Triflate, Indium

The reaction of stilbene oxide with a variety of aniline derivatives in the presence of indium triflate and the chiral bipyridine ligand 22 provided aminoalcohols in generally good yields and enantioselectivity <07SL2136>. 

The ring-opening reaction can also be induced by attack of oxygen-centered nucleophiles, as demonstrated by the facile ring cleavage of the unsymmetrical bicyclic aziridine 60 with methanol in the presence of boron trifluoride etherate to give the product of attack at the more-substituted aziridine carbon <2002T7355>. Indium triflate... 

It was first reported in 1996 that indium trichloride catalyzes the Diels-Alder reaction in water [131]. The reaction of acrolein with cyclopentadiene in the presence of 20 mol% InCl proceeds stereoselectively (endo. exo=91 9) (Scheme 8.101). Without catalyst the reaction only goes to 60% completion (endo exo=74 26). The InCh-catalyzed Diels-Alder reaction works with either cyclic or non-cyclic dienes. InCh can be recovered for reuse after the reaction is completed. Indium triflate is also an effective catalyst for intramolecular Diels-Alder reactions of furans under microwave irradiation (Scheme 8.102) [132]. 

Indium trichloride and indium triflate are good catalysts also for the imino Diels-Alder reactions. With 20 mol% InClj, N-benzylideneaniline reacts with cyclopentadiene to give the corresponding tetrahydroquinoline derivative (Scheme 8.104) [134]. Similar InCls-catalyzed imino Diels-Alder reachons proceed with 3,4-dihy-dro-2H-pyrane, indene [135], and cychc enamides [136]. In contrast, cyclohexen-2-one gives no phenanthridinone, but azabicyclo [2.2.2]octanone is isolated (Scheme 8.105) [137]. The reachon seems to proceed through the formahon of die-nolate ion by strong coordinahon of InCb with the enone. 

At a loading as low as 0.5 mol%, indium triflate catalyzes the three-component coupling reaction between aldehydes, amines, and Danishefsky s diene to afford tetrahydropyridine derivatives (Scheme 8.106) [138]. In fhe presence of indium triflate, 3,4-dihydro-2H-pyran reacts with in sitw-generated chromone Schiff s bases to give the endo cycloadducts (Scheme 8.107) [139]. 

Indium chloride and indium triflate are efficient catalysts for the synthesis of bis-indolylmefhane and indolylquinoline derivatives. Indium triflate needs lower catalytic loading (Scheme 8.113) [156]. 

A catalytic amount (5 mol%) of IrifJ, alters the ratio of 1,2- to 1,4-addition of Grignard reagents to a,/ -unsaturated carbonyl compounds [177]. In the presence of indium triflate or gallium chloride coupling of internal alkynes and aldehydes proceeds (Scheme 8.134) [178]. 

In the presence of indium triflate, O-H insertion reactions of a-diazo... 

Indium trichloride induces rearrangement of aryl-substituted epoxides to the respective aryl-substituted acetaldehydes via an exclusive hydride shift As phenyl group migration occurs more readily than hydride migration, stilbene oxide is converted to diphenylacetaldehyde (Scheme 8.140) [184]. N-Tosyl aziridines react smoothly with carboxylic acids in the presence of a catalytic amount of indium triflate to afford the corresponding -aminoacetates and benzoates (Scheme 8.141) [185]. Indium trichloride and indium bromide catalyze regio- and diastereoselective azidolysis, bromolysis, and iodolysis of a, -epoxycarboxylates in water (Scheme 8.142) [186]. 

Indium triiodide catalyzes transesterification processes, e.g. the acylation of alcohols or amines and the conversion of THP ethers to acetates (Scheme 8.143) [187]. Indium triflate is also an efficient catalyst for the acylation of alcohols and amines (Scheme 8.144) [188]. Carboxylates are hydrolyzed to the corresponding carboxylic acids in high yield by microwave-assisted reaction on the surface of moistened silica gel in the presence of indium triiodide (Scheme 8.145) [189]. 

HSCH2CH2OH, ZrCLt, CH2CI2, 55-97% yield. Aldehydes react much faster than ketones. Indium triflate can be used as a catalyst (70-92% yield). ... 

A significant improvement in the catalyst adds further utility to this strategy. Using 1-5 mol% of indenyl complex 15 along with indium triflate and camphorsulfonic acid (CSA) as cocatalysts, the redox isomerization in Equation 1.19 was completed in 20 min compared with 1.5 h under the indium chloride cocatalyst conditions. 

Indium triflate-mediated synthesis of oligomeric 1,6-linked galacto-oligosaccharides. 

There has been a review of asymmetric Friedel—Crafts reactions. It has been shown that the rhodium-catalysed reaction of potassium phenyltrifluoroborate with A-tosyl ketimines may lead to products such as (18) with high enantioselectivity. (g) The reaction of A-t-butanesuUinylimino esters with arenes to give products (19) is catalysed by Lewis acids, such as indium triflate, and may lead to enantio-metrically enriched a-glycines. The intramolecular alkylation of hydrogenated 0 tetralins, shown in Scheme 2, using iron(III) or aluminium(III) catalysts yields cis-hexahydrobenzophenanthridines. Formation of a stabilized carbocation intermediate is... 

Most of indium triflate s apphcations exploit its properties as a mild, chemoselective, and versatile Lewis acid. It has been shown to activate various functional groups containing oxygen, halogens, or carbon-carbon u--bonds, enhancing the electrophilicity or nu-cleophUicity of the complexed species. 

Scheme 8.6 Reactivity patterns of monovalent indium triflate (a) and halide (b) salts and their crown ether complexes (X = Cl, Br, I R = H, Cl). With permission from Ref [88],...

Indium Lewis acids have garnered attention due to their mild reactivity and air and water stability. Both Li et al. and Chan and Loh have shown that In(III) complexes are suitable Lewis acids for Prins cyclizations [81, 82]. These reports prompted Loh and coworkers to embark on a synthesis of (+)-SCH 351488 that utilized this strategy (Scheme 40) [83]. Condensation of homoallylic alcohol 147 and aldehyde 148 in the presence of indium ttibromide and TMSBr gave 4-bromo THP 149 in 65 % overall yield as an inconsequential mixture of diastereomers (2,A-cisP,4 trans = 75 25). Complete retention of the homoallylic alcohol stereochemistry is responsible for the key 2,6-cis relationship in the product. Initial attempts to apply these same conditions to the B ring resulted in acetonide deprotection and no THP formation. Subsequent optimization revealed that indium triflate and TMSCl were competent additives to effect cyclization. Careful temperature control was required to suppress an undesired Prins side reaction. The combination of homoallylic alcohol 150 and aldehyde 151 in the presence of the appropriate Lewis acids at 78 °C, followed by warming to 0 °C for 4 h, led to the desired monomer precursor 152 in 42 % yield. 

The coupling reaction of A-methylindole (747) with alkyl and aryl phosphonates (746) in the presence of indium triflate furnished bis(indo-lyl)methanephosphonate (748) in good yield and high selectivity (Scheme 190). ... 

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