REACTIVITY OF TRIETHYL PHOSPHITE WITH TETRACHLOROMETHANE : ELECTRON TRANSFER VERSUS IONIC SUBSTITUTION ON "POSITIVE" HALOGEN

Article abstract of DOI:10.1016/S0040-4020(01)89983-X

The reaction <1> of triethyl phosphite (1) with tetrachloromethane (2) has been studied from a mechanistic point of view. 1 reacts at 80 deg C with 2 to form diethyltrichloromethanephosphonate (3) (85-90percent yield) and chloroethane (4) (80percent yield).Several results hint at a radical chain mechanism (like S_RN1).Trichloromethyl radical is trapped by 2,6-di-t-butyl-4-cresol (BHT), the reaction may be initiated with UV radiation (254 nm) and a charge transfer complex (CTC) is formed between 1 and 2 ; furthermore, the reaction is inhibited by 7,7',8,8' tetracyanoquinodimethane (TCNQ).Tris(cyclopropylmethyl)phosphite (12a) and tri(1-hexene-6-yl) phosphite (7a) are used as potential radical clocks in these reactions.The first leads inter alia to 3-chloro-1-butene (17) and the second to 5-chloro-1-hexene (11), the first therefore suggests a radical mechanism but not the second.However in this particular case even the results obtained with the tris(cyclopropylmethyl)phosphite may be rationalized also by an ionic mechanism.For the photostimulated reaction <1>, the overall quantum yield is 0.1.The electrochemical oxidation of 1 with added CCl4 does not account for a radical chain process as the main pathway.Furthermore, the application of Marcus analysis to reaction <1> viewed as an electron transfer leads to a calculated rate constant in the range of 10^-20 M^-1s^-1.The synergy of the techniques that we used lead us to conclude that the thermal reaction is in fact an S_NCl⁺ substitution.The radical intermediates would mainly be derived from the electron-transfer reaction between CCl3^- and CCl4 the importance of which increases when special conditions such as hν activation are applied.Reaction <1> therefore provides an example where the observed paramagnetic species during a D/A interaction could deceptively suggest an electron-transfer between D and A whereas they originate from an interaction between A and an electron donor formed after or during the first step of the reaction.