Observation of a Betaine Lithium Salt Adduct During the Course of a Wittig Reaction

The Wittig Reaction with Pyridylphosphoranes

“…[6] In our previous work on the mechanism of the Wittig reaction we have shown NMR-detectable dynamic equilibria of oxaphosphetanes which are complexed by the lithium ions in solution. [7] In the special case [8] of a Wittig reaction between triphenylmethylenephosphorane (1) and dipyridylketone 2, however, we have demonstrated [9] the occurrence of a betaine salt adduct 3, since, in the presence of lithium salts the corresponding oxaphosphetane 4 is destabilized by complexation of the lithium ion to the two pyridyl rings (Scheme 1).…”

A Wittig Reaction with 2-Furyl Substituents at the Phosphorus Atom: Improved (Z) Selectivity and Isolation of a Stable Oxaphosphetane Intermediate

“…We have been studying the influence of heteroaromatic substituents on Wittig reactions since the reported failure of a Wittig reaction between triphenyl(methylene)phosphorane and 2,2′‐dipyridyl ketone in the presence of lithium salts 12. This was due to the formation of stable betaines, which could be observed in the presence of lithium ions, when two or more 2‐pyridyl substituents were bound either to the carbonyl compound,13 or to the phosphorus atom 14. Interestingly, improved ( Z )‐alkene selectivities could be observed in the latter case.…”

New possible ionic structures in Wittig reaction: Analysis of stability and rotation barriers by semiempirical PM3 method

“…This intermediate may be formed from the ylide and the carbonylic compound, before the formation of the stable oxaphosphetane intermediate. On the other hand, the betaine intermediate may also be formed in the step of the end product formation from the oxaphosphetane intermediate 1–20. The Wittig reaction can take place through an oxaphosphetane intermediate directly formed through an asymmetric [2 + 2] cycloaddition between the ylide and the carbonylic compound 8, 9, 11, 20–22.…”

“…This intermediate may be formed from the ylide and the carbonylic compound, before the formation of the stable oxaphosphetane intermediate. On the other hand, the betaine intermediate may also be formed in the step of the end product formation from the oxaphosphetane intermediate 1–20.…”

“…2) have not been met with success 27. As in other similar cases, a valid approach to bring information on the formation and stability of the three hypothetic intermediates is using quantum chemical methods 9, 10, 12–17, 20, 29–32.…”