We report herein ane fficient methodt os ynthesize triptycenes by the reaction of benzynes and anthranoxides, whicha re electron-richa nd readily prepared from the corresponding anthrones. Using this method, 1,9-synsubstituted triptycenes were regioselectivelyo btained employing 3-methoxybenzynes. This methodw as also applied to synthesize pentiptycenes. AD FT study revealed that the cycloaddition of lithium anthranoxide and benzyne proceeds stepwise. Triptycenes 1, [1] whichc onsist of an arene-fused bicyclo[2,2,2]octatriene bridgehead system,a re aromatic compounds with at hree-dimensional rigid molecular structure. Based on this unique structure, triptycenes are widely used in the field of molecular recognition, [2] molecular machine, [3] supramolecular chemistry, [4] materials science [5] and as metal ligands. [6] To synthesize triptycene 1a,c ycloaddition reactions between anthracenes 2 and benzynes 3 [7] are commonly utilized (Scheme 1a1). The conventionalm ethod focusesm ainly on the preparation of arynes, [8] and the easily availableb enzyne precursors do not alwaysp rovide triptycenes efficiently. [8a-f] Therefore, the use of more reactive arynophiles is one way to resolve this drawback, but this was not sufficiently investigateds of ar.I nl iterature, af ew electron-rich anthracenes [9] such as 9-acetoxyanthracenes, [9c,e] 9-methoxyanthracenes, [9a,d,f] and 1,4-dioxysulfonylanthracenes [9i] were appliedi nt riptycene syntheses. These anthracenes,h owever,u sually requireamultistep preparation, are less reactive [9e,h] or are easily oxidized and not isolated in pure form. [9g,i] Thus, it is desirable to develop anthracenes which are sufficiently reactive and easily handled. Recently,w edevelopedanovel one-pot synthesis of triptycenes 1b using at riple cycloaddition of benzynes to ynolates [10] 4 (Scheme 1b). [11] The key of this reactioni st he in situ generation of anthranoxides 6 by ad ouble [2+ +2] cycloaddition of ynolates 4 with two equivalents of benzynes followed by fragmentation of the resulting Dewar anthracenes 5.T he anthranoxides 6 subsequently reactedw ith the third equivalent of benzyne in a[ 4 + +2] cycloaddition.I na ddition, using 3-methoxy-[11a] or 3-silylbenzynes, sterically congested 1,8,13-trifunctionalized triptycenes 1b were selectively obtainedw here three methoxy or silyl groups were located at the same side of the C9-hydroxyl group. Duringt hese studies, we noticed that anthranoxide 6 is ah ighly reactive arynophile for the [4+ +2] cycloaddition due to the strong electron-donating effect of the Scheme1.Synthetic methods of triptycene.
This study finds that triptycenes can undergo ring opening solely after acid treatment. Triptycenes have ar igid and stable framework that enables them to act as the core structure for multifunctional molecules. Thus, their skeletal transformation is unusual and remains untapped in organic synthesis. We have found that ring opening of the triptycene scaffold occurs under acidic conditions in ar etro-Friedel-Crafts fashion to give the corresponding anthrone product. This simple yet overlooked reaction allows triptycene to serve as a" reactive unit". We also used this ring-opening reaction to synthesize tetracene, further demonstrating the potential of the reaction for the preparation of functionalized acenes.
The triptycene scaffold has been ring-opened by using a retro-Friedel-Crafts-type reaction under acidic conditions to give its corresponding anthrone product. 9-Hydroxytriptycenes and unsubstituted triptycene undergo ring-opening reaction under strongly acidic conditions, such as with TfOH. An investigation of the substitution effect has revealed that the electron-donating group on the arene moiety allows the reaction to proceed in the presence of a weaker acid, such as TFA. In addition, the reaction has been successfully applied toward the synthesis of tetracene.
Polysubstituted isocoumarins were synthesized by the reaction of substituted 2‐(trimethylsilyl)aryl triflates with trifluoromethylated β‐diketones in the presence of CsF. The reaction proceeded through carbon‐carbon bond insertion of aryne and intramolecular cyclization to form intermediates of alcohol anions, which extruded trifluoromethyl anion to afford isocoumarins. By using CuBr as a catalyst, 2 eq. of aryne reacted with β‐diketones to afford phenanthrenes and 1,2‐diarylethanones. Although reaction of 2‐(trimethylsilyl)phenyl triflate with 1,1,1‐trifluoro‐4′‐methylbenzoylacetone in the presence of CsF gave 3‐(4′‐methylphenyl)isocoumarin in 67% yield, addition of 0.2 eq. of CuCN resulted in the formation of 9‐(4‐methylbenzoyl)‐10‐trifluoromethylphenanthrene in 35% yield.
The triptycene scaffold has been ring‐opened by using an acidic retro‐Friedel‐Crafts‐type reaction to give its corresponding anthrone product. 9‐Hydroxytriptycenes and unsubstituted triptycene were ring‐opened by treatment with TfOH. The electron‐donating group on the arene moiety allows the reaction to proceed in the presence of weaker acids. Additionally, the reaction has been applied toward the synthesis of tetracene. More information can be found in the Research Article by T. Iwata, M. Shindo et al. (DOI: 10.1002/chem.202104160).
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