A multistep single-crystal-to-single-crystal bromodiacetylene dimerization

Packing constraints and precise placement of functional groups are the reason that organic molecules in the crystalline state often display unusual physical or chemical properties not observed in solution. Here we report a single-crystal-to-single-crystal dimerization of a bromodiacetylene that involves unusually large atom displacements as well as the cleavage and formation of several bonds. Density functional theory computations support a mechanism in which the dimerization is initiated by a [2 + 1] photocycloaddition favoured by the nature of carbon–carbon short contacts in the crystal structure. The reaction proceeded up to the theoretical degree of conversion without loss of crystallinity, and it was also performed on a preparative scale with good yield. Moreover, it represents the first synthetic pathway to ( E )-1,2-dibromo-1,2-diethynylethenes, which could serve as synthetic intermediates for the preparation of molecular carbon scaffolds. Our findings both extend the scope of single-crystal-to-single-crystal reactions and highlight their potential as a synthetic tool for complex transformations. The photochemical-induced dimerization of bromine-terminated oligo(ethynylene)s in the solid state is shown to give 1,2-dibromoeneynes on a preparative scale. This single-crystal-to-single-crystal transformation proceeds through a multistep reaction that involves the making and breaking of several bonds in addition to large atom displacements. The reaction represents an atom-efficient and catalyst-free pathway towards functional carbon-rich molecular scaffolds.