Substitution reactions with benzene

While complexes 1-5 did not promote saturated alkane or fluorocarbon activation, the lessons learned from die synthetic and reactivity studies have led to the synthesis of the tris(triflate) complex 13, which is considerably more reactive due to the labile triflate ligand sphere. A promising example of the enhanced reactivity of 13 is its electrophilic substitution reactions with benzene and pyridine to form titanium-carbon bonds. These "titanations" are formal carbon-hydrogen bond activation processes and should allow access to the many useful organic reactions that titanium-carbon bonds are known to undergo (37,38). 

Unlike benzene, pyridine undergoes electrophilic aromatic substitution reactions with great difficulty. Halogenation can be carried out under drastic conditions, but nitration occurs in very low yield, and Friedel-Crafts reactions are not successful. Reactions usually give the 3-substituted product. 

In the case of 1,2-dibenzoylbenzene systems 3, however, it is possible to obtain the 1,4-diazocine derivatives 4 on reaction with benzene-1,2-diamines.13 - 18 The eight-membered ring exists in a tub conformation, which exhibits such a high inversion barrier that in the case of unsym-metrically substituted systems it is possible to isolate the corresponding enantiomers. 

Arenes are unsaturated but, unlike the alkenes, they are not very reactive. Whereas alkenes commonly take part in addition reactions, arenes undergo predominantly substitution reactions, with the TT-bonds of the ring left intact. For example, bromine immediately adds to a double bond of an alkene but reacts with benzene only in the presence of a catalyst—typically, iron(III) bromide—and it does not affect the bonding in the ring. Instead, one of the bromine atoms replaces a hydrogen atom to give bromobenzene, C H Br ... 

For many years phenol was made on a large industrial scale from the substitution reaction of benzene sulfonic acid with sodium hydroxide. This produced sodium sulfite as a by-product. Production and disposal of this material, contaminated with aromatic compounds, on a large scale contributed to the poor economics of the process, which has now been replaced by the much more atom economic cumene route (see Chapter 2, Schemes 2.2 and 2.3). 

In a different study, anthracene, phenanthrene, perylene 93 (Fig. 31), and 2,7-di-tert-butylpyrene underwent regioselective oxidative-substitution reactions with iodine(III) sulfonate reagents in dichloromethane to give the corresponding aryl sulfonate esters. The use of [hydroxy(tosyloxy)iodo]benzene, in conjunction with trimethylsilyl isothiocyanate, led to thiocyanation of the PAH nucleus. 

The trend observed with the polycyclic hydrocarbons (see preceding section), namely that the product radical anions (ArNu ) are more stable than those derived from the simple benzene analogs, is even more evident with the heteroaromatic substrates and, as a consequence, fragmentation processes are minimized.41 For example, 2-chloroquinoline is the only substrate of many studied to undergo a substitution reaction with PhCH2S- ion without fragmentation of the benzyl-S bond,103 and to react with diphenyl-arsenide ion without scrambling of the aryl moieties.25... 

Historically, the cyclic structure of benzene with symmetry, as shown in Fig. 7.3.15, was deduced by enumerating the derivatives formed in the mono-, di-, tri-substitution reactions of benzene. The structure can also be established directly using physical methods such as X-ray and neutron diffraction, NMR, and vibrational spectroscopy. We now discuss the infrared and Raman spectral data of benzene. 

Energy profiles with a deactivating group. Nitrobenzene is deactivated toward electrophilic aromatic substitution at any position, but deactivation is strongest at the ortho and para positions. Reaction occurs at the meta position, but it is slower than the reaction with benzene. 

Since we introduced conjugate addition in Chapter 10, a number of new reactions have been covered and a number of new nucleophiles introduced. Some of these can lead to conjugate addition. One important new reaction is electrophilic aromatic substitution, which we met in the last chapter. Michael acceptors can combine with Lewis acids to provide electrophiles for reactions with benzene derivatives. 

Benzene can undergo substitution reactions with halogens, HNO3, all l halides (RX), and H2SO4. 

---