A team of Princeton University researchers has tinkered with the ligand design of an iron catalyst to overcome a long-standing reactivity barrier for coupling alkenes to make cyclobutanes. The developments by Jordan M. Hoyt, Paul J. Chirik, and coworkers provide chemists new options for converting abundant olefin feedstocks such as propylene and 1-octene into diverse sets of chemical building blocks.
The Princeton team focused on [2+2] cycloaddition reactions that weld together a pair of alkenes or an alkene and a diene to make cyclobutanes. [4+2] Cycloadditions, which are used to make six-membered rings, proceed readily under mild reaction conditions. In contrast, [2+2] cycloadditions require an extra kick to overcome the electron orbital restraints of producing a smaller four-membered ring.
To give the reaction a nudge, chemists often use light to stimulate at least one of the coupling partners, but simple alkenes are not amenable to this photoactivation. Metal catalysts have also been used, but poor substrate selectivity typically leads to mixtures of products and yields remaining too low to be useful.
Chirik’s group previously reported an iron pyridine(diimine) catalyst that facilitates [2+2] cycloadditions of unactivated dienes to prepare bicyclic compounds. With further work, they have now discovered how subtle changes in the alkyl and aryl substituents around the edges of the iron complex help control the oxidation state and steric environment of the iron center to influence catalyst activity toward simple alkenes (Science 2015, DOI: 10.1126/science.aac7440).
One of their new catalysts bearing bulky ligands promotes cyclodimerization of propylene or terminal alkenes to form various cyclobutanes. When the catalyst is outfitted with smaller ligand groups, it promotes tail-to-tail alkene dimerizations, for example, converting two propylene molecules into 2,3-dimethylbutene. The team introduced other alterations in the ligand system to achieve alkene-diene cross-cycloadditions, leading to more diverse cyclobutanes.
The Princeton method “brings unprecedented scope to this ostensibly simple yet deceptively challenging transformation,” write Myles W. Smith and Phil S. Baran of Scripps Research Institute California in an accompanying perspective article. “This compelling discovery will no doubt spur development of more robust and versatile catalyst platforms for [2+2] cycloadditions of commodity olefins and beyond.”