Chloro substituent

Table 9.7 contains recent data on the nitration of polychlorobenzenes in sulphuric acid. The data continue the development seen with the diehlorobenzenes. The introduetion of more substituents into these deactivated systems has a smaller effect than predicted. Whereas the -position in ehlorobenzene is four times less reactive than a position in benzene, the remaining position in pentachlorobenzene is about four times more reactive than a position in 1,3,4,5-tetraehlorobenzene. The chloro substituent thus activates nitration, a circumstance recalling the faet that o-chloronitrobenzene is more reactive than nitrobenzene. As can be seen from table 9.7, the additivity prineiple does not work very well with these compounds, underestimating the rate of reaction of pentachlorobenzene by a factor of nearly 250, though the failure is not so marked in the other cases, especially viewed in the circumstance of the wide range of reactivities covered. 

Reactions with strongly basic nucleophiles such as potassium amide in liquid ammonia may prove much more complex than direct substitution. 2-Chloro-4,6,7-triphenylpteridine reacts under these conditions via an S ANRORC mechanism to form 2-amino-4,6,7-triphenylpteridine and the dechlorinated analogue (78TL2021). The attack of the nucleophile exclusively at C-4 is thereby in good accord with the general observation that the presence of a chloro substituent on a carbon position adjacent to a ring nitrogen activates the position meta to the chlorine atom for amide attack. 

In pyrazoles the two most important effects are produced by nitro groups and by quaternization (Section 4.02.3.1.7). Both enhance considerably the reactivity of a second substituent, for example, that of a methyl group towards aldehydes or of a chloro substituent towards amines. 

The 4-chloro substituent markedly stabilizes the 3-ethylenedioxy group in (35) thereby permitting the removal of the 20-ethylenedioxy group which is only partially hindered by the 17a-hydroxyl. ... 

Polymers containing three-coordinate sulfur(IV) are generally hydrolytically sensitive even when the chloro substituents are replaced... 

Vancomycin, ristocetin A and teicoplanin are produced as fermentation products of Streptomyces orientalis, Nocardia lurida and Actinoplanes teichomyceticus, respectively. All three of these related compounds consist of an aglycone basket made up of fused macrocyclic rings and pendant carbohydrate moieties (Fig. 2-1). The macrocycles contain both ether and peptide linkages. The aglycones of vancomycin and teicoplanin contain two chloro-substituted aromatic rings, while the analogous portion of ristocetin A contains no chloro substituents. 

By a suitable choice of chloro substituent and starting glycoside such as in 88, a tetrachloro tetradeoxy glycoside (90) was synthesized (26). 

Several authors studied the influence of substituents on activation parameters. Bad-dar et al.315 who studied the polyesterification of y-arylitaconic anhydrides and adds with 1,2-ethanediol found that in the non-catalyzed reaction a p-methoxy substituent decreases both the activation enthalpy and the entropy whereas an increase is observed with a p-chloro substituent. On the other hand, Huang et al., who studied the esterification of 2,2-dimethyl-l,3-propanediol with benzoic, butanedioic, hexanedioic, decanedioic and o-phthalic add found the same values since the activation enthalpy is 64 kJ mol-1 for the first reaction and 61 kJ mol-1 for the others. 

H2S04). For the former compound the chloro substituent should increase the amount of free amine in solution and hence speed up the rate so that a significant difference in the relative rates in the two acid media should be observed if reaction occurs on the free amine the relative rates were, however, 40 and 37. 

The second method is termed ad eosdem competition— to the same (products, plural) . Consider the solvolysis of 4-chlorobenzyne by methoxide ions and methanol. The nucleophile adds to one end or the other of the triple bond. Because of the 4-chloro substituent, both meta and para isomers are formed. Thus, there are four parallel reactions ... 

Note A surprising aminolytic displacement of a phenylethynyl group has been observed when it occupies a position adjacent to a chloro substituent on the quinoxaline ring. 

Chloro-2-quinoxalinecarbaldehyde oxime (143) gave 3-oxo-3,4-dihydro-2-quinoxalinecarbonitrile (144) (K2CO3, EtOH, reflux, 3 h 82% note concomitant hydrolysis of the chloro substituent). ... 

An approach to the preparation of asymmetrically 1,2-disubstituted 1,2-diamines has been reported the zinc-copper-promoted reductive coupling of two different N-(4-substituted)phenyl aromatic imines, one bearing a 4-methoxy and the other a 4-chloro substituent, in the presence of either boron trifluoride or methyltrichlorosilane, gave a mixture of the three possible 1,2-diamines, where the mixed one predominated [31 ]. Low degrees of asymmetric induction were observed using 1-phenylethylamine, phenylglycinol and its 0-methyl ether, and several a-amino acid esters as the chiral auxiharies meanwhile the homocoupling process was not avoided (M.Shimizu, personal communication). 

The relationship between structure and enhancement of sweetness in chlorodeoxysucroses was studied by Hough and Khan. Examination of a range of mono-, di-, tri-, and tetra-chlorodeoxy derivatives of sucrose and galacto-sucTOse suggested that C-4, C-1, and C-6 appear to be important in the enhancement of sweetness when substituted with chloro substituents. The increase in sweetness is substantial if a combination of two of these hydroxyl groups is replaced for example, the 4,l -dichloride (55) and the r,6 -dichloride (56) were reported to be respectively 120 and 76 times... 

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