Application of Hydrogen Peroxide
The preparation of diethyl ester 518 was carried out starting with Wittig reaction of benzyl bromide 515 with 5-bromopentanal, followed by catalytic reduction of the resulting olefin 517. It was found that a residual triphenylphosphine carried over from the Witting reaction can poison the catalyst in the subsequent hydrogenation step.
The use of hydrogen peroxide can oxidize the residual triphenylphosphine used in the preparationof phosphonium salt. Thus, an oxidative protocol was developed by treating the Wittig reactionmixture with 30% H2O2 under reflux to oxidize the residual triphenylphosphine completely to triphenylphosphineoxide.
• Charge 651 g (1.63 mol) of 515 into a reactor and then add 477 g (1.82 mol) of triphenylphosphine and MeCN (5.4 L).
• Heat the batch at reflux for 1 h.
• Cool the content down to 40oC.
• Charge 296 g (1.79 mol) of 5-bromopentanal and 8.2 L (95.2 mol) of 1,2-epoxybutane.
• Heat at reflux for 18 h.
• Add 272 mL of 30 wt % H2O2 over 10 min.
• Heat the resulting mixture at reflux for 2.5 h while maintaining a N2 flow.
• Cool to rt and isolate the product, and then go to Stage (c).
(a) The authors suggested that Cs2CO3 should be used to replace 1,2-epoxybutane as acidscavenger on a large-scale preparation.
(b) Oxidation using H2O2 in flammable solvent is potentially hazardous. It is important tomaintain a constant flow of inert gas to prevent oxygen from building up in the reaction vessel.