Thomas A. Emokpae

Kinetic and equilibrium studies of σ-adduct formation and nucleophilic substitution in the reactions of 2-phenoxy-3,5-dinitropyridine and 2-ethoxy-3,5-dinitropyridine with aliphatic amines in dipolar aprotic solvents

The reactions of aliphatic amines with 2-phenoxy-3,5-dinitropyridine, 4, and 2-ethoxy-3,5-dinitropyridine, 5, in DMSO result in the rapid reversible formation of anionic sigma-adducts at the 6-position. Kinetic studies show that proton transfer from the initially formed zwitterions to base may be rate-limiting. Slower reactions result, except in the case of 5 and piperidine, in displacement of the 2-substitutent via intermediates which have lower thermodynamic stabilities than their 6-isomers. Base catalysis of the substitution process is attributed in the case of 4 to rate-limiting proton transfer from zwitterionic intermediates, but in 5 to acid catalysis of ethoxide departure (SB-GA mechanism). X-Ray crystallography of 5 shows a planar non-strained structure although the structure of 2-piperidino-3,5-dinitropyridine, 10c, shows distortion resulting from steric interactions of the 2- and 3-substituents. Kinetic and equilibrium results are compared with those for related reactions of the more sterically strained 2,4,6-trinitrobenzene derivatives. Results for the reactions of 4 and 5 with pyrrolidine in three dipolar aprotic solvents are compared. Values of equilibrium constants for sigma-adduct formation decrease in the order DMSO > DMF >> Acetonitrile, while values of rate constants for proton transfer are in the reverse order.

Steric and electronic effects on the mechanism of nucleophilic substitution (SNAr) reactions of some phenyl 2,4,6-trinitrophenyl ethers with aniline and N-methylaniline in acetonitrile and in dimethyl sulfoxide

Rate data are reported for the reactions of a series of 3′- and 4′-substituted phenyl 2,4,6-trinitrophenyl ethers, 4, with aniline in acetonitrile, leading to 2,4,6-trinitrodiphenylamine. The main reaction flux occurs through a base catalysed pathway involving formation of a zwitterionic intermediate, equilibrium constant K1, and rate-limiting proton transfer to base, rate constant kAn. The effects of ring substituents on values of rate and equilibrium constants are discussed. In contrast the corresponding reactions in DMSO involve both uncatalysed and base catalysed pathways. Reactions of 4 with N-methylaniline are extremely slow, but values of rate constants for reaction of 4-nitrophenyl 2,4,6-trinitrophenyl ether were measured using 1H NMR spectroscopy. The value of the parameter K1kAn is lowered by a factor of 105 for N-methylaniline relative to aniline in both acetonitrile and in DMSO. This reduction is attributed to increased steric hindrance both in formation of the intermediate and to proton transfer.

The Structure of a Novel Copper(I) Cyanide Complex with Dimethylsulphoxide, (CuCN)2.2DMSO

Abstract The title complex has been isolated and its structure determined by X-ray crystallography.

Aromatic reactivity. Part LVII. Detritiation of o- and p-Me3Si[CH2]nC6H4·3H in trifluoroacetic acid

The following rate factors f(the specific rates relative to that for [3H]benzene) have been obtained for detritiation in trifluoroacetic acid at 70° of o- and p-Me3Si[CH2]n·C6H4·3H with n= 1–4 : (n= 1) 1, o- 9300, p- 82,000; 2, o- 450, p- 810; 3, o- 270, p- 580; 4, o- 270, p- 690. Some cleavage of the C–Si bonds also occurs, but it is concluded that this does not significantly influence the detritiation rates. The values of the (log f0) : (log fp) ratio all fall within the range 0·865 ± 0·05 previously observed to apply to a wide range of monosubstituted benzenes in electrophilic aromatic substitutions. The rate factors correlate quantitatively with the corresponding factors for cleavage of the compounds o- and p-Me3Si[CH2]n·C6H4·SiMe3 by aqueous methanolic perchloric acid except for the o-isomers with n= 1, and it is suggested that an unusual steric effect reduces the rate of the protodesilylation in this case. The significance of the rate factor for p-Me3SiCH2·C6H4·3H for understanding the ease of and cleavage of ArSiMe3 compounds is discussed, with reference to the importance of hyperconjugative electron-release from Me3Si–CH2bonds. It is argued that, contrary to a recent conclusion, such hyperconjugation can be significant in ground states as well as carbonium ions and transition states.

The kinetics of the reactions of picryl chloride with some substituted anilines. Part 7. 2-Substituted and 2,5-disubstituted anilines

Arrhenius parameters have been measured for the reactions of picryl chloride with the following substituted anilines in acetonitrile: 4-X-(X = F, Cl, Br, I, Me, Or OMe); 3,4-X2-(X = Cl, Br, Me, or OMe); 4-X-3-nitro-(X = F, Cl, or Me); 3-X-4-methoxy-(X = F, Cl, Br, or I); 3-fluoro-4-nitro-; and 3,4,5-trimethoxy-aniline. In the 3,4-disubstituted series the effect of two methyl groups on the free energy of activation is strictly additive, but for the rest the measured rate constants are greater than those calculated on the assumption of additivity. The results are rationalized in terms of the positions of the transition states on the reaction co-ordinate.