HETEROCYCLES

Paper | Special issue | Vol. 77, No. 2, 2009, pp. 1261-1268
Received, 9th September, 2008, Accepted, 17th November, 2008, Published online, 18th November, 2008.
DOI: 10.3987/COM-08-S(F)112

■ Effect of Fused Benzene Ring on Rotational Barriers of 2,2’-Bifuran, 2-Phenylfuran, Biphenyl, and Their Benzo Analogues

Naoto Hayashi,* Yoko Saito, and Hiroyuki Higuchi*

Department of Chemistry, Faculty of Science, Toyama University, Gofuku 3190, Toyama 930, Japan

Abstract

Molecular orbital (MO) calculations of torsional potentials of 2,2’-bifuran (F-F), 2-(2-furyl)benzofuran (F-BF), 2-phenylfuran (Ph-F), 2-(2-naphthyl)furan (Naph-F), 2-phenylbenzofuran (Ph-BF), biphenyl (Ph-Ph), 2-phenylnaphthalene (Ph-Naph), 2-phenylbenzo[1,2-b:4,5-b’]difuran (Ph-BDF), and 2-phenylbenzo[1,2-b:5,4-b’]difuran (Ph-BDF’) by using HF/6-31G** and B3LYP/6-31G** methods have revealed that the rotational barriers are increased by 0.4-0.9 kcal/mol by replacement of the aryl moiety on the furan ring (furyl group of F-F and phenyl of Ph-F) with its benzo analogue. In contrast, increase of the rotational barriers is slight when the aryl moiety on the benzene ring (phenyl groups of Ph-F and Ph-Ph) is replaced with its benzo analogue and more extended π moiety.

INTRODUCTION
Planarity is one of the most important structural feature of biaryls. For example, oligoarenes comprised of planar biaryl are very frequently employed in material chemistry due to the exteneded π conjugation systems.1 In biochemistry, the molecular planarity is relating to the cell toxicity because planar molecules can penetrate into the cell and bind to DNA more facilely than non-planar ones.2 The planarity of biaryl is generally evaluated by energetically favorable torsional angle (minimal point of the potential curve) and the magnitude of the rotational barrier (ΔErot, which is defined as the energy difference between potential minimum and maximum). In this respect, biphenyl (Ph-Ph) is regarded as a non-planar molecule,3 which bears the torsional angles (φ) of 42.50° at the potential minimum and the relatively large rotational barrier of 2.17 kcal/mol (φ = 0) calculated by B3LYP/6-311+G* methods3a due to steric repulsion of ortho hydrogen atoms. The twisted structure of Ph-Ph was supported by electron diffraction study, where φ was observed to be 44.1±1.2°.4 In contrast, 2,2'-bifuran (F-F) appears to be more planar because the steric repulsion of ortho hydrogens is significantly reduced therein. In fact, molecular orbital (MO) calculations predicted F-F to be almost planar with φ = 180º at the potential minimum.5-7 It also bears the potential minimum at φ = 16º (HF/6-31G*)5 ~ 37º (MP2/6-311++G**),6 at which two furan moieties are twisted due to β-H/β-H as well as weaker O/O steric repulsions. It may be noted that molecular structure of 2,2'-bithiophene (T-T) is more twisted at the potential minimum than that of F-F.6-8 MO calculations showed that T-T bears two potential minima at φ = 138º ~ 163º and 32º ~ 44º. The less planarity of T-T compared to F-F should be due to the larger atomic radius of sulfur than oxygen, which gives rise to more significant steric repulsions.
In addition to the steric factors, electronic factors can affect the molecular planarity. In this respect, we recently reported that the rotational barriers of 2,2'-bibenzofuran (BF-BF) is by 1~1.5 kcal/mol larger than that of F-F.9 Similar effects of fused benzene moiety on the rotational barrier were found in comparison of T-T and 2,2'-bibenzothiophene and of 2,2'-bipyrrole and 2,2'-biindole. The increase of the rotational barriers should not be due to additional steric repulsions as seen in comparison of F-F (ΔErot = 5.72 kcal/mol) and 2,2'-bi(isobenzofuran) (10.59 kcal/mol),6 but to other factors.

In the present study, effects of fused benzene moiety on the torsional potentials and the rotational barriers of Ph-Ph and 2-phenylfuran (Ph-F) have been investigated by using MO calculations, and compared to that of F-F.

RESULTS AND DISCUSSION
Torsional potentials (Figure 1) of F-F, 2-(2-furyl)benzofuran (F-BF), Ph-F, 2-(2-naphthyl)furan (Naph-F), 2-phenylbenzofuran (Ph-BF), 2-phenylbenzo[1,2-b:4,5-b']difuran (Ph-BDF), 2-phenylbenzo[1,2-b:5,4-b']difuran (Ph-BDF'), Ph-Ph, and 2-phenylnaphthalene (Ph-Naph) were calculated by HF/6-31G** and B3LYP/6-31G** methods. The potential energy, ΔEx, is defined as the energy gap of the conformer whose geometrical parameters are fully optimized and that with fixed φ (to be xº). The energies at the global minimum (ΔEmin), local minimum (ΔEmin'), and maxima (ΔEmax) are shown in Table 1.

