Charged thiourea derivatives containing one and two methylated or octylated pyridinium ion centers and a tetraarylborate or triflate counteranion are reported. These novel catalysts are much more active in the Friedel-Crafts reactions of trans-β-nitroalkenes with N-methylindoles than the privileged N,N′-bis(3,5-bis(trifluoromethyl)phenyl)thiourea (i.e., Schreiner's thiourea) by up to 2-3 orders of magnitude. A previously reported UV-vis spectroscopic method by Kozlowski et al. was exploited to rationalize their reactivity order along with noncharged analogues. These results offer a new design strategy for organocatalysts by introducing positively charged centers without adding additional N-H, O-H, or S-H hydrogen bond donor sites.
Bibliographical noteFunding Information:
Generous support from the National Science Foundation (CHE-1665392) the Petroleum Research Fund (55631-ND4) as administered by the American Chemical Society, and the Minnesota Supercomputer Institute for Advanced Computational Research are gratefully acknowledged.
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