Iodine promoted oxidation

The hypervalent iodine-promoted oxidation of enamide 257 in the presence of a base leads to the spiroe-namide 258, which is a key intermediate product in the total synthesis of annosqualine (Scheme 3.106) [325], A similar oxidative cyclodearomatization approach has been utilized in the total synthesis of a proaporphine alkaloid, ( )-stepharine [326],... 

Harikrishna and collaborators devised an iodine-promoted oxidative transformation of o-vinyl diaryl ketones into o-acetyl diaryl ketones, which then were used to prepare l-methyl-4-arylphthalazines 16 (Scheme 10) (14EJO1066). The ori/io-vinyl-substituted ketones were obtained in moderate-to-good yields via treatment ofWeinreb amide 17 with either... 

Metal-free iodine-promoted oxidative cyclization domino process was successfully utilized by Jiang et al. [15] to synthesize poly substituted oxazoles. Similarly, very efficient iodine-promoted oxidative cyclization domino process was reported by Wu et al. [16] for the synthesis of 2-acylbenzothiazoles 38 via a sp C-H functionalization (Scheme 9.8). 

The effectiveness of the halogen compounds depended on the particular halogen concerned the promoting effect on oxidation increased in the order chlorine compounds < bromine compounds < iodine compounds. Although iodine compounds were the most effective in promoting oxidation, their effect on ignition was often less than that of bromine compounds, and as mentioned earlier, they sometimes inhibited ignition. 

The most convenient route to TSF was reported by Cava in 1987 [64]. The procedure utilizes a novel iodine-promoted coupling of 2-methylene-1,3-diselenole (28). The starting 1,2,3-selenadiazole (29) is easily obtained from the oxidation of the semicarbazone of acetaldehyde with selium dioxide. 

AJthaugh various propiisals for the ni chani m of methanol homologation exist, the course of the reaction is still not fully understood. This is especially true for the activation of methanol with a concomitant C-0 bond scission. Also, the folc of the iodine promoter and of ligands remains unclear. This situation is controversial to the closely-related carbonylation of methanol to acetic acid with rhodium catalysts, where the oxidative addition of the intermediate methyl iodide to a rhodium (1) is a generally-accepted reaction path [SR]. 

A number of other photo-oxidations involving aromatic substrates have been reported, some of which again have been carried out in non-traditional reaction environments. A light-promoted oxidative cleavage of the olefinic bond in aromatic alkenes, which takes place in mesoporous Si02 (FSM 16) and involves a catalytic amount of I2, has been reported (Scheme 23). The photochemical involvement appears to extend no further than the formation of the iodine atoms. The suggestion that an a-iodoketone is an intermediate in the process is based on NMR evidence from the reaction where R = Bu, but it is not clear how this intermediate is converted to the final product. A similar transformation of aromatic alkenes has been carried out in zeolite NaY. Irradiation of styrene,... 

Scheme 6. Mercuric acetate/oxide-iodine-promoted reactions cascade involving regioselective 1,5-and 1,6-hydrogen transfers...

The first step is the oxidative carbonylation of aniline to form the intermediate diphenyl urea shown in Eq. (38). This reaction is carried out with a noble metal catalyst and an iodine promoter. A palladium catalyst with a sodium iodide promoter has been used successfully. The intermediate, diphenyl urea is oxidatively carbonylated in ethanol in the presence of the palladium catalyst to form ethyl phenyl carbamate (EPC) as shown in Eq. (39). Reactor conditions are 160°C (320°F) and 80 bars (1175 psig). 

A. Kirschning, Md. A. Hashem, H. Monenschein, L. Rose, and K.-U. Schoning, Preparation of novel haloazide equivalents by iodine(III) promoted oxidation of halide ions, /. Org. Chem., 64 (1999) 6522-6526. 

Canesi and coworkers have developed several synthetically useful tandem rearrangements on the basis of hypervalent iodine-promoted phenolic oxidation [328-331]. An oxidative Prins-pinacol tandem process mediated by a hypervalent iodine reagent allows the stereoselective tfansformation of simple phenols 261 into highly elaborate spirocyclic dienone cores 262 containing several quaternary carbon centers (Scheme 3.108). 

