Observation and Stereochemistry of Betaine Intermediates in the Reaction of Phosphonium Ylide Containing a Phosphaboratatriptycene Skeleton with Benzaldehyde.

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Betaine intermediates, rather than the corresponding 1,2-oxaphosphetanes, were observed in the reaction of the phosphonium ylide containing a phosphaboratatriptycene skeleton with PhCHO in THF. Their chemical shifts were -4.56, -4.92, -7.32, and -11.1 ppm at -10 °C in variable temperature (VT) 31P{1H} NMR experiments. These signals were observed by the deprotonations of β-hydroxyalkylphosphonium salts with lithium hexamethyldisilazide (LiHMDS) and sodium hexamethyldisilazide (NaHMDS). Their signals were assigned to betaine lithium complexes and salt-free betaines, respectively. The betaine lithium complexes were thermodynamically stable even at 25 °C and were converted to β-hydroxyalkylphosphonium salts by protonation, whereas the salt-free betaines were unstable, providing the corresponding ethylphosphonium salt and PhCHO at 0 °C, instead of olefins and phosphine oxide. The potential structures of the betaine lithium complexes and salt-free betaines were investigated by optimizing the expected stereoisomers and estimating the phosphorus chemical shifts by density functional theory (DFT) calculations. The results indicated that the erythro-syn- and threo-syn-betaines were more stable than their anti-forms. Erythro-syn-betaines were the most thermodynamically stable among the expected stereoisomers in both lithium complexes and salt-free betaines. The estimated phosphorus-31 signals were in good agreement with those experimental values.