Olefins halohydrins

The reductive elimination of halohydrins provides a means of introduction of double bonds in specific locations, particularly as the halohydrin may be obtained from the corresponding a-halo ketone. This route is one way of converting a ketone into an olefin. (The elimination of alcohols obtainable by reduction has been covered above, and other routes will be discussed in sections IX and X.) An advantage of this method is that it is unnecessary to separate the epimeric alcohols obtained on reduction of the a-bromo ketone, since both cis- and tran -bromohydrins give olefins (ref. 185, p. 251, 271 cf. ref. 272). Many examples of this approach have been recorded. (For recent examples, see ref. 176, 227, 228, 242, 273.) The preparation of an-drost-16-ene (123) is illustrative, although there are better routes to this compound. 

It is not unlikely that the actual reagent is HOCl attack on the 9,11-olefin by Cl+ would occur from the less hindered side to give the 9a,lla-chloronium intermediate. Attack of OH to give diaxial ring opening would lead to the observed halohydrin. 

Peroxidases, in particular haloperoxidases, catalyze the oxidation of olefins in the presence or absence of halide ions. In the latter case, epoxides are produced directly (Fig. 6, route A), while in the former process halohydrins are formed (Fig. 6, route B), which may be transformed to epoxides either enzymatically by halohydrin epoxidase [128] or chemically by base treatment. 

Table 9. CPO-catalyzed formation of halohydrins from olefins ...

Below are described five approaches to epoxide synthesis by way -.if haLohydrins. These halohydrina may be isol ble purifi ble intermediates or they may be transient, unstable species that undergo spontaneous ring closure under the conditions used to generate them. Jho former are typical of (1) addition of hypobalous acids to olefins 111 chemical reduction of a-h Iocarbonyl compounds, and (3) addition if organometallic reagents to o-halocarbonyl compounds the latter,... 

Conversion of olefins into epoxides has been achieved in a great ifnbfiT of cases through halohydrin intermediates generated by " kiition of hypoholous acids across the olefiiuc double bond (Eq, 127). 

No attempt will be made here to cite every recorded instance of the preparation of a halohydrin from an olefin. It will be sufficient to consider (1) the various moans employed to generate hypohakuw acids (2) the mechanism of the reaction and (3) a few representative examples of its application. 

Reference has already been made to the addition of chromyl chloride to olefins, which gives abnormal halohydrins 72 Propylene, 1-butene, 1-pentone, and 1-hexene, for example, all yield primary alcohols, as shown in Eq. (141). 

Halohydrins are /I-halogenated alcohols. They can be obtained in H20-containing solvents from olefins and reagents, which transfer Hal+ ions. TV-Bromosuccinimide (transfers Br+ Figures 3.33 and 3.34 as well as 3.36), chloramine-T (transfers Cl+ Figure 3.35), and elemental iodine (transfers I+ Figure 3.36) have this ability. Bromonium and chloronium ions are then attacked by H20 according to an SN2 mechanism. This furnishes the protonated bromo- or chlorohydrins, which are subsequently deprotonated. 

CHAPTER 8 Ionic Addition of HX to an Alkene 332 Free-Radical Addition of HBr to Alkenes 334 Acid-Catalyzed Hydration of an Alkene 338 Oxymercuration of an Alkene 340 Hydroboration of an Alkene 345 Addition of Halogens to Alkenes 350 Formation of Halohydrins 352 Epoxidation of Alkenes 360 Acid-Catalyzed Opening of Epoxides 362 Olefin Metathesis 376... 

The formation of halohydrins can be promoted by peroxidase catalysts.465 Recently 466 it has been shown that photocatalysis reactions of hydrogen peroxide decomposition in the presence of titanium tetrachloride can produce halohydrins. The workers believe that titanium(IV) peroxide complexes are formed in situ, which act as the photocatalysts for hydrogen peroxide degradation and for the synthesis of the chlorohydrins from the olefins. The kinetics of chlorohydrin formation were studied, along with oxygen formation. The quantum yield was found to be dependent upon the olefin concentration. The mechanism is believed to involve short-lived di- or poly-meric titanium(IV) complexes. 

The high steric demand of the catalytically active species in molybdenum-catalyzed epoxidations is also demonstrated by the epoxidation of an estrene derivative 25 5-7 5. In contrast to the peracid epoxidation, and similar to epoxidation via halohydrin formation68, the molybdenum-catalyzed reaction shows a strong preference for the less hindered convex face of the olefinic system. 

When the above reagents are combined with olefins in the presence of a reactive solvent like an alcohol or acid, the corresponding halohydrin... 

Oxiranes are usually prepared from halohydrins. A general method of obtaining chloroalcohols from olefins is by the addition of hypochlorous acid formed in situ form -chloramides or fert-butyl hypochlorite. Bromoalcohols are produced from alkenes with Ai-bromosuccinimide or iV-bromoacetamide, while iodoalcohols are prepared with iodine in the presence of oxidants (iodic acid, oxygen, and nitrite). 

See Larock, R.C. Comprehensive Organic Transformations, 2nd ed., Wiley-VCH, NY, 1999, pp. 263-267, for reagents that produce olefins from P-halo ethers and esters, and from halohydrins. 

Epoxides are usually prepared in the laboratory by the reaction of olefins with organic peracids or by the reaction of halohydrins with alkali. The mechanism of the first of these processes has not been completely elucidated,170 but the second involves an ionic reaction whose mechanism has already been discussed (p. 96). The synthesis of phenyl-methyl glycidic ester176 appears to proceed by a related sequence ... 

Steroid trans-halohydrins are similarly converted into the corresponding olefinic products. Cornforth employed the combination zinc dust-AcOH-Nal-NaOAc for conversion of epoxides to olefins via the intermediate iodohydrins. 

The epoxides were prepared from the corresponding olefins either by oxidation with various peracids such as peracetic acid or m-chloroper-benzoic acid or in a two-step procedure via the halohydrin (SCHEME IV). The direct epoxidation procedure is preferred due to the generally low yields of halohydrins obtained in the second method. The halohydrins are active herbicides and will be discussed in a future publication (9). 

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