The site-specific modification of amines has been a highly sought-after objective in organic synthesis. Despite the rapid advancement of carbon-hydrogen (C-H) bond functionalization methods, effective strategies for carbon-carbon (C-C) bond functionalization of amines remain elusive. Here we report a borane-catalysed method for the selective insertion of alkynes into alkyl C-C bonds of amines, resulting in the ring expansion of cyclic amines and chain elongation of acyclic amines. This approach begins with the cleavage of C-H bonds in amines, then transitioning to C-C bond functionalization upon reaction with alkynes. This method is effective with amines lacking an activating or leaving group and is suitable for late-stage functionalization of pharmaceuticals through C-C bond modification. Furthermore, by coupling this reaction with hydrolysis and hydrogenation steps, successive alkyne insertions are achieved, enabling modular and iterative alkyl growth of amines.