[544-25-2]  · C7H8  · 1,3,5-Cycloheptatriene  · (MW 92.14)

(precursor in the synthesis of troponoids,1a,b polycyclic compounds,1b,c tropylium salts1a,b)

Physical Data: mp -79.5 °C; bp 117 °C/749 mmHg; d422 0.888 g cm-3.

Solubility: sol EtOH, Et2O, C6H6, CHCl3.

Form Supplied in: slightly colored liquid, purity about 90% (usually contains toluene).

Preparative Methods: 1,3,5-cycloheptatriene (tropylidene) (1) is synthesized through benzene ring expansion,2a or pyrolysis of 7,7-dibromobicyclo[4.1.0]heptane.2b,c These reactions represent general synthetic methods to produce derivatives of (1).2c Reaction of thiophene S,S-dioxides with cyclopropenes is extremely advantageous for the synthesis of highly substituted cycloheptatrienes.2d

Purification: via transformation into tropylium tetrafluoroborate.

Analysis of Reagent Purity: 1H NMR, GLC.

Handling, Storage, and Precautions: resinifies in air; flammable liquid; toxic. This reagent should be handled in a fume hood.

Synthesis of Cycloheptatriene Derivatives and Other Monocyclic Compounds.

Several one-step methods for the transformation of (1) into potentially more useful functionalized monocyclics have been developed. 1,4-Addition of Bromine to (1) (CCl4, 0 °C) quantitatively affords 1,4-dibromo-2,5-cycloheptadiene,3 while reduction of (1) with alkali metals (Lithium, Sodium) in ether or NH3 proceeds with 1,6-addition to the conjugated system and produces 1,3-cycloheptadiene after quenching with N-methylaniline.4 Intermediate C(5)-metalated 1,3-cycloheptadienes react with alkyl halides to give 5-alkyl-1,3-cycloheptadienes.4 These dienes can also be prepared by electrochemical reductive alkylation of (1) on a Pt cathode.5

Friedel-Crafts acylation of (1) is very sensitive to the reaction temperature and nature of Lewis acid. In Aluminum Chloride-catalyzed reactions, the chloro ketones initially formed via 1,6-addition of acyl chlorides are transformed into 1-acyl-1,3,5-cycloheptatrienes or are ring-contracted into benzylic ketones, depending on the workup procedure (eq 1).6a With Tin(IV) Chloride as a catalyst, only benzylic ketones are produced.6b

Stepwise anodic oxidation of (1) in MeOH delivers 7-methoxy-1,3,5-cycloheptatriene and then tropone,7a which can also be prepared by direct oxidation of (1) with Selenium(IV) Oxide-KH2PO4.7b

Synthesis of Bicyclic Compounds.

Monocyclopropanation of (1) with carbenes affords bicyclo[5.1.0]octa-2,4-dienes8,9 and can be accompanied by seven-membered ring contraction, yielding benzocyclobutenes.9

Cycloheptatriene reacts in [4 +2] and [6 + 4] cycloadditions.10a An interesting cycloaddition which is an anionic analog of [6 + 4] cycloadditions has been described (eq 2).10b

Synthesis with Complexed 1,3,5-Cycloheptatrienes.

The reactivity of (1) in complexes with various transition metals1c,11 is usually considerably higher than that of noncomplexed (1). Tricarbonyl(h4-cycloheptatriene)iron1c is acylated (1. KH; 2. RCOCl) at C(7) with very high diastereoselectivity (exo/endo = 35-70).12 Cycloaddition of this complex with tetrazine affords a novel pyridazino[2,3-d]cycloheptatriene heterocyclic system.13 Regiocontrolled and stereocontrolled functionalization of Mn complexes in reactions with sodium enolates has been used in the synthesis of 6-substituted cyclohepta-2,4-dien-1-ols.14 Either thermally15a or photochemically15b induced stereospecific [6p + 4p] and [6p + 2p] cycloaddition reactions have been performed with the chromium(0) complex (2) (eq 3).15b The same transformations can also be accomplished directly for (1) employing only a catalytic quantity of (2) as a source of Cr0.15a

Tropylium Salts.

