This is by design, something you can see when you navigate to c:\windows\microsoft.net\framework with Windows Explorer. Note that you'll only see a subdirectory named v4.0.30319, there is no v4.5 subdirectory. Or in other words, .NET 4.5 is a true in-place update for version 4.0 and the C# v5 compiler replaces the v4 compiler.
You'll need to specify "v4.0".
