Fragmentation – Fragmentations are controlled by stereochemistry (Cis isomer)

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The cis isomer clearly undergoes a fragmentation reaction. Numbering the atoms can help to identify the bond that breaks. The nitrogen lone pair pushes, and the departing tosylate pulls, resulting in the formation of an iminium ion which then hydrolyses to the aldehyde product.

The fragmenting bond is trans to the leaving group–indeed, it is both parallel and trans, in other words anti-periplanar to the leaving group. Electrons can flow smoothly from the breaking σ bond into the σ* of the carbon-mesylate bond, forming as they do so, a new π bond.

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U. Burckhardt, C. A. Grob and H. R. Kiefer, Helv. Chim. Acta, 1967, 50, 231–244.


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