Adams catalyst stabilities

The stability of the sydnone ring is shown by its resistance to catalytic hydrogenation. For example, 3-phenylsydnone is only reduced to A-phenylglycine with the very reactive Adams catalyst (57QR15). 

The aromaticity of the imidazole nucleus ensures stability towards reduction, and when benzimidazole (27) is hydrogenated over Adams catalyst in acetic acid the carbocyclic ring is reduced first to give the tetrahydrobenzimidazole (28). However, if the solvent is changed to acetic anhydride, A(-acylation promotes the reduction of the heterocycle and the 1,3-diacetylbenzimidazoline (29) is then formed (Scheme 1). Imidazole (30) under these conditions gives 1,3-diacetylimidazoline (31). Imidazolium salts (32) are easily reduced and treatment with excess sodium borohydride in 95% aqueous ethanol culminates in the formation of 1,2-diamines, (33) or (34). Either N—C bond may cleave, although if the substituent R is benzyl the major products are benzylamines (33 R = Bn). ... 

The wider utility of palladium metal catalysts and hydride cleavage of allylic systems has more or less replaced interest in using platinum-based catalysts such as Adams catalyst. Where this has been used for conjugation-stabilized allylic centers good yields have been achieved." The use of platinum for de-benzylation at low H2 pressure is effective." ... 

Scheeren, C. W. Domingos, J. B. Machado G. Dupont, J. (2008). Hydrogen reduction of Adams catalyst in ionic liquids formation and stabilization of Pt(0) nanoparticles. The Journal of Physical Chemistry C, Vol.112, No.42, (September 2008), pp 16463-16469, ISSN 1932-7447... 

Cyclopentadiene adducts (mono-, tetra- and hexa-adducts) of were stabilized against retro-reaction by selective hydrogenation and bromination of the pendant groups [21]. Utilization of Adam s catalyst and dilute bromine solutions exclusively leads to an addition to the cyclopentene double bonds, because itself is inert towards these reagents. The increased stability of the reduced cycloadducts can be demonstrated by mass spectrometry [21]. 

Polymerisation in emulsion, in which the monomer is (a) dispersed in monomer droplets stabilized by an adsorbed layer of soap molecules (Fryling and Harrington, 1944, Kolthoff and Dale, 1945, Price and Adams, 1945, Siggia et ah, 1945, Vinograd etal, 1944) (b) solubilised in the soap micelles (Harkins, 1945, McBain, 1942, McBain and Soldate, 1944) which exist in an aqueous soap solution of sufficient concentration and (c) molecularly dissolved in the water. The amount of polymer formed in the droplets, in the micelles, and in solution will depend upon the way in which the monomer and catalyst are distributed in the three existing phases the monomer phase, the soap micelle phase, and the water phase - and possibly also upon the accessibility and reactivity of the monomer in these three phases. In certain aqueous soap emulsions, such as styrene, dichlorostyrene, or isoprene, the amount of molecularly dissolved monomer is small and, therefore, the reaction will occur preponderantly either in the monomer droplets or in the soap micelles. If the polymer formation occurs preponderantly in the micellar phase, one is inclined to speak of a typical emulsion polymerisation. If, however, polymerisation takes place to a considerable extent both in the monomer droplets and the soap micelles, the case is intermediate between suspension and emulsion polymerisation. There also exist emulsion... 

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