Addition to aromatic rings

Organomagnesium compounds add to unsubstituted arenes only under forcing or Barbier conditions [1]. On the other hand, aromatic rings substituted by electron-withdrawing groups are surprisingly susceptible to attack by organomagnesium compounds. The early work on reactions of hindered aryl ketones [2] has been little further developed [3], despite some reports which should surely be followed up, e.g. [4], 

Reactions in which methoxy groups, ortho to keto, ester or oxazoline groups, are displaced by organomagnesium compounds presumably proceed by an addition-elimination pathway, and are covered in Section 8.2). 

Early investigations of reactions of organomagnesium compounds with nitro compounds led to mixtures of products, and gave little indication that they might be useful [5]. However, it has now been established that with two equivalents of an alkylmagnesium halide in THF, nucleophilic addition to the ring of a variety of nitroaromatic compounds occurs, as summarized in Table 4.1. The product of the addition is a nitronate anion, which may be converted into various products, as illustrated by the example of 2-nitronaphthalene. 

---

Part B of Table 12.2 gives some addition reaction rates. Comparison of entries 19 and 20 shows that the phenyl radical is much more reactive toward addition than the benzy 1 radical. Comparison of entries 22 and 23 shows that methyl radicals are less reactive than phenyl radicals in additions to an aromatic ring. Note that additions to aromatic rings are much slower than additions to alkenes. 

For quite some time, a variety of transition metal salts have been a useful reaction system in the facilitation of phosphorus addition to aromatic rings In addition to the general approaches that have been so available, recent developments also exist. These latter reaction systems have to an extent been noted previously in this volume when considering particular "reaction types," but will be noted again here in a discussion aimed toward a particular application. 

HO -Addition to Monocyclic Aromatic Compounds and Biphenyls. The rate constants for HO addition to aromatic rings are simply calculated by using the rate constant for the unsubstituted compound and the Hammett-cr+ constants. Thus, kHO, (Ar) for addition to a substituted benzene ring can be estimated by ... 

Additions to aromatic rings can become useful when radicals and acceptors are electronically paired. The additions of electrophilic radicals to electron rich aromatic rings are growing in importance and the additions of nucleophilic radicals to electron poor alkenes have long been of preparative value. This chapter can provide only a few representative examples of each class. Giese s book is recommended as a more thorough overview of additions to aromatic rings.232... 

The recent resurgence in the application of radical reactions to organic synthesis has provided only a few examples of additions to aromatic rings however, radical cyclizations should be applicable for con-... 

The method is based on the observations that gas-phase OH radical reactions with organic compounds proceed by four reaction pathways, assumed to be additive H-atom abstraction from C-H and O-H bonds, OH radical addition to >C=C< and -C=C-bonds, OH radical addition to aromatic rings, and OH radical "interaction" with N-, S-, and P-atoms and with more complex structural units such as ->P=S, >NC(0)S- and >NC(0)0- groups. The total rate constant is assumed to be the sum of the rate constants for these four reaction pathways (Atkinson, 1986). The OH radical reactions with many organic compounds proceed by more than one of these pathways estimation of rate constants for the four pathways follow. Section 14.3.5 gives examples of calculations of the OH radical reaction rate constants for the "standard" compounds lindane (y-hexachlorocyclohexane), trichloroethene, anthracene, 2,6-di-ferf-butylphenol, and chloropyrofos. 

The rate constants for OH radical addition to aromatic rings are calculated using the correlation between the OH radical addition rate constant and the sum of the electrophilic substituent constants Xo (Zetzsch, 1982 Atkinson, 1986). As Zetzsch (1982) discussed, Xo is calculated by assuming that steric hindrance can be neglected Xo+ is the sum of all of the... 

As examples of the calculation of OH radical reaction rate constants using the method discussed above (Kwok and Atkinson, 1995), the OH radical reaction rate constants for lindane [y-hexachlorocyclohexane cyclo-(-CHCl-)6], trichloroethene (CHC1=CC12), 2,6-di-tert-butylphenol, and chloropyrofos appear below. As the section dealing with OH radical addition to aromatic rings mentions, at present the rate constant for the reaction of the OH radical with anthracene (and other PAH) cannot be estimated with the method of Kwok and Atkinson (1995). In carrying out these calculations, one first must draw the structure of the chemical (the structures are shown in the appendix to Chapter 1). Then one carries out the calculations for each of the OH radical reaction pathways which can occur for that chemical. 

Useful one carbon additions to aromatic ring systems HCHO/HCl/ZnCl2... 

Atkinson77 and Kwok et al.95 have proposed and discussed methods for the estimation of OH radical reaction rate constants for the PCBs, PCDDs and PCDFs, based on the correlation of the rate constants for OH radical addition to aromatic rings, kOH, with the sum of the electrophilic substituent constants, Ecr+.77,100 Based on a review of the literature rate constants for OH radical addition to a wide range of aromatic compounds, Atkinson77 derived the correlation,... 

Electrophilic addition to alkenes carrying oxygen substituents (enols and enolates) ch21 Electrophilic addition to aromatic rings ch22... 

Unstable dihydro compounds formed by addition to aromatic rings. 

Hydrogen Abstraction Reaction with N, S and -OH = Addition to Triple Bonds = Addition to Olefinic Bonds = Addition to Aromatic Rings = Addition to Fused Rings =... 

---