Windows 2012 R2 is now a supported operating system after the long awaited release of Exchange 2013 SP1 on February 25th, 2014.
Exchange 2013 is currently supported on:
Windows 2008 R2 SP1
Windows 2012 R2 (Exchange 2013 SP1 only)
** Follow this link to Install Exchange 2013 prerequisites on Windows 2012 or Windows 2012 R2
Exchange 2013 OS Support Chart: