Oxidative addition solvent polarity, effect

In the first step, the carbon centered radical is generated. The second step involves the addition of this radical to the protonated ring. The third step consists of the rearomatization of the radical adduct by oxidation. The rates of addition of alkyl and acyl radicals to protonated heteroaromatic bases are much higher than those of possible competitive reactions, particularly those with solvents. Polar effects influence the rates of the radical additions to the heteroaromatic ring by decreasing the activation energy as the electron deficiency of the heterocyclic ring increases. 

DDQ ( red = 0.52 V). It is noteworthy that the strong medium effects (i.e., solvent polarity and added -Bu4N+PFproduct distribution (in Scheme 5) are observed both in thermal reaction with DDQ and photochemical reaction with chloranil. Moreover, the photochemical efficiencies for dehydro-silylation and oxidative addition in Scheme 5 are completely independent of the reaction media - as confirmed by the similar quantum yields (d> = 0.85 for the disappearance of cyclohexanone enol silyl ether) in nonpolar dichloromethane (with and without added salt) and in highly polar acetonitrile. Such observations strongly suggest the similarity of the reactive intermediates in thermal and photochemical transformation of the [ESE, quinone] complex despite changes in the reaction media. 

The effect of solvent polarity on the rate of the individual steps was also deduced from a comparison of the kinetics determined by IR. It was concluded that, comparing MeOH/Mel (80 20 v/v) with CH2Q2/MCI (80 20 v/v), the overall increase in rate of reaction of [Rh(CO)2l2] with Mel to give [Rh(C(0)Me)( CO)I] included contributions due to enhancement of the forward rates of both oxidative addition (ca. 50%) and migratory insertion (ca. 100%). 

In this study, two Deloxan Metal Scavengers were investigated. The first, THP II, is a thiourea functionalized polysiloxane while the second, MP, is mercapto functionalized. Both resins have been tested in solutions containing 20 - 100 ppm Pd(II), Pd(0) or Ru(II). In addition to different metals and oxidation states, the effects of solvent (polar vs. nonpolar), temperature (25 - 80 °C) and mode (fixed bed vs. batch) were explored. These resins were found to reduce precious metal concentrations in process solutions to levels at or below the target concentration of 5 ppm, even at room temperature in the case of Pd(II) and Pd(0). The results of this study will be discussed. 

The variable regiochemistry observed in the collapse of [Ar, Os04 ] to the cycloadduct A1OSO4 underscores the importance of CIP structures in determining the course of electron-transfer oxidation. Since CIP structures are not readily determined as yet, the structural effects induced by qualitative changes in solvent polarity, salts, additives and temperature are reaction variables that must always be optimized in the synthetic utilization of electron-transfer oxidation by either thermal or photochemical activation. 

Nucleophilicity at the metal promotes oxidative addition, but it is not dominant in determing polar displacement vs. radical mechanisms. Steric and electronic properties of the halide and solvent effects are more important. 

Detailed studies of the mechanism of these reactions have been performed by Mattay and by Kochi . The former has shown that the endo/exo regiochemistry of the ring closure reaction can be controlled either by variation of the silyl group or by addition of polar molecules such as alcohols (probably the source of hydrogen in equations 37a-c). Based on solvent and salt effects, Kochi has proposed that the oxidation of enols to ketones in the presence of activated chloranil proceeds via photoactivation of chloranil which reacts with the silyl enolate through two competing pathways, namely oxidative elimination to the ketone and oxidative addition to the adduct 51 (equation 38). Non-polar solvents such as dichloromethane favour the oxidative eliminations, while polar solvents such as acetonitrile direct the reaction towards the oxidative addition. More strikingly. 

This potential catalyzing effect of polar solvents is supported by the discovery a few years ago of neighboring group participation, or chelate assistance, in aiding a variety of oxidative additions 195). Reactions of HCl, MeCl, MeBr, Mel, CCU, CI2, and PhCOCl with phosphine complexes of Rh(I), Ir(I), or Pt(II) are all enhanced when the phosphine is PMe2(o-MeOC6H4), which contains a nucleophilic MeO group, com-... 

The solvent effects on oxidative addition reactions to square planar iridium(I) complexes have drawn some comment. The general acceleration of the addition reactions of hydrogen, oxygen and methyl iodide to trans-[lrX(CO)(FFhQ)2] (where Xis a. halogen) by polar solvents, such as dimethylformamide, is taken by Chock and Halpern to be evidence for a polar transition state. ° Perhaps more interesting is the stereochemical result of the addition of alkyl and hydrogen halides to these iridium(I) complexes. "... 

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