Heteroarenes hydrogenation

The SULPHOS-containing rhodium and ruthenium complexes retained their catalytic activity in heteroarene hydrogenation when supported on styrene-divinylbenzene polymer [180] or on silica [181], and showed even higher activity than in homogeneous solution. This effect is attributed to the diminished possibility of dimerization of the active catalytic species to an inactive dimer on the surface of the support relative to the solution phase. The strong hydrogen bonds between the surface OH-groups on silica and the -SO3 substituent in 31 withheld the catalyst in the solid phase despite the rather drastic conditions (100 °C, 30 bar H2). 

Initial investigations showed that the direct hydrogenation of pyrazine carboxylic acid derivatives 14—16 was possible [15] (Fig. 13). This is one of the few examples known today for a homogeneous heteroarene hydrogenation [16]. With [Rh(NBD)Cl]2 and a variety of standard ligands, for example Chiraphos, Norphos, Duphos, Deguphos, PPm, BDPP, BINAP, BPPFOAc no reaction was observed. Similarly no reaction could be observed with Ir catalysts. The positive results are given in Tab. 6. 

M A K O S Z A Vicarious nucleoptiiHc substitution Synthesis of akyl substituted hetsrocydss by vicai1ous nucleophilic substitution ol hydrogen in heteroarenes, nitrodenvatives ol heleroarenes. 

Silylations of aromatic analogs including heteroarenes are known, although in some instances a small amount of non-directed product is formed. The addition of a hydrogen scavenger was found to increase the yield of product (Equations (98) and (99)).93... 

Falck has recently reported dehydrogenative silylation of heteroarenes with triethylsilane (18) [97]. Coupling with the Si-H bond of triethylsilane, rather than the disilane Si-Si bond, in conjunction with the use of norbomene that presumably acts as a hydrogen acceptor, gives good yields with indoles, thiophenes, and furans, under relatively mild condition (80°C). Unlike the reaction shown in Scheme 7, silylation of indole did not require protection of the N-H group. 

M A K 0 S Z A Vicarious nucleophilic substitution Synthesis of afcyl substituted heterocydes by vicarious" nucleophilic substitution of hydrogen in heteroarenes, nitroderivatives of heteroarenes. 

Aryl groups - Groups derived from arenes by removal of a hydrogen atom from a ring carbon atom. Groups similarly derived from heteroarenes are sometimes subsumed in this definition. 

An important role play carbonyl-bearing heterocycles, e.g. 2H- and 4//-pyranones, pyridones, diazinones and the benzo systems derived thereof, as well as hydrogenated heteroarenes like tetrahydropyrane, chromane, piperidine, morpholine and piperazine. 

Makosza M, Wojciechowski K (2013) Nucleophilic substitution of hydrogen in arenes and heteroarenes. In Top Heterocycl Chtan. SpringCT, Berlin, Heidelberg. doi 10.1007/ 7081 2013 115... 

In the foUowing sections we will present some recent examples of introduction of various substituents into heteroarenes via nucleophilic substitution of hydrogen. The full account of the previous results was given in our preceding reviews [4, 89, 90]. 

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