This work demonstrates that the accurate control of the reaction parameters realized within microreactor systems allowed for a taming of the reactivity of thermally unstable intermediates such as haloalkyllithiums. The first example of effective external trapping of a reactive carbenoid such as the chloromethyllithium is described. By using microreactor systems, a continuous flow synthesis of chloro alcohols and chloro amines could be achieved with high yields. By controlling the residence time the highly reactive chloromethyllithium could be generated and reacted with electrophiles at temperatures much higher than in batch-mode and without internal quenching. The developed continuous-flow process matches the requirements for sustainability.

This work demonstrates how the directing ability of the azetidine ring could be useful for regioselective ortho-C-H functionalization of aryl compounds. Robust polar organometallic (lithiated) intermediates are involved in this synthetic strategy. The reagent n-hexyllithium emerged as a safer, yet still effective, basic reagent for the hydrogen/lithium permutation relative to the widely used reagent nBuLi. Two different reaction protocols were discovered for regioselective lithiation at the ortho positions adjacent to the azetidine ring, which served as a toolbox when other competing directing groups were installed on the aromatic ring. The coordinating ability of the azetidine nitrogen atom, as well as the involvement of dynamic phenomena related to the preferential conformations of 2-arylazetidine derivatives, were recognized to be responsible for the observed reactivity and regioselectivity. A site-selective functionalization of the aromatic ring was achieved for aryl azetidines with either coordinatively competent groups (e.g. methoxy) or inductively electron-withdrawing substituents (e.g. chlorine and fluorine). By fine-tuning the reaction conditions, regioselective introduction of several substituents on the aromatic ring could be realized. Several substitution patterns were accomplished, which included 1,2,3-trisubstitution, 1,2,3,4-tetrasubstitution, and 1,2,3,4,5-pentasubstitution, up to the exhaustive substitution of the aromatic ring.

Flow chemistry: A flow-microreactor-mediated synthesis of 1,2,3,4-tetrahydroisoquinolines (THIQs) is reported (see scheme). Starting from a laterally lithiated aziridine, a tetrahydroisoquinoline lithiated at C4 was generated by thermally induced isomerization. Because the reaction temperature is a crucial parameter, the exquisite thermal control possible in a flow-microreactor system allowed for fast, efficient, and highly reproducible synthesis of functionalized aziridines or THIQs.

The regioselective lithiation-functionalization of 2-arylazetidines has been explored. The nature of the N-substituent is mainly responsible for a regioselectivity switch. ortho-Lithiation occurred, using hexyllithium as a greener base, in N-alkylazetidines, while α-benzylic lithiation has been observed with N-Boc azetidines.

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