Protonation of 2<i>H</i>‐Azaphosphirene Complexes: PN Bond Activation and Ring‐Expansion Reactions

Abstract PN bond activation of 2 H ‐azaphosphirene complexes 1 and 2 by using triflic acid led to ring expansion in the presence of nitriles. In the absence of nitriles, the reaction surprisingly afforded two haptomeric N‐protonated 1‐aza‐3‐phospha‐butadiene complexes in the case of complex 1 , whereas the N‐protonated 2 H ‐azaphosphirene complex [H‐ 2 ] + was characterized by NMR spectroscopy. magnified image Protonation of 2 H ‐azaphosphirene complexes 1 and 2 using trifluoromethanesulfonic acid in the presence and in the absence of nitriles was studied. Reactions with nitrile derivatives of different nucleophilicity and steric demand ( 3 a – f ) led to highly bond‐ and regioselective ring‐expansion reactions of complexes 1 and 2 and yielded 2 H ‐1,4,2‐diazaphosphole complexes 4 a – f and 5 a after deprotonation with NEt 3 . Upon protonation of complex 1 in the absence of nitriles a mixture of two haptomeric N‐protonated 1‐aza‐3‐phosphabuta‐1,3‐diene complexes 13 and 14 was obtained, which upon addition of Me 2 NCN ( 3 a ) and NEt 3 afforded 2 H ‐1,4,2‐diazaphosphole complex 16 a bearing a P‐substituent with only one SiMe 3 group. Protonation of complex 2 led to the 2 H ‐azaphosphirenium complex [H‐ 2 ] + , unambiguously identified by NMR spectroscopy at low temperature. All isolated products were characterized by multinuclear NMR spectroscopy, IR, MS, and single‐crystal X‐ray crystallography in the cases of complexes 4 a , 4 e , 6 a , and 16 a . Furthermore, ESI‐MS/MS studies on primarily formed reactive intermediates and DFT studies on the reaction mechanism are presented.