Thermal racemisation of hepta-, octa-, and nonahelicene

Martin, R. H.; Marchant, Marie-Jeanne

doi:10.1016/s0040-4020(01)91469-3

Cited by 203 publications

(142 citation statements)

References 8 publications

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“…For the assignment of the enantiomers, CD-spectra were acquired and the absolute sign of the optical rotation at 590 nm was determined and compared with the literature, where in turn the enantiomers were assigned with the help of X-ray crystallography. 9 The helicene deposition was performed with a homemade Knudsen cell. 14 The temperature in the cell was held at 160°C for sublimation of [7]H and the molecular beam was directly subjected to the Cu(111) crystal surface, which was cooled prior to deposition to 150 K, if not mentioned otherwise.…”

Section: Methodsmentioning

confidence: 99%

“…This leads to outstanding optical activity, e.g., a value of 5,900°was measured for the molecular optical rotation for the pure enantiomers in solution. 9 Because of the high polarizability and hyperpolarizability of -electrons delocalized over the helical molecular frame, one can expect relatively high intensities in the nonlinear optical effects. A significant second-order electro-optic response for [7]H was calculated recently.…”

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confidence: 99%

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Two‐dimensional separation of [7]helicene enantiomers on Cu(111)

et al. 2001

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The adsorption of heptahelicene, a helically shaped polyaromatic hydrocarbon (C(30)H(18)), on a Cu(111) surface was studied by means of thermal desorption mass spectrometry (TDMS) and low energy electron diffraction (LEED) at temperatures between 130-1,000 K under ultrahigh vacuum (UHV) conditions. The molecule in the monolayer remains intact up to 400 K. Above that temperature it decomposes in several steps into carbon and hydrogen, desorbing subsequently as H(2). In the saturated monolayer of the racemate the enantiomers are separated into two different domains on the surface which are mirror images of each other. After adsorption of one enantiomer only, no mirror domains were observed.

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Section: Methodsmentioning

confidence: 99%

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confidence: 99%

Two‐dimensional separation of [7]helicene enantiomers on Cu(111)

et al. 2001

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“…[1b] In the case of [6]helicene (19), such a conformational process is characterized by a free energy barrier of 154.5 kJ mol À1 at 188 8C. [15] However, the corresponding data for pyridohelicenes had not previously been determined. Thus, we attempted the thermal racemization of single enantiomers of 2 as well as of 3 while monitoring the ee values by HPLC analysis on a Chiralcel OD-H column.…”

Section: In Memory Of Otto Exnermentioning

confidence: 99%

A Straightforward Route to Helically Chiral N‐Heteroaromatic Compounds: Practical Synthesis of Racemic 1,14‐Diaza[5]helicene and Optically Pure 1‐ and 2‐Aza[6]helicenes

Míšek¹,

Teplý²,

Stará̈³

et al. 2008

Angew Chem Int Ed

169

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Helicenes have attracted attention as unique inherently chiral three-dimensional aromatic compounds for several decades. Despite significant recent progress in the synthesis and applications of carbohelicenes and thiaheterohelicenes, [1] the potential of the aza analogues with a pyridine unit (pyridohelicenes) have not been explored. [2,3] There are only scattered examples of the preparation of pyridohelicenes, [4] but with no general synthetic methodology, since the photochemical approach can fail with pyridohelicenes [4b,c,e] while non-photochemical alternatives can be difficult to adapt to the synthesis of N-heteroaromatic compounds. The properties and chemical behavior of pyridohelicenes are practically unknown apart from their basicities [4e,g] and the self-assembly [5] of certain derivatives. Nevertheless, promising applications of pyridohelicenes in various branches of chemistry and material science might be envisaged and, therefore, further research in the field is required.Recently, we observed remarkably high proton affinities of pyridohelicene derivatives, as measured by mass spectrometric techniques.[6] In the gas phase, helically chiral 1-aza[6]helicene (2, Scheme 1), for example, exhibits a comparable proton affinity (1000 kJ mol À1 ) to 4-(dimethylamino)-pyridine (DMAP, 997 kJ mol À1 ). Herein, we report the practical syntheses of 1,14-diaza[5]helicene (1), [7] 1-aza[6]helicene (2), [8] and 2-aza[6]helicene (3).[9] Moreover, we have succeeded in resolving racemates of 2 and 3 into their enantiomers, assigning their absolute configuration, determining the energy barriers to racemization, and obtaining Xray structures of their corresponding silver complexes. The strategy for the preparation of 1-3 relies on a [2+2+2] cyclotrimerization of an aromatic triyne in the presence of a Co I catalyst to build the helical scaffold. We have already proven that such a methodology is robust and reliable for the preparation of helicenes and their analogues.[10] Thus, the straightforward synthesis of 1 started from the readily accessible bromopyridine 4, [11] which was treated with lithiated 1-(triisopropylsilyl)-1-propyne to yield 5 which contained an attached alkyne side arm (Scheme 2). Sonoga-

