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

(Tropon-2-ylimino)pnictoranes of the general structure RN=MPh3 (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 crys talline 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 thei r 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 Angstrom) lie below the sum of the van der Waals radi i (3.37 Angstrom), and thus, there is appreciable bonding interaction betwe en the arsine and the oxygen atoms. With a view to constructing a series of cyclohepta-annulated heterocycles and in order to gain a better understand ing of a series of iminopnictoranes, compounds 4-6 were allowed to react wi th heterocumulenes such as carbon disulfide, phenyl isothiocyanate, phenyl isocyanate, and diphenylcarbodiimide, in an aza-Wittig/electrocyclization o r a formal [8 + 2] type cycloaddition eliminating triphenylpnictorane oxide to give 2H-cycloheptaoxazol-2-one, its thione, and imine derivatives. On t he other hand, the reaction of compounds 4 and 5 with dimethyl acetylenedic arboxylate (DMAD) gives postulated dimethyl cyclohepta[b]pyrrole-2,3-dicarb oxylate, which subsequently reacts with DMAD to result in the formation of tetramethyl 2H-cyclohepta[gh]pyrrolizine-1,2,4,5-tetracarboxylate, while th e reaction of 6 gives only intractable tarry materials. The reactivity of t he compounds 4-6, which contain a formal N=M (M = As, Sb, and Bi) double bo nd, 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)].