Iodohydrin alkenes

Iodine was found to be an efficient catalyst for the aziridination of alkenes (Scheme 6) utilizing chloramine-T (A-chloro-A-sodio-p-toluenesulfonamide) as the nitrogen source. For example, when 2 equiv. of styrene (45a) were added to chloramine-T in the presence of a catalytic amount of iodine (10mol%) in a 1 1 solvent mixture of acetonitrile and neutral buffer, the corresponding aziridine (46) was obtained in 91% yield. The reaction proved to work with other acyclic and cyclic alkenes, such as oct-l-ene and cyclohexene. The aziridination of para-substituted styrene derivatives (45b-e) demonstrated that, as expected for an electrophilic addition, electron-rich alkenes reacted faster than electron-poor alkenes. However, with 1 equiv. of I2, mainly iodohydrin (47) was formed. A catalytic cycle has been proposed to account for the fact that only a catalytic amount of iodine is required (Scheme 1) ... 

An unusual incorporation of iodine derived from the Na(lO ) used in RuClj/aq. Na(10 )/CCl -CH3CN was noted in the oxidation of the terminal alkene 2-allyl-2,5-dichloro-4-morpholino-cyclopent-4-ene-l,3-dione giving the iodohydrin 5 3,7-dichloro-l-P-hydroxy-3 3-iodomethyl-8-morpholino-2-oxabicyclo[3.3.0]-oct-7-en-6-one and its 3a-epimer. The iodine apparently derives from the formation of or T from the lOj" to which IO is reduced after the RuClj/IO " reaction (Fig. 3.21) [236]. 

Deoxygenation of epoxidesEpoxides are converted to alkenes with retention of configuration by treatment with this combination in hexane at 0 -> 25°. The reaction involves anti-opening of the epoxide to an iodohydrin, which undergoes trans-elimination to an alkcnc. 

Since the reaction of I2 + H20 with alkenes is an equilibrium process, the use of oxidizing agents allows one to shift the equilibrium by oxidizing the I-formed.148 149 Substituted cyclohexenes can be converted to iodohydrins in regio-selective and stereoselective maimer with I2 and pyridinium dichromate.149 High yields of iodohydrins may be achieved by reacting alkenes with iodine in the presence of moist tetramethylene sulfone—CHCI3.150... 

Iodohydrins.1 This combination (1 2) presumably generates hypoiodous acid, IOH, since it converts alkenes into iodohydrins in moderate to high yield. Bromohy-... 

Treatment of epoxide (34) with Bu3SnH/AIBN in the presence of Mgl2 first forms iodohydrin synthon, which rapidly reacts with Bu3Sn to form a cyclohexanol derivative (35) via 6-exo-trig ring closure of (3-hydroxyl radical as shown in eq. 3.12 [50-54]. Since the epoxides can be obtained from alkenes with peroxides, this is an indirect radical cyclization method of alkenes. 

Epoxides may be prepared from alkenes by the action of a peroxy acid such as m-chloroperbenzoic acid (Scheme 2.20a) or via the formation of a bromohydrin or iodohydrin and the treatment of this with base (Scheme 2.20b). Since the initial electrophile, the bromine or the iodine, is displaced in the second step when the epoxide is formed, the stereochemistry of this epoxidation is likely to differ from that of the reaction with peroxy acid. 

Organophosphorus reagents based on triphenylphosphine, or trimethylsilyl iodide, may be used to deoxygenate epoxides to re-form the parent alkene. Reactions based on this, or on a related scheme using the reduction of an iodohydrin, have been used in the synthesis and protection of alkenes as their epoxides (Scheme 2.23). 

Ghlorofluorination with T-chlorosaccharin-HF/pyridine <1995SL327>, iodofluorination with T-iodosaccharin-HF/pyridine, and iodomethoxylation with A -iodosaccharin <2000SL544> of alkenes, alkynes, and activated aromatic compounds have been described. Bromohydrin and iodohydrin derivatives were prepared from electron-deficient alkenes and A -halosaccharins <2005EJO2349>. 

