Theoretical Studies on the Mechanism of C−P Bond Cleavage of a Model α-Aminophosphonate in Acidic Condition

Abstract

Various reaction paths of the P-C bond cleavage of alpha-aminophosphonates in acidic media, resulting in the derivatives of phosphonic acid, has been investigated using density functional level of theories in the gas phase as well as in aqueous medium. Dimethyl (alpha-anilinobenzyl)phosphonate has been used as the model molecule and our investigation confirms a three steps process including protonation, P-C bond cleavage, and the transformation of the products from the final transition state (imine cation and H-phosphonate) through hydrolysis. The most favorable reaction path starts from the amino group protonation, followed by a proton transfer through N-H...O(P) hydrogen bond, and the P-C bond cleavage from the resulting protonated structure. Explicit inclusion of water molecules indicated that two waters are needed for the P-C bond cleavage, and the calculated mechanistic paths in this hydrated model are similar to those of the aqueous solvation model.

Associative vs. dissociative mechanism of P-C bond breaking in α-aminophosphonates leading to phosphoric acid [P(V)] derivatives.

“…Both forms facilitate the elimination to P III (increased concentration of non-protonated Nphenyl amine nitrogen) and lower the reaction rate of decomposition to P V (decreased concentration of the protonated pyridine nitrogen which is necessary to form a good leaving group). 28,47 This observation has been reported independently by Boduszek et al 26,45 and by Kiersnowska. 27 In this case the phosphorus III and phosphorus V derivatives are easily distinguished, the former appears in its 31 …”

“…This study confirmed that a three-step process including amino-group protonation, proton transfer through a hydrogen-oxygen bond NH•••O(P), 43 and P-C bond cleavage, leading to a protonated imine and derivatives of H-phosphonate is the most probable mechanism. 6,27,28,43,44 It should be noted that for N-alkyl aminophosphonates the decomposition is not observed as the amine nitrogen in this case is protonated and formation of imine (leaving group) is prevented. Lowering the basicity of the amine as is the case in the N-benzhydryl derivative, again facilitates the elimination process 47 Decomposition to P V .…”

“…For decomposition of aminophosphonate to P III derivatives. 6,24,27,35,36,43 we have proposed the retro Kabachnik elimination mechanism. In this mechanism we assumed that in aminophosphonate (N-aromatic, or with bulky groups) the presence of a nitrogen of relatively low basicity causes the equilibrium between species protonated on nitrogen and phosphoryl oxygen to be significantly shifted toward the latter.…”

“…We have compared also the P-C bond breaking processes leading to P III and P V decomposition products. 24,27,28 Results and Discussion…”

“…34 In the case of α-aminophosphonates there are two centers of basicity, the amine nitrogen and the phosphoryl oxygen. 35 We have performed calculations of the possible equilibria that could exist in α-aminophosphonates 36,37 and we have found that the presence of a basic amine group in close proximity to the phosphoryl oxygen helps in protonation of the latter by the neighboring group effect. In this case the calculated energy barrier for the shift of a proton from the vicinity of nitrogen toward the vicinity of the phosphoryl oxygen via the hydrogen bond interaction was found to be about 9 kcal/mol.…”