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The Surface catalyzed racemization of 1, 1’-Binaphthyl

University of British Columbia

The surface catalyzed racemization of a simple organic reaction - the racemization of 1,1'-binaphthyl - has been studied using carbon, Raney Nickel, and platinum (Adams' Catalyst) as solid catalysts. In each case the rate of the catalyzed reaction was studied using polarimetric methods. The dependence of the rate on various parameters was then used to determine features of the catalyzed reactions. In the case of the carbon catalyzed reaction, 4,4'-disub-stituted-1,1'-binaphthyls were synthesized and their catalyzed reaction rates determined. All the reactions followed smooth first order kinetics. Using a Hammett (σ[sup P]) plot for the catalyzed and uncatalyzed reactions, catalysis was found to proceed by way of electron donation into the binaphthyl ring system. Oxidation or reduction of the catalyst, which would modify disorganized areas on the surface, had no effect on its activity. Halogenation of the catalyst, which is believed to occur on the organized basal planes of the carbon, dramatically increased its activity and led to new solvent effects. The effect of halogenation was explained by considering chemisorbed solvent molecules to be a type of inhibitor whose desorption rate is increased when the sites of adsorption (the basal planes) are disrupted by bound halogen. A potassium-graphite intercalate was used as a model for the carbon surface and led to the conclusion that carbon catalysis may involve the formation of the radical anion of 1,1' binaphthyl on the basal planes of the carbon surface. Raney Nickel catalyzed both the racemization and the re-, duction of 1,1'-binaphthyl. In addition, an extensive adsorption process occurred. These three interactions could be controlled independently by careful poisoning with elemental sulfur or dodecanethiol. This suggested that there were three different types of surface sites on Raney Nickel, each responsible for a single type of interaction with 1,1'-binaphthyl. Also, it allowed the three processes to be studied independently. The reduction proceeded through three intermediates to give 5,6,7,8,5',6',71,81-octahydro-1,1'-binaphthyl as the final product. NMR studies showed this molecule to exist as two enantiomers which could interconvert at a slow rate (t[sub 1/2] = 17 hr) The racemization process showed good first order kinetic plots after an initial curved portion. The curved portion of the plots is due to the concurrent adsorption process. Problems of reproducibility hampered further kinetic studies. The adsorption process showed a Langmuir-type adsorption isotherm, as well as an adsorption isobar and kinetics which are typical of other slow chemisorptions. The platinum (Adams' catalyst) catalyzed racemization of 1,1'-binaphthyl also showed good first order kinetics. The observed rate constant, k[sub obs] was inversely proportional to the concentration of substrate. The reaction rate was also independent of the concentration of the catalyst. This peculiar kinetic effect could not be accounted for by the intervention of diffusion-controlled processes and remains unexplained. The poisoning effect of air and the inhibitory effect of cyclohexene and cyclohexane indicate that reduction and racemization occur on the same site. Continued studies were not possible because of an uncontrollable decrease in catalyst activity.

Text

2010-03-26

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http://hdl.handle.net/2429/22527

Chemistry

University of British Columbia

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