Hypoiodite irradiation

Hypohalites IRO-Hal) are similar to nitrates (see p. 155 in their photochemical behaviour. Ultraviolet irradiation gives an (n,Ji > excited state that cleaves to form an alkoxy radical and a halogen atom. The radical may undergo alpha-cleavage before recombination with the halogen atom occurs, and this accounts for the formation of 5-iodopentanal (5.69) from the hypoiodite of cydopentanol such hypoiodites are generated in situ from the alcohol. Iodine and merturyfll oxide. In open-chain systems the alkoxy radical can... 

Ring expansion via the hypoiodite reaction.1 Irradiation of 1, formed by... 

Oxygen heterocycles have also been obtained on photolysis of hypo-iodites.441 Irradiation of the hypoiodite (503), for example, gave the tetra-hydropyran (504) by the route shown in Scheme 35.442... 

Oxasteroids. Hydroxy steroids are converted by this system into hypoiodites, which on irradiation are cleaved to formyl esters. These products are cyclized to oxasteroids with sodium borohydride. 

Of great synthetic importance is the generation of aikoxy radicals from alcohols DIB reacts first with elemental iodine to form acetyl hypoiodite which in the presence of an alcohol is converted into alkyl hypoiodite upon irradiation with visible light the aikoxy radical is generated ... 

The scission reaction is carried out by irradiation of the hypoiodite of the lactol. The process is formulated for conversion of 7-cholesianone (1) into 7-oxacholestanc (4). 

A related reaction occurs on irradiation of a hypochlorite (Scheme 2). The alkoxyl radical can again abstract a hydrogen atom in a l,S-shift, and the final chloro alcohol can be cyclized to form a tetrahydro-furan. Some processes lead directly to the cyclic product For instance, an alcohol with an accessible 8-hydrogen can be directly converted to a tetrahydrofuran on refluxing with lead tetraacetate. In a related reaction, treatment of an alcohol with silver carbonate and bromine can lead to the cyclic ether by initial formation of a hypohalite. The cyclization occurs when the rearranged radical is converted to a cation, either by oxidation with Pb(OAc)4 or by silver-assisted loss of halide ion. Hypoiodites are also fiequent-ly used, generated in situ. ... 

Alkoxy radicals produced by homolysis of hypohalites can attack non-activated C-H bonds, and lead to tetrahydro-furanoid structures after basic treatment of the intermediate halo-alcohols. Hypochlorites of [2y,2g] and C(20)-tertiary alcohols [28,2gJ gave 6 8,19- and 18,20-oxido derivatives respectively on irradiation and hydrolysis, but the more reactive hypobromites and hypoiodites are now preferred. Akhtar ef al. [30] distinguish between two modes of reaction of a 6jS-hypobromite (2), generated in situ by treating the alcohol with bromine and silver acetate. Experiments with... 

When a molecule is irradiated, particularly with ultraviolet light, some bonds within the molecule can absorb this energy and undergo homolyt-ic cleavage to generate free radicals. These reactions include the formation of alkoxyl (RO ) radicals from alkyl hypoiodites (RO) or nitrites (RONO) or the generation of bromine atoms from bromine or A-bro-mosuccinimide. 

Examples of the generation and use of oxyl radicals, obtained by photolysis of hypoiodites, have again been described. The conversion of 7-cyano-2,3-benzobicyclo[4.2.0]octa-2,4-dien-6-ol (204) into the homotropone (205) on irradiation in benzene in the presence of HgO-l2 can be rationalized in this way a possible explanation is shown in Scheme 8. 

Rather rare examples of the oxidation of tertiary alcohols are the degradation of 3-ethyl-3-pentanol to 3-pentanone and iodoethane in respective yields of 90 and 84% by acetyl hypoiodite in acetic acid and chlorobenzene under irradiation for 1 h at 20-25 °C [780 and the conversion of trflnj-9,10-dihydroxy-l,4,5,8,9,10-hexahydronaphthalene into 3,8-cyclodecadiene-l,6-dione and its tetramethylacetal by lead tetraacetate (equation 274) [447. ... 

A convenient method for generating the alkoxy radical is by fragmentation of hypoiodites prepared in situ from the corresponding alcohol. This can be accomplished by treatment of the alcohol with iodine and lead tetraacetate or mercury(II) oxide, or with iodine with iodobenzene diacetate. For example, irradiation of the alcohol 29 under these latter conditions gave a high yield of the tetrahydrofuran 30 (4.29). In another apphcation of this chemistry, for the specific deprotection of benzyl ethers, irradiation of the alcohol 31 and A-iodosuccinimide (NIS) gave the cyclic acetal 32 (4.30). ... 

Oxidation Reactions. Hypoiodite intermediates may be generated from the reaction of simple alcohols with NIS. When conducted under photochemical irradiation, the products of Barton-type or fragmentation reactions of alkoxyl radical intermediates may be obtained. Aldehydes are oxidized to methyl esters via hemiacetal intermediates by reaction with NIS in methanol at rt. However, such conditions are not effective for the oxidation of simple alcohols. The combination of NIS and Tetrabutylammonium Iodide in dichloromethane has been developed for the oxidation of a variety of alcohols to the corresponding carbonyl corrpounds (eq 8). This reagent system is most widely used for the oxidation of lactols to lactones, in which near-quantitative yields are generally obtained under mild conditions (eq 9). ... 

The Yokoyama group found that irradiating 3-phenyl-l-propanol and its derivatives 59 and 60 with phenyliodine(III) diacetate and I2 formed a hypoiodite. Homolytic O—I bond cleavage results in an oxygen-centered radical which can undergo HAS to synthesize a chromane. Once formed, however, the now-activated electron-rich aromatic ring is primed for a classical EAS under tlie reaction conditions. AcOI acts as the electrophile for this tandem reaction to place an iodine substituent para to the new oxygen substituent in products 61 and 62. ... 

Extending an investigation of the double )S-scission reaction of alcohols with nitrogen substitution at the )3-carbon (Scheme 37), Japanese authors have reported that the intermediate alkoxyl radical can be generated more effectively photochemically, using hypoiodite, or oxidatively, with lead tetraacetate, rather than by the earlier method of nitrite ester irradiation. 

Thus, two groups of investigators reported that the irradiation of certain oxygen heterocycles having a hydroxyalkyl side chain in cyclohexane in the presence of (diacetoxyiodo)benzene and iodine at room temperature gave bis-spiroacetals in good yields (Scheme 30). Subsequent work showed that similar reaction conditions could be applied to the synthesis of a number of five- and six-membered cychc acetals from y- and e-hydroxyethers (Scheme 31). The formation of a cychc acetal was also found in the hypoiodite reaction of hydroxysteroid with mercury(ll) oxide and iodine (Scheme 32). 

Functionalized bicyclo [x.y.0] alkanes (x = 5 or 6 y = 7 or 8) can be synthesized by the irradiation of hypoiodites prepared by [2+2] -photoaddition of cyclic enones with the trimethylsilyl enol ethers of cycUc ketones (as outlined in general in Scheme 70). Scheme 71 outlines the new total synthesis of ( )-a-himachalene based on this method as the key step. ... 

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