Dehydrogenation, base-induced

A nice feature of reductive as well as oxidative coupling reactions has been presented recently in the synthesis of quateranthene 132 (Scheme 32) [82]. After addition of the lithiated species of 128 into the bianthrone and dehydrogenation, base induced cyclization gave the intermediate 130. Grignard addition to the ketone and subsequent reduction delivers 131, which was then fully aromatized by a Scholl-reaction involving DDQ as an oxidant in the presence of Sc(OTf)3. 

The heterocyclic cycloproparenes l//-4-oxacyclopropa[/]indene (15a) and l//-4-thiacyclo-propa[/]indcnc (15b) °° were synthesized from the appropriate dienes mand 1,2-dibromocy-clopropene via dehydrogenation of the cycloadducts with 2,3-dichloro-5,6-dicyanobenzo-quinonc and base-induced elimination. Both compounds decompose rapidly in the solid state and in solution. 

An acetoacetate side-chain provides the four carbon atoms required to form a pyridine ring by this base-induced cyclization [2034,2081] a better yield (90%) was later obtained by heating the acetoacetate (74.3) in xylene [2356]. DMSO has also been used as solvent [3252]. A pyridin-2-one ring is formed by heating the ester (74.4) with first ammonia and then 4-toluenesulphonic acid. Dehydrogenation with DDQ gives an aromatized ring [3545]. 

AT-Ray analysis has shown that the aromatic azatropolone ring (11 R = COaEt) is, surprisingly, not planar, and it has chirality. A general route to a range of these interesting compounds is provided by the base-induced ringopening of (10) followed by dehydrogenation. "... 

Among the reactions of 4,5-dihydroisoxazoles, base-induced or reductive ring-opening as well as dehydrogenation are synthetically relevant [321],... 

Alternatively, 2-aryl-4-quinolones (124, R =Ar) are obtained from (2-amino)aceto-phenones 125 by base-induced aldol condensation with aromatic aldehydes (—>-126), Lewis acid-promoted cyclization to the 2,3-dihydro-4-quinolones 127, and their dehydrogenation with PhI(OAc)2 (127 -> 124, R=Ar) [186],... 

In a base-free medium (dry MeCN), Fe Ch activates HOOH to form a reactive intermediate that oxygenates alkanes, alkenes, and thioethers, and dehydrogenates alcohols and aldehydes. Table 11 summarizes the conversion efficiencies and product distributions for a series of alkene substrates subjected to the Fe Cfi/HOOH/MeCN system. The extent of the Fe Cb-induced monooxygenations is enhanced by higher reaction temperatures and increased concentrations of the reactants (substrate, Fe Cls, and HOOH). For 1-hexene (representative of all of the alkenes), a substantial fraction of the product is the dimer of 1-hexene oxide, a disubstituted dioxane. With other organic substrates (RH), Fe Cb activates HOOH for their monooxygenation the reaction efficiencies and product distributions are summarized in Tables 11(b). In the case of alcohols, ethers, and cyclohexane, a snbstantial fraction of the product is the alkyl chloride, and with aldehydes, for example, PhCHO, the acid chloride represents one-half of the product. In the absence of snbstrate the Fe Cls/MeCN system catalyzes the rapid disproportionation of HOOH to O2 and H2O. 

Nafta reforming and alkane dehydrogenation processes are directly connected on platinum alumina based catalysts. The stability and selectivity requirements for industrial purposes induced the addition of a second metal like Sn, Ge or Re. In fact, these promoters coupled to other acidity controllers added on the support enhanced the catalytic efficiency [1-3]... 

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