Surprising acidity for the methylene of 1,3-indenocorannulenes?

Shi Liu; Märt Lõkov; Sofja Tshepelevitsh; Ivo Leito; Kim K Baldridge; Jay S Siegel

doi:10.3762/bjoc.20.260

Surprising acidity for the methylene of 1,3-indenocorannulenes?

Beilstein J Org Chem. 2024 Dec 2:20:3144-3150. doi: 10.3762/bjoc.20.260. eCollection 2024.

Authors

Shi Liu¹, Märt Lõkov², Sofja Tshepelevitsh², Ivo Leito², Kim K Baldridge¹, Jay S Siegel¹

Affiliations

¹ School of Pharmaceutical Science and Technology, Tianjin University, Tianjin, PR China.
² University of Tartu, Institute of Chemistry, Ravila 14a, Tartu 50411, Estonia.

Abstract

Quantitative assessment of the first acidity constant (pK _a) for BFC (27.6 in CH₃CN) and FIC (27.8 in CH₃CN) shows the methylene protons to be significantly more acidic than those in related cyclopentadiene (32 in CH₃CN), indene (34 in CH₃CN), or fluorene (37 in CH₃CN) and comparable to the methine of 9-perfluorophenylfluorene (28.14 in CH₃CN). This work reports quantitative pK _a values of BFC and FIC, places those values in a broadened context of CpH-cognate hydrocarbon acidity and presents a Clar-Loschmidt graph perspective to help understand the "surprises".

Keywords: Clar sextet; Loschmidt group element; acidity; aromatic hydrocarbon; molecular graph.

Grants and funding

Experimental work at Tartu was carried out using the instrumentation at the Estonian Center of Analytical Chemistry (TT4, https://www.akki.ee) and was supported by the Estonian Research Council grant (PRG690) and by the Estonian Ministry of Education and Research (TK210).