Peter B. D. de la Mare

The kinetics and mechanisms of additions to olefinic substances. Part XII. Kinetics of addition initiated by chlorine acetate

The kinetics of the reactions of electrophilic chlorine acetate with methyl p-nitro-trans-cinnamate in acetic acid and in acetic acid containing various proportions of water, and of the similar reactions of methyl p-chloro-trans-cinnamate, of trans-1-nitro-2-phenylethylene, and of acrylic acid in aqueous acetic acid are described. The results confirm that the principal reagent effective under these conditions is molecular chlorine acetate. Catalysis by added mineral acid, indicating reaction through a protonated species, is found also. The products of the reactions of chlorine acetate and of chlorine with trans-1-nitro-2-phenylethylene are described also.

The kinetics and mechanisms of aromatic halogen substitution. Part XXXI. The reaction pathway involved in the chlorination of N-acetylcarbazole

We have confirmed that the chlorination of N-acetylcarbazole in anhydrous acetic acid involves direct electrophilic chlorination, contrary to a recent proposal.

Naphthalene tetrachlorides and related compounds. Part IV. Photochemical chlorination of 1-chloronaphthalene

The photochemical chlorination of 1-chloronaphthalene gives a mixture of dichlorides, which on further chlorination give several new 1-chloronaphthalene tetrachlorides. The structures of the latter have been elucidated by using 1H n.m.r. spectroscopy and by isomerisation with aluminium trichloride as the 1,1,r-2,t-3,c-4- and 1,1,r-2,-c-3,t-4-pentachlorotetralins; and the r-1,t-2,t-3,c-4,5-; r-1,t-2,c-3,c-4,5-; and r-1,t-2,c-3,t-4,5-pentachlorotetralins. Evidence is presented to suggest that the last compound, because of internal non-bonding strain, exists predominantly in an unusual ‘half-boat’ conformation. The results are compared with those obtained by heterocyclic chlorination, from which another isomer, r-1,c-2,t-3,t-4,5-pentachlorotetralin, is a minor product. The course and products of alkaline dehydrochlorination have been used to support some of the assignments of structure. The heterolytic chlorination of 1,2-dichloronaphthalene has been examined also; the major product is r-1,t-2,c-3,t-4,5,6-hexachlorotetralin, which also exists in a ‘half-boat’ conformation.

Naphthalene tetrachlorides and related compounds. Part V. Chlorination of 1,5-dichloronaphthalene; conformations affected by intramolecular non-bonding repulsions

The chlorination of 1,5-dichloronaphthalene in chloroform or in dichloromethane is heterolytic in character, but gives little or no trichloronaphthalene. The main tetrachloride product is 1,1,r-2,t-3,c-4,5-hexachlorotetralin, previously thought to be its t-4-isomer. The latter compound has been prepared by isomerisation of the former with aluminium trichloride. The structures have been established by using 1H n.m.r. spectroscopy together with reactions with aluminium trichloride and with sodium methoxide. The former tetrachloride exists in a deformed conformation as the result of intramolecular non-bonding repulsions. trans-1,2,4,8-Tetrachloro-1,2-dihydronaphthalene is an intermediate in the chlorination.

The kinetics and mechanisms of additions to olefinic substances. Part X. Stereochemistry of the chloro- and bromo-hydrins derived from cinnamic acid, and of some derivatives thereof

The racemic chlorohydrins and bromohydrins derived from cinnamic acid, together with their methyl esters, the acetates thereof, and the derived epoxides, have been prepared by routes which, when associated with their physical and chemical properties, establish their structures, including their stereochemistry. The results allow correction of some assignments of structure given in the literature.

The kinetics and mechanisms of additions to olefinic substances. Part XI. Stereochemistry of addition of chlorine acetate and of chlorine to some unsaturated compounds

The proportions of diastereoisomeric acetoxy-chlorides formed by addition of chlorine acetate to methyl cinnamate and to some of its derivatives, to phenanthrene, to acenaphthylene, and to cyclohexene have been estimated in acetic acid as solvent under a number of conditions. In a number of cases the ratios differ markedly from those of the same products formed by addition initiated by electrophilic chlorine in the same solvents. Different carbonium ion intermediates must be involved in the reactions initiated by the two reagents; the mechanisms of these reactions are discussed. It is concluded that the reactions initiated by chlorine acetate involve sequences in which the products come largely from chloronium ions, whereas those initiated by chlorine involve sequences in which the products often come from zwitterionic intermediates.

