Addition to aromatic systems

The high reactivity of azomethine ylides allows addition to aromatic systems (71TL481). For example, trans-aziridine (30) adds to phenanthrene to give the fran5-phenanthropyr-rolidine (31). The reversal of expected stereochemistry is again attributed to azomethine ylide interconversion being allowed by the low reactivity of the aromatic system. 

Other radical reactions not covered in this chapter are mentioned in the chapters that follow. These include additions to systems other than carbon-carbon double bonds [e.g. additions to aromatic systems (Section 3.4.2.2.1) and strained ring systems (Section 4.4.2)], transfer of heteroatoms [eg. chain transfer to disulfides (Section 6.2.2.2) and halocarbons (Section 6.2.2.4)] or groups of atoms [eg. in RAFT polymerization (Section 9.5.3)], and radical-radical reactions involving heteroatom-centered radicals or metal complexes [e g. in inhibition (Sections 3.5.2 and 5.3), NMP (Section 9.3.6) and ATRP (Section 9.4)]. 

Addition to C-Heteroatom Multiple Bonds 5.4.93 Addition to Aromatic Systems... 

Addition to aromatic systems is probably the primary step in a number of more complex nitrene reactions such as the formation of azepines from carbethoxynitrene and benzene and a number of... 

Intramolecular addition to aromatic systems also occurs and De Keukeleire and coworkers have described a fascinating example of such a process. In their example the pyrimidone unit adds to the 1,2-positions of a benzene ring. This reaction takes place in the molecule (170) and occurs with full chemo-, stereo-and regio-selectivity affording the single stable adduct identified as (171). Intramolecular addition is also reported for the pyrimidine derivative (172) which on irradiation at 300 nm in acetone/acetonitrile yields two adducts in a ratio of 4 1 of the general structure (173). The major isomer was isolated by transesterification and was identified as (174). This result is claimed to be the first synthesis of a cis-jy/i-furanoside (2+2)-cycloadduct. ... 

Older work on reversible radical additions to aromatic systems has been reviewed in Ref. 158. 

Additions to carbon-carbon double bonds have already been mentioned. Carbenes also add to aromatic systems, but the immediate products rearrange. 

Catenated Organic Compounds of the Group IV Elements, 4,1 Conjugate Addition of Grignard Reagents to Aromatic Systems, 1, 221 Cyclobutadiene Metal Complexes, 4, 95 Cyclopentadienyl Metal Compounds, 2, 365 Diene-Iron Carbonyl Complexes, 1, 1... 

Conjugate Addition of Grignard Reagents to Aromatic Systems, 1, 221... 

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. 

Addition of 0- to double bonds and to aromatic systems was found to be quite slow. Simic et al. (1973) found that O- reacts with unsaturated aliphatic alcohols, especially by H-atom abstraction. As compared to O, HO reacts more rapidly (by two to three times) with the same compounds. In the case of 1,4-benzoquinone, the reaction with O consists of the hydrogen double abstraction and leads to the 2,3-dehydrobenzoquinone anion-radical (Davico et al. 1999, references therein). Christensen et al. (1973) found that 0- reacts with toluene in aqueous solution to form benzyl radical through an H-atom transfer process from the methyl group. Generally, the O anion-radical is a very strong H-atom abstractor, which can withdraw a proton even from organic dianions (Vieira et al. 1997). 

In addition to the described reduction of double bonds conjugated to a carbonyl group, sodium hydrogen telluride and phenyltellurol reduce double (and triple) bonds conjugated to aromatic systems. " ... 

Additions to carbon-carbon double bonds have already been mentioned. Carbenes also add to aromatic systems, but the immediate products rearrange, usually with ring enlargement (see 5-50). Additions of carbenes to other double bonds, such as C=N (6-61 and 6-62), and to triple bonds have also been reported. 

The 1,4-addition of heterocycles to aromatic systems has been reported. Photolysis of piperidine in benzene, for example, leads to the formation of the -substituted piperidine (222).204 Pyrrole, on photolysis in benzene, behaves differently and yields the 2-substituted pyrrole (223).208 In both instances, excitation of benzene, probably to the triplet, appears to be the initial step in the photolysis. The photolysis of iV-nitrosopiperidine in the presence of anthracene also results in 1,4-addition and the formation of an anthrone oxime,... 

For a review of conjugate additions of Grignard reagents to aromatic systems, see R. C. Fuson, Adv. Organomet. Chem., 1964,1, 221. 

P. M. Treichel and F. G. A. Stone Conjugate Addition of Grignard Reagents to Aromatic Systems Reynold C. Fuson... 

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

However, not only the protonating ability of IIGeCh or systems derived from it determine the addition to aromatic carbon-carbon bonds, in contrast to the behavior of other HX acids. The specific features of HGeCl3 are probably manifested at the step of the cyclohexadiene derivative formation. Energy is obviously lost during the conversion from a-complex to cyclohexadiene. The formation of the cyclohexadiene-GeCl2 molecular complex (the GeCl2 present in the reaction mixture is a result of a well-known reaction, cf. Section III) is likely to be responsible for the equilibrium shift in the direction of the cyclohexadiene. It is likely that application of some other compounds which provide such shift by complexation with cyclohexadiene will enhance the addition of other HX acids to aromatic double bonds. 

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