Ipso attack of aromatic systems (1993)
AuthorsGordon, J. L. M.show all
This thesis concerns the chlorination reactions of a series of methyl and chloro substituted 4-chloro-6-methylphenols. The regioselectivity and mode of the chlorine addition reaction to the phenolic ring is examined with respect to the position of the substituents on the phenolic ring, and their properties of either donating or withdrawing electrons. It is shown that suppression of nucleophilic mechanisms of chlorine addition, which used to be achieved through the use of sodium acetate, can be accomplished more efficiently by the choice of acetic anhydride as a solvent. Chlorine reacts with electron rich 2,6-dimethyl phenols by electrophilic mechanisms, with 2,3-addition appearing to be the favoured mode of reaction. This is illustrated by the reactions of two substituted phenols in Chapter Two. Chapter Three describes the reactions of a dichloro methyl phenol which reacts with chlorine preferably by nucleophilic pathways. Electrophilic addition can be forced by the use of acetic anhydride, or by the addition of sodium acetate to the solvent, and the greater effectiveness of the former at suppressing nucleophilic addition is illustrated in this section. The stereochemistry of the chlorine addition is examined through the chlorination reactions of 2,4-dibromo-6-methyl phenol. In Chapter Four, it is shown that nucleophilic attack of chloride ion is the preferred reaction mode of electron deficient trichloro methyl phenols. The two phenols discussed in this Chapter also illustrate the necessity of a vacant C3 ring position for nucleophilic addition to occur. Blocking this site with a chloro substituent slows the reaction drastically. The behaviour of chlorine towards two dichloro dimethyl phenols is examined in Chapter Five. Again, the necessity of an unsubstituted C3 site for nucleophilic addition is demonstrated, with the switching of mechanism from nucleophilic with C3 vacant, to electrophilic with a methyl substituted C3 position. Finally, use of Semi-Empirical calculations is made throughout this thesis, to estimate the relative stabilities of intermediate species and for a theoretical study of the interconversion of various cyclohexadienones by [1,5]-sigmatropic shifts of chlorine.