Addition of HCl accelerated the photocyclodimerization of trans-4-styrylpyridine 1a in methanol and increased the yield of syn-head-to-tail (syn-HT) dimer 2a through the effect of cation-π interactions between the pyridinium ion of one molecule and the phenyl group of the other. We examined the photoirradiation products of derivatives of 1a having alkyl substituents on the phenyl group (1b-1f). The effect of the alkyl substituent on product distribution was rather limited for the photoreaction in MeOH solutions. However, the substituents had a distinct effect on the product distribution for the photoreaction of the inclusion complexes of hydrochloride salts of trans-4-styrylpyridines with cucurbit[8]uril in aqueous solutions. Introducing an alkyl group at the 2- or 3-position of the phenyl group completely shifted the major product from the syn-HT dimer to the syn-head-to-head (syn-HH) dimer. By adjusting the balance of host-guest interactions and cation-π interactions between guest molecules through systematic changes in the substituents on the phenyl ring of trans-4-styrylpyridine, we could change the orientation of the reactant molecules in the host cavity, resulting in a change of the major regioisomer of the photocyclodimerization products.

Synthesis and characterization of a new ruthenium complex, [Ru(acac)2(mhmk)] (1) with coordinated β-ketiminate ligand are described. The complex has been obtained by the reaction of acetone on coordinated acetonirile in [Ru(acac)2(CH3CN)2], the first such reaction to be observed. X-ray structure of 1 is similar to that of [Ru(acac)3]. Cyclic voltammetric studies of 1 show a negative shift for both reduction and oxidation indicating that there is large electron density on ruthenium containing β-ketiminate ligand when compared to ruthenium containing only β-diketonato ligands, [Ru(acac)3].

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