Although we have not yet tested on .NET 4.5 and do not officially support it, you can run a Sitefinity project on a machine that has .NET 4.5 with no problems. You can also use Visual Studio 2012. The only thing you should have in mind is that all projects should use target framework .NET 4.0, because the Sitefinity assemblies are built for 4.0. Robert, what exactly did not work in your case? Any details? Here is a Sitefinity project open in VS2012 on my machine, running with Cassini (not IIS express). Note the properties screen and target framework for the webapp.
Kind regards, Slavo the Telerik team