Surface enhanced Raman spectroscopic (SERS) behavior of phenylpyruvates used in heterogeneous catalytic asymmetric cascade reaction.

The strength and geometry of adsorption of substituted phenylpyruvates on silver surface was studied by means of surface enhanced Raman spectroscopy (SERS) using silver sol. 2'-nitrophenylpyruvates were used as starting materials in a newly developed heterogeneous catalytic asymmetric cascade reaction to produce substituted quinoline derivatives. Substituents on the aromatic ring of the starting materials had significant influence on the yield of the desired quinoline derivatives. Product selectivity of the transformation of nitrophenylpyruvates were enhanced by the acid added. The geometry and the strength of the adsorption are assumed to play an important role in the outcome of this reaction, so we have tried to find correlation between the structure of adsorbed phenylpyruvates and their catalytic performance. Based on the results of our spectroscopic measurements, the enol form is predominant in the series of phenylpyruvates in solid form and methanol solutions. Stronger adsorption of phenylpyruvates in acidic media through oxygen atoms was indicated by the increased enhancement in the SERS spectrum. The nitro group of 2'-nitrophenylpyruvates has no direct role in the adsorption on Ag surface. This observation has explained why the hydrogenation of the keto group (presumably via the enol form) occurs preferentially and why the formation of the undesired indole derivatives requiring reduction of the nitro group is suppressed. The SERS behavior has helped to get a closer look on the first step of adsorption of starting materials contributing to a better understanding of the cascade reaction studied, thus providing a better flexibility in catalyst design.