The so far accepted mechanism of the Stille reaction (palladium-catalyzed cross-coupling of organotin reagents with organic electrophiles) is criticized. Based on kinetic studies on catalytic reactions, and on reactions with isolated intermediates, a corrected mechanism is proposed. The couplings between R, and [AsPh 3 ] ) 0-0.07 mol L -1 , at 322.6 K in THF. The only organopalladium(II) intermediate detected under catalytic conditions is 3a. The apparent activation parameters found for the coupling of 1 with 2a support an associative transmetalation step (∆H q obs ) 50 ( 2 kJ mol -1 , ∆S q obs ) -155 ( 7 J K -1 mol -1 in THF; and ∆H q obs ) 70.0 ( 1.7 kJ mol -1 , ∆S q obs ) -104 (). The reactions of 2a with isolated trans-[PdR 1 X(AsPh 3 ) 2 ] (X ) halide) show rates Cl > Br > I. From these observations, the following mechanism is proposed: Oxidative addition of R 1 X to PdL n gives cis-[PdR 1 XL 2 ], which isomerizes rapidly to trans-[PdR 1 XL 2 ]. This trans complex reacts with the organotin compound following a S E 2 (cyclic) mechanism, with release of AsPh 3 (which explains the retarding effect of the addition of L), to give a bridged intermediate [PdR 1 L(µ-X)(µ-R 2 )SnBu 3 ]. In other words, an L-for-R 2 substitution on the palladium leads R 2 and R 1 to mutually cis positions. From there the elimination of XSnBu 3 yields a three-coordinate species cis-[PdR 1 R 2 L], which readily gives the coupling product R 1 -R 2 .

AuR 1 (tht)] (3a) (R 1 ) 3,5-C 6 Cl 2 F 3 , tht ) tetrahydrothiophene) very efficiently catalyzes the isomerization of trans-[Pd(R 1 ) 2 (tht) 2 ] (1a) to cis-[Pd(R 1 ) 2 (tht) 2 ] in CDCl 3 . The 19 F NMR kinetic study leads to the first-order rate law r iso )and 3a, evidencing that the catalyzed isomerization takes place with aryl-group exchange between Pd(II) and Au(I). An associative mechanism passing through R-bridged intermediates [(tht)(R 1 ) 2 Pd(µ-R 2 )Au(tht)] and a donor-acceptor activated complex [(tht)(R 1 ) 2 (R 2 )PdfAu(tht)] q is proposed. The results suggest that the associative displacement of tht from 1a by the nucleophilic arylgold(I) complex to give [(tht)(R 1 ) 2 Pd(µ-R 2 )Au(tht)] is the rate-determining step (k 1 ). This is supported by the typical bimolecular activation parameters that were found: ∆H 1 q ) 56.4 ( 1.6 kJ mol -1 and ∆S 1 q ) -46 ( 6 J K -1 mol -1 .

The relative importance of the factors contributing to the accelerating effect of CuI on [PdL 4 ]-catalyzed couplings of R 1 I and R 2 SnBu 3 (copper effect) has been quantitatively evaluated in THF for R 1 ) 3,5-C 6 Cl 2 F 3 ; R 2 ) vinyl, C 6 H 4 -4-OMe; L ) AsPh 3 , PPh 3 , using spectroscopic and kinetic methods. The 19 F NMR kinetic data show that the rate enhancement produced by addition of CuI is strongly related with the "autoretardation" effect intrinsic to [PdL 4 ] catalysts and is almost independent of the organotin reagent (vinyl, aryl). The "autoretardation" is due to the release of 2 equiv of L during the oxidation of [PdL 4 ] to yield trans-[PdR 1 IL 2 ], which is the species undergoing transmetalation. CuI does not promote the dissociation of L from trans-[PdR 1 IL 2 ], but it captures part of the free neutral ligand L and therefore mitigates the autoretardation produced by the presence of free L on the ratedetermining associative transmetalation. In the conditions studied (Pd:Cu ) 1:2; T ) 322.6 K; THF as solvent), for L ) AsPh 3 the CuI added captures about 25% of the free AsPh 3 and the copper effect compensates only ca. 1% of the autoretardation, whereas for L ) PPh 3 the CuI captures about 99% of the free PPh 3 and the compensation is about 30%. This remarkable variation is caused by the combined effect of two independent factors: (i) The catalyst [Pd-(PPh 3 ) 4 ] is more autoretarded than [Pd(AsPh 3 ) 4 ]; and (ii) CuI is a more effective scavenger for PPh 3 than for AsPh 3 .