Nature of the catalytic cycle in iridium-complex catalysed hydrogenation of unsaturated alcohols |
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Authors: | John M Brown Andrew E Derome Stephen A Hall |
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Institution: | Dyson Perrins Laboratory, South Parks Road, Oxford OX1 3QY. UK |
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Abstract: | The face specific reduction of -6-methylenebicyclo2,2,2]octan-2-ol and -6-methybicyclo 2,2,2]-oct-5-en-2-ol with D2 and iridium catalysts is accompanied by deep-seated isotopic redistribution through an intramolecular mechanism, although only two deuterium atoms are incorporated on average. Individual isotopomers of the product may be identified in the 13C N.m.r. spectrum at 125 MHz. and their ratio is generally consistent with a mechanism in which product is formed by breakdown of an alkyliridium trihydride. Iridium (and rhodium) catalysts part-isomerise the exocyclic olefin to its endocyclic isomer an Ir-allyl intermediate without incorporation of deuterium. The reduction of 3-methylcyclohex-2-enol is likewise accompanied by considerable scrambling, with isotopic enrichment occurring at C2, C3, C4 and C5 of the product, -3-methylcyclohexanol. Deuteration occurs exclusively on the hydroxyl-bearing face of the molecule. |
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