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The electrochemical oxidation properties of enamines of the cyclic ketones cyclo-pentanone, hexanone,heptanone, and -0ctanone with the cyclic amines pyrrolidine, piperidine, 1 -methylpiperazine (1 -MP), morpholine (MO), hexa-, and hepta-methyleneimine are investigated with the aid of cyclic voltammetry. The oxidations are totally irreversible. The lifetime of the intermediate cation radicals are shorter than 0.2 ms as determined by double potential step chronoamperometry. The anodic peak potentials depend on the amine component in the order of ring sizes 5 < 7; 8 < 6 < 1 -MP < MO. The variation of the ketone component shows no significant influence on the peak potentials. The ionization potentials of the enamines were measured and correlated with the anodic peak potentials.Since Stork et al.'*2 utilized enamines as reactive intermediates for the alkylation and acylation of carbonyl compounds, their synthetic utility has grown rapidly as documented in a variety of pertinent reviews and monograph^.^ Of great interest are the (2) Ep = ~ E,, is the peak potential, a the transfer coefficient for the

pm-l.' It should nevertheless be noted that the radiationless decay rate of S2 (1, R = H), k,, z 2 X 10" s-l, calculated from = ( 5 f 1) X lo4 and k , z 1 X lo8 s-l, is over 2 orders of magnitude larger than that predicted from an energy gap relation empirically parametrized for polyacene~.~' Inasmuch as this correlation may be considered valid for the heterocycles 34 and 1, this discrepancy may be taken as an indication for the presence of an additional n r * state(s) between the rr* states SI and "S2". Not surprisingly, the protonated forms of 1 exhibit normal Sl fluorescence only, since the S2-SI energy gap is further reduced upon protonation. E.g., a solution of 1 H+ (R = Me) in 0.1 N aqueous sulfuric acid gave rise to a red emission ( A, , ,Another unusual feature is the near degeneracy of the lowest r r * singlet and triplet states of 1 (R = H), ( E @ , ) -E(Tl)( < 10 kJ/mol, and the short lifetime of the latter, T(T], 300 K)

R2 R3Electrochemical investigations on a selection of methylenephosphines 1 and related systems are reported. They are shown to possess an irreversible oxidation and reduction wave.An exception is diaminodiphosphene, which reversibly forms a radical anion. A relative ordering of frontier orbitals is possible, based on the redox properties of these systems. The remarkable elaboration of the synthetic utility of the chemistry of P(II1) double-bond systems, methylenephosphines 1') and iminophosphines 22), forms one of the most pleasing chapters in modern inorganic chemistry. The P(II1) double-bond systems, 1 -3, reveal an ambident behaviour, due to two energetically closely spaced occupied frontier orbitals3). i. e., their electron demand*'. This problem will be dealt with in the present report. We present in detail the first electrochemical investigations of a selection of phosphorus doublebond systems, especially of methylenephosphines 1. We will relate our findings to substituted ethenes, i. e., to typical electron-rich (enamines) and electron-poor (tetracyanoethene) olefins. Results and Discussions Electrochemical InvestigationsA typical cyclic voltammogram of a methylenephosphine 1 is shown in Figure 1. The anodic oxidation as well as the cathodic reduction were irreversible. This is a common feature of all methylenephosphines 1. Even at temperatures of -100°C the redox reactions remain irreversible. The peak potentials for various substituted methylenephosphines 1 are collected in Table 1.This can be seen in the plethora of metal complexes of methylenephosphines 1, iminophosphines 2, and diphosphenes 34-61. In addition, iminophosphines 2 show distinct self-addition behaviour7' alternatively leading either to For completeness we include the first vertical ionization potentialsg) in Table 1.To a first-order approximation, the reduction and oxidation potentials are a quantitative measure of the frontierorbital energies of HOMO and LUMO, even

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