Methylation of Mesitylene Chromium Tricarbonyl – An Undergraduate Advanced Synthesis Experiment

A multistep experiment in transition-metal organometallic chemistry for an advanced synthesis laboratory course is described. In the first part, students synthesize and characterize mesitylene chromium tricarbonyl. In the second part, students react the mesitylene chromium tricarbonyl with n-butyllithium followed by a methyl iodide quench. The two steps can be used independently or connected in a two-step process that involves four laboratory sessions. Overall, the two-step scheme provides a straightforward exercise that illustrates basic principles for safely handling pyrophoric reagents and organometallic compounds, fundamental inert atmosphere techniques, and use of low-temperature baths. The experiment highlights bonding theory in transition metal complexes, with emphasis on IR and proton NMR spectroscopy. In the second part of the scheme, students follow a discovery-based approach, analyzing the product by proton NMR spectroscopy to determine which of two possible reaction routes occurred. Students can achieve high yields and high selectivity of a single final isomer even though this might be their first time working with organometallic compounds, inert atmosphere techniques, and pyrophoric reagents.