January 2017   (Technical report   )

STM Study of Ketopantolactone/(R)-1-(1-naphthyl) ethylamine Complexes on Pt (111): Comparison of Prochiral and Enantiomeric Ratios and Examination of the Contribution of CH… OC Bonding

Yi Dong, Katrine Louise Svane, Jean-Christian Lemay, Michael N. Groves, Peter McBreen

We report a comprehensive model surface science study, using scanning tunneling microscopy (STM), of the regioselective and stereospecific complexation of a prochiral substrate molecule to a chiral modifier molecule on a metal surface. The system is chosen so as to compare the prochiral ratio (pr) measured directly by STM for the model system with the reported enantiomeric ratio (er) for the hydrogenation of the prochiral molecule using the same chiral modifier and metal under true reaction conditions. Specifically, diastereomeric complexes formed between ketopantolactone (KPL) and (R)-1-(1-naphthyl)ethylamine ((R)-NEA) on Pt(111) were studied as a function of the ratio of KPL to (R)-NEA. Only KPL molecules in complexes are detected in the STM experiments, performed between 237 and 250 K, due to rapid diffusion of free KPL on the surface. The prochirality of KPL in almost all bimolecular and termolecular, (KPL)2/(R)-NEA, complexation configurations is assigned as pro-R or pro-S using the contrast within the submolecularly resolved STM motifs. While the overall pr value is relatively constant over wide ranges of ratios of KPL to (R)-NEA, pr values specific to individual complexation configurations vary strongly. The overall pr measured at low to medium ratios closely matches er values reported in the literature for the hydrogenation of KPL on a (R)-NEA-modified Pt catalyst at atmospheric pressure. The large set of data collected also permits an investigation of the contribution of arene CH···O interactions to the formation of abundant complexes.

http://pubs.acs.org/doi/abs/10.1021/acscatal.6b02590