Indium bromide, cyclization

Intramolecular cyclization of tethered phenyl ketones (58 X = Br, SiMe3) show contrasting stereochemical outcomes for indium catalysis of the alkyl bromides and fluoride ion-induced reaction of the allylsilanes. The reactions thus allow complementarity in product diastereoselectivity, and the difference appears to be related to an... 

In order to increase the Lewis acidity of the indium centers in compounds such as 52, the preparation of derivatives that incorporate a tetrafluorophenylene backbone has also been pursued. l,2-Bis(halomercurio)tetrafluorobenzene (halide = chloride (53) or bromide (54)) reacts with two equivalents of the corresponding indium(I) halide in THF at 25 °C to afford the tetrakis(T i ) adduct of the respective 9,10-dihalo-9,10-dihydro-9,10-diinda-octafluoroanthracene (halide = chloride (55) or bromide (56)) (Scheme 22). Compound 56 is also prepared by the reaction of (o-C6F4Hg)3 (4) with InBr in refluxing toluene followed by treatment with THF. The formation of the diindacycles 55 and 56 in the reaction of 53 and 54 with two equivalents of the corresponding indium(I) halide is surprising since, in principle, bis(indiumdihalide) complexes would be the expected products. This cyclization... 

Highly enantioselective Rh-catalyzed intramolecular Alder-ene reactions for the synthesis of chiral 3-alkylidene-4-vinyltetrahydrofurans were reported by Zhang, as illustrated below <02AG(E)3457>. Metallic indium was also shown to mediate the intramolecular cyclization of tethered allyl bromides onto terminal alkynes to afford 3-methylene-4-vinyltetrahydrofurans in 50-62% yield <02SL2068>. 

Several indium-mediated intramolecular carbonyl allylation reactions have been investigated, and it has been found that these reactions provide an easy access to a variety of cyclic compounds. The intramolecular cyclization of 49a-c mediated by indium in aqueous media proceeds smoothly to afford carbocyclic systems containing y-hydroxy-Q -methylene esters 50a-c, which either spontaneously or readily cyclize to give fused o -methylene-y-butyrolactones 51a-c (Scheme 52). The same cyclization of 49d is too slow to compete with the side-reaction, in which the bromide is substituted by a hydroxy group. The ring junction stereochemistry of fused lactones 51 has been found to be cis in all cases. Of the two possible transition states, the one leading to the m-fused compounds is preferred, because the chair-chair conformation is favored over the chair-boat conformation.209... 

The intramolecular cyclization of tethered allyl bromides onto terminal alkynes mediated by metallic indium proceeds smoothly and cleanly in a mixture of THF and water to give unsaturated carbocycles and heterocycles in good yields. The reaction does not proceed efficiently under rigorously anhydrous conditions (Scheme 72). 

Dichloroindium hydride HlnCla, prepared from indium(m) chloride and DIBAL-H, works as a radical mediator. This compound reduces aryl iodides and bromides in the presence of Et3B as a radical initiator. Reduction of iodoacetals provides cyclized products in good yields (Equation (99)).385 Only a catalytic amount of InCl3 is necessary for complete consumption of the starting compound with stoichiometric DIBAL-H as a hydride source (Equation (100)). 

The same group reported indium-mediated tandem radical addition-cyclization-trap reactions in aqueous media [52c], The use of water-soluble radical initiator 2,2 -azobis[2-(2-imidazolin-2-yl)propane], water-soluble chain carrier 1-ethyl-piperidinium hypophosphite (EPHP) and surfactant cetylammonium bromide (CTAB) allowed the C-C bond-forming radical reactions of highly hydrophobic substrates in water [53], Similarly, the use of CTAB and EPHP in presence of 4,4 -azobis(4-cyanovaleric acid) promoted the indium-mediated radical addition to P-substituted conjugated alkenes in water [54]. 

Similar reactions involving an allylic fragment proceed with formation of the Sn2 type products. Indium(III) salts (InCb predominantly investigated) have been found to be effective Lewis acids to catalyze conversion of 13 at room temperature in the presence of molecular sieves. The cyclization is accompanied by elimination of hydrogen bromide, leading to the vinyl derivatives of tetrahydronaphthalene 14 and tolerates electron-withdrawing groups. 

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