Showing papers by "George M. Whitesides published in 1971"

Thermal decomposition of vinylic copper(I) and silver(I) organometallic compounds

Abstract: The thermal decomposition of cis-and tans-l-propenylcopper(I) and cis-and. trctns-l-propenyl(tri-n-butylphosphine)copper(t) yields copper(O) and 2.4-hexadienes with i.t.niion of configuration at the olefinic double bcnds. This stereochemical result, when interpreted in light of the known rate of inversion of configuration of vinylradicirl, is sufhcic'nt to establish that free propenyl radicais are not intermediates in these thermal coupling re-ttctions. A comparison of the retention of stereochemistry observed on decomposition of the analogous 2-butenyl-copper(t) and-silver([) compounds with the complete loss of stereochcmistry; observed on reduction of cis-and rruns-2-bromo-2-butene with tri-n-butyltin hydride undcr free-radical conditions qualitatively supports a similar conclusion. A sltrvey of the yietd and stcreochcmistry of the conversion of trun.t-propt-'nyllithium to 2.4-hexadienes on oxidation rvith a variety of transition metalsalts suggests that frec propenytradicais atio arc not inv,olved in these co uplings. fhe reaction of arylrlagnesium and-lithiunr re-I *gents wirh transition nretal halides is widely usecl s.v\"ntheticerlly as a nlerhod lor preparing biarvls.r-; Thesc reactions are believed in most instances to irrvolve transitiorr metal aryls as internrecliates; the isolatecl products result fronr the rapid decomposition of these thermally unstable intermediates under the conditions of the reactions. Alttrough idenrification of the inrer-nrediare orgirnometallic conlpounds in these reactions and independent derlonstration that their thernral decomposition yields coupleci proc-luct have proved possible in special cases,','6 in general the thermal stability of transition metal organometallic compounds has ap-pearc'd suftrciently low to discourage attenrpts to examine the mechanisnrs of this interesting crass of coupling reactions in detail. A number of studies have establisheci copper(l) and silv'er([) as among the nlost usefur oi the transition nretals for the oxidative coupling of arvl and vinylic Grignard reagents.s'7-rr During these studies several-qroups have prepared and characterizecl authentic aromatic and vinylic organonretallic compounds of these me[als.'-r'r The thernial stability of this class of compounds , although low conrpared with the stabilities ol nrain group organometallic cotltpounds, appeared tu Lrs to be sullicient to make a derailed investigation ot' the ir thernral decom position a practical enterprisc. Therefore. in consequence of our in[erest in the mechanisms and synthetic applicarions oi the Ullman and related coupling reactions,r{-r6 as well as in the variety ttt-other organic reactions catalyzeci by compounds of copper .''t'17 we have examined the mechanism of thermal decomposition of vinylic copper(l) and silver(l) organo-metallic conrpounds to butadiene s. A question of fundamental interest in discussion ot' these reactions concerns the mechanism of the step in which the carbon-metal bond is broken and the …