A New Photochemistry Experiment: A Simple 2+2 Photocycloaddition that Poses an Interesting NMR Problem

The cycloaddition of ß-nitrostyrene to 2,3-dimethyl-1,3-butadiene provides an extremely clean example of 2 + 2 cycloaddition. Evaporation of the solvent, hexane, and the excess diene gives the two isomeric products. Together with a Diels-Alder reaction, which is performed routinely in all organic labs, this 2 + 2 cycladdition provides a platform for learning about orbital-symmetry control of cycloaddition. The 200-MHz 1H NMR spectrum of the crude product provides the student with an excellent intellectual challenge. Students can deduce the regiochemistry of the process and the configuartion of each product. The can identify major and minor products and using integration can determine the ratio of products. The NMR spectrum also provides good examples of interesting magnetic resonance phenomena: accidental coincidence of peaks, a geminal coupling with J = 0, spin-spin splitting patterns, and damagnetic shielding of methyl protons by a cis-phenyl group. This laboratory exercise combines theory, technique, spectroscopy, and data interpretation.