Chlorination regioselective

GUY LEMAIRE GUETTE Phenol Chlorination Regioselective ortho or para halogenaMon ol phenols and the corresponding ethers (only para) by hexachloro cyciohexadene 2 or 3 respectively. 

In principle, the direct hydride addition or catalytic hydrogenation, which did not give chlorins, was replaced by an electrocyclic intramolecular addition which is much easier with the above system. Complete regioselectivity was also achieved since electrocyclization did not occur with the resonance-stabilized ring C. 

Other interesting regioselective reactions are carried out within the synthesis of nitrofurantoin. Benzaidehyde semicarbazone substitutes chlorine in chloroacetic ester with the most nucleophilic hydrazone nitrogen atom. Transamidation of the ester occurs with the di-protic outer nitrogen atom. Only one nucleophilic nitrogen atom remains in the cyclization product and reacts exclusively with carbonyl compounds. 

The high regioselectivity ( stereoelectronic control ) in the ring cleavage by chlorination of sulfur was anticipated. It had been found before that in corresponding bicyclic systems such as in the scheme below oxidation of the sulfur atom always led to the undesired cleavage of the S—Cg bond. This was rationalized through the observation on molecular models that... 

As for chlorins and bacteriochlorins, reactions at the chromophore periphery of porphyrins can be used to prepare isobacteriochlorins. The lack of regioselectivity in these reactions necessitates the use of highly symmetric porphyrins as starting materials. In some cases, intramolecular reactions have been used to solve the problems of regioselectivity. 

Similiar problems of regioselectivity as in reduction reactions are encountered in oxidation reactions of porphyrins and chlorins. The oxidation of chlorins to isobacteriochlorins can be directed by insertion of zinc(II) or nickel(II) into the macrocycle. Again here, the metal-free chlorins give the bacteriochlorins whereas the metal chlorins, e.g. 1, give isobacteriochlorins, e.g. 3.15a,b I 7... 

The radical chain mechanism of the sulfochlorination is very similar to that of the chlorination. Accordingly, in normal cases the regioselectivities of the sulfochlorination and the chlorination are equal. For example, (-1) substituents decrease the reactivities of the adjacent C-H bond. This influence can even be observed at the y position. Thus, the consecutive second sulfochlorination affords no geminal or vicinal disulfochlorides in the product. Where there are differences between the regioselectivities of sulfochlorination and chlorination (as in the case of isoalkanes), it is because under the conditions of sulfochlorination, chlorination also takes place to a considerable extent. Figure 6 shows the main components of a sulfochlorination mixture. Today the kinetics and the regioselectivity of the sulfochlorination of /z-alkanes are so well known that the kinetic modeling of the concentration-conversion curves is possible for all partners of the reaction [12]. 

Alkoxybenzenes were highly regioselectively halogenated by use of copper(II) halides supported on alumina to give 4-halo-alkoxybenzenes in high yield. Bromination of alkoxybenzenes with alumina-supported copper(II) bromide occurred at lower temperature than chlorination with alumina-supported copper(II) chloride (ref. 14). 

Our own earlier work on the chlorination of toluene had been subject to similar constraints. In this case, chlorination with ferf-butyl hypochlorite had proved to be advantageous. In the presence of silica gel as catalyst the yield of chlorotoluenes was quantitative but the regioselectivity was more or less statistical (ref. 8). However, the use of proton-exchanged zeolite X allowed the production of chlorotoluenes with a para-selectivity of more than 90 % (Fig. 4) (ref. 9). No HCl is generated in this process since the by-product is tert-butanol, and there is no inhibition of the catalyst. Indeed, the catalyst can be reused if necessary. 

It was of interest to see if the use of tm-butyl hypobromite would provide similar opportunities for regioselectivity to those found in the case of chlorination reactions with te/t-butyl hypochlorite. The initial indications were favourable use of H zeolite X as catalyst in dichloromethane solution gave an almost quantitative yield of bromotoluenes with a para ortho ratio of 81 19 (Fig. 5) (ref. 10). 

It has been shown that it is possible to compel regiospecific para substitution by enclosing the substrate molecules in a cavity from which only the para position projects. Anisole was chlorinated in solutions containing a cyclodextrin, a molecule in which the anisole is almost entirely enclosed (see Fig. 3.4). With a high enough concentration of cyclodextrin, it was possible to achieve a para/ortho ratio of 21.6 (in the absence of the cyclodextrin the ratio was only 1.48). This behavior is a model for the regioselectivity found in the action of enzymes. 

The bromine atom is much more selective than the chlorine atom. As indicated on page 906, it is often possible to brominate tertiary and benzylic positions selectively. High regioselectivity can also be obtained where the neighboring-group mechanism (p. 899) can operate. 

When chlorination is carried out with N-haloamines and sulfuric acid (catalyzed by either UV light or metal ions), selectivity is much greater than with other reagents. In particular, alkyl chains are chlorinated with high regioselectivity at the position next to the end of the chain (the to -1 position). Some typical selectivity values are ... 

The direct bromination of 2-hydroxypyridine is regioselective at C-3 whereas chlorination is not. Qu6guiner has developed a halide exchange reaction. Heating 3-bromo-2-hydroxypyridine in pyridine hydrochloride at reflux gives 3-chloro-2-hydroxypyridine in good yield <96TL(37)6695>. The directed aminomethylation of 3-hydroxy-2(lH)-pyridones and 3-hydroxy-4(lfl)-pyridones has been studied <96T(52)1835>. 

Dong, C., Flecks, S., Unversucht, S. et al. (2005) Tryptophan 7-halogenase (PrnA) structure suggests a mechanism for regioselective chlorination. Science, 309, 2216-2219. 

The oxidation of 2,2-dichloro-3-cyclobuten-l-ones 241 with VO(OR)Cl2 results in the regioselective formation of alkyl 2,4,4-trichloro-3-butenoates 242 with chlorination at the a-position of 243 [140]. (Scheme 97)... 

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