Theoretical Studies On The Mechanisms Of Allylation Of Benzaldehydes Catalyzed By Pyridine N-oxides

Allylation of aldehydes with allylsilanes is one of the most useful tools in or-ganic synthesis. In this dissertation, a series of theoretical studies have been per-formed for allylation of aldehydes with pyridine N-oxides as catalysts. And we used density functional theory at the molecular level to clarify the detailed reaction mecha-nisms and different reaction paths. This will play an important role on exploring the essence behind the experiment phenomenon and presenting the powerful theory and guidance for the research on the development of new organic reactions. The content of this paper summarized as follows:1. Mechanisms for the allylation of aromatic aldehydes with allyltrichlorosilanes catalyzed by QUINOX. The research was investigated at the B97-D/TZV(2d,2p) level of DFT theory. Three solvents (Toluene, CH3CN, and CH2Cl2) have been considered to address the effect of solvent polarity on predicted barriers and enantioselectivity. Three different pathways namely associative, non-bonded and dissociative mecha-nisms were explored, and it was found that the preferred pathway depends on the na-ture of the solvent. Moreover, we also found that the associative pathway leads to preferential formation of the (R)-enantiomer, while the dissociative route leads to formation of the (S)-enantiomer. The predicted stereoselectivity for the non-bonded pathway was highly dependent on the choice of continuum solvent model, so no clear predictions can be drawn regarding this mechanism.In the gas phase, a non-bonded mechanism is predicted to dominate, while in tol-uene the associative mechanism is competitive with the non-bonded mechanism. However, in CH2Cl2, allylation is predicted to proceed via the associative mechanism, while in CH3CN, the dissociative mechanism becomes the most favorable pathway. However, due to the possible participation of CH3CN, the mechanism for these allyla-tion reactions may be far more complex in this solvent than in others. This is poten-tially why CH3CN has been found to be a poor solvent for QUINOX-catalyzed allyla-tions. In each of these solvents, differential π-stacking interactions between the ben- zaldehyde and the naphthyl group of the catalyst contribute substantially to the ob-served selectivity.Regarding substituent effects, computed reaction barriers indicate that both tri-fluoromethyl and methoxy substituted groups result in enhanced selectivity compared to non-substituted benzaldehyde, even though experimentally -OMe has been shown to lead to drastically reduced enantioselectivity. However, computational datas do predict a lower reaction barrier for p-trifluoromethylbenzaldehyde compared to either benzaldehyde or p-methoxybenzaldehyde, in agreement with experiment. Although the computational data are not in accord with experiments on all fronts, the present results should provide a guide for designing improved catalysts, and finding optimal reaction conditions for formation of the desired stereoisomer in QUINOX catalyzed allylation reactions.2.Theoretical research for the Mechanisms of allylation of aromatic aldehydes with allyltrichlorosilanes catalyzed by axial chiral dipyridine N,N’-bioxides. The study used density functional B97-D/TZV(2d,2p) method, combining with the polar-izable continuum model(PCM) to calculate the solvation effect for allylation of aro-matic aldehydes with allyltrichlorosilanes catalyzed by axial chiral dipyridine N,N’-bioxides((S)-3) in different dielectric constant of solvent. The calculation results show that, the solvent polarity play an important role on the reaction mode between benzaldehyde with allyltrichlorosilanes and the formation of the transition state con-figurations. We also verified that the reaction can be proceed catalyzed by axial chiral dipyridine N,N’-bioxides((S)-3) through three pathways mentioned earlier, such as: dissociative, non-nonded, associative mechanisms. It is revealed that the reaction mechanism depend on the solvent polarity. In polar solvent, a dissociative mechanism is predicted to dominate, while in nonpolar solvent, the associative mechanism is competitive with the non-bonded mechanism but the non-bonded mechanism based. Moreover, the chiral of product was also effected by the solvent polarity, such as:po-lar solvent leads to preferential formation of the (R)-enantiomer, while the nonpolar solvent leads to formation of the (S)-enantiomer. In addition, we also researched the influence of the substituted group(using -OMe and -CF3 as an example) with different dielectric properties on the steroselectivity of the reaction.