Mechanism of the Palladium-Catalyzed Metal−Carbon Bond Formation. A Dual Pathway for the Transmetalation Step

Abstract

The mechanism of the transmetalation step in the metal-carbon bond-formation process catalyzed by palladium complexes has been studied by spectroscopic and kinetic methods. The reaction of properly designed model complexes [structure: see text], resulting from oxidative addition of a Mo-I moiety to a palladium center, with aryltributyltinacetylides Bu(3)Sn-C [triple bond] C-(p-XC(6)H(4)) (11a, X = H; 11b, X = Cl) yields the products of transmetalation [structure: see text] (5a,b). The reaction, which shows a strong dependence on the nature of the phosphine ligand PR(3) (Ph > Bu > Me) and less so on the nature of the p-substituent X group, proceeds through two competing pathways, depending on the initial concentration of substrate. At high [3] (approximately equal to 10(-2) M), the transmetalation proceeds through an intermediate species (12) formed by the interaction of complex 3 with 11a. This associative complex accumulates in the presence of added PPh(3) and has been characterized spectroscopically. At low [3] (approximately equal to 10(-4) M), the reaction rate shows an inverse dependence on the concentration of the complex. This is due to the formation of a solvent-coordinate species (13), in which PPh(3) has been substituted by a dimethylformamide (DMF) molecule, as shown by UV-vis and (31)P NMR spectroscopy. Values of k(obs) depend on the concentration and nature of the aryltributyltinacetylides, in agreement with the existence of a kinetically detectable intermediate. A dimeric iodide bridged complex [structure: see text](14) has been obtained during attempts at isolating 13, which changes quantitatively into 13 upon dissolution in DMF and reacts with 11a to give the transmetalation product.

The Mechanisms of the Stille Reaction

“…[198] This reaction is an example of palladium-catalyzed metal--carbon bond formation. [198][199][200][201] Complex 56 is a model of the transition state for the cyclic transmetalation and was shown to evolve to give 57 by first eliminating PPh 3 .…”

“…[198] The involvement of this complex in a parallel transmetalation with the organostannane is another example of the competition of open and cyclic mechanisms. [198] …”

Mechanistic Aspects of Metal‐Catalyzed C—C and C—X Bond‐Forming Reactions

“…The alkenyl stannane then substitutes the DMF ligand to form complex 62, which undergoes transmetallation (associative cyclic mechanism) as shown in Scheme 1-26. A similar process might be operating in the system studied by Lo Sterzo (see Scheme 1-23) [139].…”

“…Pentacoordinated palladium complex 45 was detected spectroscopically and shown to evolve to 46 by first eliminating PPh 3 . Complex 44 exchanged PPh 3 in DMF to form complex 47 and the corresponding iodo-bridged palladium dimer [139]. The involvement of this complex in a parallel transmetallation with the organostannane was proposed to support the dissociative mechanism for the transmetallation reaction [139].…”

“…Lo Sterzo has observed a different precoordination complex in the transmetallation of bimetallic complex 44 with alkynyl stannanes (Scheme 1-23) [139], one of the key steps of the palladium-catalyzed metal-carbon bond formation [139,140]. Pentacoordinated palladium complex 45 was detected spectroscopically and shown to evolve to 46 by first eliminating PPh 3 .…”