) > 5 (M = Sb) > 4 (M = As) > [the corresponding iminophosphorane derivative 3 (M = P)].