Studies on the Solvolysis of Hindered Benzylic Substrates and Substituted Acyl Chlorides

• Main Authors: Chen Hung I, 陳宏亦
• Other Authors: Liu Kwang Ting
• Format: Others
• Language: zh-TW
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/74439635431443202438

博士 === 國立臺灣大學 === 化學研究所 === 87 === In the solvolysis of the 1-aryl-2,2-dimethylpropyl substrates (I-T-X，I-B-X，I-C-X), the effect of ortho-methyl group on the reaction center could be reflected in two aspects. One was electron-donating effect and the other was steric hindrance. Introduction of two ortho-methyl groups in the phenyl ring, the steric hindrance will cause the deviation from the coplanarity of the aryl ring and the reaction center, and will change the extent of solvation between solvents and leaving groups (electrophilic pull). From the rate data, the deviation from the coplanarity was the largest for tosylates, which had the earliest transition state of solvolysis among three substrates in this study. The ortho effect in solvents containing trifluoroethanol was smaller because of the better solvation effect, whereas in aqueous methanol, ethanol, and acetone it was larger. On the other hand, the ortho effect was smaller for bromides and was the smallest for chlorides, which was consistent to the later transition state for bromides and the latest transition state for chlorides. The reaction center would be more electron-deficient by introducing the strongly electron-withdrawing trifluoromethyl group at the a position and thus the electron-donating effect of the aryl ring in the transition state of solvolysis would be more important. In the solvolysis of 1-bromo-1-aryl-1-phenyl-2,2,2-trifluoroethane with strong electron-donating substituents in the aryl ring (e.g. II-B-1, II-B-2, and II-B-3), the log ks of the substrates gave excellent linear correlation with YBnBr. Although II-B-4∼II-B-10 showed better linear correlation with YxBnBr, only II-B-9 gave excellent R value. However, the other substrates, such as II-B-5, II-B-6, and II-B-10, excellent linear correlation with II-Q-19 was found. These results suggested the charge distributions in the solvolytic transition state for II-B-X were highly extended. The theoretical calculation of charge distributions in 1-(4''-phenoxyphenyl)-1-phenyl-2,2,2-trifluoroethyl cation indicated a possibility of the existence of non-canonical resonance in the transition state. The substituent constant s+ and the solvent ionizing power YBnCl could also be used to study the mechanism of the solvolysis of aroyl chlorides. The mechanism of the solvolysis of aroyl chlorides would be substituent- and solvent-dependent. We found that III-C-2 solvolysed with limiting SN1 mechanism. The solvolysis for III-C-3, III-C-4, and III-C-5 involved nucleophilic solvent participation. The extent of solvent participation was increasing while the electron-donating ability of the substituent reduced. The solvolysis of III-C-10, the substrate containing strong deactivating substituent, was likely to proceed with with the addition-elimination mechanism. In highly ionizing-power solvents, e.g. 50E, 50A, 60M, and 100T, linear correlations between log ks of III-C-4, III-C-5, III-C-8, and III-C-9 against s+ constants were observed with r values about -2∼-3, which suggested the limiting SN1 mechanism. Moreover, the log ks for III-C-4, III-C-5, and III-C-8 in those solvents showed linear relationship against dDE. A dissociation mechanism could also be deduced. The log ks of N,N-diphenylcarbamoyl chloride were found to give linear correlation with the dual-parameter Grunwald-Winstein equation, mYxBnCl+lNOTs. Obviously, the partial positive charge in the solvolytic transition state delocalized throughout both phenyl rings. The calculation of theoretical charge distributions indicated the same result. Therefore, the possibility of the non-canonical resonance at the transition state could be proposed.