Efficient 1‑Hexene Trimerization with Triazacyclohexane Chromium Catalysts and Detailed Product Analysis by 13C NMR

Triazacyclohexane chromium precatalysts are described which catalyze the selective trimerization of 1-pentene, 1-hexene, and 1-octene with unprecedented activity, reaching nearly 5000 turnovers in 1 h. The isomer distribution of the trimers has been analyzed in detail by application of quantitative 13C NMR spectroscopy to 13C labeled samples. All endocyclic elimination products predicted by the metallacyclic mechanism have been characterized, and many have been confirmed by their independent syntheses. Significant variations in the isomer distribution are observed on variation of the ligand bulk. Quantification gave considerable insights into the relative abundance of metallacyclic intermediates, with 1,3-substituted chromacyclopentanes being by far the most abundant resting state. Ring expansion selectivity favors 1,2-insertion into the less hindered Cr–C bond, but some 2,1-insertion products have also been identified.