Epoxidation of geraniols

SCHEME 88. Mn-salen catalyzed epoxidation of geraniol and nerol using H2O2... 

Allylic alcohols can also be epoxidized with methyltrioxorhenium (MTO). However, in contrast to the early transition metal catalysts, metal-alcoholate binding does not appear to be operative, but rather straightforward hydrogen bonding, as demonstrated by the epoxidation of geraniol (20)... 

Another example is the asymmetric epoxidation of geraniol to give (2S,3-S )-2,3-epoxy-3,7-dimethyl-6-octenol (2, for assignment, see p 448)". 

Catalytic Reactions. Certain catalytic reactions show a considerable departure from the linear relationship between the ee of a chiral source and the extent of the asymmetric induction (Scheme 42) 68). The Sharpless epoxidation of geraniol using Ti(IV) tetraisopropoxide modified by enantiomerically pure diethyl tartrate (DET) (Ti DET =1 1) gives the epoxide in 94% ee. Kagan and Agami first found that, by using the tartrate auxiliary in 50% ee, the optical yield varied to 70% ee. This optical yield is considerably higher than the expected value, 47 % ee... 

Epoxidation catalysts were prepared from these polymer-linked tartrates by combination with 0.5 equiv. of Ti(0-i-Pr)4, based on the weight of tartrate ester, which had been added to the polymer. Epoxidation of geraniol with 4 or 5 gave epoxy alcohol with 49 and 65% ee, respectively. Recycling of the polymer-linked tartrate was possible, but the subsequent epoxidation suffered from significant loss in enantioselectivity [21 ]. 

Likewise, the epoxidation of geraniol with a stoichiometric amount of the complex gave epoxide (entry 3) in 77% yield (95% ee), which was improved to 95% yield (91 % ee) when a catalytic amount of complex was used (entry 4). 

We showed in 1986 that the Sharpless epoxidation of geraniol gives a well-defined (+)-NLE, although the amplification is moderate (Scheme 6).2 Curiously, however,... 

Obviously, the way to heterogeneous enantioselective catalysis is open when chiral ligands are introduced into the polymers. This idea was first proposed by Okamoto and Still (231). However, the first catalytic results were published by Cazaux and Caze (235). These authors use the asymmetric Mo(VI) complex (9) covalently bound to a cross-linked polystyrene resin for the enantioselective epoxidation of geraniol. Unfortunately, the enantiomeric excess obtained was low—far below that obtained with the Ti-tartrate catalyst (Section II,B). Moreover, the polymeric structure appeared to be unstable. 

The Sharpless epoxidation uses ferf-butyl hydroperoxide, titanium(IV) isopropoxide, and a dialkyl tartrate ester as the reagents. The following epoxidation of geraniol is typical. 

Dimethylallyl aloteol was epoxidized with >90% ee (Table 7, entry 1) but in low yield when a stoichiometric amount of tee titanium tartrate conqtlex was used. However, when a catalytic amount of tee complex was used and in situ derivatization employed, tee p-nitrobenzoate (>98% ee after recrystalli-zation) and p-toluenesulfonate (93% ee) were isolated in yields of 70% and 55%, respectively. Likewise, tee epoxidation of geraniol with a stoichiometric amount of tee conqtlex gave the epoxide (Table 7, entry 3) in 77% yield (95% ee) which was improved to 95% yield (91% ee) when a catalytic amount of complex was used (entry 4). 

The representative procedure, given in the original work32, for epoxidation of geraniol is modified according to the above discussion to include a 20 30mol% excess of ( + )-di-ethyl L-tartrate and a 20-30 minute aging period prior to the addition of tert-butyl hydroperoxide. 

Green Epoxidation of Geraniol Using 3% Hydrogen Peroxide... 

The conversion of alkenes to epoxides is covered in most introductory organic chemistry texts, often exemplified by the use of m-chloroperoxybenzoic acid (MCPBA) as the epoxidizing reagent. Bradley et al. compared the enantioselective Sharpless epoxidation of geraniol with the classical MCPBA method, which gives a racemic product 21). Hoye and Jeffrey have illustrated a... 

EXPERIMENT 9.4 VO(acac)2 CATALYZED EPOXIDATION OF GERANIOL USING f-BuOOH... 

In considering the mono-epoxidation of geraniol there are four possible products, as shown in Figure 9.10. Because the olefinic protons on C6 and C2 of the products have different chemical shifts, the two sets of regioisomers are easily distinguished using NMR spectroscopy. The hydrogen on C2 has a chemical shift of 5.404 ppm while that on C6 is... 

An optional experiment—Sharpless epoxidation of geraniol—is given in the instructor notes. 

Results Summary for the VO(acac)2-catalyzed Epoxidation of Geraniol Attach labeled NMR and IR spectra of your product to this page. 

What product was formed from the VO(acac)2 epoxidation of geraniol ... 

---