The bent bond model developed by Pauling and Slater in the early 20th century is not a common tool for the modern chemist. However, it holds an incredibly large potential in terms of chemical applications. The bent bond model, or tau bond, is a description of unsaturated systems that involves a completely different geometric and electronic character than the one mainly used today, the Hückel model. The most important characteristic of the tau bond applied in this study is the energetic non-equivalence of the tau orbitals. This distinctive aspect allows to introduce a desymetrisation of the electron density in one of the two bent bonds by hyperconjugation from neighbouring groups following the antiperiplanar hypothesis. This whole concept can be applied easily and with a perfect fit to numerous known chemical phenomena. The Cieplak effect or the Inomata' Syn effect are two examples among many where the bent bond model provides a clearer understanding, both visually and fundamentally. This study focuses on the SN1 glycosylation reaction of rigid and simplified furanoside and pyranoside analogues for which the substitution in terms of electronegative groups is controlled both in nature and location. The transition state of such a reaction proceeds through an oxocarbenium ion to which the bent bond model can be applied to rationalise the resulting anomeric ratio. Model substrates for the reaction are synthesized in a way that minimizes the structural and stereoelectronic factors influencing the selectivity at the anomeric center in order to emphasize the role played by hyperconjugation of a tau orbital with the σ* orbital of an electron withdrawing or donating group.

Bent bond and the antiperiplanar hypothesis: A novel approach in the comprehension of glycosides reactivity at the anomeric center

256th National Meeting (2018)

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