Indium based catalysts

Aldehydes and ketones have been alkynylated using indium(III) and Hunig s base (PrjNEt) as catalysts.213 IR and NMR evidence support a dual-activation role for indium it is a Lewis acid for the hard electrophile (carbonyl compound), and has sufficient n -coordination ability for a soft nucleophile such as a terminal alkyne. For the latter substrate, the amine then assists proton abstraction. 

With the exception of Yb(OTf)3 3H2O, indium salts and Bronsted acids, there are several metal-based Lewis acid catalysts available for these Michael reactions, such as a CeCl3 7H20-NaI combination supported on... 

The reaction of tri- -butylindium with tetrachloroethylene affords 3-methyl- 1,1,2-trichloro-l-pentene and indium trichloride without the use of any catalyst and solvent. Other trialkylindium gives similar results (Table 29). The yield of the product based on the amount of trialkylindium used reaches 287%. This fact shows that all three alkyl groups attached to indium are effectively transferred to the product. Several haloalkenes lead to the selective formation of cross-coupling products under similar conditions. A stereospecific cross-coupling with ( )-/3-bromostyrene gives ( )-/ - -butylstyrene. In contrast, starting from both (E)- and (Z)-l,2-dichloroethylene, a mixture of (E)- and (Z)-l-chloro-3-methyl-l-pentene has been obtained in a ratio of 6 4.398... 

The influence of the oxide support (i.e. AI2O3, Nb20s, Si02 and Ti02) on the surface properties, reduction and oxidation properties and acid-base properties of supported indium oxide catalysts has been investigated by temperature pro-... 

Indium chloride is also an excellent catalyst for ionic Diels-Alder reactions. Acyclic and cyclic olefinic acetals undergo reactions with isoprene and cyclopentadiene in the presence of 20 mol% IrifJ, to form the corresponding cyclic adducts in good yield with good selectivity (Scheme 8.103) [133]. With cyclopentadiene, the endole-xo ratio is fairly good and comparable with that for the LiClO4- and Nafion-H-based reactions. With other Lewis acids, e.g. Yb(OTf)3 or Sc(OTf)5, no cycloadduct is formed. 

The Henry reaction is an aldol-type reaction between a nitroalkane and an aldehyde in the presence of a base. Since basic reagents are also catalysts for the aldol condensation, the nitroaldol reactions must be strictly controlled. An interesting alternative lies in the use of surfactants to perform the reaction in an aqueous medium [63], The Reformatsky reaction, which involves a-haloketones and aldehydes, can be mediated by zinc, tin or indium in water in the latter case the proportion of undesirable reduction products could be strongly reduced [64]. 

Tan et al. [44] succeeded in developing a new urea-sulfmimide catalyst that promotes the indium mediated allylation of acyUiydrazones (Scheme 9.9). Incorporation of Lewis base functionality in proximity to the urea moiety is designed to promote the addition of organometallic reagents to the C=N bond of acylhydrazones through dual activation. 

In some cases, aUc5me hydrothiolation can be achieved in the absence of transition metal catalysts. Examples include the use of indium halides, selenium halides and salts, bases and p-cyclodextrin [215-226]. In particular, Cesium bases yield exclusively the anti-Markovnikov product and fi equently give high regioselectivity for the Z-linear olefin, which is complementary to transition metal catalysis. While these approaches as yet lack the generality of the transition metal-catalyzed systems, the ability to achieve hydrothiolation without the need for a metal catalyst is attractive. Undoubtedly, this area of research will continue to yield promising results. 

The encapsulation of metalloporphyrins in the cavities of an indium imidazoledicarboxylate-based rho-zeolite-like metal-organic framework (rho-ZMOF) has been reported by Eddaoudi and coworkers [71]. The catalytic activity of this material was assessed by cyclohexane oxidation with TBHP as the oxidant, with cyclohexane conversion reaching 91.5% after 24 h at 65 °C. Cyclohexanol and cyclohexanone were the only observed products, suggesting that the investigated oxidation reaction is selective toward the desired products. Furthermore, upon reuse of the catalyst, no loss of crystaUinity, reactivity, and selectivity in up to 11 cycles was observed, while no leaching of the encapsulated metalloporphyrin into the product solution was detected by the UV-vis spectra. 

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