Observation of a Hidden Intermediate in the Stille Reaction. Study of the Reversal of the Transmetalation Step

Abstract

A study of the reaction of cis-[PdRf2(AsPh3)2] (Rf = 3,5-C6Cl2F3) with ISnBu3 (that is the reversal of the natural Stille reaction of [PdRfI(AsPh3)2] with RfSnBu3) allows for the observation of cis-[PdRf2(AsPh3)(ISnBu3)], the expected intermediate from a cyclic transmetalation in the direct Stille reaction, thus providing experimental support to the operation of cyclic transmetalation pathways.

Stille Coupling Involving Bulky Groups Feasible with Gold Cocatalyst

“…This is a well‐known effect in Stille reactions when transmetalation is the rate‐determining state, and results from the easier displacement of arsane ligands 9. 16 So, AsPh 3 was chosen for this study, and was complemented with some experiments with PPh 3 . Stille coupling reactions, with and without a gold cocatalyst, and with or without added LiCl, were tested using p ‐CF 3 C 6 H 4 I ( 1 ), which allows for easy quantification of the products by 19 F NMR spectroscopy.…”

The Stille Reaction, 38 Years Later

“…26 Study of the retro-transmetalation reaction in Scheme 3, combining kinetic experiments and monitoring by 19 F NMR spectroscopy with DFT calculations, has provided definitive additional confirmation of the cyclic mechanism. 27 This coupling frustration allows for several interesting observations: i) the reaction in Scheme 3 proceeds smoothly, confirming that the transmetalation step is, as expected, reversible; ii) the Stille transmetalation (from the initial reagents to [PdRf 2 L 2 ]) is counter-thermodynamic (3+4 is more stable than 1+2), and for this reason it needs the irreversible coupling step to take place so as to make the overall process thermodynamically favorable; iii) a slow reductive elimination step can be rate determining (or, as in this case, frustrate the coupling): this is an important complication as commented above, because slow coupling will propitiate formation of undesired byproducts from undesired mistaken group exchanges in the reversible transmetalation. This explains, for instance, the observation of Bu 3 SnPf (Pf = C 6 F 5 ) in the Stille coupling of PfI and Bu 3 Sn(vinyl); 9 iv) monitoring the retrotransmetalation allows for experimental observation of the intermediate complex [PdRf 2 (AsPh 3 )(ISnBu 3 )] (I1), which precedes the rate determining state in the retro-Stille evolution and becomes observable.…”

Strong Metallophilic Interactions in the Palladium Arylation by Gold Aryls

“…Inspired by the microscopic reversibility principle, we chose cis‐ [Pd(C 6 Cl 2 F 3 ) 2 (AsPh 3 ) 2 ] ( 1 ), because it is resistant to coupling and allows study of the transmetalation in the reverse sense of the usual coupling reaction. This approach has been used before in the study of the Stille reaction 7. At 50 °C the reaction in Figure 1 was complete in less than 5 min, but the kinetic studies were carried out at −10 °C to make the reaction slower.…”