(525b) Ring-Closing Metathesis (RCM) – a New Route for Producing Nylon 11, 12 and 13 Precursors from Oleic Acid
The production and use of biomass-derived renewable materials, such as polymers, has been a topic of intense research in recent years, as alternatives to petroleum-derived products are increasingly sought. Nylons 11, 12 and 13 are high-strength materials that have use across many industries. The amino-acid precursor of Nylon 12 is conventionally manufactured from petroleum resources in 6 steps. Syntheses of Nylon 11 and 12 precursors from plant-derived fatty acids have also been reported, but these too involve multiple steps (4–6 steps) and the feed material used is restricted to ricinoleic acid, a fatty acid available only from castor beans. Our program focuses on development of new and simple synthetic strategies towards these nylon precursors from oleic acid, an abundant natural fatty acid available from common plant sources as well as oleaginous microorganisms including microalgae.
Our strategy to Nylons 11–13 involves first converting the oleic acid to alkenyl amides by reaction with allyl, homoallyl, and bis-homoallyl amines, respectively. Each amide is then subjected to “ring closing metathesis” generating an unsaturated ene-lactam, which is then hydrogenated to a lactam – the usual precursor for Nylon 12 and Nylon 13 synthesis. Production of Nylon 11 is from a linear amino ester, which can be readily accessed by hydrolysis of the lactam.
The first step, amide formation and the third hydrogenation step occurred smoothly under conventional conditions giving the desired products in excellent yields. Thus, our focus has been the optimization of ring-closing metathesis steps. When homoallyl oleate or bis-homoallyl oleate were subjected to ring-closing metathesis conditions, the desired lactams were obtained in good to excellent yield after tuning of several reaction parameters (temperature, solvent, reaction time, concentration, and mode of catalyst addition). However, allyl oleate only gave a very low conversion and produced significant amount of oligomers. This issue was resolved by the attachment of a protecting group to the amide. Upon systematic screening, we found the best protecting group that reverses the selectivity of oligomerization to macro-cyclization, which may have general value for other macro-lactam cyclization reactions.
Our approach to nylon precursors from oleic acid represents one of the shortest approaches to date. This strategy features better selectivity than other cross coupling methods and allows use of stoichiometric amount of coupling partners (vs excess of materials), leading to less complex product mixtures. Full details of our optimization efforts, including screening of reaction parameters and amide protecting groups, will be presented.