Unlocking Acyclic π-Bond Rich Structure Space with Tetraethynylethylene–Tetravinylethylene Hybrids

Abstract

Literature reports describe tetraethynylethylene (TEE) as unstable but tetravinylethylene (TVE) as stable. The stabilities of these two known compounds are reinvestigated, along with those of five unprecedented TEE-TVE hybrid compounds. The five new C10 hydrocarbons possess a core, tetrasubstituted CC bond carrying all possible combinations of vinyl and ethynyl groups. A unified strategy is described for their synthesis, whereupon cross-conjugated ketones are dibromo-olefinated then cross-coupled. Due to an incorrect but nonetheless widely held belief that acyclic π-bond rich hydrocarbons are inherently unstable, a standardized set of robustness tests is introduced. Whereas only TVE survives storage in neat form, all seven hydrocarbons are remarkably robust in dilute solution, generally surviving exposure to moderate heat, light, air, and acid. The first X-ray crystal structure of TEE is reported. Subgroups of hybrids based upon conformational preferences are identified through electronic absorption spectra and associated computational studies. These new acyclic π-bond rich systems have extensive, untapped potential for the production of stable, conjugated carbon-rich materials.

Substituted Tetraethynylethylene–Tetravinylethylene Hybrids

“…Two-fold Suzuki–Miyaura coupling of dibromide 20 with excess E -styrenylboronic acid 22 using [PdCl 2 (dppf)] as the precatalyst gave tetraphenyl TVEE 23 . Tetraphenyl Z -DVDEE 27 was accessed from dibromide 20 by a two-step sequence that involved, first, a Z -selective Negishi coupling with E -styrenylzinc bromide 24 catalyzed by [Pd­(PPh 3 ) 4 ] to afford monobromide 25 . , A subsequent stereoretentive Negishi coupling under Fu conditions with alkynylzinc reagent 26 , derived from phenylacetylene, gave Z -DVDEE 27 , which was isomerized cleanly into E -DVDEE 28 with a catalytic amount of molecular iodine. The origins of selectivity in cross-couplings of 1,1-dibromoalkene 20 and bromotrienyne 25 are discussed later, along with an extension of this methodology to other substrates.…”

“…We recently designed and prepared a family of five C 10 acyclic π-bond rich molecular components that represent the simplest hybrid structures of TVE and TEE . While these hybrid molecules 4 – 8 (Figure , bottom) are manageable in dilute solution at −20 °C, they are insufficiently robust to be stored neat at ambient temperature.…”

Enantioselective Synthesis of Axially Chiral Styrene‐Carboxylic Esters by Rhodium‐Catalyzed Chelation‐Controlled [2+2+2] Cycloaddition

“…The absolute configuration of (+)‐ 15 ab was determined to be S , S configuration by an X‐ray crystallographic analysis (Figure S3) [20] . These asymmetric syntheses are highly practical because symmetric endynes 14 a and 14 b could be readily synthesized by the double Sonogashira coupling of the corresponding dibromides 13 a and 13 b [21] with prop‐1‐yne.…”

An Integrated Building Block for Cascade Diels–Alder and Hetero-Diels–Alder Reactions

“…For example, the bisketene acts as an equivalent of DA diene giving convergent access to highly substituted p -hydroquinones, and the lactam could act as precursor of dienamine for late-stage DA cycloaddition toward stereo- and regioselective synthesis of functionalized isoquinuclidines . Usually, the oligoenes, including dendralenes, 1,1-divinylallenes, and tetravinylthylene (TVE), were disclosed as multiple DA dienes that lead to iterative DA reaction and rapid generation of remarkably molecularly complex pericycles (Figure A) . Despite these advances, the integration of DA and hetero-Diels-Alder (HDA) reaction in a molecule in an iterative sequence has rarely been reported …”