[771-29-9] · C10H12O2 · a-Tetralyl Hydroperoxide · (MW 164.22)
(weak electrophile which delivers oxygen to alkenes when activated by Lewis acids2)
Alternate Names: tetralin hydroperoxide; THPO; 1,2,3,4-tetrahydro-1-naphthalenyl hydroperoxide.
Physical Data: mp 54-54.5 °C.
Solubility: sol toluene, cyclohexane, CHCl3.
Form Supplied in: not commercially available.
Analysis of Reagent Purity: assay using iodometry.3
Preparative Methods: oxygen is passed through pure tetralin maintained at 70 °C until the peroxide content of the reaction mixture is 25-30% (24-48 h). Unreacted tetralin is separated by fractional distillation in an all-glass apparatus in high vacuum (0.2-0.4 mmHg); the pot temperature is not allowed to exceed 70 °C. Crystallization of the pot residue from toluene furnishes THPO (1) in 44-57% yield.4
Purification: by recrystallization from toluene.
Handling, Storage, and Precautions: remains colorless without appreciable decrease in active oxygen for several months if stored in the dark at or below 0 °C. Since the reagent is a peroxide it is potentially explosive and care is required in handling it (use safety shield). During workup, check for peroxides before solvent removal.
When THPO (1) is heated in the presence of cyclohexyl metaborate (CHMB) (2) it undergoes facile decomposition at 80 °C.2 This system (THPO-CHMB) epoxidizes alkenes in high yields. For example, 1-methylcyclohexene (3) is thus epoxidized in quantitative yield (eq 1).2 Epoxidation of cis-2-octene at 80 °C furnishes only cis-2,3-epoxyoctane in almost quantitative yield. Competitive epoxidations of alkenes have been studied. It has been observed that the rate of epoxidation increases with the degree of substitution; trisubstituted alkenes react faster than disubstituted alkenes. The reactivity pattern is comparable to that of organic peroxy acids. It has been suggested that the epoxidation proceeds through the transition state (4).2
When cyclodecyne was heated with THPO-CHMB in cyclohexane (reflux, 24 h), a mixture of (5), (6), and (7) was obtained in a 47:23:31 ratio.5 Similar results are obtained when cyclodecyne is reacted with m-Chloroperbenzoic Acid.
Asymmetric epoxidation of the allyl alcohol (8) has been carried out with six different peroxides at -20 °C with L-(+)-DIPT/Titanium Tetraisopropoxide as catalyst; the yields and ee of the resulting epoxy alcohol (9) have been determined.6 Best results are obtained with t-Butyl Hydroperoxide (TBHP), which furnishes (9) having 90% ee in 45-50% yields. When THPO is used, (9) is obtained in 35% yield but the ee of the product is only 14%.
For preparing epoxides from alkenes lacking directing groups, the widely used reagents are TBHP/Mo7 and m-CPBA.
2-Thienol (10) has been prepared in 10% yield by reacting 2-thienyllithium with THPO;8a the same procedure applied to phenyllithium furnishes phenol in 75% yield. THPO oxidation of b-carotene in CHCl3 at rt for 20-22 h furnishes a number of allylic oxidation products, including 4,4´-dihydroxycarotene (11) and 4,4´-dioxocarotene (12).8b
A. Somasekar Rao & H. Rama Mohan
Indian Institute of Chemical Technology, Hyderabad, India