2- 2/TBHP

The first practical method for asymmetric epoxidation of primary and secondary allylic alcohols was developed by K.B. Sharpless in 1980 (T. Katsuki, 1980 K.B. Sharpless, 1983 A, B, 1986 see also D. Hoppe, 1982). Tartaric esters, e.g., DET and DIPT" ( = diethyl and diisopropyl ( + )- or (— )-tartrates), are applied as chiral auxiliaries, titanium tetrakis(2-pro-panolate) as a catalyst and tert-butyl hydroperoxide (= TBHP, Bu OOH) as the oxidant. If the reaction mixture is kept absolutely dry, catalytic amounts of the dialkyl tartrate-titanium(IV) complex are suflicient, which largely facilitates work-up procedures (Y. Gao, 1987). Depending on the tartrate enantiomer used, either one of the 2,3-epoxy alcohols may be obtained with high enantioselectivity. The titanium probably binds to the diol grouping of one tartrate molecule and to the hydroxy groups of the bulky hydroperoxide and of the allylic alcohol... 

Isobutane. Isobutane can be oxidized noncatalyticaHy to give predominantly /-butyl hydroperoxide [75-91-2] (TBHP) (reactions 2 and 3). The... 

A significant outlet for TBHP is the molybdenum-complex cataly2ed production of propylene oxide, a process developed by Oxirane (221—224). 

The tert-huty hydroperoxide is then mixed with a catalyst solution to react with propylene. Some TBHP decomposes to TBA during this process step. The catalyst is typically an organometaHic that is soluble in the reaction mixture. The metal can be tungsten, vanadium, or molybdenum. Molybdenum complexes with naphthenates or carboxylates provide the best combination of selectivity and reactivity. Catalyst concentrations of 200—500 ppm in a solution of 55% TBHP and 45% TBA are typically used when water content is less than 0.5 wt %. The homogeneous metal catalyst must be removed from solution for disposal or recycle (137,157). Although heterogeneous catalysts can be employed, elution of some of the metal, particularly molybdenum, from the support surface occurs (158). References 159 and 160 discuss possible mechanisms for the catalytic epoxidation of olefins by hydroperoxides. 

After epoxidation a distillation is performed to remove the propylene, propylene oxide, and a portion of the TBHP and TBA overhead. The bottoms of the distillation contains TBA, TBHP, some impurities such as formic and acetic acid, and the catalyst residue. Concentration of this catalyst residue for recycle or disposal is accompHshed by evaporation of the majority of the TBA and other organics (141,143,144), addition of various compounds to yield a metal precipitate that is filtered from the organics (145—148), or Hquid extraction with water (149). Low (<500 ppm) levels of soluble catalyst can be removed by adsorption on soHd magnesium siUcate (150). The recovered catalyst can be treated for recycle to the epoxidation reaction (151). 

Limonene (15) can be isomerized to terpiaolene (39) usiag Hquid SO2 and a hydroperoxide catalyst (/-butyl hydroperoxide (TBHP)) (76). Another method uses a specially prepared orthotitanic acid catalyst with a buffer such as sodium acetate (77). A selectivity of about 70% is claimed at about 50% conversion when mn at 150°C for four hours. 

Recently (79MI50500) Sharpless and coworkers have shown that r-butyl hydroperoxide (TBHP) epoxidations, catalyzed by molybdenum or vanadium compounds, offer advantages over peroxy acids with regard to safety, cost and, sometimes, selectivity, e.g. Scheme 73, although this is not always the case (Scheme 74). The oxidation of propene by 1-phenylethyl hydroperoxide is an important industrial route to methyloxirane (propylene oxide) (79MI5501). 

The remarkable stereospecificity of TBHP-transition metal epoxidations of allylic alcohols has been exploited by Sharpless group for the synthesis of chiral oxiranes from prochiral allylic alcohols (Scheme 76) (81JA464) and for diastereoselective oxirane synthesis from chiral allylic alcohols (Scheme 77) (81JA6237). It has been suggested that this latter reaction may enable the preparation of chiral compounds of complete enantiomeric purity cf. Scheme 78) ... 

SHARPLESS Asymmetne Epoxidation EnanlioselectK/e epoxidation of altyl alcohois by means of Irtanlum a5(oxide, (+) or () diethyl tartarate (OET) and t butyl hydroperoxide (TBHP)... 

A great deal of kinetie information on the AE reaetion has been obtained. The rate of reaetion is first order in allylie aleohol, Ti(0-iPr)2(tartrate), and TBHP. In addition, the rate is inversely-square dependent on isopropoxide. This refleets the required replaeement of two isopropoxide ligands on Ti(0-iPr)2(tartrate) with TBHP and the allylie aleohol. The rate-determining step is oxygen transfer from the peroxide to the olefin. 

A number of reaction variables or parameters have been examined. Catalyst solutions should not be prepared and stored since the resting catalyst is not stable to long term storage. However, the catalyst solution must be aged prior to the addition of allylic alcohol or TBHP. Diethyl tartrate and diisopropyl tartrate are the ligands of choice for most allylic alcohols. TBHP and cumene hydroperoxide are the most commonly used terminal oxidant and are both extremely effective. Methylene chloride is the solvent of choice and Ti(i-OPr)4 is the titanium precatalyst of choice. Titanium (IV) t-butoxide is recommended for those reactions in which the product epoxide is particularly sensitive to ring opening from alkoxide nucleophiles. ... 

The use of heterogeneous catalysts in the liquid phase offers several advantages compared with homogeneous counterparts, in that it facilitates ease of recovery and recyclidg. A chro-miiun-containingmediiun-pore molecular sieve fSi Cr> 140 1, CrS-2, efficiently catalyzes the direct oxidadon of various primary amines to the corresponclmg nitro compounds using 70% r-butylhy operoxide (TBHP. ... 

When tertiary butyl hydrogen peroxide (TBHP) was used alone as the radical initiator, no grafting of methylmethacrylate (MMA) onto wool was observed. However, TBHP in conjunction with mineral acids, such as H2SO4, HNO3, or HCIO4 afforded good results [26]. Protonation of TBHP by the acid aided in the dissociation of TBHP to yield free radicals, which initiated grafting reaction. 

One molecule of acid first protonates TBHP to give a protonated complex (I), which is stabilized by the second molecule of H2SO4 involving hydrogen bonding in the following manner ... 

The formation of the complex is expected to decrease the free energy of activation for the homolysis of the peroxide bond, and the decomposition of TBHP would occur at a lower temperature. It was further observed that at a higher concentration of mineral acid, the decomposition of TBHP occurs via an ionic pathway, as reported by Turner [27]. 

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