A Stabilized β-Oxaphosphoniumbetaine: An Elusive Zwitterion

A β-oxaphosphoniumbetaine stabilized by two tert-butyl groups at the phosphonium part of the
betaine and two trifluoromethyl groups at the carbon adjoined to the oxa part of the betaine was isolated
and structurally characterized. Additional stabilization results from the solvation of the betaine by a parent
phosphonium salt, as in 4, or by protonation with methanol, as in 10. According to X-ray analysis, the
betaine exhibits a sterically strained “gauche” conformeric form, with torsion angles in the P−C−C−O
moiety of 32.2° for 4 and 28.1° for 10. The P···O separations of 3.121 Å for 4 and 3.086 Å for 10 are just
under the sum of the O and P van der Waals radii (3.32 Å). The fast H/D exchange of α hydrogen atoms
to the phosphonium center was observed in the solution of 10 in deuterium oxide and MeOH-d4. The
β-oxaphosphoniumbetaine has long been suspected as an intermediate in the Wittig reaction, but this is
the first time a stabilized derivative has been isolated.