Unsuccessful/successful attempts to produce penta(heteroaryl)phosphoranes/-arsoranes R5E ( E = P, As; R=2-furyl, 2-thienyl)

Tri(2-thienyl) phosphine (1) has been transformed into chlorotri(2-thienyl) phosphonium chloride (3) in the reaction with hexachloroethane, into tetra(2-thienyl) phosphonium bromide (4) in a NiBr2-catalyzed quaternization with 2-bromothiophene, and into the p-tolylsulfonyliminotri(2-thienyl) phosphorane (6) using chloramine T. Attempts to generate the homoleptic penta(2-thienyl) phosphorane (2-C4H3S)(5)P (5) by treating 3, 4, 6 or the known (PhO)(3)P = NSO2C6H4-2-Me (9) with 2-thienyllithium were unsuccessful. Tri(2-furyl) phosphine (2) was converted into the related imine 7, but the reaction of 7 or of 9 with 2-furyllithium failed to give (2-C4H3O)(5)P (8). It was only with the strained phosphorane Ph(C12H8) P = NSO2C6H4- 4- Me (C12H8 = 2,2'-biphenylylene) (10) that with 2-C4H3OLi the corresponding phosphorane Ph(C12H8) P(C4H3O-2)(2) (11) could be obtained (P-31 NMR: delta - 106.7 ppm). In the arsenic series, tri(2-thienyl)- and tri(2-furyl) arsine (12, 13) were converted into the tosylimino compounds (14, 15) and successfully transformed into the homoleptic arsoranes with 2-C4H3E - Li: penta(2-thienyl)- (16) and penta(2-furyl)-arsorane (17) are stable colourless crystalline solids, the NMR spectra of which indicate rapid pseudo-rotation in solution. The single crystal structure analysis of 17 shows an only slightly distorted trigonal-bipyramidal configuration. In crystals of the phosphine 2 and the arsine 13 the molecules have a propeller configuration with approximate C-3v symmetry for the former, but C-s symmetry for the latter. The crystal structures of the precursors or intermediates 3, 4, 6, 9 and 10 have also been determined.