Coupled oxidation steric effects

Recently, Fu and coworkers have shown that secondary alkyl halides do not react under palladium catalysis since the oxidative addition is too slow. They have demonstrated that this lack of reactivity is mainly due to steric effects. Under iron catalysis, the coupling reaction is clearly less sensitive to such steric influences since cyclic and acyclic secondary alkyl bromides were used successfully. Such a difference could be explained by the mechanism proposed by Cahiez and coworkers (Figure 2). Contrary to Pd°, which reacts with alkyl halides according to a concerted oxidative addition mechanism, the iron-catalyzed reaction could involve a two-step monoelectronic transfer. 

A consideration of these relationships reveals8 that because E° is a thermodynamic parameter and represents an energy difference between two oxidation states and in many cases the spectroscopic or other parameter refers to only one half of the couple, then some special conditions must exist in order for these relationships to work. The special conditions under which these relationships work are that (a) steric effects are either unimportant or approximately the same in both halves of the redox couple and (b) changes in E° and the spectroscopic or other parameters arise mainly through electronic effects. The existence of many examples of these relationships for series of closely related complexes is perhaps not too unexpected as it is likely that, for such a series, the solvational contribution to the enthalpy change, and the total entropy change, for the redox reaction will remain constant, thus giving rise to the above necessary conditions. 

The redox behavior of chemically and electrochemically prepared PABA in acidic solution (0.5 M HCl) is reportedly similar to that observed for unsubstituted polyaniline [41]. Two sets of redox waves are observed, at 0.18 and 0.74 V, suggesting facile conversion of leucoemeraldine to emeraldine and subsequent conversion to pernigraniline oxidation states. These results suggest that the boronic acid substituent and polymerization conditions have no detrimental influence on the electronic properties of the polymer. This is in contrast to sulfonated polyaniline where the redox couples are more closely spaced than for polyaniline due to the electronic and steric effects of the -SOa" groups on the backbone of the polymer (for details see, Chapter 2, section 2.5.3). 

Taking steric effects into consideration, the introduction of bulky substituents close to the coupling positions may prevent polymerization from proceeding [61]. This fact may be attributed in part to a reactivity decrease, but it seems more likely that the steric constraints produce scarcely conjugated sequences that are difficult to oxidize and, therefore, more susceptible to degradation. 

Microwave irradiation accelerates the rate of Mg(II)-catalysed 1,3-dipolar cycloaddition between mesitonitrile oxide and jS-hydroxy-2-methylene esters, but has little effect on the diastereoisomeric excess. FMO interactions and regiochemical drift due to steric effects have been used to determine the regiochemistry of 3 + 2-cycloadditions of nitrile oxides with a,/3-unsaturated amides. The gas-phase 1,3-dipolar cycloaddition of fulminic acid to ethyne has been investigated using valence-bond theory in the spin-coupled form and using intrinsic reaction... 

Steric effects could be exploited to deter diarylation consequently, 3-substituted pyridine V-oxides garnered the best selectivity for monoarylation (20 1). Notably, the V-oxides of quinoline and benzo[/t]quinoline exclusively underwent C2-arylation this reactivity diverged from the standard preference of benzo[/2]quinoline for C 10-functionalization. Homocoupling between arene coupling partners could be minimized (<5%) using pyridine and excess p3iridine V-oxide. 

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