Computational and 31P NMR studies of moisture-metastable cyclic diaminophosphine oxide preligands

Recently, cyclic heteroatom-substituted secondary phosphine oxides (cyclic HASPOs: (X^X)PH(O), X = NR or O atom) have been applied to various catalytic reactions as preligands and with notable success. Similar to secondary phosphine oxides (SPOs: R2PH(O)), cyclic HASPOs are conventionally regarded as air- and moisture-stable compounds. Nevertheless, as reported herein, cyclic diaminophosphine oxides (cyclic N–P–N type HASPO: (RN^NR)PH(O), where (RN^NR) represents 1,2-diamino group) are susceptible to hydrolysis. The hydrolysis process of two P–N bonds in cyclic diaminophosphine oxides, 1a and 1b, has been thoroughly investigated by 31P NMR experiments. Density functional theory (DFT) methods have also been employed for elucidating the reaction mechanisms of hydrolysis for diaminophosphine oxides, 1c and 1b, respectively. The results pointed to a stepwise-concerted (SN2I@P4–SN2@P4) mechanism for consecutively double hydrolysis of 1c. 1c has been employed as a model for 1a and other diaminophosphine oxides with smaller substituents on nitrogen atoms. In contrast, a concerted-stepwise (SN2@P4–SN2I@P4) mechanism for 1b, bearing two bulky –tBu groups, was validated. Accordingly, kinetic stability which brought about by two bulky substituents indeed prevents 1b from being hydrolyzed. The results from 31P NMR experiments also showed that hydrolysis of 1a proceeded faster in acidic solution than in neutral condition. Unexpectedly, 1a exhibited unusual resistance toward hydrolysis in alkaline solution. This implies that the addition base is crucial to cyclic diaminophosphine oxides against hydrolysis in catalytic reaction in the presence of trace amount of water. Moreover, the effect of Neighboring Group Participation (N.G.P.) is presumed to account for the notable stability of preligand 1a in basic condition.

Hydrolysis process and mechanisms of two P–N bonds in N–P–N type HASPO pre-ligands were extensively investigated by 31P NMR experiments and DFT calculations. Since HASPOs have been shown as efficient pre-ligands for various cross-coupling reactions where water is possibly presented, it is crucial to learn the plausible ligand transformation in the catalyst for advanced ligand design.Figure optionsDownload as PowerPoint slide