Estudo mecanístico da reação de Morita-Baylis-Hillman e aplicações em síntese orgânica

Abstract

The C-C bond formation reactions are among the key methods for the development of chemistry, since they form the backbone of virtually all organic molecules. Among the methods employed, we can highlight the Morita - Baylis - Hillman reaction that represents an organocatalyzed transformation in which polyfunctionalized molecules are accessed. Thereafter, this work describes the mechanistic study of two protocols developed to overcome limitations of this reaction as low yields against aldehydes rich in electrons. The first protocol studied was the MBH / DABCO-AcOH reaction. The increase in the reactivity of these electrophiles coupled to second-order kinetics with respect to the aldehyde and the mono-protonated Lewis base leads us to propose that the rate determining step of the reaction becomes the elimination step and not the aldol addition, since the electronic effect of the aldehyde in this methodology seems to be not very expressive. The second protocol studied was the MBH / DABCO-MeOH reaction. The interception and characterization of cationic intermediates consistent with the one-pot process of transesterification / MBH reveals that even in the early stages of the reaction, the 1.4 addition step to the acrylic system, complex equilibria are involved. The proposal for the transesterification mechanism occurs, according to the control experiments, in a basic medium. The methoxide ion present is added to the carbonyl of the activated acrylic system followed by the displacement of the ethoxide ion. This protocol also served as a platform for the kinetic study of the decomposition of the MBH adduct assisted by DABCO and showed that such a process is of first order regard to the adduct. The elaboration of a photocatalytic methodology for trifluoromethylation of deactivated double bonds using the MBH adduct as substrate was also developed. The best condition for the hydrotrifluoromethylation reaction, until now, is the use of 3 mol% of the photocatalyst of [Ru(bpy)3](PF6)2, 1.1 equivalent of the Umemoto reagent in 1.0 ml of dichloromethane under constant magnetic stirring for 24 hours against a blue LED panel of 30W. Considering the modes of activation commonly observed in photocatalysis, we proposed three catalytic cycles for the mechanism of the studied reaction, which may occur through energy transfer or electron transfer.