Unprecedented Metal-Free C(sp³)−C(sp³) Bond Cleavage: Switching from <i>N</i>-Alkyl- to <i>N</i>-Methyl-1,3,5-triaza-7-phosphaadamantane

N-Alkyl-1,3,5-triaza-7-phosphadamantane halides [PTA-R]X (X = I−, Br−) with more than two carbon atoms in the alkyl chain are shown to easily convert, under mild conditions in aqueous medium, into the corresponding N-methyl derivatives, thus representing a novel type of metal-free C(sp3)−C(sp3) bond cleavage that affects inert, unstrained, and unbranched aliphatic groups. This unusual transformation is promoted by Cu ions and is proposed to occur via a postulated ammonium ylide. In the presence of Cu ions, this C−C bond dissociation is also a key point for the formation of a unique 2D Cu(I) coordination polymer with a rare P,N-coordination mode of the PTA cage.