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“…Also, the racemization barriers of helical systems have been of interest to both experimentalists 17 and computational chemists alike, with one of the first computational studies on such systems being carried out with semi-empirical molecular orbital (MO) theory already in 1975. 18 The majority of papers on this topic have been published in the 1990s; [19][20][21] however, also in very recent times, joint experimental-computational studies have been conducted.…”

Section: Introductionmentioning

confidence: 99%

The INV24 test set: how well do quantum-chemical methods describe inversion and racemization barriers?

Goerigk

Sharma

2016

Can. J. Chem.

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For years, there has been ongoing interest in experimentally and theoretically understanding inversion and racemization processes. However, to the best of our knowledge, there has been no computational study that systematically investigated how well low-level quantum-chemical methods predict inversion barriers. Herein, we provide an answer to this question and we present the INV24 benchmark set of 24 high-level, ab initio inversion barriers. INV24 covers inversion in triatomics, in pyramidal molecules, in one cyclic system, and in various helical and bowl-shaped compounds. Our results indicate that previously applied DFT approximations combined with small basis sets are not reliable enough and that at least a triple-basis is needed for meaningful results. Moreover, we show that intramolecular London dispersion influences the barriers by 2 kcal/mol or more and that dispersion corrections should always be applied to DFT results. With our analysis of 34 DFT approximations, we can reproduce the well-known Jacob's Ladder scheme with (meta-)generalized-gradient-approximation methods underestimating barriers and global-hybrid DFT functionals performing better. Range-separated hybrids or Minnesota-type hybrids are not particularly superior to more conventional methods, such as B3LYP-D3. The by far best results are achieved with dispersioncorrected double hybrids, which give results below the chemical accuracy target of 1 kcal/mol. They also outperform wavefunction second-order perturbation theory approaches and we recommend using them whenever possible. Given that our systematic study of INV24 is the first of its kind, our findings have the potential to change common practice in this field and they will guide future investigations of inversion processes.Key words: barrier heights, chemical inversion, racemization barriers, benchmarking, density functional theory, wave-function theory.Résumé : Depuis des années, il y a un intérêt pour comprendre de manière expérimentale et théorique les processus d'inversion et de racémisation. Cependant, à notre connaissance, aucune étude computationnelle n'a évalué de manière systématique la précision des méthodes de chimie quantique de bas niveau pour prédire les barrières d'inversion. Dans le cadre des présents travaux, nous offrons une réponse à cette question et présentons la base de référence INV24 de 24 barrières d'inversion ab initio de haut niveau. L'application de la base INV24 couvre l'inversion des systèmes triatomiques dans les molécules pyramidales, dans un système cyclique et dans divers composés hélicoïdaux et de forme concave. Nos résultats indiquent que les approximations de DFT appliquées antérieurement, combinées avec des bases de petite taille, ne sont pas assez fiables et qu'une base tripleest minimalement requise pour que les résultats soient significatifs. De plus, nous montrons que la dispersion intramoléculaire de London a une incidence de 2 kcal/mol ou plus sur les barrières et que les résultats de DFT doivent toujours être corrigés pour tenir compte...

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Thermal racemisation of hepta-, octa-, and nonahelicene

Cited by 203 publications

References 8 publications

Two‐dimensional separation of [7]helicene enantiomers on Cu(111)

Two‐dimensional separation of [7]helicene enantiomers on Cu(111)

A Straightforward Route to Helically Chiral N‐Heteroaromatic Compounds: Practical Synthesis of Racemic 1,14‐Diaza[5]helicene and Optically Pure 1‐ and 2‐Aza[6]helicenes

The INV24 test set: how well do quantum-chemical methods describe inversion and racemization barriers?

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