The support for .NET 4.5 is introduced in the Q2 2012 SP1 release of RadControls for ASP.NET AJAX. It is recommended to upgrade at least to this version in order to ensure the proper operation of the controls in your .NET 4.5 project.
As for upgrading the RadControls, I would suggest checking this blog post
as it explains the main steps that you need to follow in this case.
All the best,
the Telerik team
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