Unusual Reactivity of Bent Acenes: Reactions of [6](1,4)Naphthalenophane and [6](1,4)Anthracenophane with Electrophiles

Yoshito Tobe, Akihiro Takemura, Mamoru Jimbo, Tohru Takahashi, Kazuya Kobiro*, Kiyomi Kakiuchi

*Corresponding author for this work

Research output: Contribution to journalArticle

31 Scopus citations

Abstract

The unusual reactivity of [6](1,4)naphthalenophane (2) and [6](1,4)anthracenophane (3), the smallest-bridged acenophanes hitherto known, in electrophilic reactions has been disclosed. These reactions include (i) acid-catalyzed telomerization, (ii) peracid oxidation, and (iii) addition with dienophiles. Semi-empirical molecular orbital calculations (MNDO and MND0/PM3) were performed in order to compare the geometries, energies, and bonding characters of 2 and 3 with those of [6]paracyclophane (1). The calculations predict that the deformation of the bridged aromatic ring increases in the order of 1 < 2 < 3. On the other hand, the strain energy decreases in this order. Also, the calculations predict the following structural features (i-iii) in the deformed aromatic rings of 1-3, although these predictions should be viewed with appropriate caution. (i) The π bond order of the bridgehead bond a of [6]paracyclophane (1) is smaller than that of the reference compound 4. (ii) On the other hand, the corresponding π bond orders of 2 and 3 are larger than those of the planar molecules 5 and 6, respectively, despite the fact that an overlap of the π orbitals is less favorable because the aromatic ring is bent into a boat shape, (iii) Moreover, inspection of the π bond orders and the bond lengths of the unbridged aromatic rings of 2 and 3 suggests that the bridged rings of 2 and 3 are disconnected as 4π systems from the unbridged rings which have more 6π and 10π character than 5 and 6, respectively. Treatment of 1 with an acid (H2SO4 or trifluoroacetic acid) afforded the isomers 7 and 8. When the reaction was performed under concentrated conditions, the dimer 9 formed along with 7 and 8. On the other hand, naphthalenophane 2 afforded the dimers 10 and 11 predominantly, along with a small amount of the trimers 12 and 13. In the case of anthracenophane 3, only the trimers 14 and 15 were obtained even when the reaction was undertaken under dilute conditions. The structure of 14 was elucidated by X-ray crystallographic analysis. The stabilities of the arenium ion intermediates 21-23, formed by ipso protonation of 1-3, and those of 24-26, derived by the 1,2-shift of the bridge of 21-23, respectively, and the proton affinities of 1-3 were estimated by PM3 calculations. The anomalous reactivity of 2 and 3 toward acid is explained in terms of (i) the relative stabilities of 21-26 and (ii) the affinities in nucleophilic attack of 1-3 to the carbocation intermediates like 21-23. The molecular structure of the naphthalenophane dimer 10, which possesses a [6](1,3)naphthalenophane subunit, was determined by single-crystal X-ray analysis, and the geometries were compared with those of the [5]metacyclophane 30 and the oxa[6]metacyclophane 31 as well as those calculated by the MNDO and PM3 methods. Oxidation of 2 and 3 with m-chloroperbenzoic acid readily took place to give the bridged dienones 34 and 35, their overoxidation products 36 and 38, and the hydroxy esters 37 and 39, respectively, while cyclophane 1 gave only the dimer of the bridged dienone 33. The inefficiency toward bridge migration in the epoxides 40 and 41, the precursors of 34, 35, 37, and 39, is ascribed to the difference in the relative stabilities of the cation intermediates involved in the isomerization and/or addition processes. The dienones 34 and 35 showed no hint of enolization to their bridged phenol isomers. Treatment of 1 with tetracyanoethylene (TCNE) gave the Diels-Alder adduct 42 as the sole product. On the other hand, naphthalenophane 2 gave the [2 + 2] adduct 43 with TCNE. In the case of anthracenophane 3, the [2 + 2] adduct 44 was obtained as the sole product in CH2Cl2, while in benzene the [4 + 2] adduct 45 was also obtained as a minor product. X-ray crystallographic analysis of 45 was undertaken in order to provide structural information for the [6]paracyclophane system free of perturbation by the carbonyl substituent(s). With dicyanoacetylene (DCNA), 1 gave the (4 + 2] adduct 47, while the adduct 48, a [4 + 2] adduct between [6]metacyclophane (27) and DCNA, was obtained as a minor product when the reaction was done in CH2Cl2. In contrast, naphthalenophane 2 and anthracenophane 3 afforded the [2 + 2] adducts 49 and 51 as sole products in CH2Cl2. The cyclopropane-containing products 50 and 52 were also obtained in benzene along with the [2 + 2] adducts. The [2 + 2] cycloadducts 43, 44, 49, and 51 are probably derived through zwitterion intermediates like 46. The formation of the cyclopropane-containing products 50 and 52 is explained in terms of an ene-like reaction or through diradical intermediates like 53. Naphthalenophane 2 reacted with dimethyl acetylenedicarboxylate (DMAD) in CH2Cl2 to give the lactone 54, while 1 did not react with DMAD. Reaction of 3 with DMAD yielded the lactone 55 and the 2:1 adduct 56 in CH2Cl2. In benzene, the cyclopropane-containing product 57 was also obtained along with 55 and 56. The formation of the lactones 54 and 55 and the 2:1 adduct 56 is explained in terms of a mechanism involving zwitterion intermediates like 58. The anomalous reactivity of 2 and 3 toward dienophiles is ascribed to the high double bond character of the 1,2-position of the aromatic rings and to the low ionization potentials.

Original languageEnglish
Pages (from-to)3479-3491
Number of pages13
JournalJournal of the American Chemical Society
Volume114
Issue number9
DOIs
StatePublished - 1 Apr 1992

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