Diels-Alder Reactions of Inert Aromatic Compounds within a Self-Assembled Coordination Cage

• Published in: Chemistry, an Asian journal Vol. 6; no. 7; pp. 1839 - 1847
• Main Authors: Horiuchi, Shinnosuke, Murase, Takashi, Fujita, Makoto
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 04-07-2011; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Chemistry; Diels-Alder reaction; entropy control; host-guest systems; preorganization; self-assembly
• ISSN: 1861-4728; 1861-471X
• DOI: 10.1002/asia.201000842

A self‐assembled coordination cage serves as a nanometer‐sized molecular flask to promote the Diels–Alder reactions of aromatic hydrocarbons with N‐cyclohexylmaleimide. The coordination cage accelerated the Diels–Alder reaction of anthracene at the electronically unfavorable, terminal benzene ring to give a compact, cavity‐restrained syn‐adduct. Activation‐parameter measurements for the reactions revealed considerable reduction in the entropy cost, and preorganization of the substrates is a dominant factor in the enhanced reactivity. Owing to this entropy‐cost reduction, otherwise‐unreactive aromatic compounds, such as naphthalenes or triphenylene, also underwent Diels–Alder reactions in a regio‐ and stereocontrolled fashion. In the naphthalene Diels–Alder reaction, X‐ray crystallographic analysis of the guest‐inclusion complex clarified the reinforced orientation and proximity of the substrate pairs before the reaction. A perylene Diels–Alder adduct was stabilized inside the cage and protected from aerial oxidation.
Cage against the machine: A self‐assembled coordination cage can accommodate a pair of aromatic compounds and dienophiles to promote the Diels–Alder reactions of otherwise inert aromatic molecules. Preorganization of the substrate pair within the cage was quantified as a significant decrease of entropy cost and clearly illustrated by X‐ray crystallographic analysis.