Ipso adducts, formation

Since the measured yields of the phenolic products corresponded to 30% OH yield, the ipso adduct formation is limited to 10% OH yield. Since the combined G value of m- and -hydroxybenzaldehydes in NjO-saturated solutions in the presence of IrCl is 1.7, only 30% OH seems to add to the ring. The reason for low phenolic yields in the case of benzaldehyde is the possibility of another reaction channel leading to the formation of the exocyclic OH adduct (structure 5) from the addition of OH to the carbonyl group of benzaldehyde [reaction (4)]. 

Several computational studies have been conducted on (Reaction lb) to examine the relative importance of the initial OH addition to the ipso, ortho, meta, and para sites on the aromatic ring. A summary of the computational approaches used by different authors and the reaction energies calculated for the OH-adduct formation for the case of toluene appear in Table 14.3. The data indicate that... 

These features will not be dealt with here. Two points of importance should be stressed however. First, the formation of an ipso adduct may give rise to either nitro group rearrangement... 

Formation of phenol very likely requires an ipso addition of C02 to chlorobenzene, as shown in reaction (R35). The ipso position was also reported to be the preferred site for hydroxyl radical addition to chlorobenzene." The bimolecular reaction of the ipso adduct C02 CHD with HO-CHD radicals of chlorobenzene is an alternative reaction channel yielding benzoic acid, HCI and chlorophenol, reaction (R36). Elimination of HC02 from the ipso adduct, reaction (R37), and subsequent reaction with HO-CHD radicals leads to the formation of phenol, HCI and chlorophenol," reaction (R38). 

For some reason, Dickeson et a/.15 were unable to obtain 4-methoxy-3,5-dinitropyridine from the 4-chloro compound and obtained adduct 5 instead. This failure led them to suppose that adduct 5 would directly form from a C—Cl adduct, as derived from the 4-chloro compound on ipso attack of MeO" to position 4, by solvolysis, according to a previous suggestion by Farmer.50 Actually, 4-methoxy-3,5-dinitropyridine does form from the chloro compound, as described by us,37 and can eventually be converted in situ to 5 in the presence of an excess of MeO". To date there is no evidence for the detection of the C—Cl adduct, even though its intermediacy in the formation of the 4-methoxy compound is expected. 

Several factors have been considered to play a role in determining the stability of a geminally substituted dimethoxy adduct and/or its rate of formation by ipso attack on a methoxy substituted substrate provided with two nitro groups flanking the methoxy group. Such factors include... 

Chloro-l,7-naphthyridine (110 X = Cl) gives on animation with KNH2/ NH3 the tele product 2-amino-1,7-naphthyridine (53) in addition to the ipso product 8-amino-l,7-naphthyridine (54).10 25 The formation of 53 involves as intermediates anionic cr-adduct 111 (X = Cl) (its existence has been proved by NMR spectroscopy see Section I1,B,1) and probably 2-amino-2,8-dihydro-8-chloro-1,7-naphthyridine (112). The latter undergoes a base-catalyzed dehydrochlorination, yielding 53. Because there are four atoms between position 2 and 8. the reaction is called an even tele substitution. 

TABLE 14.3 Reaction energies (kcal mol ) for the formation of the ipso-, ortho-, meta-, and poro-toluene/OH adducts... 

The reaction mechanism reported in the reaction of OH with benzaldehydes is shown in Scheme 2. The OH radical attacks the ring [reaction (2)] and the -CHO group [reaction (4)]. The attack of OH on the ortho-position of benzaldehyde was considered negligible because of the lack of formation of salicylaldehyde under steady-state conditions. The two hydroxycyclohexadienyl radicals (structures 2 and 3) were oxidized to the corresponding phenols in the presence of an oxidant [reaction (3)], while the ipso OH adduct (structure 4) was not. 

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