Synthesis and Reactions of Triptycenes and Their Derivatives

In order to verify if the triptycyl (the analog of triphenylmethyl in which the three phenyl groups were united to a CH) free radical 1a (Fig. 2.1) should be more instable than the triphenylmethyl radical itself, Bartlett et al. [1] first synthesized triptycene 1 in a low total yield by a multistep route starting from the Diels–Alder addition reaction of anthracene and benzoquinone (Scheme 2.1) in 1942. Several years later, Bartlett et al. [2] also reported the synthesis of 9-bromotriptycene (2) from the 9-bromoanthracene and benzoquinone according to the similar synthetic strategy. However, it was noted that the target compound 2 could be immediately afforded by the deamination of the diamine with NaNO2 in the presence of 50 % H3PO2 at −3 ◦C (Scheme 2.2). Craig and Wilcox [3] provided a new route to the synthesis of triptycene 1 based on the reduction of triptycene monoquinone, the adduct between anthracene and p-benzoquinone, with LiAlH4 or NaBH4. The resulting reduction product was treated with ethanolic hydrochloric acid, and then followed by the chromatography on acid alumina to give the target product 1 in 15 % overall yield. In 1956, Wittig and Ludwig [4] developed a convenient and one-pot method for the synthesis of triptycene 1 by the addition reaction between benzyne and anthracene. During the next few years, various routes to triptycene via benzyne had been reported. For example, Wittig et al. [4–6] synthesized the triptycene in 30 % yield by the addition reaction between anthracene and organomagnesium reagent generated by the reaction of o-bromochloro benzene with magnesium in a THF or ether solution. Soon after, they also reported the synthesis of triptycene 1 by the reaction of chlorobenzene with anthracene in the presence of butyl lithium [7] or LiSb(C6H5)6 [8], respectively. Similarly, Sharp and co-workers [9, 10] synthesized triptycene by treating halogenated benzene with potassium t-butoxide, followed by the treatment of anthracene [11]. By the addition reaction between tetrafluorobenzyne and anthracene, in 1968, Heaney and co-workers [12, 13] reported the synthesis of tetrahalogenated triptycene 3 in a yield of 42 % (X= F) or 54 % (X=Cl), along with the subsidiary product 4 (Scheme 2.3).