Observation of an Equilibrium between a Betaine and a 1,2-Oxaphosphetane in a Phosphaboratatriptycene System

The deprotonations of β-hydroxyalkylphosphonium salts containing a phosphaboratatriptycene skeleton with MHMDS (M = Li, Na, and K) were monitored by variable-temperature (VT) ³¹P{¹H} NMR spectroscopy. Upon using LiHMDS, the corresponding betaines were observed at 25 °C, whereas the betaines were detectable below -40 °C, upon using NaHMDS. In contrast, the corresponding 1,2-oxaphosphetanes were observed upon using NaHMDS in the presence of crown ether and KHMDS. The results indicated that the equilibrium between betaines and 1,2-oxaphosphetanes depends on the interaction of the oxide ion of betaines with alkali metal ions. The relative stabilities of betaines and 1,2-oxaphosphetanes were estimated using density functional theory (DFT) calculations with or without alkali metal ions, and their ³¹P NMR signals were calculated using the gauge-including atomic orbital (GIAO) method. The interaction between an alkali metal ion and the oxide ion of betaine significantly affected betaine stability in the order of Li > Na > K. However, it had little effect on the stability of betaines and 1,2-oxaphosphetanes in the vicinity of the phenylborata ion.