Oxidative-addition ligand steric properties

Barrios-Landeros F, Carrow BP, Hartwig JF (2009) Effect of ligand steric properties and halide identity on the mechanism for oxidative addition of haloarenes to trialkylphosphine Pd(0) complexes. J Am Chem Soc 131 8141-8154... 

The rhodium(III) complexes can be prepared either by oxidative addition to the corresponding rhodium(I) complexes or by direct reaction of the ligands with rhodium(ITI) salts. Normally the reducing properties of tertiary polyphosphines ensure that rhodium(I) complexes are formed hence the rhodium(III) complexes of these ligands have been prepared via oxidative addition reactions. However, the sterically hindered ligand (105) fails to reduce hydrated rhodium trichloride even when allowed to react with the latter in refluxing ethanol (equation 260).235... 

The rhodium complexes are more resistant towards oxidative addition than their iridium counterparts and this is believed to be linked to steric crowding, especially when employing bulky tertiary phosphine ligands. Work by Wilkinson explored several aspects of the steric and electronic properties of the rhodium(i) analogues, but definite crystal structural confirmation of the reaction... 

The excellent performance of P(t-Bu)3 as ligand in cross-coupling reactions is attributed to its electron-rich nature and impressive steric demand [17]. Thus, the oxidative addition step in cross-coupling reactions is facihtated by its electronic-donating properties, and it is therefore understandable why challenging substrates can be activated at mild reaction temperatures using P(t-Bu)3. 

Steric properties of the metal center tend to affect ttie rates of oxidative addition and reductive elimination in opposite ways, and these trends, again, result from thermody-nanuc effects. Reductive elimination tends to be faster from complexes possessing more sterically hindered ancillary ligands than from complexes possessing less sterically hindered ancillary ligands. Similarly, with some exceptions, reductive eliminations also tend... 

Certain reaction conditions and properties of the nickel complexes promote hydrocyanation. The oxidative addition of HCN requires an open coordination site. Catalysts containing P(0-o-Tol)3 as ligand are particularly reactive because the imsatu-rated L Ni complex is the most stable form of the Ni(0) complex, whereas the saturated 18-electron L Ni complex is the most stable form of the catalysts containing smaller phosphite ligands. The need to imderstand the steric and electronic properties of the ligand on the dissociation of phosphine led to the classic work of Tolman on cone angles and electronic parameters. ... 

One might imagine that the steric and electronic properties of the ligand would accelerate one step but retard other steps of the catalytic cycle. However, steric hindrance appears to accelerate the rate of both oxidative addition and reductive elimination. The sterically hindered ligands accelerate oxidative addition because they favor generation of the unsaturated intermediate that reacts with the aryl halide (Figure 19.3 compare the rates of Equations 19.55 and 19.56), and they accelerate reductive elimination because reductive... 

Examples of the formylation of aryl halides with synthesis gas catalyzed by palladium complexes are summarized in Equation 19.90. These reactions relied upon the development of ligands with particular steric and electronic properties. The dia-damantyl-n-butyl phosphine shown in the equation, in combination with palladium acetate, leads to the formation of aromatic aldehydes in high yields from electron-rich and electron-poor aryl bromides. Reactions of nitroarenes and 2-bromopyridine provided the aldehydes in low yield, but other examples occurred in satisfactor) yield with only 0.1-0.75 mol % catalyst. The identity of the base is important in this process, and TMEDA was the most effective base. The mechanism of this process was not proposed in the initial work, but is likely to occur by oxidative addition of the aryl halide, insertion of the carbon monoxide into the palladium-aryl bond, and a combination of hydrogenolysis of the acyl intermediate and elimination of hydrogen halide to regenerate palladium(O). The base would then be involved in the hydrogenol5 sis and consumption of hydrogen halide. 

Several studies have been conducted aiming ai the separation of steric and electronic effects [71-73]. For a single step process such as an oxidative addition or one-electron change in electrochemical processes this may be useful, but for multi-step reactions as we are dealing with in catalysis, this technique will encounter many problems. There will be different effects on the distinct steps and linear free-energy relationships will be an exception rather than the rule. When for instance both an oxidative addition step and a reductive elimination step are involved volcano curves must be expected for reactivity versus a ligand property, as in a series of metal oxides when... 

Oxidative addition is still one of the key processes observed in the literature. In fact, a better understanding of the steric and electronic properties of complexes that can promote C—H bond activation throughout this mechanism has been widely studied. The recent work of Conejero and coworkers [13] highlights this interest by the alteration of the environment of the V-heterocyclic carbene (NHC) ligands on a series of stable T-shaped [Pt(NHC )(NHC)][BAFj, where NHC represents the cyclometallated ligand, species (Fig. 25.6). 

Several studies reported the application of established five-membered ring (imidazol-2-ylidene type) NHC ligands in Heck chemistry. Whilst most authors generally focus on catalytic activity, Jutand and co-workers examined the mechanism of oxidative addition of aryl halide to NHC-Pd complexes. Complexes [Pd L2] (L = saturated NHC) were generated electrochemically and tested in situ for oxidative addition. They proposed that either an associative or dissociative mechanism could operate, depending on steric and electronic properties of the NHC ligand. 

---