The interplay between hydrogen bonding and π–π stacking interactions in the crystal packing of N1-thyminyl derivatives, and implications for the photo-chemical [2π + 2π]-cycloaddition of thyminyl compounds

Abstract

The solid-state photo-chemical dimerisation of thyminyl derivatives occurs when two thyminyl units are aligned in such a way that the olefinic moieties are separated by a distance of less than 4.2 Å. When irradiated with >270 nm UV, the thyminyl olefinic groups undergo [2π + 2π]-cycloaddition to form a dimeric cyclobutane derivative. However, the design and execution of [2π + 2π]-cycloaddition reactions can be challenging due to the requirement to produce molecular crystals with the necessary olefinic alignment. In this investigation, the crystallographic and solid-state photo-chemical reactions of six N1-thyminyl derivatives are studied. Only one derivative, thyminyl propanamide (7), was found to undergo [2π + 2π]-cycloaddition in the crystalline state. As such, quantum chemical methods were employed to study the photo-chemical transition states of the derivatives, as well as the strengths of typical intermolecular interactions that were observed in their crystal structures (such as π–π stacking between the thyminyl rings, Watson and Crick style hydrogen bonding and hydrogen bonding between functional groups of N1 substituents). These results were used to rationalise the solid-state photo-reactivity of more complex bis-thyminyl monomers.