C-X bond-forming reductive elimination

Neutral Pd complexes are typically octahedral and contain two L-type and four X-type ligands. As summarized in the following Unes, four general mechanisms are most commonly proposed for carbon-heteroatom (C-X) bond-forming reductive elimination from these species. 

Dissociative Neutral (Dm). Dm transformations proceed via initial dissociation of an L-type ligand to form a neutral five-coordinate Pd adduct, which is followed by C-X bond-forming reductive elimination. 

This reaction typifies the two possibilities of reaction routes for M-catalyzed addition of an S-X (or Se-X) bond to alkyne (a) oxidative addition of the S-X bond to M(0) to form 94, (b) insertion of alkyne into either the M-S or M-X bond to provide 95 or 96 (c) C-X or C-S bond-forming reductive elimination to give 97 (Scheme 7-21). Comparable reaction sequences are also discussed when the Chalk-Harrod mechanism is compared with the modified Chalk-Harrod mechanism in hydrosily-lations [1,3]. The palladium-catalyzed thioboratiori, that is, addition of an S-B bond to an alkyne was reported by Miyaura and Suzuki et al. to furnish the cis-adducts 98 with the sulfur bound to the internal carbon and the boron center to the terminal carbon (Eq. 7.61) [62]. 

Two possible routes are envisioned for X = B in Scheme 7-21. The authors favored a path involving the oxidative addition of the S-B bond to Pd(0), insertion of the alkyne into the Pd-S bond followed by C-B bond-forming reductive elimination. On the other hand, Morokuma et al. studied the mechanism of the addition of HSB(0CH2)2 (99) to acetylene (C2H2) using Pd(PH3)2 (100) as a catalyst to produce 101 using hybrid density functional calculations (Eq. 7.62) [5]. 

As we have seen, reviewing catalytic S-X bond activations, some reactions complete their catalytic cycles by C-S bond-forming reductive elimination from C-M-S complexes. Hartwig et al. have reported on the mechanism of the C-S bond-forming reductive elimination from Pd(L)(R)(SR ) 122 (Eq. 7.72) [69]. 

In 2002, Yamamoto published the first account of C-O bond-forming reductive elimination from an isolated Pd(IV) complex (11) [37]. As shown in Eq. 14,11 was synthesized by the reaction of Pd2(dba)3 (dba = dibenzylideneacetone), tetra-chloro-l,2-benzoquinone, and benzonorbomadiene, followed by the addition of pyridine. This Pd adduct is stable in the solid state to >100°C and was characterized by H and C NMR spectroscopy as well as X-ray crystallography [38]. 

Most recently, our group has studied C-O bond-forming reductive elimination from Pd complexes of general structure 16 (Eq. 17) [39]. Complex 16 was synthesized by reaction of (phpy)2Pd (pbpy = 2-phenylpyridine) with the hyper-valent iodine oxidant PhI(OAc)2 in CH2CI2 for 1 min at room temperature. This Pd species was stable for at least 12 months at —35°C and was characterized by NMR and IR spectroscopy as well as X-ray crystallography. 

Let us consider the general trends of the reactivity of C-C, C-S, and C-Q (Q = Cl, Br, I) bonds towards oxidative addition and reductive elimination (Scheme 7-25). In many cases, either C-C bond-forming reductive elimination from a metal center (a) or the oxidative addition of a C-Q bond to a low-valent metal center is a thermodynamically favorable process (c). On the other hand, the thermodynamics of the C-S bond oxidative addition and reductive elimination (b) lies in between these two cases. In other words, one could more easily control the reaction course by the modulation of metal, ligand, and reactant Further progress for better understanding of S-X bond activation will be achieved by thorough stoichiometric investigations and computational studies. 

The first case involves direct arylation of arenes with aryl halides via a Pd(0)/Pd (II) cycle (Scheme 8). After initial oxidative addition of Pd(0) into the aryl halide, the C-H bond activation event takes place at the Ar-Pd(II)-X species 4 and is followed by C-C bond forming reductive elimination and regeneration of Pd(0). 

Abstract This work provides a comprehensive review (1986-2010) of the synthesis, characterization, and reactivity of palladium(rV) complexes that undergo car-bon-heteroatom bond-forming reductive elimination reactions. In cases where mechanistic information is available, the molecular pathway for C-X bond formation is described in detail. Examples of catalytic transformations that may involve this mechanistic manifold are also presented. 

Upon standing in acetone solution at 0°C (complex 2) or — 10°C (complex 3), these Pd species underwent competing carbon-chalcogen and C-C bond-forming reductive elimination to form ethane and CHa-XPh (X = S or Se). The yields of nongaseous organic products were determined by NMR spectroscopy and GCMS and are shown in Eq. 8. 

Based on the extensive proposals of carbon-halogen bond-forming reductive elimination from Pd in catalysis, many groups have pursued model studies to investigate the viability and mechanism of such transformations. These studies are detailed in the following lines, and are arranged on the basis of the type of C-X bond that is being constructed (C-1, C-Br, C-Cl, and C-F, respectively). 

Several recent studies have begun to uncover the molecular mechanisms of carbon-heteroatom bond-forming reductive elimination from Pd centers. This recent work has addressed such fundamental questions as the electronic requirements of C-X coupling, the effects of ancillary ligands, the influence of solvent and additives, and the relative rates of competing transformations. The future of this field is bright, as there are stiU many outstanding mechanistic questions to be... 

An account of the redox chemistry of binuclear palladium complexes and the role of binuclear intermediates in Pd-catalysed oxidation reactions has been provided. Stoichiometric organometallic studies of the oxidation of binuclear Pd(II) complexes to binuclear Pd(III) complexes and subsequent C-X reductive elimination from the resulting binuclear Pd(III) complexes, which confirmed the viability of C-X bond-forming reactions mediated by binuclear Pd(III) complexes, has been described. The effect of ligand modification on the structure and reactivity of binuclear Pd(III) complexes has been presented to highlight the impact that ligand structure can exert on the structure and reactivity of binuclear Pd(III) complexes." ... 

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