I am really struggling to find any details on the rearangement of isonitriles to nitriles (isocyanides to cyanides). I have done extensive patent searching with no results and also comprehensive engine searches with no luck.
All I have is a small passage in an organic chemistry book saying that, on heating isonitriles undergo rearrangement, forming nitriles.
Another interesting thing I found :
A newer method of synthesizing nitriles is to pass the vapour of the carboxylic acid or its ester, mixed with ammonia, over heated alumina at 500°c.
John Lennon - Working Class Hero