Peroxy acids hydroxylation

The preparation of Pans-1,2-cyclohexanediol by oxidation of cyclohexene with peroxyformic acid and subsequent hydrolysis of the diol monoformate has been described, and other methods for the preparation of both cis- and trans-l,2-cyclohexanediols were cited. Subsequently the trans diol has been prepared by oxidation of cyclohexene with various peroxy acids, with hydrogen peroxide and selenium dioxide, and with iodine and silver acetate by the Prevost reaction. Alternative methods for preparing the trans isomer are hydroboration of various enol derivatives of cyclohexanone and reduction of Pans-2-cyclohexen-l-ol epoxide with lithium aluminum hydride. cis-1,2-Cyclohexanediol has been prepared by cis hydroxylation of cyclohexene with various reagents or catalysts derived from osmium tetroxide, by solvolysis of Pans-2-halocyclohexanol esters in a manner similar to the Woodward-Prevost reaction, by reduction of cis-2-cyclohexen-l-ol epoxide with lithium aluminum hydride, and by oxymercuration of 2-cyclohexen-l-ol with mercury(II) trifluoro-acetate in the presence of ehloral and subsequent reduction. ... 

In the field of enzyme catalysis, heme-proteins such as cytochrome P450, for example, exhibit both types of 0-0 bond cleavages in organic hydroperoxides and peroxy acids (178). Heterolytic cleavage of HOOH/ROOH yields H20 or the corresponding alcohol, ROH and a ferryl-oxo intermediate (Scheme 4). Homolytic 0-0 bond cleavage results in the formation of a hydroxyl (HO ) or an alkoxyl (RO ) radical and an iron-bound hydroxyl radical. 

Other examples of oxidant-iron(III) adducts as intermediates in iron porphyrin-catalyzed reactions have been published as listed in references 54a-k. Competitive alkene epoxidation experiments catalyzed by iron porphyrins with peroxy acids, RC(0)00F1, or idosylarenes as oxidants have been proposed to have various intermediates such as [(porphyrin)Fe (0-0-C(0)R] or [(porphyrin)Fe (0-I-Ar)]. Alkane hydroxylation experiments catalyzed by iron porphyrins with oxidant 3-chloroperoxybenzoic acid, m-CPBA, have been proposed to operate through the [(porphyrin)Fe (0-0-C(0)R] intermediate. J. P. CoUman and co-workers postulated multiple oxidizing species, [(TPFPP )Fe =0] and/or [(TPFPP)Fe (0-I-Ar)] in alkane hydroxylations carried out with various iodosylarenes in the presence of Fe(TPFPP)Cl, where TPFPP is the dianion of me50-tetrakis(pentafluorophenyl)porphyrin. ... 

Hydroxyl groups favor syn stereoselectivity.127 Amino groups have a similar directing effect.128 This is similar to the substituent effects observed for peroxy acids and suggests that the substituents may stabilize the transition state by acting as electron donors. 

The similarity of the structure of peroxynitrous acid to the simplest peroxy acid, per-oxyformic acid, immediately raised the question as to its relative reactivity as an oxygen atom donor. This became particularly relevant when it was recognized that the 0—0 bond dissociation energy (AG° = 21 kcalmoR ) of HO—ONO was much lower than that of more typical peroxides. Consequently, peroxynitrous acid (HO-ONO) can be both a one- and two-electron oxidant. Since the 0-0 bond in HO-ONO is so labile, its chemistry is also consistent in many cases with that of the free hydroxyl radical. 

Only much later was the hydroxylation of pure trans-307 and cis-307 performed, and the corresponding ci.s-diols were obtained in 22-26% yield. The compound obtained from t ran 4-307 was converted199 into ert/fhro-pentopyranos-4-ulose hydrate by successive alkaline de-benzoylation, and hydrolysis of the acetal in the presence of Dowex W-50 resin. An attempt at direct tran.s--hydroxylation of trans-307 by Woodward s or Prevost s method failed, as have attempts at its epoxi-dation with peroxy acids.200 Nevertheless, a number of 3,4-epoxides... 

With one exception—when the substituent is a hydroxyl group. When an allylic alcohol is epoxidized, the peroxy-acid attacks the face of the alkene syn to the hydroxyl group, even when that face is more crowded. For cyclohexenol the ratio of syn epoxide to anti epoxide is 24 1 with m-CPBA and it rises to 50 1 with CF3CO3H. 

Stabilizing the transition state when the epoxidation is occurring syn. This hydrogen bond means that peroxy-acid epoxidations of alkenes with adjacent hydroxyl groups are much faster than epoxidations of simple alkenes, even when no stereochemistry is involved. 

Before we move on, we leave you with one final example. Stereoselectivity in the epoxidation of lactone-bridged alkenes related to those we saw earlier (p. 874) can be completely reversed if the lactone is hydrolysed, revealing a hydroxyl group. In this bicyclic example, the hydroxyl group delivers the peroxy-acid from the bottom face of the alkene. First, the lactone bridge is used to introduce the alkene as before. 

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