If the oxidation is performed in the presence of an external dienophile, the respective products of [4+2] cycloaddition are formed [351-356]. Typical examples are illustrated by a one-pot synthesis of several silyl bicyclic alkenes 283 by intermolecular Diels-Alder reactions of 4-trimethylsilyl substituted masked o-benzoquinones 282 generated by oxidation of the corresponding 2-methoxyphenols 281 [351] and by the hypervalent iodine-mediated oxidative dearomatization/Diels-Alder cascade reaction of phenols 284 with allyl alcohol affording polycyclic acetals 285 (Scheme 3.118) [352]. This hypervalent iodine-promoted tandem phenolic oxidation/Diels-Alder reaction has been utilized in the stereoselective synthesis of the bacchopetiolone carbocyclic core [353]. 

Numerous examples of hypervalent iodine-promoted fragmentations or rearrangements at electron-deficient centers have been reported. Several examples of oxidative fragmentations are shown below in Schemes 3.139-3.142. A mild and efficient fragmentation reaction of p-amino alcohols 349 and a-amino acids 350 upon treatment with [bis(trifluoroacetoxy)iodo]pentafluorobenzene leading to N,0-acetals 351 has been developed (Scheme 3.139). This method has been utilized in an improved synthesis of the key intermediate of discorhabdins [455,456]. 

Intramolecular N-arylations of amidines 83 led to benzimidazoles 84 mediated by potassium hydroxide (140L536). Multi-substituted benzimidazoles 86 were obtained from amidines 85 via iodine(III)-promoted oxidative C(sp )-C(sp ) and C(sp )-N bond formation (140L2822). Direct... 

Mixed oxides, typically containing bismuth and molybdenum, are used as catalysts and these have been improved over the years so that the conversion of propylene to acrylonitrile is now over 80% per pass through the reactor. HCN is produced in a side reaction and is used mainly to make acetonitrile or methyl methacrylate [23]. Acetic acid and acetic anhydride, which are made in high yields by the carbonylation of methanol and methyl acetate, respectively, using an iodine promoted rhodium catalyst, can now be made in a variable ratio, to match market demand, using the same plant [24]. 

Oxazolines 151 and 153 were respectively obtained via intramolecular cyclization of P-hydroxyethanamides 150 with titanium tetrachloride (13TL2315) and iodine(III)-promoted oxidative cyclization of N-aUyl-amides 152 (13TL2960). 

Stoichiometric intermolecular amination reactions of arenes have been explored to a larger extent recently. Of major interest had been the investigation on the behavior of phthalimide in the presence of hypervalent iodine as oxidation promoter, which was simultaneously reported by DeBoef and Chang (Scheme 6) [20, 21], The respective reactions of p-xylene 26 to 2-phthaloyl xylene 27 and arenes 28 to... 

The rearrangements described in parts 2-5 involve iodine(III) reagents which can be obtained commercially or prepared in situ in a catalytic fashion. However, some authors have described rearrangements promoted by an iodine(V) oxidant. For example, when reacted with IBX in the presence of Et4NBr, amides 136 undergo a Hofmann-type rearrangement to produce cyanides 137 (Scheme 39) [32]. 

Zhao et al. utilized the remarkable oxidative property of FIFA in a C-C bond formation to prepare an array of 3-arylquinolin-2-ones 120 (Scheme 26). FIFA-promoted oxidation of N-methyl-N-phenylcinnamamides 119 in the presence of a Lewis acid afforded 3-arylquinolin-2-ones in good yields. Formation of 3-arylquinolin-2-ones was realized through the FIFA-promoted simultaneous C-C bond formation and 1,2-aryl migration. This simple and metal-free FIFA-mediated green approach for the biologically important 3-arylquinolin-2-ones also widen the synthetic utility of hypervalent iodine reagents [42]. 

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