![]() ![]() 1a).Ĭonsidering the significance of the transformation, tuning the regioselectivity of the Diels–Alder reaction of anthracenes is an attractive problem. ![]() The selectivity is governed by thermodynamic factors given that the twofold stabilization energy of the benzene moieties (2 × 35 = 70 kcal/mol) is greater compared to that of the naphthalene moiety (50 kcal/mol) as well as the kinetic factor for having the largest frontier molecular orbital coefficients at the 9,10-positions in the Highest Occupied Molecular Orbitals (HOMO) of 1a (Fig. Extensive synthetic and mechanistic studies reveal that the natural preference for -cycloadditions of unsubstituted anthracene is at its 9,10-positions 1, 2. Being an easily available conjugated π-electron-rich carbocyclic system, anthracene ( 1a) has been widely exploited as a classic diene in Diels–Alder reactions wherein its chemical reactivity and transformational effectiveness are subsidized by the partial loss of aromaticity. The -cycloaddition of a conjugated diene with 4 π-electrons and a dienophile (an alkene or an alkyne) with 2 π-electrons-commonly known as the Diels–Alder reaction-is one of the most emblematic pericyclic reactions and is deservedly celebrated due to its synthetic reliability and atom-economic approach for the facile construction of various complex 6-membered ring systems in a regio- and stereoselective fashion. A mechanistic rationale is offered with the aid of detailed computational studies, and finally, synthetic applications are presented. Likewise, the regioselective functionalization of the terminal anthracene ring in electrophilic substitution reactions is demonstrated. The developed synthetic strategy does not require any premeditated engagement of the 9,10-positions either with any sterically bulky or electron-withdrawing substituents and allows delicate calibration of the regioselectivity by modulating the electron-donating strength of the substituents on the terminal rings. We, herein, report a 1,4-selective -cycloaddition strategy of 9,10-unsubstituted anthracenes by installing electron-donating substituents on the terminal rings which is heretofore unprecedented to the best of our knowledge. Anthracenes are well-known to undergo -cycloadditions with dienophiles at their 9,10-positions (central ring) over 1,4-positions (terminal ring) guided by the relative aromatic stabilization energy of the two possible products, and also by harboring the largest orbital coefficients of the highest occupied molecular orbital (HOMO) at the 9,10-positions. Reversing the regioselectivity of the renowned Diels–Alder reaction by overriding the usual thermodynamic and kinetic governing factors has always been a formidable challenge to synthetic organic chemists. ![]()
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