The calculated torsional potential of Ph-Ph was identical to the reported ones in principle3: the potential minima are at φ ~ 40º, and by HF calculations the rotational barrier at φ = 0º (that is, ΔE0) is larger than that at φ = 90º (ΔE90), whereas ΔE0 to be smaller than ΔE90 in B3LYP calculations. The reported torsional potential of F-F5-7 was also reproduced by the HF and B3LYP calculations, where in contrast to Ph-Ph, the molecular structure of F-F was almost planar at the potential minima.
In both HF and B3LYP calculations the rotational barriers of Ph-Ph and Ph-Naph are very close, the deviations of which are less than 0.13 (HF) and 0.11 kcal/mol (B3LYP). Small effects of fused benzene ring on the torsional potential were reported in comparison of Ph-Naph and 2-(2-naphthyl)naphthalene,10 Ph-Ph and Ph-Naph,11 and Ph-Ph and 2-(2-naphthyl)naphthalene.12 In contrast, the torsional potential curves of F-F and F-BF are significantly different. The rotational barriers of F-BF are larger than those of F-F by 0.89 (ΔE100 - ΔEmin) and 0.56 kcal/mol (ΔE100 - ΔEmin') for HF, and by 0.92 (ΔE90 - ΔEmin) and 0.70 kcal/mol (ΔE90 - ΔEmin') for B3LYP calculations. These results are similar to our previous report9

Noteworthily, the torsional potentials of Ph-F and Ph-BF are very close with slightly different rotational barriers by 0.05 (HF) and 0.04 kcal/mol (B3LYP), while that of Naph-F is considerably different. Comparing to that of Ph-F, one rotational barrier (ΔE90 - ΔEmin) of Naph-F is much larger (by 0.42 kcal/mol for HF and B3LYP calculations), whereas the other one (ΔE90º - ΔEmin') is comparable with the difference of -0.01 (HF) and -0.02 kcal/mol (B3LYP).
The aforementioned results may be summarized as follows. When the aryl substituent on the furan ring is replaced with its benzo analogue, the rotational barrier increases significantly. This will give rise to the coplanar conformer to be more favorable. In contrast, replacement of the aryl substituent on the benzene ring with its benzo analogue affects the rotational barrier slightly. The latter effect is small, but not zero. Table 2 shows that the rotation barriers of Ph-F, Ph-BF, Ph-BDF', and Ph-BDF become larger in this order (Ph-F < Ph-BF ~ Ph-BDF' < Ph-BDF) albeit slightly, indicating large π group may increase the rotational barrier even if it is substituted on the benzene ring.
Obviously, the increased rotational barrier should not be due to additional steric repulsions but to electronic factors. It, however, is not meant to be enhanced double bond character of aryl-aryl bond because as shown in Figure 2 aryl-aryl bond length (dAr-Ar) is intact on substitution of aryl group with its benzo analogue in Ph-Ph, Ph-F, and F-F. The effect of fused benzene ring on the rotational potentials is presumably ascribable to dipole-dipole and/or dipole-induced dipole interactions of the aryl moieties. The existence of those interactions is suggested by the fact that ΔE0 and ΔE180 of Naph-F differ by 0.43 (HF/6-31G**) and 0.34 kcal/mol (B3LYP/6-31G**). The dipole-induced dipole interaction appears to be more important because, although the magnitude of dipole moment differs less significantly between phenyl and naphthyl groups than between 2-furyl and 2-benzofuryl groups, deviation of the torsional potentials is larger in Ph-F and Naph-F than in Ph-F and Ph-BF.

In conclusion, this paper described the effects of fused benzene ring on heterobiaryl stabilizing planar conformations based on HF/6-31G** and B3LYP/6-31G** calculations. The stabilization effect is larger when aryl moiety substituted on furan ring is replaced with its benzo analogue than when aryl substituent on benzene ring is replaced. The origin of this is not yet clear, albeit the dipole moments of aromatic moiety are likely concerned. The present results will provide a guideline to design oligo(heteroarene)s as a π linker or a molecular wire and useful information to understand structure-properties relationship between heterobiaryls and bioorganism.

EXPERIMENTAL
MO calculations were conducted with Spartan PC '04 software package on Microsoft Window XP . In all calculations HF/6-31G** and B3LYP/6-31G** levels were employed and for F-F and Ph-Ph C2 molecular symmetry was postulated. For all compounds, fully relaxed single-bond torsional potentials were calculated; i.e. for each fixed torsional angle (φ) around the central single-bond, all remaining internal degrees of freedom were optimized. A 10º grid of points was applied. Fully geometrical optimizations were also conducted to find global and local potential minima.

ACKNOWLEDGMENTS
The authors express their gratitude to Prof. Bommanna Loganathan at Murray State University (Kentucky, USA) for his suggestion about health effects of biaryl compounds.


Dedicated to Professor Emeritus Keiichiro Fukumoto on the occasion of his 75th birthday

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