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

Octafunctionalized Biphenylenes: Molecular Precursors for Isomeric Graphene Nanostructures

“…Moreover, using two equivalents of n ‐butyllithium led to the instantaneous formation of [ n ]phenylenes 6 , with n =3, 4, 5, and 6. Unlike the multistep approach of Vollhardt,10a–e,g the described method enabled the synthesis of [ n ]phenylenes in one step, and thus allowed the anticipated East–West expansion. DFT calculations14 confirmed the crucial influence of the TMS groups for the [2+2]‐cycloaddition by a π‐backbonding from the aryne to the d orbitals of the adjacent silicon atoms, which increased the electron density of the aryne intermediate (see the Supporting Information).…”

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

“…Mobius bond shifting converting 1 to 2b, followed by conformation change to 2a, was proposed 30 to explain the observed chemistry of 1 28 and was invoked to rationalize later results on annelated derivatives. 39 Our CCSD(T) results indicate an 18.3 kcal/mol ZPEcorrected barrier for the Mobius bond-shifting step 1 → 2b, with the process being 7.3 kcal/mol uphill. The transition state for electrocyclic ring closure is computed to be just slightly lower than that for bond shifting.…”

Controllable carbonyl-assisted C(sp3)–C(sp3) bond reduction and reorganization

“…6,9 For example, despite the importance of the formal [2 + 2 + 2]-and [2 + 4]-cycloreversion of functionalized carbocycles in the latestage diversification of bioactive carbocycles 2f,11 and organic synthesis, successful examples are currently limited to the ring-expansion isomerization of highly strained substrates such as tri[cyclopropane]-and tri(cyclobutene)-fused cyclohexane derivatives under harsh conditions. 12 As compared to late transition metal counterparts that are overwhelmingly employed as catalysts in the activation of C-C bonds, alkaline and redox-neutral organometallic complexes of rare earth metals (Ln) readily deprotonate organic substrates and thus favor a catalytic cycle through a straightforward deprotonation-protonation pathway. 13 It is tempting to specu-late that the formation of Ln enolate intermediates from ketones can serve as a transient carbonyl protector.…”