The kinetics and mechanisms of additions to olefinic substances. Part 13. Reactions of 3-substituted cholest-5-enes with sources of electrophilic bromine

The kinetics of second-order bromination in acetic acid, and of third-order bromination in chlorobenzene, have been examined for a series of 3β-substituted cholest-5-enes; the results confirm that the rates of these reactions are subject to polar influences approximately of the magnitude expected for a substituent insulated by two saturated carbon atoms from the site of development of the carbocationic centre. The products of reactions of a number of these substrates with sources of electrophilic bromine and chlorine, in chlorobenzene, chloroform, and acetic acid as solvents, have been investigated by 1H n.m.r. spectroscopy. In most cases the main products are the expected 5α-halogeno-6β-substituted adducts. 5β,6α-Adducts are formed in minor amounts also in the bromination of 3β-trifluoroacetoxycholest-5-ene, and in the reactions of a number of 3β-substituted cholest-5-enes with bromine chloride in deuteriochloroform and in chlorobenzene. Some of the mechanistic implications of these findings are discussed.

The kinetics and mechanisms of additions to olefinic substances. Part 14. Reactions of cholest-5-en-3-one with electrophilic brominating agents

Electrophilic brominations of cholest-5-en-3-one with molecular bromine and with bromine chloride have been studied in several solvents. The major products are 5α,6β-adducts and the 6-bromocholest-4-en-3-ones; in some circumstances a small proportion of 5β,6α-adduct was recognised. Results obtained by using bromine chloride establish that the diaxial adducts arise by electrophilic attack on both the α- and the β-face of the steroid. 6α-Bromocholest-4-en-3-one is formed by proton loss from the ionic α-bromonium intermediate; study of 4β-H/D isotope effects on the rates and products of bromination establish that 6β-bromocholest-4-en-3-one is obtained not only by this route, in which the 4α-proton is preferentially released, but also by way of a concerted, stereoselectively syn, SE2′ mechanism. In the presence of excess of halide ion, substitution is reduced in proportion through catalysis of an AdE3 mechanism involving electrophilic attack on the α-face of the molecule to give the diaxial adduct. The pathways available for substitution and addition in cholest-5-en-3-one are compared with those found for the 3β-substituted cholest-5-enes (accompanying paper).

The kinetics and mechanisms of aromatic halogen substitution. Part XXXIII. Kinetics and products of chlorination of some aryl acetates

The rates and products of reaction of 4-methylphenyl acetate, 3,4- and 2,6-dimethylphenyl acetate, and 2,3,5,6-tetramethylphenyl acetate with molecular chlorine in acetic acid have been studied. ‘Abnormal’ reaction paths play a large part in the determination of the products of the chlorination of the first two of these compounds, probably through electrophilic attack on an activated ‘ipso’-position. The observed rates of reaction are compared with those calculated on the basis of additivity of substituent effects; they allow an estimate to be made of the effect of steric inhibition of resonance on the directing power of the acetoxy-group. The product of chlorination of the corresponding phenols are recorded also.

The kinetics and mechanisms of aromatic halogen substitution. Part XXXII. The directive power of the acetoxy-group

The rates and products of chlorination of acetoxybenzene, the diacetoxybenzenes, and 4-acetoxyacetanilide by molecular chlorine in acetic acid at 25° have been examined. The results allow calculation of partial rate factors for chlorination directed by the acetoxy-group to the ortho-, meta- and para-positions. The extent to which the observed rates and products for chlorination of the diacetoxybenzenes accord with estimates calculated by using the ‘ additivity principle ’ is discussed. The response of change in rate to change in structure for the chlorination of 4-substituted anilides (4-RC6H4NHAc, where R is an electron-withdrawing group) is less than would be expected from the rates of chlorination of monosubstituted benzenes containing electron-releasing groups. Possible reasons for this difference are discussed in terms of factors which may affect the electron-releasing power of activating substituents. Further evidence that molecular chlorination of simple aromatic compounds is accompanied by minor, but significant, contributions from sequences initiated by addition of chlorine is presented. Little indication of contributions from chlorodeacylation has been found under the conditions used in the present investigation.