Definite Proof of Spiroconjugation in [1.2]Spirenes

Abstract

Linkage of conjugated segments of polyenes (ribbond']) via a formally sp3-hybridized C atom should lead to a spirene exhibiting an interaction designated as spiroconju-gationf2]. HMO['] and CND0/2 calculations[31 on [ 1.2]spirene[**] predict stabilization of the H O M O by interaction between the x ribbons while the energy of the LUMO remains largely unaffected[', ' I. This should result in a hypsochromic shift of the longest wavelength UV band compared with that of the partially hydrogenated system (7). However, both electronic and steric substituent effects can perturb the system and mask the spiroconjugation. Thus the spirene (5c) provides no unequivocal evidence for spiroconjugation[21, whereas ( 5 d ) and ( 5 e ) exhibit a marked hypsochromic shift in the UV spectrum[4]. These differing results prompted us to synthesize the alkyl-substituted [1.2]spirenes ( 3 a ) and ( 3 b ) in order to substantially eliminate substituent effects. R R Compounds ( 3 a ) and ( 3 b ) were prepared by irradiating diazocyclopentadiene ( I ) in 2-butyne ( 2 a ) or 3-hexyne ( 2 b ) with long wave UV light (?i>360nm) until evolution of nitrogen had ceased (cf. ref. IS]). ( 3 a ) and ( 3 b ) are pale yellow oils (yield 47 and 35% respectively) which decompose on attempted gas chromatography but could be purified by chromatography and vacuum rnicrodistillation at room temperature. The structure follows from the spectra. IR: ( 3 a ) : 1938; ( 3 6 ) : 1925 cm-' (cyclopropene stretching mode); NMR: ( 3 a ) : .r=3.8-4.0 (m) and 3.3-3.5 (m) (cyclopentadiene ~~~ -[*] Prof.

Synthesis and properties of 1-methylenespiro[2.4]hepta-4,6-diene. A facile and bond-selective sigmatropic ring expansion

“…Under these conditions virtually no dimer was produced and a single 1:1 adduct was isolated. The structure of the adduct was assigned as 14 on the basis of consistent spectral and analytical data: ]H NMR (CDC13) 2.12-3.14 (m, 14 H), 4.57 (d, t,/ = 5.5, 1.3 Hz, 1 H), 5.26 (t, 7 = 2.6 Hz, 1 H), 7.78 (d, p, 7 = 16, 2.5 Hz, 1 H). The NMR data was so similar to that of the dimer 12 that the presence of a common structural unit seems unavoidable.…”

Oligounsaturated Five‐Membered Carbocycles – Aromatic and Antiaromatic Compounds in the Same Family

“…This carbene has, for example, been trapped with alkynes to form spiro-annelated cyclopentadiene derivatives 30 (Scheme 5) [20]. It has been proved by UV spectroscopy [21] and supported by calculations [22] interaction, so-called spiroconjugation, across their central sp 3 carbon atom in spite of the orthogonal orientation of the two p-systems.…”

Thermal Rearrangements of Spiro[2.4]hepta-1,4,6-trienes

“…Preparation of 1,2-Dimethylspiro[2.4]hepta-1,4,6-triene (1b). A solution of diazocyclopentadiene (1 g, 0.0109 mol) in 2-butyne (30 mL) was cooled to 0 °C in a Pyrex tube and purged with argon for 15 min. The contents of the tube were irradiated at 0 °C for 2 h using a 450-W lamp in a quartz immersion apparatus (Diaza filter, λ = 366 nm).…”

“…The synthesis of spiro[2.4]hepta-1,4,6-triene 1a is illustrated in Scheme . Photolysis of a solution of diazocyclopentadiene in (2-bromovinyl)trimethylsilane yielded the desired precursor 2 as a mixture of trans and cis isomers. ,9a A coproduct identified as 3 was isolated by preparative gas chromatography. The origin of 3 , although formally the HBr addition product of the silane, has not been determined.…”

“…Low-temperature distillation provided 1a in 68% yield. Spirenes 1b and 1c were prepared by photolysis of diazocyclopentadiene in the presence of the appropriate alkyne (Scheme ) 1 …”