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

Abstract

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)₅P (5) by treating 3, 4, 6 or the known (PhO)₃PNSO₂C₆H₄-2-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)₅P (8). It was only with the strained phosphorane Ph(C₁₂H₈)PNSO₂C₆H₄-4-Me (C₁₂H₈ = 2,2′-biphenylylene) (10) that with 2-C₄H₃OLi the corresponding phosphorane Ph(C₁₂H₈)P(C₄H₃O-2)₂ (11) could be obtained (³¹P NMR: δ −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.