Abstract
Whereas two equivalents of base are typically required to prepare carboxylate (CO2
– ) ylides [Ph3 P+ C– (H)-alk-CO2
– ] (alk = alkanediyl) from carboxy (CO2 H) phosphonium salts [(Ph3 PCH2 -alk-CO2 H)+ ] X– , we reveal, for the first time, that carboxy ylides [Ph3 P+ C– (H)-alk-CO2 H] can be generated with one equivalent of NaHMDS at 0 °C, and that the Wittig reaction of simple aliphatic aldehydes (1 equiv) with these carboxy ylides (1.5–2 equiv) in THF at –95 to –90 °C for one hour, then at warming temperatures to 0 °C over two hours affords (Z )-alkenoic acids. Phosphonium salts containing (CH2 )
n
alkanediyl chains (n = 2–5) showed adequate reactivity and high Z -selectivity, whereas shorter or longer alkanediyl chains resulted in a low Z -selectivity and/ or a low yield. On the basis of these results with different (CH2 )
n
chains and that obtained with a rigid methylene group, we propose that a rapid equilibrium between Ph3 PCH
2 -alk-CO2
– and Ph3 P+ C– (H)-alk-CO2
H , through an intramolecular hydrogen exchange, accounts for the success of the Wittig reaction.
Key words Wittig reaction - carboxy phosphonium salt - carboxy ylides - carboxylate ylides - alkenoic acids