Stable carbenium ions abstract hydride ion from C(7) of (1), yielding the 6p-electron aromatic system of tropylium salts (3).16 Tropylium salts are also formed by mild pyrolysis of the adduct of (1) with Br217 or by heating norcaradienecarboxylic acid azide in C6H6.18 Salts of type (3) have been involved in several redox transformations, such as reductive dimerization into bitropyl,19a oxidative ring contraction producing aromatics,19b and disproportionation on heating in MeCN or DMSO with formation of (1) and tropone.19c

Undoubtedly the salts (3) are the most powerful synthons to incorporate the cycloheptatrienyl moiety into organic substrates even with moderate reactivity. They effectively alkylate amides,17 tosylureas,20 H2S,17 and react with RLi to produce 7-alkyl- or 7-arylcycloheptatrienes.3,21 Aldehydes are a-alkylated by (3; X = Br) directly22 or after preliminary transformation into enamines.23 The latter approach is especially successful for ketones.23 The salts (3) readily react with carbanions8,24 to provide a route to heptafulvenes (eq 4).24b

1. (a) Nozoe, T. Non-Benzenoid Aromatic Compounds; Ginsburg, D., Ed.; Interscience: New York, 1959; pp 339-464. (b) Lloyd, D. Non-Benzenoid Conjugated Carbocyclic Compounds; Elsevier: Amsterdam, 1984; pp 89-125. (c) Pietra, F. CRV 1973, 73, 293.
2. (a) Müller, E.; Fricke, H. LA 1963, 661, 38. (b) Lindsay, D. G.; Reese, C. B. T 1965, 21, 1673. (b) Anciaux, A. J.; Demonceau, A.; Noels, A. F.; Hubert, A. J.; Warin, R.; Teyssie, P. JOC 1981, 46, 873. (c) Reinhoudt, D. N.; Smael, P.; van Tilborg, W. J. M.; Visser, J. P. TL 1973, 3755.
3. Doering, W. von E.; Knox, L. H. JACS 1954, 76, 3203.
4. Hafner, K.; Rellensmann, W. CB 1962, 95, 2567.
5. Shono, T.; Nozoe, T.; Yamaguchi, Y.; Ishifune, M. TL 1991, 32, 1051.
6. (a) Blair, J. A.; Tate, C. J. JCS(C) 1971, 1592. (b) Dran, R. B.; Decock, P.; Decock-Le Reverend, B. CR(C) 1971, 272, 1664.
7. (a) Shono, T.; Nozoe, T.; Maekawa, H.; Yamaguchi, Y.; Kanetaka, S.; Masuda, H.; Okada, T.; Kashimura, S. T 1991, 47, 593. (b) Radlick, P. JOC 1964, 29, 960.
8. Korte, F.; Büchel, K.-H.; Wiese, F. F. LA 1963, 664, 114.
9. DeCamp, M. R.; Viscogliosi, M. R. JOC 1981, 46, 3918.
10. (a) Houk, K. N.; Woodward, R. B. JACS 1970, 92, 4143. (b) Bower, D. J.; Howden, M. E. H. JCS(P1) 1980, 672.
11. (a) King, R. B.; Stone, F. G. A.; JACS 1959, 81, 5263. (b) Müller, J.; Mertschenk, B. JOM 1972, 34, 165. (c) Kalsotra, B. L.; Multani, R. K.; Jain, B. D. JOM 1971, 31, 67. (d) Pauson, P. L.; Todd, K. H. JCS(C) 1970, 2315.
12. Williams, G. M.; Rudisill, D. E. TL 1986, 30, 3465.
13. Ban, T.; Nagai, K.; Miyamoto, Y.; Harano, K.; Yasuda, M.; Kanematsu, K. JOC 1982, 47, 110.
14. Pearson, A. J.; Bruhn, P.; Richards, I. C. TL 1984, 25, 387.
15. (a) Rigby, J. H.; Short, K. M.; Ateeq, H. S., Henshilwood, J. A. JOC 1992, 57, 5290. (b) Rigby, J. H.; Ateeq, H. S. JACS 1990, 112, 6442.
16. (a) Dauben, H. J.; Honnen, L. R.; Harmon, K. M. JOC 1960, 25, 1442. (b) Volz, H.; Volz de Lecca, M. J. TL 1965, 3413.
17. Doering, W. von E.; Knox, L. H. JACS 1957, 79, 352.
18. Dewar, M. J. S.; Pettit, R. JCS 1956, 2021.
19. (a) Olah, G.; Ho, T.-L. S 1976, 798. (b) Jutz, C.; Voithenleitner, F. CB 1964, 97, 29. (c) Reingold, I. D.; DiNardo, L. J. JOC 1982, 47, 3544.
20. Aumüller W.; Muth, K. LA 1962, 655, 36.
21. Badejo, I. T.; Karaman, R.; Fry, J. L. JOC 1989, 54, 4591.
22. Vol'pin, M. E.; Akhrem, I. S.; Kursanov, D. N. JGU 1959, 29, 2855 (CA 1960, 54, 12 016i).
23. Watanabe, T.; Soma, N. CPB 1970, 18, 1595.
24. (a) Conrow, K. JACS 1959, 81, 5461. (b) Ostrowski, S.; Makosza, M. LA 1989, 95.

Emmanuil I. Troyansky

Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, Russia

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