Spannungsvermittelte Bildung chiraler, optisch aktiver Buta‐1,3‐diene

Abstract

Die formalen [2+2]-Cycloadditionen-Retroelektrocyclisierungen (CA-RE) zwischen Tetracyanoethylen (TCNE) und gespannten, elektronenreichen, dibenzoanellierten Cyclooctinen wurden untersucht. Die Auswirkung der Spannung auf die Reaktionskinetik wurde quantifiziert; dabei waren die Geschwindigkeiten der Cycloadditionen unter Verwendung gespannter, cyclischer Alkine bis 5500-mal hçher (298 K) als mit ungespannten Alkinen. Cyclobuten-Intermediate sowie Buta-1,3-dien-Produkte wurden isoliert und ihre Strukturen mittels Rçntgenstrukturanalyse untersucht. Die Isolierung eines seltenen Beispiels für ein optisch aktives, konfigurationsstabiles Buta-1,3-dien wird beschrieben. Theoretische Studien des Enantiomerisierungsmechanismus der Buta-1,3-diene zeigten, dass die Inversion des Achtrings über einen Ring-Flip des Boot-Konformers stattfindet. In Übereinstimmung mit den berechneten Werten betrugen die experimentell gemessenen freien Aktivierungsenthalpien der Racemisierung bis zu 26 kcal mol À1 .

Porphyrin Donor and Tunable Push–Pull Acceptor Conjugates—Experimental Investigation of Marcus Theory

“…Most CA‐RE‐based products give rise to interesting physical properties, such as tunable intramolecular charge‐transfer (CT) bands expanding into the near infrared (NIR), large third‐order optical nonlinearities, high two‐photon absorption cross sections, thermal and environmental stability up to 300 °C, and in many cases sublimability . Products from CA‐RE reactions have already proven useful for the synthesis of tetracenes, with applicability to singlet fission, and chiral optically active buta‐1,3‐dienes, and found application as waveguides . In addition, CA‐RE‐derived products stand out among all electron acceptors as a unique set of photoactive molecular materials for solar cells, in general, and electron‐transfer reactions, in particular …”

On the Mechanism of the Formal [2+2] Cycloaddition – Retro‐electrocyclization (CA‐RE) Reaction

“…Due to the short lifetime of intermediate 2 , the signal to noise ratio was less than desireable but we were still able to assign carbon 6 resonating at 139.00 ppm from the HMBC spectrum and carbon atoms 2 and 3 resonating at 114.61 and 128.60 ppm, respectively, from the HSQC spectrum. For comparison, the sp 2 ‐carbon atoms of cyclobutene resonate at 162.3 ppm, [8] while they resonate at 130.57 ppm for a tetracyano‐cyclobutene derivative in which the C=C bond is part of a dibenzo‐fused cyclooctatrienyne ring [9] . Upon ring‐opening of 2 , the signals for these carbon atoms move significantly downfield, 183.13 and 168.74 ppm (C5 and C6 of 3 ).…”

Direct Cross‐Couplings of Propargylic Diols

“…[4] Accordingly,n umerous techniques to synthesize the 1,3-diene motif featuring various substitution patterns have been devised. [5][6][7][8][9][10][11][12][13] Cross-coupling reactions grant facile access to many butadiene substitution patterns.T hus,B çhmer and Grigg employed twofold Stille or Suzuki-Miyaura reactions to convert propargylic dicarbonates into 2,3-diaryl-1,3-butadienes bearing up to two additional substituents (Scheme 1a). [9a] General and efficient methods for the synthesis of more highly substituted 1,3-butadienes remain elusive, however, with existing methods being severely limited in scope and/or involving lengthy preparations.…”