Chemistry - Action of hot alkaline KMnO4 on tert butyl benzene

Solution 1:

A similar reaction with chromium oxide in alkaline solution has been documented.

The acidic chromium oxide serves the same role as alkaline KMnO4 i.e a strong oxidizing agent. As pointed out by @Waylander in the comments permanganate oxidizes double bonds even in aromatic compounds.

Reference:
$\large{•}$ Chromium Oxidations in Organic Chemistry; By G. Cainelli, G. Cardillo

Solution 2:

For reaction with $\ce{KMnO4}$, the given compound must have a carbon atom attached directly to the ring and that carbon atom must have at least one hydrogen atom. But in case of tert-butylbenzene this condition isn't followed and hence is a special case. The final product obtained on reaction of tert-butylbenzene with $\ce{KMnO4}$ is Pivalic acid $\ce{(CH3)3CCOOH}$ (Common name: Dimethylpropanoic acid).