Alkenes continued hydration

This chapter begins, thus, with a short introduction to the chemical reactivity of epoxides. We continue with a description of the epoxides hydrolases and their biochemistry, and devote most of its length to a systematic discussion of the substrates hydrated by these enzymes. Arene oxides and diol epoxides will be presented first, followed by a large variety of alkene and cy-cloalkene oxides. 

However, the enzymology of alkene and alkyne hydration is not well known. Recently, Meckenstock et al. (1999) discovered that the enzyme responsible for anaerobic hydration of acetylene contains a tungsten atom and an [Fe-S] cluster. This may hint that the enzyme uses the tungsten as a Lewis acid to activate the double bond. Possibly, the [Fe-S] cluster then serves to deliver a hydroxide as known in many common metabolite hydrations (Flint and Allen, 1996). Having introduced an oxygen moiety in an initial hydration, anaerobic bacteria may now be able to continue the biodegradation of such compounds. 

Yates and coworkers have examined the mechanism for photohydration of o-OH-8. The addition of strong acid causes an increase in the rate of quenching of the photochemically excited state of o-OH-8, and in the rate of hydration of o-OH-8 to form l-(o-hydroxyphenyl)ethanol. This provides evidence that quenching by acid is due to protonation of the singlet excited state o-OH-8 to form the quinone methide 9, which then undergoes rapid addition of water.22 Fig. 1 shows that the quantum yields for the photochemical hydration of p-hydroxystyrene (closed circles) and o-hydroxystyrene (open circles) are similar for reactions in acidic solution, but the quantum yield for hydration of o-hydroxystyrene levels off to a pH-independent value at around pH 3, where the yield for hydration of p-hydroxystyrene continues to decrease.25 The quantum yield for the photochemical reaction of o-hydroxystyrene remains pH-independent until pH pAa of 10 for the phenol oxygen, and the photochemical efficiency of the reaction then decreases, as the concentration of the phenol decreases at pH > pAa = 10.25 These data provide strong evidence that the o-hydroxyl substituent of substrate participates directly in the protonation of the alkene double bond of o-OH-8 (kiso, Scheme 7), in a process that has been named excited state intramolecular proton transfer (ESIPT).26... 

We expect the reactions complementary to equations (1) and (2), namely electrophilic attacks, to be faster for alkenes than for alkynes. Thus, reactivity ratios (/-ii and rj2) for corresponding alkynes and alkenes (PhC CH, PhCH=CH and BuC CH, BuCH=CH2) in radical copolymerizations favour the alkene over the alkyne . Electrophilic additions of Br, CI2, ArSCl and H3O+ to alkenes are usually much faster than those to alkynes . However, A (C=C)/A (C=C) can vary from 10 to < 1 for the different electrophilic processes and by 10 for one process (Br2 addition) when the solvent is changed from HjO to HOAc . This unexpected trend in reactivity continues undiminished in the rates of acid-catalysed hydration... 

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