Elsevier

Tetrahedron: Asymmetry

Volume 10, Issue 11, 4 June 1999, Pages 2153-2164
Tetrahedron: Asymmetry

Chiroptical properties of 12,15-dichloro[3.0]orthometacyclophane—correlations between molecular structure and circular dichroism spectra of a biphenylophane

https://doi.org/10.1016/S0957-4166(99)00220-7Get rights and content

Abstract

The valence excited electronic states and the circular dichroism (CD) spectra of the recently synthesized 12,15-dichloro[3.0]orthometacyclophane 1 are discussed by means of quantum chemical calculations which combine density functional theory with the single-excitation configuration interaction approach (DFT/SCI). The X-ray structure of this highly strained biphenylophane is presented. In order to investigate the influence of the cyclophane-type distortions on the CD spectrum of 1, the CD spectra of three model geometries (2a–2c) are also calculated. It appears that the CD spectrum of the biphenylophane 1 differs substantially from that of the corresponding unstrained biphenyl 2c. Furthermore, it is found that the pyramidalization of the bridging atoms of the cyclophane ring is an important factor for the red shift of the first band with respect to that of an unstrained benzene chromophore.

Introduction

The circular dichroism (CD) spectra of cyclophanes, e.g. ring substituted [2.2]paracyclophanes[1]and [n]paracyclophanes,2, 3hetera[2.2]metacyclophanes[4]and triphenylenicene,[5]have been the subject of special interest. Recently, in the quest for highly strained yet isolable [n]metacyclophanes, the synthesis and reactivity of 12,15-dichloro[3.0]orthometacyclophane 1 (see Fig. 1) has been described.[6]

This molecule can be derived from 8,11-dichloro[5]metacyclophane by formally replacing two methylene groups of the cyclophane bridge by an ortho-substituted benzene ring. Thereby the structural features of a biphenyl and an [n]metacyclophane are combined in this highly strained molecule: the dichloro-substituted benzene ring (atoms C(10)–C(15), henceforth denoted as ring I) shows the typical boat-type distortions of [n]metacyclophane, whereas the ortho-substituted benzene ring (atoms C(1)–C(6), henceforth denoted as ring II) is only slightly distorted except for the elongation of the C(1)–C(6) bond (see below). Since ring II is incorporated into the cyclophane bridge, the biphenyl axis is strongly bent from linearity. The biphenylophane 1 can be considered as having axial or, depending on the point of view, planar chirality. In the present context, the absolute conformation of 1 is always denoted with reference to the axial chirality of the biphenyl moiety.

In the present paper we discuss the CD spectrum of 1. Quantum chemical calculations of the CD spectrum have been carried out in order to assign the measured bands and to determine the absolute conformation of 1. Furthermore, we want to compare the chiroptical properties of the strongly distorted biphenyl moiety of 1 with those of unstrained biphenyls whose chiroptical properties have been studied earlier by several authors.7, 8, 9, 10, 11, 12, 13For this purpose we have calculated the CD spectra for different geometries of a model system (see Fig. 2).

In this model system the cyclophane bridge is broken in order to change the geometry from that corresponding to the framework of 1 (geometry 2a) to that of the unstrained biphenyl 2c. It will be seen that the CD spectrum of the biphenylophane 1 differs substantially from the CD spectrum of the corresponding biphenyl.

Section snippets

Experimental details

The preparation of 1 was performed as described previously.[6]Subsequently 1 was separated into its enantiomers using high performance liquid chromatography (HPLC) on a chiral stationary phase (cellulose-tris(3,5-dimethylphenyl)carbamate on silica gel); although failing in baseline separation, the enantiomers could be enriched.

Computational details

The CD spectra (excitation energies and rotatory strengths) were calculated with a recently developed method[14]combining Kohn–Sham density functional theory (DFT) with the single-excitation configuration interaction (SCI) approach. The DFT calculations were performed employing the hybrid B3LYP exchange-correlation functional.15, 16The Gaussian AO basis sets employed were of split-valence (SV) quality (i.e. [3s2p] for carbon, [4s3p] for chlorine and [2s] for hydrogen),[17]augmented with

Geometry of the biphenylophane 1

The X-ray data indicate that 1 adopts the endo conformation (atom C(7) points toward C(12), see Fig. 3). This confirms previous density functional results[6]indicating that the endo conformation is preferred over the exo conformation by 3.3 kcal/mol. Selected experimental geometrical parameters of endo-1 are compared with calculated ones obtained at the PM3 and the RI-MP2 level of treatment in Table 1.

The most pronounced deformations of ring II with respect to benzene occur in the strong

Summary

The DFT/SCI method has been applied in order to discuss the CD spectrum and the valence excited states of the biphenylophane 1. The absolute conformation has been assigned on the basis of the calculated CD spectrum. By calculating the CD spectra of three model geometries, it was found that the CD spectrum of the biphenylophane 1 differs substantially from that of the corresponding unstrained biphenyl 2c. Furthermore it was found that the pyramidalization of the bridging atoms C(10) and C(14)

Crystal structure determination

C15H12Cl2, Mr=263.15 g mol−1, pale yellow needles, 0.82×0.44×0.13 mm3, orthorhombic, Pca21, a=16.4769(4) Å, b=10.4623(2) Å, c=7.2599(1) Å, V=1251.51(4) Å3, Z=4, ρ=1.397 g cm−3, 12 085 measured reflections, 2476 unique reflections (Rint=0.0943), R (I>2σ(I)): R1=0.0256, wR2=0.0660. R (all data): R1=0.0267, wR2=0.0669, S=1.052. Intensities were measured on a Nonius Kappa CCD diffractometer with rotating anode (Mo-Kα, λ=0.71073 Å) at a temperature of 150 K. The structures were solved with the

Acknowledgements

The services and computer time made available by the Sonderforschungsbereich 334 (`Wechselwirkungen in Molekülen') have been essential to this study which was financially supported by the Deutsche Forschungsgemeinschaft. This investigation was also supported (M.J.v.E., M.L., A.L.S.) by the Council of Chemical Sciences (GCW) with financial aid from the Netherlands Organization for Scientific Research (NWO). S.G. thanks the Land Nordrhein-Westfalen for financial support through the

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