Chiral Induction from Allenes into Twisted 1,1,4,4‐Tetracyanobuta‐1,3‐dienes (TCBDs): Conformational Assignment by Circular Dichroism Spectroscopy

Abstract

Dedicated to Professor Yitzhak Apeloig on the occasion of his 65th birthdayCircular dichroism (CD) spectroscopy can be regarded as one of the most powerful techniques for stereochemical analysis: it is sensitive to the absolute configuration as well as to conformational features, which are often completely obscured in ordinary absorption spectra.[1] The analysis of CD spectra within the exciton coupling framework is extremely useful in a number of situations.[2] Nevertheless, when a molecule contains three or more different excitonically coupled chromophores with several transitions, the resulting CD spectrum is rather complicated due to the overlap of several exciton couplets. In such cases, full CD calculations using quantum-mechanical methods are necessary, and comparison with the experimental spectra is more reliable than just considering the CD signals arising from all possible pairwise terms.[3] Furthermore, one may want to explore the CD spectra as a source of more complex information, such as the assignment of Cotton effects to specific transitions, determination of absolute configuration (AC), and even investigation of conformational preferences. Typically, a conformational analysis is performed and further evaluated by the Boltzmann equation to ascertain the relative population of all the conformers.[3] For systems with conformational stability, it is possible to deduce the preferred conformation in solution by comparing the calculated CD spectrum of the representative conformer with the experimental one.We have been involved in the preparation of ethynylated allenes [4] as building blocks for the construction of linear [5] and macrocyclic [6] carbon-rich scaffolds through oxidative acetylenic coupling. Following the synthesis of a first series of stable 1,3-diethynylallenes (DEAs), we proceeded with the preparation of shape-persistent macrocycles with allenoacetylenic carbon backbones.[6] Although DEAs are axially chiral, racemic mixtures were previously used in our group for acetylenic scaffolding. Recently, we reported the synthesis of nearly enantiopure DEA (M)-1 in 69 % yield with an enantiomer ratio (e.r.) of 96:4 by a Pd-mediated syn-S N 2'-type cross-coupling of 3-methyl-1-butyn-3-ol with an optically pure bispropargylic ester (Figure 1). [7] Full optical purity was achieved by subsequent HPLC separation. We showed that 4-N,N-dimethylanilino (DMA) donor-substituted optically active DEAs react in a facile [2 + 2] cycloaddition with tetracyanoethene (TCNE) to form, after retro-electrocyclization, optically active, photostable 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) [8] derivatives. Interestingly, CD spectroscopy revealed a chiral induction from the chiral allene moiety into the sterically crowded TCBD chromophores.Herein, we report time-dependent density functional theory (TDDFT) [9] as well as p-electron dipole velocity molecular orbital (SCF-CI-DV MO) [10] calculations to predict UV/Vis and CD spectra of 1,3-diethynylallene (DEA) (M)-2 (Figure 1). The exciton couplet in allene (M)-2 is in agre...

All‐Carbon Scaffolds by Rational Design

“…[128,129] Their absolute configuration was unambiguously assigned based on calculations of the experimental CD spectra and X-ray crystallographic analysis. Macrocyclic allenoacetylene 34 (Fig.…”

Amplification of Chirality in Monodisperse, Enantiopure Alleno‐Acetylenic Oligomers

“…[5][6][7] Recently, we reported the synthesis of (P)-(+)-1 (Scheme 1) and (M)-(À)-1 with an enantiomer ratio of 96:4 by a palladium-mediated enantioselective syn-S N 2'-type cross-coupling reaction of an alkyne with an optically pure bispropargylic ester. [8] The DEA derivatives were obtained in completely enantiopure form by HPLC resolution on a chiral stationary phase and subsequently used to induce chirality into sterically crowded donorsubstituted 1,1,4,4-tetracyanobuta-1,3-diene chromophores [9] and to prepare enantiopure, shape-persistent alleno-acetylenic macrocycles. [10] Chiroptical methods, such as electronic circular dichroism (ECD) and optical rotatory dispersion (ORD), are highly sensitive to conformational features, and they are commonly used for the characterization of chiral secondary structures.…”

Optische Stabilität axial‐chiraler push‐pull‐substituierter Buta‐1,3‐diene: Effekt einer einzelnen Methylgruppe auf der Oberfläche von C60

“…[5] Im Verlauf der letzten Jahre haben wir eine Reihe nichtplanarer push-pull-substituierter Buta-1,3-diene durch [2+2]-Cycloaddition von Tetracyanethen (TCNE) oder 7,7,8,8-Tetracyan-p-chinodimethan (TCNQ) mit elektronendonorsubstituierten Alkinen und anschließende Cycloreversion synthetisiert. [6][7][8] In mehreren dieser Systeme sind die Buta-1,3-dien-Einheiten sterisch stark überladen, und das Auftreten von Konformeren, die sich durch Rotation um die zentrale C-C-Einfachbindung nur langsam ineinander umwandeln, konnte NMR-spektroskopisch nachgewiesen werden.…”