From Metacyclophanes to Cyclacenes: Synthesis and Properties of [6.8]₃Cyclacene

Abstract

In this article we show synthetic pathways to [6.8](n)cyclacenes demonstrated by the de novo synthesis of [6.8](3)cyclacene as the first purely hydrocarbon cyclacene and of precursors for [6.8](4)cyclacene. The design of the de novo synthesis by exploring alternative pathways is discussed and various precursors are shown. Crucial to the synthesis of [6.8](3)cyclacene were two cyclization steps. The first is a Wittig trimerization reaction which yielded the hexamethyl substituted all-cis-[2(3)]metacyclophanetriene. For the second cyclization step the methyl groups were converted to aldehyde functionalities by two subsequent oxidation steps of N-bromosuccinimide (NBS) bromination and oxidation with 2-iodoxybenzoic acid (IBX). The final cyclization of the second set of double bonds was achieved by a McMurry-coupling reaction. Towards the synthesis of [6.8](4)cyclacene different synthetic pathways to methyl substituted all-cis-[2(4)]metacyclophanetetraenes were explored. Insights into the structures of [2(3)]metacyclophanetri- and [2(4)]metacyclophanetetraenes were gained by X-ray crystallographic investigations on various intermediates. A crystallographic analysis of [6.8](3)cyclacene revealed a D(3h) symmetrical structure with planar benzene rings and a formation of tubular structures in the solid state.

On the cation–π capabilities of small all sp2‐carbon host structures. Evaluation of [6.8]3cyclacene from relativistic DFT calculations

“…The calculated 13 C‐NMR spectrum for 1 exhibits a 3‐peak pattern with an 1:2:2 intensity located at 125.7, 134.8, and 135.9 ppm, accounting for chemically different carbon atoms denoted as C1, C2, and C3, as denoted in Figure and Table . Such results are in good agreement with the experimental data denoting 3 peaks located at 125.2, 133.1, and 136.1 ppm . Upon formation of 1 ‐1Ag + , a consequent splitting of the 3 peaks is observed, owing to the coordination to one face of the nanobelt, leading to C1 in the coordinating face and C1′ in the bottom face, which is also observed for C2 and C3.…”

“…The [6.8] 3 cyclacene ( 1 ), characterized by Gleiter and coworkers, exhibits an almost D 3h symmetry involving an all‐sp 2 ‐hybridized carbon backbone, in a small nanobelt structure (Figure ) with a closed‐shell electronic structure. The calculated bond lengths are in good agreement with the experimental data, denoting carbon—carbon distances within the 6‐membered ring of 1.400 and 1.409 Å for the shared vertex (experimental values of 1.392 and 1.407 Å, respectively). At the 8‐membered ring section, the observed distances were 1.486 and 1.341 Å (experimental values of 1.481 and 1.322 Å, respectively).…”

“…In this venue, a convenient approach is to provide segments that can adopt a boat‐like conformation into the belt‐like structure, as proposed by Gleiter through the evaluation and synthesis of smaller prototypical example, namely, [6.8] 3 cyclacene ( 1 ) based on an alternate fusion of 6‐ and 8‐membered rings . Such system represent one of the first examples of a pure‐fused hydrocarbon cyclacene with a resulting closed‐shell electronic structure, which envisages the characterization of higher members of the [6.8] n cyclacene family, as pointed by the authors …”

Chemistry of the highly stable hindered cobalt sandwich compound (η5-Cp)Co(η4-C4Ph4) and its derivatives

“…Examples of (η 5 -RCp)Co stabilized cyclacenes [146][147][148][149][150][151] Synthesis of a wide range of [N]phenylenes, having a unique alternating arrangement of formally antiaromatic four-membered and aromatic six membered rings has been developed by…”

“…A range of (η 5 -RCp)Co stabilized cyclacenes, some of them belt-like (beltenes) have been prepared by the elegant use of multialkynyl compounds by Gleiter and coworkers and structurally characterized (Chart 6). [146][147][148][149][150][151].…”