Hydrogenolysis of benzylic alcohols on rhodium catalysts

Alumina-supported catalysts from various sources and with different rhodium dispersions predominantly yield the hydrogenation product perhydro-1-indan ol in the liquid-phase hydrogenation of 1-indanol, while carbon-supported c atalysts mainly give the C-O bond scission - hydrogenation product perhydro indane. Addition of organic or inorganic bases to the reaction mixture supp resses C-O bond scission. To distinguish between the two possible pathways for C-O bond scission of direct hydrogenolysis or dehydration followed by h ydrogenation, deuteration studies have been carried out with carbon-support ed catalysts, Not only 1-indanol but also indane and indene (the two possib le mechanistic intermediates in the C-O bond scission routes) were deuterat ed, Information about the actual pathway has been obtained by determining t he degree of deuteration and the positions at which deuterium is incorporat ed in the resulting perhydroindane product by means of mass spectrometry an d C-13 NMR spectroscopy, The results prove that C-O bond scission takes pla ce primarily through the direct hydrogenolysis pathway on the carbon-suppor ted catalysts. Direct hydrogenolysis occurs on the carbon support because o f the formation of a better leaving group (OH2+) from the benzylic hydroxy group and its subsequent substitution by spillover hydrogen.