Effect of the presence of oxygen on the photochemical dehalogenation of dehydropyridines and isophthalonitrile derivatives

G. FIRPO; M. V. COOKE; E. N. RUIZ PEREYRA; G. M. CHANS; W. J. PELÁEZ; E. GÓMES; C. ALVAREZ-TOLEDANO; G. A. ARGÜELLO

Villa Carlos PAz

Encuentro; XIII ELAFOT; 2017

XIII ELAFOT

As part of our ongoing project on the reactivity of halogenated compounds, we are exploring halogenated systems such as isophthalonitrile (IPN) and hydropyridine (HYP) derivatives, figure 1. We are interested in these compounds since pyridines emerge as one of the most prevalent structural units in pharmaceutical and agrochemical targets, as well as in material science [1], while chlorinated isophthalonitrile derivatives have a potencial usage as fungicides[2]. Our study aims to analize and comprehend the reactivity after irradiation ( = 254nm), the identity of the photoproducts generated under different enviroments (presence or absence of O2) and the knowledge of all possible reaction paths of the excited state, scheme 1 i-v. We present the analysis of the disappearance rate constants for the compounds of figure 1, under different conditions. All photolysis reactions had a linear decay with time, so it can be concluded that the process is of first order. For IPN derivatives, even though at the wavelength of photolysis the samples have a considerable molar absorptivity coefficient (), the kr at which some of them are being photolized are rather small and they resulted to be higher in the absence of oxygen in the medium. For HYP derivatives in contrast, no significant differences were observed in terms of the presence of oxygen but the kr were significantly higher. These facts indicate that the excited state is taking different paths for IPNs and HYPs. The presence of O2 plays a significant role in the photodegradation mechanism of IPNs. A possible role could be the formation of 1O2 via an energy transfer process which has been probed with an indirect approach. It is well known that L-cysteine in the presence of 1O2 forms L-cystine through a soft oxidation. Therefore, a solution of reagents/L-cysteine was irradiated in the presence of O2 and the formation of a white solid identified as cystine was observed confirming the occurrence of 1iv with high quantum yield. Other possible path that was tested, is fluorescence. Therefore, we measured the fluorescence spectra but the quantum yields determined were of the order of 10-3 in all cases. Measurements were done relative to Ru(bpy)32+ as standard [3]. Therefore, for HYPs decomposition is the path with the highest quantum yield as singlet oxygen formation is the prevailing path for IPNs. In relation to the photoproducts formed, for IPN derivatives dechlorination and decyanation took place with small quantum yields, while for HYPs the loss of the -SO2-CF3 moiety as well as re-aromatization processes occurred with high quantum yields.