Aryl groups, electronic effects

Extensive information concerning the operation of stereoelectronic effects on the photoreactivity of homobenzylic halides has also been obtained for dibenzo[2.2.2]-systems223-228 such as 51-54. The photo-Wagner-Meerwein rearrangements and photo-solvolyses of these compounds also occur via exclusive223 or preferential224 reaction of the p-C—Cl bond anti to the excited aryl group. Electron transfer between these two moieties is considered to be the key step here too. Indeed, the thermodynamics of such a step seem... 

They display the considerable trans-influence of hydride and aryl groups (the Pt-Cl bond lengths should be compared with the value of c. 2.31 A in PtCl4 ). The Pt-P bond lengths are more insensitive to the phosphine, but by synthesis of ds-Pt(PR3)(PR3)Cl2 [150] complexes, it has been possible to study the effect of electron-withdrawing substituents on the Pt-P bond, as well as on the trans-influence of the phosphine (Figure 3.94). 

Although alkyl and aryl groups attached to heterocycles are still called nonfunctional substituents, in fact they do undergo a variety of reactions and do have important electronic and steric effects on the molecule as a whole. 

The carbonyl n orbital is also assumed to be unsymmetrized arising from the out-of-phase interaction of the orbital attached to the more electron-donating aryl group (9 and 10). These unsymmetrizations of the carbonyl k orbital correspond well to syn addition (9) and anti addition (10), respectively. Thus, the electron-donation of the p-a orbitals controls the facial selectivities. The cyclopentane system was more sensitive to stereoelectronic effects, showing larger induced biases, than the adamantanone system [63]. 

The structural arrangement of group 11 amidinate complexes is determined by the substituents on the amidinate aryl groups as well as on the NCN carbon [14,19]. The electronic vs. steric effect of the substituents on the molecular arrangement of gold(I) amidinate complexes have been studied in detail in the Fackler laboratory. 

Table 5 Electronic effects of R and aryl groups on v(CO) of rhodium(I) carbonyl complex (18).

The arylation of electron-rich arenes, such as azulene (55)206 and heteroarenes, has been sporadically described. Under similar conditions phenols undergo arylation, which is preferably directed at the ort/zo-positions, probably due to the involvement of palladium phenolate intermediates.188,207 Polysubstitution occurs readily.208 The para-position can be attacked only with the sterically hindered 2,6-di-t-butylphenol.209 Similar ortho-diarylation of arenes bearing carbonyl groups (acetophenone, anthrone, benzanilide, etc.) shows that the or//zo-di reeling effect of the substituent is more important than its other electronic effects.189... 

More recently, reductive elimination of aryl ethers has been reported from complexes that lack the activating substituent on the palladium-bound aryl group (Equation (55)). These complexes contain sterically hindered phosphine ligands, and these results demonstrate how steric effects of the dative ligand can overcome the electronic constraints of the reaction.112,113 Reductive elimination of oxygen heterocycles upon oxidation of nickel oxametallacycles has also been reported, but yields of the organic product were lower than they were for oxidatively induced reductive eliminations of alkylamines from nickel(II) mentioned above 215-217... 

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