The thermal decomposition of cis-and ?ra«.s-l -propenylcopper© and cis-and trans-1 -propenyl(tri-nbutylphosphine)copper(I) yields copper(O) and 2,4-hexadienes with retention of configuration at the olefinic double bonds. This stereochemical result, when interpreted in light of the known rate of inversion of configuration of vinyl radical, is sufficient to establish that free propenyl radicals are not intermediates in these thermal coupling reactions. A comparison of the retention of stereochemistry observed on decomposition of the analogous 2-butenylcopper(I) and -silver© compounds with the complete loss of stereochemistry observed on reduction of cisand f/ww-2-bromo-2-butene with tri-n-butyltin hydride under free-radical conditions qualitatively supports a similar conclusion. A survey of the yield and stereochemistry of the conversion of /razH-propenyllithium to 2,4-hexadienes on oxidation with a variety of transition metal salts suggests that free propenyl radicals also are not involved in these couplings.he reaction of arylmagnesium and -lithium reagents with transition metal halides is widely used synthetically as a method for preparing biaryls.3-5 These reactions are believed in most instances to involve transition metal aryls as intermediates; the isolated products result from the rapid decomposition of these thermally unstable intermediates under the conditions of the reactions. Although identification of the intermediate organometallic compounds in these reactions and independent demonstration that their thermal decomposition yields coupled product have proved possible in special cases,5•6 in general the thermal stability of transition metal organometallic compounds has appeared sufficiently low to discourage attempts to examine the mechanisms of this interesting class of coupling reactions in detail.A number of studies have established copper(I) and silver(I) as among the most useful of the transition metals for the oxidative coupling of aryl and vinylic Grignard reagents.3•7-11 During these studies several groups have prepared and characterized authentic aro-(1) Supported in part by the National Science Foundation, Grants

A referee has suggested an alternative interpretation for the MCE results on CuOEP. This is that the peak at 6912 Á is from the trip-doublet (2T,) designated 2EU In our text while the 6820 A signal arises from the second trip-doublet (2T2) arising from coupling with the B state triplet and thought also to be in this energy region.12 Field-Induced mixing of these states might show similar effects to those observed. We do not pursue this argument here but hope to clarify this question in a future publication describing MCE results on CuOEP at temperatures below 4.2 K

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