UNSUBSTITUTED 1-PHOSPHABUTADIENES 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 o f the (chloromethyl)vinylphosphine 8 and 1-phosphadienes 1 by 1,4-dehy drochlorination of the corresponding (chloroallyl)phosphine 13a (1a) a nd by FVT of diallylphosphines 15a,b via a retro-ene reaction (1a,b). All the dehydrohalogenations occurred either in solution at low temper ature with a Lewis base or in the gas phase (VGSR). Whichever the meth od used, only the opened structures 1a,b and 3a have been observed bot h in solution as well as in the ps-phase. 2-Phosphabutadiene 3a has be en unambiguously characterized in solution by H-1 and P-31 NMR. Adduct s 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-pha se dehydrochlorination of 8 and 13a and thermolysis of the diallylphos phine 15a). Other structural evidence for 1a,b and 3a has been given b y coupling the VGSR or FVT apparatus with the IR, MS, and PE spectrome ters. In particular, the PE spectra of the opened chains 1a and 3a hav e been qualitatively estimated (Koopman's approximation and direct cal culation (CIPSI)). These results are consistent with the experimental IP values [9.28 eV (pi(CP) - pi(CC)), 9.96 eV (n(P)), 11.14 eV (pi(CP) + pi(CC)) for 3a, 9.00 eV (pi(CP) - pi(CC)), 10.13 eV (n(P)), 11.47 e V (pi(CP) + pi(CC)) for 1a]. The possibility of ring closure of la and 3a to dihydrophosphetes 2a and 4a is discussed.