Unsubstituted 1- and 2-phosphabutadienes: preparation and spectroscopic characterization

The present work is devoted to the preparation of unsubstituted 1- and 2-phosphabutadienes. 2-Phosphadiene 3a is formed by HCl-elimination of the (chloromethyl)vinylphosphine 8 and 1-phosphadienes 1 by 1,4-dehydrochlorination of the corresponding (chloroallyl)phosphine 13a (1a) and by FVT of diallylphosphines 15a,b via a retro-ene reaction (1a,b). All the dehydrohalogenations occurred either in solution at low temperature with a Lewis base or in the gas phase (VGSR). Whichever the method used, only the opened structures 1a,b and 3a have been observed both in solution as well as in the gas-phase. 2-Phosphabutadiene 3a has been unambiguously characterized in solution by 1H and 31P NMR. Adducts of 1a and 3a were isolated when 2-propanethiol was introduced either with the Lewis base and chlorophosphine precursors 8 and 13a or with the condensed products from the VGSR and FVT apparatus (vacuum gas-phase dehydrochlorination of 8 and 13a and thermolysis of the diallylphosphine 15a). Other structural evidence for 1a,b and 3a has been given by coupling the VGSR or FVT apparatus with the IR, MS, and PE spectrometers. In particular, the PE spectra of the opened chains 1a and 3a have been qualitatively estimated (Koopman's approximation and direct calculation (CIPSI)). These results are consistent with the experimental IP values [9.28 eV (πCP - πCP), 9.96 eV (nP), 11.14 eV (πCP + πCP) for 3a, 9.00 eV (πCP - πCP), 10.13 eV (nP), 11.47 eV (πCP + πCP) for 1a]. The possibility of ring closure of 1a and 3a to dihydrophosphetes 2a and 4a is discussed. © 1993 American Chemical Society.