| Abstract
| - The so far accepted mechanism of the Stille reaction (palladium-catalyzed cross-coupling of organotinreagents with organic electrophiles) is criticized. Based on kinetic studies on catalytic reactions, and on reactionswith isolated intermediates, a corrected mechanism is proposed. The couplings between R1I (1) (R1 = C6Cl2F3 = 3,5-dichlorotrifluorophenyl) and R2SnBu3 (R2 = CHCH2, 2a; C6H4-4-OCH3, 2b), catalyzed bytrans-[PdR1I(AsPh3)2] (3a), give R1−R2 and obey a first-order law, robs = a[3a][2a]/(b + [AsPh3]), with a =(2.31 ± 0.09) × 10-5 s-1 and b = (6.9 ± 0.3) × 10-4 mol L-1, for [1] = [2a] = 0−0.2 mol L-1, [3a] =0−0.02 mol L-1, and [AsPh3] = 0−0.07 mol L-1, at 322.6 K in THF. The only organopalladium(II) intermediatedetected under catalytic conditions is 3a. The apparent activation parameters found for the coupling of 1 with2a support an associative transmetalation step (ΔH⧧obs = 50 ± 2 kJ mol-1, ΔS⧧obs = −155 ± 7 J K-1 mol-1in THF; and ΔH⧧obs = 70.0 ± 1.7 kJ mol-1, ΔS⧧obs = −104 ± 6 J K-1 mol-1 in chlorobenzene, with [1]0 =[2]0 = 0.2 mol L-1, [3a] = 0.01 mol L-1). The reactions of 2a with isolated trans-[PdR1X(AsPh3)2] (X =halide) show rates Cl > Br > I. From these observations, the following mechanism is proposed: Oxidativeaddition of R1X to PdLn gives cis-[PdR1XL2], which isomerizes rapidly to trans-[PdR1XL2]. This trans complexreacts with the organotin compound following a SE2(cyclic) mechanism, with release of AsPh3 (which explainsthe retarding effect of the addition of L), to give a bridged intermediate [PdR1L(μ-X)(μ-R2)SnBu3]. In otherwords, an L-for-R2 substitution on the palladium leads R2 and R1 to mutually cis positions. From there theelimination of XSnBu3 yields a three-coordinate species cis-[PdR1R2L], which readily gives the coupling productR1−R2.
|
