MRT - Ring Closure — Synthesis of Methylenecyclopentane

By: Matthias Junkers, Aldrich ChemFiles 2009, 9.4, 14.

Aldrich ChemFiles 2009, 9.4, 14.

Benefit from MRT & Flow Chemistry: complete suppression of sidereactions.

The cyclization of 1,5-hexadiene to exo-methylenecyclopentane (251356) poses multiple challenges: the batch reaction affords an inseperable mixture of the endo- and exo-cyclization product (Scheme 6), the desired kinetically favoured product isomerizes under reaction conditions in presence of the catalyst; separation of the two isomers is difficult since both have virtually the same boiling point, and last but not least, the transition metal catalyst used for this transformation requires a minimum working temperature of 55 °C, as at this temperature the starting material evaporates.

Scheme 6: Production of exomethylenecyclopentane in a microreactor (251356)(128554)

Microreactor Technology proved to solve all these problems. Insertion of a pressure valve into the product channel allows the reaction to be performed at an elevated pressure of 2 bar. This raises the boiling point of the reactant sufficiently to meet the perfect working temperature of the catalyst at 58 °C without gas formation. Isomerisation of the desired product is completely suppressed by immediate transport of freshly formed product out of the reaction zone and an instant quench of the product stream. A simple T-piece suffices to mix the product stream with an excess of water and induce the fast quenching of the reaction. This procedure affords a highly pure product (>99%, 700g / 8 h) without the need of a time-consuming workup.

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