New and facile substitution reactions at tertiary carbon. Displacement of a nitro group from a saturated carbon atom

Substitution reactions which proceed via radical anion intermediates. 17. Synthesis of highly branched, .beta.-arylated nitroparaffins

“…Ten years ago it was discovered that the aliphatic nitro group of a,p-dinitrocumene ( 1) is readily displaced by a wide variety of nucleophiles as is shown in eq l. evidence now exists in support of the view that these are electron-transfer processes in which radical anions and free radicals are intermediates.3 When the chemistry of a,p-dinitrocumene (1) was first described it was emphasized that the p-nitro group facilitates the displacements of eq 1; in no instance did «-nitrocumene (2) react with nucleophiles under conditions which resulted in complete reaction when a,pdinitrocumene was employed. 2 We have now found that the aliphatic nitro group of anitrocumene (2), and of substituted -nitrocumenes and homologues thereof, can be displaced by nitroparaffin salts, albeit at a distinctly slower rate than when a,p -dinitrocumene is used. What is required is the use of hexamethylphosphoramide (HMPA) as the solvent, rather than the DMF or Me2SO originally employed, and a relatively long reaction time (see Table I).…”

“…Thus, in 45 h cv-nitrocumene and the lithium salt of nitroethane react smoothly (eq 2). While the matter -CH.CHNOxLi (2) has not been studied extensively, it appears that electronwithdrawing substituents facilitate these substitutions; for example, the reaction of eq 3 takes only 8 h and gives 94% yield. This type of reaction also proceeds at 25 °C when the salts of secondary nitroparaffins are employed.…”

“…The relative insensitivity to steric hindrance of the processes of Table I is consonant with the view that they are radical anion reactions (vide infra) and serves to emphasize, once again, the utility of radical anion reactions for the preparation of highly branched structures.3 Several of the transformations listed in Table I have been studied in regard to the matter of mechanisms; in each case the characteristics of electron transfer substitution processes have been observed. Thus, the reaction of the lithium salt of nitroethane with a-nitrocumene (2) requires 45 h to proceed to completion and produces pure 2-phenyl-2-methyl-3-nitrobutane (3) in 74% yield (eq 2). But if di-tert-butyl nitroxide is present at the 9 mol % level the reaction is completely inhibited for 45 h. m -Dinitrobenzene (20 mol %) also retards this reaction; after 45 h it proceeds only 4% to completion, m-Dinitrobenzene is recognized as a diagnostic for radical anions,1 di-tert-butyl nitroxide is a free-radical scavenger1,6•7 and clearly the reaction of eq 2 is a chain process.…”

“…All the transformations of Table I are conducted with exposure to the illumination of two ordinary 20-W fluorescent lamps. However, when a-nitrocumene (2) and the lithium salt of nitroethane are brought together under conditions which result in the reaction of eq 2, except that now the system is maintained in total darkness, no reaction occurs and 83% of the -nitrocumene is recovered. Nor is this a unique result; the reactions of eq 3 and 4 also do not occur in the absence of light.…”

“…One is the dimer of the cumyl radical (9), the other the reduction product of the ß-arylated nitroparaffin (10). In this connection the reaction of a-nitrocumene (2) with the lithium salt of 2-nitropropane is of interest. This reaction is very slow; after 6 days NMR analysis indicates that 22% of the -nitrocumene is still unreacted, that the /3-phenylated nitroethane (2,3-dimethyl-2-phenyl-3-nitrobutane) is formed in 30% yield, and that the bicumyl is produced in 48% yield.…”