Phenanthrene electrophilic addition

Arene radical anions, particularly from polynuclear aromatic hydrocarbons (e.g., phenanthrene, anthracene, and pyrene), generated by ET using amines as the electron donor has been shown [165] to undergo carboxylation reaction (e.g., Phen->189) by the electrophilic addition of CO2, followed by the termination of the resultant radical species by H-abstraction from the solvent (Scheme 39). A laser flash photolysis study [166] has recently confirmed the involvement of arene radical anions in this reaction. 

Phenanthrene has more Kekule resonance structures than does naphthalene, so we expect it to be more aromatic, and the data in Table 4.4 indicate that it does have a larger resonance energy. However, naphthalene does not undergo electrophilic addition, and phenanthrene readily undergoes electrophilic addition across the 9,10 bond. 

Substituted benzene compounds belong to a class of conjugated compounds called arenes. Examples include benzene, naphthalene, anthracene, and phenanthrene. The common structural feature of arenes is a monocyclic or polycyclic system of k electrons that results in a special stability called aromaticity. As a result, aromatic compounds are much less reactive in electrophilic addition reactions than we would expect based on the reactivity of polyenes. 

Although naphthalene, phenanthrene, and anthracene resemble benzene in many respects, they are more reactive than benzene in both substitution and addition reactions. This increased reactivity is expected on theoretical grounds because quantum-mechanical calculations show that the net loss in stabilization energy for the first step in electrophilic substitution or addition decreases progressively from benzene to anthracene therefore the reactivity in substitution and addition reactions should increase from benzene to anthracene. 

Electrophilic substituent factors for the -C6H5 XC1X and -OC6H5 XC1X groups, needed to calculate the OH radical addition rate constants for polychlorinated biphenyls (PCBs), polychlorodibenzo-p-dioxins (PCDDs), and polychlorodibenzofurans (PCDFs), appear in Atkinson (1996), with discussion of an approach to calculating the rate constants for the PCDDs and PCDFs. The room temperature rate constants for the reactions of the OH radical with phenanthrene and anthracene, recently measured by Kwok et al. (1994,1997), are lower by factors of 2.5-8 than the previous recommendations and rate data (Biermann et al., 1985 Atkinson, 1989), casting doubt on the previously proposed correlation between the OH radical addition rate constant and ionization potential (Biermann et al., 1985). 

Both phenanthrene and anthracene have a tendency to undergo addition reactions under the conditions involved in certain electrophilic substitutions. For example, an addition product can be isolated in the nitration of anthracene in the presence of hydrochloric acid. This is a result of the relatively close balance in resonance stabilization to be regained by elimination (giving an anthracene ring) or addition (resulting in two benzene rings). 

Both phenanthrene and anthracene have a tendency to undergo addition reactions under the conditions involved in certain electrophilic substitutions. Halogenation and nitration may proceed in part via addition intermediates ... 

Acrylic esters, thioesters and A-acryloyl pyrrole have been identified by Dixon and Rigby as elfective electrophiles in the enantioselective Michael addition reaction with p-keto esters catalysed by a cinchona alkaloid bearing a bulky phenanthrene group (Scheme 1.27). High yields combined with excellent enantioselectivities of up to 96% ee were obtained in almost all cases of substrates. 

The same principles of resonance, steric considerations, and directing power of substituents apply to larger polycyclic systems, derived from naphthalene by additional benzofusion, such as anthracene and phenanthrene (Section 15-5). For example, the site of preferred electrophilic attack on phenanthrene is C9 (or CIO) because the dominant resonance contributor to the resulting cation retains two intact, delocalized benzene rings, whereas all the other forms require disruption of the aromaticity of either one or two of those rings. 

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