Taming m‐Quionodimethanes by Dispersion Force: Cyclodimerization of (Trialkylsilyl)Ethynyl‐Substituted Indeno[2,1‐b]Fluorenes and Fluoreno[2,3‐b]Fluorenes

Indeno[2.1-b]fluorenes and fluoreno[2,3-b]fluorenes having (trialkylsilyl)ethynyl groups dimerize spontaneously to yield the cyclodimers in the yields ranging from virtually zero to quantitative depending on the alkyl group. The regioselectivity of the bond formation (Head versus Tail) is also dependent on the alkyl group. These remarkable effects are attributed to the attractive London dispersion force between the π- system and the alkyl groups.

Abstract

We present herein a systematic study on the cyclodimerization of (trialkylsilyl)ethynyl-substituted derivatives of indeno[2.1-b]fluorene and fluoreno[2,3-b]fluorene, which incorporate an open-shell meta-quinodimethane (m-QDM) unit. Our investigation focuses on the influence of different alkyl groups (methyl, isopropyl, or cyclohexyl) attached to the silyl moiety. Upon generation from the precursor diols via treatment with SnCl₂, these m-QDM derivatives yielded cyclodimers with an anti-configuration. The reaction yields varied significantly, ranging from virtually zero to quantitative, depending on the steric bulk of the alkyl group. Furthermore, the regioselectivity of bond formation (Head versus Tail) with respect to the ethynyl–allenyl radical resonance forms was also notably affected by the choice of alkyl group. These remarkable observations are attributed to attractive London dispersion forces operating between the π-system and the alkyl groups. Crucially, the degree of size matching between these components dictates both the dimerization efficiency and the regioselectivity of the bond-forming process.

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