Arenes reactivity correlations

In contrast to oxidations with Mn(III) acetate, the oxidation of alkylbenzenes by the stronger oxidant, Co(III) acetate, appears to involve only electron transfer. No competition from classical free radical pathways is apparent. Waters and co-workers,239,240 studied the oxidation of a series of alkylbenzenes by Co(III) perchlorate in aqueous acetonitrile. They observed a correlation between the reactivity of the arene and the ionization potential of the hydrocarbon which was compatible with the formation of radical cations in an electron transfer process. 

Fig. 12. Li near correlations of the relative reactivity of arenes in electrophilic substitution reactions (log k/kf) with the optical transition energies in the accompanying CT complexes. Data from ref. [62b],...

The same shape of relationship as shown in Fig. 3 is valid for polynitro arenes in Fig. 5 here the impact sensitivity detected by sound correlates with C NMR chemical shifts of the bearers of the most reactive nitro groups in the molecule. According to this diagram, nitro groups in 2-positions of the... 

In the coordinate system shown in Fig. 5, the set of polynitro arenes studied falls into several classes. Classes A and C contain compounds characterised by the trinitrotoluene mechanism of primary fission in their thermal decomposition. Class B represents unsubstituted polynitro arenes (TNB, HNB and NONA) with primary homolysis of C-NO2 bond in their thermal decomposition. Correlation of HNS data with this Class may be a coincidence but it may also be a result of intermolecular interaction in its crystals. Class D contains dipicryl derivatives in which the intermolecular interaction should dominate the influence on their reactivity (primary fission by heat in NONA is different from that in DIPS). The said influence occurs occasionally in larger molecules with strong intermolecular interactions and was observed in some cases of decomposition initiated by impact [36,75], electric spark [35,36] and (depending on the measurement method applied) also heat [102]. 

Workentin et al. have recently reported the results of an extensive laser flash photolysis investigation of the reactions of the cation radicals of 9-phenyl- and 9,10-diphenylanthracene (PA and DPA, respectively) with amines. Primary amines react with both cation radicals via nucleophilic addition with rate constants which reflect both the amine basicity and a steric requirement for bond formation. Steric effects are more pronounced for addition of DPA " vs. PA ", presumably due to the presence of substituents at both the 9- and 10-position. Tertiary amines and anilines react with PA " and DPA " via electron transfer with rate constants which correlate with amine ionization potentials. Rate constants for nucleophilic addition of primary amines are faster in acetonitrile than in acetonitrile/water solution. The rate-retarding effect of water is attributed to an equilibrium between the fiee amine (reactive) and hydrated amine (unreactive). The beneficial effect of water on preparative ET-sen itized photoamination may reflect its role as a catalyst for the proton transfer processes which follow C-N bond formation (Scheme 2). Hydration of the amine also should render it less reactive in primary and secondary electron transfer processes which can compete with the formation of the arene cation radical. 

The rates of radical additions to protonated heteroarenes correlate with the nucleophihcities of the attacking radicals [112] for example, electrophihc radicals such as CH2C02H, CH2CN and CH2N02 do not react with protonated pyridines. Furthermore, the reactivity towards aromatic substitution depends on the electro-philicity of the arene moiety, with the highest rates being observed for addition to the electron-poor 4-cyanopyridinium salts (Scheme 13.15). Similar reactions with the para-methoxy derivative may be up to 3.5 x 10 times slower [112, 113]. 

In addition to directed and intramolecular C-H alkylations, palladium-catalyzed intermolecular C-H alkylations have been reported. In this case, the regioselec-tivity of the C-H bond cleavage is controlled by the natural reactivity of the (hetero)arenes, which corresponds to the longest C-H bond, often correlating with the most acidic C-H bond, in the CMD mechanism [31]. 

Contrary to heteroarenes, only few examples of intermolecular alkylations of the less reactive arenes have been reported. In a seminal example, perylene bisimides have been meta-alkylated with various alkyl halides under palladium catalysis and using CS2CO3 as the base (Scheme 19.25) [39]. The C-H activation step was again proposed to occur through the base-induced CMD mechanism (Scheme 19.18), and the exclusive functionaU2ation at the meta position can be correlated to the higher acidity of the meta C-H bond, as already shown in the related C-H arylation of electron-deficient arenes [29b,c]. 

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