Hydrolysis of styrene oxides

Styrene glycol is available from Aldrich Chemical Company, Inc. or from Eastman Organic Chemicals. Alternatively, it may be prepared by hydrolysis of styrene oxide.3 If the glycol melts at lower than 63°, it should be recrystallized before use. 

Figure 6.66 Enantioconvergent hydrolysis of styrene oxides using two biocatalysts.

In a study by Berti et al., acid-catalyzed hydrolysis of styrene oxide was reported to occur with 67% inversion and 33% retention at the benzyl carbon.45 In a later study, it was reported that the styrene glycol product formed in the acid-catalyzed hydrolysis of chiral styrene oxide is completely racemic, which would indicate an A-l mechanism.46 As these two results indicate quite different mechanisms for this reaction, the glycol product from acid-catalyzed hydrolysis of chiral styrene oxide was converted to its bis-( + )-a-(methoxy-a-trifluoromethyl)phenylacetate diester derivative, and the composition of the diastereomeric diester mixture was determined by H NMR.47 This study agreed with those of Berti et al. and showed that acid-catalyzed hydrolysis of styrene oxide occurs with 67% inversion and 33% retention at the benzyl carbon. Acid-catalyzed methanolysis of styrene oxide is reported to occur with 89% inversion at the benzyl carbon.48 The fact that the diol product from acid-catalyzed hydrolysis of chiral styrene oxide is not completely racemic demonstrates that the lifetime of the carbocation is not sufficiently long for it to become symmetrically solvated. 

A mechanistic scheme for the acid-catalyzed hydrolysis of styrene oxide (32) is given in Scheme 10. If reaction of H+ with 32 occurs with benzyl C-O bond... 

In a comparison of the rates of acid-catalyzed racemization of and lsO exchange in chiral 1-phenylethanol, it was concluded that the departing -OH2 group shields that side of the benzyl carbocation from attack by other solvent molecules.26 The lifetime of the /i-phenylethyl carbocation, which should be similar to that of the fl-hydroxy benzylic carbocation formed in the acid-catalyzed hydrolysis of styrene oxide 32, is therefore too short to allow complete solvent equilibration about the carbocation. 

The benzylic carbocation formed from the reaction of styrene oxide with H +, like the 1-phenylethyl carbocation and simple tertiary carbocations, most likely reacts with water molecules from within the inner solvent shell that are present when the carbocation is formed.30 The lifetime of the carbocation formed in the acid-catalyzed hydrolysis of styrene oxide must be similar to the time required for solvent relaxation and rotation about the Ca-Cy3 bond. 

It was originally reported that attack of hydroxide ion on styrene oxide occurs only at the primary carbon.55 Later studies on the hydroxide ion-catalyzed hydrolysis of styrene oxide in 11/80 and of chiral styrene oxide, however, showed that... 

Acid-catalyzed hydrolysis of oxiranes has been reviewed extensively (59CRV737, 60QR317, B-64MI 103-01, B-72MI104-01). The kinetics and mechanism of the acid-catalyzed hydrolysis of styrene oxides... 

Table 11.2-4. Enzymatic hydrolysis of styrene oxide-type epoxides, see Fig. 9(b).

DESs have also proven to be suitable as cosolvent for enzymatic reactions in aqueous solution. Gorke et al. [8] have reported that upon addition of 10% of the ChCl/glycerol DES, the hydrolysis of p-nitrophenyl acetate was accelerated moderately when catalyzed by several esterases a threefold increase for pig liver esterase (PLE) and Rhizopus oryzae esterase (ROE) and a 25% increase for Pseudomonas fluorescens esterase (PFE) and CALB. The presence of the same DES in aqueous solution also triggered an up to 20-fold enhancement in the rate of the epoxide hydrolase (from Agrobacterium radiobacter)-cslaiyzed hydrolysis of styrene oxide. In fact, there was a bell-shaped relationship between the reaction rate and the DES content, as shown in Figure 25.5. 

Figure 25 Conversion obtained in the hydrolysis of styrene oxide catalyzed by epoxide hydrolase ADI from Agrobacterium radiobacter as a function of the volume fraction of the ChCl/glycerol DES in aqueous solution. This figure is drawn by using the data taken from Gorke et al. [8],...

A study of the hydrolysis of styrene oxide and its / -MeO, / -Me, p-C and /7-NO2 derivatives under aeidie (HsO and basic (OH ) conditions has been carried out. The experimental results obtained in flie study are as follows ... 

The acid hydrolysis of (+)-styrene oxide yields a totally racemic diol, whereas the acid methanolysis of (+)-styrene oxide yields a product with 89% inversion of configuration at the benzyhc position. 

The second and third terms in rate expression Eq. 11.1 ( rLHsO" ] and Aoh[OH"], respectively) clearly indicate that the hydrolysis of styrene oxides is subject to both and OH" catalysis. Acid-base catalyzed reactions have characteristic profiles when plotting log Aobs versus pH (a typical profile is represented in Fig. 11.1). The plots would reflect the contribution of the acid catalysis (a), the spontaneous (uncatalyzed) mode (b) and the basic catalyzed reaction (c). ... 

With the data we have discussed until now we have plenty of information to propose a reasonable mechanism for the HsC -catalyzed hydrolysis of styrene oxides. First, we know that it is a specific acid-catalyzed reaction and second, the intermediacy of a carbocation seems very likely. When writing a reaction mecha-... 

Chen, W.-J., Lou, W.-Y. and Zong, M.-H. (2012) Efficient asymmetric hydrolysis of styrene oxide catalyzed by mung bean epoxide hydrolases in ionic liquid-based biphasic systems. Bioresour. Technol., 115,58-62. 

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