The release of ring strain in epoxides is probably responsible for the high reactivity of these special ethers. HI opens epoxides under mild conditions stereospecifically to iodohydrins (Scheme 26). The mechanism is similar to the reaction of bromide with epoxides (see Section 1.7.3.3). It should be noted, however, that reduction of epoxides to alkenes may occur if vicinal diiodides are intermediately formed, which can lose I2 under the reaction conditions. With the combination of acyl chloride and Nal unstable diiodides are avoided and 2-iodoethyl esters are formed from oxiranes (Scheme 27). ° ... 

The stereoselective reductive deoxygenation of aliphatic cw-oxiranes to the corresponding c/5-alkenes via iodohydrin intermediates has been accomplished by convenient one-pot procedures (equations 6 and... 

The generation of an alkene by the reaction of a v/c-disulfonate ester with iodide (the Tipson-Cohen reaction) has been known since 1943 and in some cases it has proved useful where other methods have failed, as in the preparation of the spirocyclic triene (54 Scheme 22). The mechanism probably involves an initial nucleophilic displacement to give an iodohydrin sulfonate, which then undergoes iodide-induced elimination to the alkene. Methanesulfonates can be used as well as arenesulfonates. 

Elimination. The highly stereoselective conversion of iodohydrin esters to alkenes [erythro ( ), threo — (Z)] by the allyltrimethylsilane-TiCl mixture enables stereoretentive deoxygenation of epoxides via the iodohydrins. 

Alkenes are converted to oxiranes upon treatment with I2 and pyridinium dichromate (pdc) in CH2C12 (Equation (41)) <83T1765>. The respective iodohydrins have been shown to be the precursors of the oxiranes formed in these reactions. 

Further proof of the intermediacy of the iodohydrins 85 in the formation of the hydroxy-tetrahydrofurans 80 came from two sources. Firstly, treatment with potassium carbonate led to formation of the corresponding epoxides. Secondly, by providing a second alkene function, suitably positioned to trap the iodohydrin hydroxyl by a 6-eto-trig iodocyclization, we have been able to intercept these species and hence define a new approach to substituted pyrans. Thus, treatment of the dienyl hydroxy-ester 90 with iodine and NaHCO, resulted in the formation of pyrans 92 in the ratio of 3.2 1. Presumably, initial iodohydrin formation 91 is followed by a relatively non-stereoselective 6-exo cyclization. Further chemistry of such products has yet to be carried out, especially efforts to distinguish the two iodine atoms and to cyclize to give furopyran systems <01M1001>. 

Asensio and coworkers reported a low temperature oxidation of iodomethane with DMDO to afford a pale yellow precipitate of iodosylmethane (20, Scheme 2.10) [104]. Upon raising the temperature to -40 °C, in the presence of moisture iodosylmethane decomposes to form the unstable hypoiodous acid, HOI, which can be trapped in situ by an alkene to afford iodohydrins. The formation of MelO has also been detected in the photochemical reaction of iodomethane with ozone in an argon matrix at 17 K [105], A similar low-temperature reaction of trilluoroiodomethane affords the unstable CF3IO, which was identified by infrared spectroscopy [106]. 

Additional examples of synthetic application of periodic acid as an oxidant include the oxidative iodination of aromatic compounds [1336-1341], iodohydrin formation by treatment of alkenes with periodic acid and sodium bisulfate [1342], oxidative cleavage of protecting groups (e.g., cyclic acetals, oxathioacetals and dithioacetals) [1315, 1343], conversion of ketone and aldehyde oximes into the corresponding carbonyl compounds [1344], oxidative cleavage of tetrahydrofuran-substituted alcohols to -y-lactones in the presence of catalytic PCC [1345] and direct synthesis of nitriles from alcohols or aldehydes using HsIOe/KI in aqueous ammonia [1346],... 

The argument is closely analogous to that used to explain the regioselectivity of formation of bromoacetoxy compounds (Table 9.2) formed in the addition of bromine to alkenes in acetic acid. Similarly, addition of bromine to alkenes in water produces bromohydrins. Although they are more difficult to synthesize, iodohydrins and fluorohydrins are also known. For a review of the synthesis and reactions of halohydrins, see Rosowsky, A. in Weissberger, A., Ed. Heterocyclic Compounds with Three- and Four-Membered Rings, Part One Wiley-Intersdence New York, 1964 p.l. 

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