Ein- und Zweielektronen-Oxidation von Phosphor-Yliden mit Kupfer(II)-Salzen / One- and Twoelectron Oxidation of Phosphorus Ylides Using Copper(II) Salts

Abstract

The reaction of CuCl₂ with equimolar quantities of an ylide R₃P=CH₂ (1a-c, R=CH₃, C₂H₅, C₆H₅) in anhydrous tetrahydrofuran at low temperature leads to oxidative coupling, with ethylidene-1,2-bis-phosphonium salts R₃P^⊕-CH₂-CH₂-P^⊕R₃ 2X^⊖ (2, 3a-c) as high- yield products. The process is interpreted as a one-electron transfer to give radical cations R₃P^⊕-ĊH₂ (II) which subsequently undergo dimerization. The initial counterions CuCl₂ ^⊖ (in 2a-c) can be replaced by PF₆ ^⊖ (as in 3a-c). (C₂H₅)₃P=CHCH₃ (1d) yields the cation (C₂H₅)₃P^⊕-^*CH(CH₃)-^*CH(CH₃)P^⊕(C₂H₅)₃ with two chiral centers as a mixture of diasteromers RR/SS, RS. The ratio of the components is 2,8:1. as evaluated by NMR spectroscopy. The analogous reaction with (C₆H₅)₃P=CHCH₃ and (C₆H₅)₃P=C(CH₃)₂ (1e, f) takes a very different course. Along with minor quantities of (C₆H₅)₃P^⊕CH₂CH₃/(C₆H₅)₃P^⊕CH(CH₃)₂ species, mainly 1-chloroalkyl-phosphonium cations are formed, which were again isolated as the hexafluorophosphates (4a, 4b: [(C₆H₅)₃P^⊕CH(CI)CH₃]PF₆ ^⊕, [(C₆H₅)₃P^⊕C(Cl)(CH₃)₂]Pf₆ ^⊖). H-radical trapping from the medium by II ist proposed for the formation of the former, while a two- electron oxidation followed by addition of Cl^⊖ is the most plausible mechanism for the generation of the latter