The Thermal Retro[2+2+2]cycloaddition of Cyclohexane Activated by Triscyclobutenannelation: Concerted All‐Disrotatory versus Stepwise Conrotatory Pathways to Fused [12]Annulenes

Abstract

The unknown C 3v -symmetric all-cis triscyclobutenocyclohexane 1[1] is of fundamental interest as a key structure on which to probe the mechanism of [2+2+2]cycloreversions of cyclopropa-and cyclobuta-fused cyclohexanes ("tris-s-homobenzenes").[ [3] which is the result of unfavorable through-bond interactions. [2c,d] In contrast, the double bonds in 1 should not only activate by introducing further strain, [4] but also enable additional orbital symmetry control in the form of stepwise conrotatory cyclobutene ring openings, conflicting stereochemically with a concerted all-disrotatory unraveling of the cyclohexane ring. While conrotatory motion is seemingly constrained in bicyclo[n.2.0]alkenes, leading to much speculation regarding the feasibility of forbidden disroratory pathways in such systems, [5] cis-bicyclo[4.2.0]oct-7-ene, the pertinent subunit of 1 (highlighted), chooses this option. [6] More generally, such cyclohexane cycloreversions constitute one strategy for gaining access to the interior of fullerenes. [7] In addition, triene 1 (and its derivatives) are of importance structurally, because of their potential central-ring planarity, [8] and synthetically, as new precursors to novel [12]annulene isomers [9] and as new 6p-electron ligands for catalysis.[10]We report the synthesis, structure, and thermal rearrangement of the first example of structural motif 1, namely the triply annelated hydrocarbon 2 and its congeners 3 and 4, and compare their behavior to that of their common precursor, the trisbenzo analogue 5, [11] for which all-disrotatory retro-[2+2+2]cycloaddition has been proposed. [11b] To assist in the interpretation of the results, DFT calculations were employed. These calculations suggested an unexpected switch-over in the mechanism of cyclohexane ring fissure along the series.Birch reduction [12] of 5 [11b] could be controlled to give 6 a, either pure (93 %), or admixed with varying amounts (depending on the quantity of lithium used) of the mono-(6 b) and bisbenzo relatives (6 c), which were separated by column chromatography.[13] Selective dihydrogenation with Wilkinsons catalyst then furnished the targets 2-4, respectively, in good yields. Inspection of the NMR spectroscopic data of the series 2-5 reveals the trends expected for progressive, peripheral aromatization. Noticeable are the fairly invariant chemical shifts of the central cyclohexane carbon atoms, namely d = 43.3, 42.3 (average), 41.5 (average), and 40.6 ppm, respectively, and the small shielding trend, reflecting that observed for the corresponding carbon atoms in cyclobutene (d = 31.4 ppm) and benzocyclobutene (d = 29.5 ppm). Because of their novelty, X-ray structural analyses

Heliphenes and Related Structures

“…Stereo-views of [12]heliphene (side and front view). lar reactions (with higher yields in the presence of CuCl 2 which acts also as oxidant, in Kovacic-type reactions (dehydro coupling of aromatic rings [87]) have been carried out by Müllen and coworkers for obtaining experimentally large claromatic benzenoids [88].…”

Mechanisms and Möbius strips: understanding dynamic processes in annulenes

“…The [12]annulene configuration‐change reaction 2 → 11 lies at the core of Houk and Vollhardt's explanation of recent experimental results (Fig. ) . Thermal isomerization of hydrocarbon 14 to yield cage compound 17 likely proceeds via a key Möbius π‐bond‐shifting step ( 15 → 16 ) …”

Calculations Predict That Heavy-Atom Tunneling Dominates Möbius Bond Shifting in [12]- and [16]Annulene

“…Relevant optimized structures are shown in Figure and in the Supporting Information. Möbius bond shifting converting 1 to 2b , followed by conformation change to 2a , was proposed to explain the observed chemistry of 1 and was invoked to rationalize later results on annelated derivatives . Oth and co-workers derived k (−40 °C) = 2.4(±0.4) × 10 –4 s –1 and Δ G ⧧ (−40 °C) = 17.4(±0.1) kcal/mol for the thermal conversion of 1 to 3 …”