2,6‐Dimethyl Semibullvalene‐2,6‐dicarboxylates

Abstract

The enolized p-0x0 ester 9 is reduced by NaB(CN)H, in weakly acidic methanol solution to afford an almost quantitative yield of a mixture of diastereomeric P-hydroxy esters 10. On treatment with phosphorus oxychloride in pyridine, 10 is converted into a mixture of the P-chloro esters exo-11, endo-11, exo-12, endo-12, and 13 which is separated in part by chromatography. Sodium methoxide in methanol eliminates hydrogen chloride from the crude mixture of P-chloro esters producing the a$-unsaturated ester ?a in 84% yield based on 9. -Acid-catalysed equilibration of the known vinyl sulphides C2-and Cs-15 is carried out on large scale furnishing a 7 3~2 7 ratio of C2-and Cs-15. This mixture is oxidized to the sulphones C2-and Cs-16 which are allowed to react with sodium cyanide supported on aluminium oxide affording the u,punsaturated dinitrile 17 which is isolated by chromatography. Thus, 17 is now available on a 30-g scale. Methanolysis of l?by a prolonged treatment with hydrogen chloride in methanol followed by hydrolysis of the Pinner salt leads to ?b. -Both a$-unsaturated esters 7 a and b are readily brominated at the allylic positions by N-bromosuccinimide to yield the y,y'-dibromo esters 19a and b. Zinc-copper couple in tetrahydrofuran converts 19a and b into the semibullvalenedicarboxylates 2 a and b. While 2 a is persistent only in dilute solutions, thus frustrating all attempts at its isolation, 2b exhibits considerable stability in solution and forms lemon-coloured crystals.The semibullvalenedicarboxylates 2 a and b undergo a very rapid degenerate Cope rearrangement in solution. In addition, 2 b exhibits reversible thermochromism in both solution and solid state. The structures of the new compounds are based on spectroscopic evidence, including mass, IR, and NMR spectra. The configurations of exo-11, exo-l2,19a and bare established by X-ray diffraction analyses.The predicted accelerating effect of n-acceptor substituents on the degenerate Cope rearrangement of bridged homotropilidenes, e.g. bullvalenes, barbaralanes, and semibullvalenes[21, was demonstrated for the first time in the barbaralane series, with 2,6-dicyanobarbaralane as the andi id ate^^,^]. Shortly afterwards, the 2,6-dicyanosemibullvalene 1 b has been which not only corroborated this resultr6] but also stimulated new interest by exhibiting exceptional properties, such as being yellow in the absence of a chromophor, showing reversible thermochromism ['], and, in the solid state, a rapid non-degenerate Cope rearrangement between valence tautomers non-equivalent only by virtue of their environment in the crystal lattice"'. The interesting colour and thermochromism of 1 b are shared by the 1,5-bridged semibullvalene-2,6-dicarbonitrile 1 c[lo,lll but not by the 3,7-dicyanosemibullvalene 4a['2-141 nor the corresponding dimethyl 3,7-dicarboxylate 4 b"2,151. Grohmann's series of tetramethyl semibullvalenetetracarboxylates 3 seems to exhibit properties similar to those of the dinitriles 1. Unfortunately, an investigation of the thermochromism of 3 is s...

Thermal Isomerisation of Substituted Semibullvalenes and Cyclooctatetraenes – A Kinetic Study

“…The semibullvalenes 1b, [19] 13a, [20] b, [21] and 17 [22] used in this study were available from previous work. 1,5-Dimethylcyclooctatetraene (2b) has been synthesized from 1b by flash vacuum pyrolysis.…”

Lifting of the Degeneracy in Semibullvalenes by Remote and Direct Substituents: A Quantitative Study Using Variable-Temperature Carbon-13 NMR Spectroscopy

“…Detailed protocols for the syntheses of the 1,5-dimethylsemibullvalenes 5a , 5b , , and 5c 20 corresponding with the (1,5)-ethylmethylsemibullvalenes 3 ⇋ 3 ‘ have been published. These protocols were followed in the syntheses of the latter semibullvalenes.…”

“…Instrumentation. ,, Preparative GC was performed with a 3 m × 2 mm glass column packed with Volaspher A2 (Merck) which was coated with 20% silicon oil SE30. NMR signals were assigned with the help of DEPT, 1 H, 1 H COSY, 13 C, 1 H COSY, NOESY, and NOE experiments.…”

“…According to the procedure reported for the preparation of the corresponding 1,5-dimethyl compounds, the reaction of diketone 6 (43.5 g, 0.24 mol) with 4-chlorothiophenol (75 g, 0.52 mol) in tetrahydrofuran as solvent (0.9 L) in the presence of titanium tetrachloride (56 mL, 0.51 mol) and triethylamine (142 mL, 1.0 mol) yielded, after flash chromatography of the crude product on silica with petroleum ether (30−50 °C)/ethyl acetate (97:3), pale yellow crystals (90 g, 86%), mp 86−91 °C. Acid-catalyzed equilibration 11 , 12 ⇋ 13 , as described for the corresponding dimethyl compounds, afforded pale yellow crystals (86 g, 95%), mp 87−91 °C, in which the desired 2,6-diene 13 was enriched to 74% (HPLC).…”

“…3,7-Bis(4-chlorophenylsulfonyl)-1-ethyl-5-methylbicyclo[3.3.0]octadienes 14−16. The equilibrated mixture of 11 − 13 (92 g, 0.21 mol) was oxidized with sodium perborate tetrahydrate (196 g, 1.28 mol), as decribed for the corresponding dimethyl compounds, , to yield a colorless powder (103 g, 98%), mp 234−236 °C.…”