On 1 July 2010, he rejected the prize of one million dollars, saying that he considered the decision of the board of the Clay Institute to be unfair, in that his contribution to solving the Poincaré conjecture was no greater than that of Richard S. Hamilton, the mathematician who pioneered the Ricci flow partly with the aim of attacking the conjecture.
In August 2006, Perelman was offered the Fields Medal (“Nobel in math”) for “his contributions to geometry and his revolutionary insights into the analytical and geometric structure of the Ricci flow”, but he declined the award, stating: “I’m not interested in money or fame; I don’t want to be on display like an animal in a zoo.”
More:
I’d have to check but I seem to remember that Perelman is mostly a misanthropist so calling him based is a bit meh. He’s definitely a very interesting mathematician and deserves great respect for his work though.
And as you mention Grothendieck (and since I’m studying in the university he did most of his research in lol) he’s indeed an anarchist but still, way more of a misanthropist than anything (seems to be a common factor about maths geniuses). Though compared to Perelman he had way more of what we could call a political expression.
Stanisław Lem, an extremely intelligent sci-fi author and technological philosopher, was another example of misanthropist. I think intelligent people are frequently misanthropists, and I can understand why - just look how the world looks like.