Ionic and radical reactions of some nitro compounds

Abstract

The thesis deals with substitution and elimination reactions of some nitro compounds. The reactions of p—(l-chloro-2,2-dimethylpropyl)— nitrobenzene and p—(2,3-dimethyl—l-nitropropyl)nitrobenzene with ambident nucleophiles of the type (Z)(Y)RC— where Z,Y are electron withdrawing substituents (e.g. CN, COzEt, Ac) were examined. The substitution reactions at the benzylic carbon proceed by the SRNl mechanism. Increased steric hindrance favours an alternate reduction process and so does the presence of acyl groups in the nucleophile. In the absence of severe steric hindrance good yields of C—alkylated products are obtained. The reversibility of the association step of the SRNl reaction was studied. The dissociation of C;alkylated products is extremely slow and it is also clear that the association step in most of the SRNl reactions studied is not reversible. The dynamic lH n.m.r. behaviour of derivatives of the C-alkylates formed in the above SRNl reactions was examined. There is no correlation between the yield and reaction time in SRNl reactions and the energy parameters, determined by dynamic 1H n.m.r., for rotation around the aryl-benzylic carbon bond. Useful information on the conformational changes involved in rotation about the spz-sp3 bond was obtained, however.The mechanisms of elimination of nitrous acid from p-(2-methyl)-2—nitropropyl)nitrobenzene and l-chloro-Z-methyl—Znitro- l-(p-nitrophenyl)propane and their deuterated analogues were studied. Deuterium isotope effects and the stereochemistry of elimination from p-(2,3,3-trimethyl-2-nitro— butyl)nitrobenzene show the elimination from p-(2—methyl-2- nitropropyl)nitrobenzene in 50% dimethylsulfoxide - methanol with sodium methoxide as base is occurring by an E2 mechanism. Comparison of the rate'of elimination from this compound with the analogous compounds in which bromide and chloride are the nucleofuges allows nitrite to be ranked as intermediate in nucleofugacity between the two halides. Determination of activation parameters show that this increased nucleofugacity of nitrite is due to a more positive entropy of activation for the nitrous acid elimination. Deuterium isotope effects and isotopic exchange experiments show that the mechanism of elimination for l-chloro-Z—methyl-2—nitro-l—(p—nitrophenyl) propane in methanol with sodium methoxide as base tends to Ech with an irreversible first step. The electrophilic addition of hydrogen halides and mercury(II) to 8,8- dimethylstyrene and its p-OMe and p—NO2 derivatives gave products that can be rationalized as arising from-the tertiary cation rather than a benzylic cation.