Photon Factory Activity Report 2016 #34 (2017) B

AR-NE3A/2015G097

Stereoisomerism in Nanohoops with Heterogeneous Biaryl Linkages of E/Z- and R/S- Geometries
Parantap Sarkar 1, Zhe Sun1,2, Toshiki Tokuhira3, Motoko Kotani1,4, Sota Sato1,2 & Hiroyuki Isobe*,1,2,3
1Advanced Institute for Material Research and Department of Chemistry, Tohoku University
2JST, ERATO, Isobe Degenerate π-integration Project and Advanced Institute for Material Research
3Department of Chemistry, The University of Tokyo
4
Mathematical Institute, Tohoku University
1 Introduction
Pervading all of chemistry, stereochemistry is anindispensible concept in any field allied with central
science of molecules. When embedded in cyclic
molecules, the stereochemistry results in an interesting
stereoisomerism (cyclostereoisomerism) accompanying
isomer degeneracies. The cyclostereoisomerism also
emerges in hoop-shaped, sp2-carbon networks ofcycloarylenes, called nanohoops, and gives rise to uniqueplanar chirality in the persistent belt-shaped structures
mimicking single-wall carbon nanotubes (SWNT). In thisstudy, cyclostereoisomerism of a cyclophenanthrenylene
nanohoop ([8]CPhen3,9) possessing both E/Z- and R/SExperimental
geometries
hasbeenclarified[1].
investigations of the dynamic behavior of the [8]CPhen3,9
further revealed two-stage isomerization processes taking
place separately at E/Z- and R/S-linkages.

structures were determined. The dihedral angles measured
with 4-3-3'-4' carbon atoms at the 3,3'-connections of(E,R,E,R,E,R,E,R)-structure in Figure 1 were 141°, 149°,
166° and 166°, and those with 8a-9-9'-8a' atoms at the9,9'-connections were –142°, –142°, –143° and –143°(see below for the descriptors and Figure 1 for the atomnumbers). Thus, deviations from the coplanar biaryl
orientations were 25° at the 3,3'-linkages and 38° at the9,9'-linkages on average.
OMeMeI2hν

HO 2C
Br
1I NOcyclohexane
HO 2Crt, 48 h
45%

N I

Cl2CHCHCl2
Br reflux, 15 h
42%
Me2Br
THF
–78 °C, 3 h; –78 °C, 1 h; rt, 15 h
61%

Br

3 Results and Discussion
As shown in Figure 1, the crystal structure of[8]CPhen3,9 existed as a racemate, and two enantiomeric

MeMe
1. PtCl 2(cod), CsF
THF[8]CPhen 3,970 °C, 24 h5%pinB
Bpin
5IBr
3pinB-Bpin
PdCl 2(dppf)•CH 2Cl2
KOAcBuLi; CuBr2; PhNO 2

42 Experiment
We synthesized [8]CPhen3,9 through a combination ofseveral transformations. The stilbene 1, synthesized via
Perkin condensation was subjected to iodine-mediated,photochemical cyclization to afford a phenanthrene
derivative 2. Photochemical conversion of carboxylic acid
to iodide with 1,3-diiodo-5,5-dimethylhydantoin was
found applicable with 2 to synthesize 9-iodophenanthrene
derivative 3. After unsuccessful attempts of Ullmanncoupling under several conditions, we found that a lowtemperature condition in nitrobenzene allowed for thesite-selective coupling reaction to give 9,9'biphenanthrenyl derivative 4. After furnishing 9,9'biphenanthrenyl with boryl moieties via Miyaura
borylation, Pt-mediated macrocyclization and subsequent
reductive elimination were performed to afford[8]CPhen3,9 (Scheme1).
A single crystal (ca. 0.10 × 0.07 × 0.01 mm3) suitablefor X-ray analysis was obtained from slow diffusion ofpentane into o-DCB solution of [8]CPhen3,9. A singlecrystal was mounted on a thin polymer tip withcryoprotectant oil and frozen at –178 °C via flash-cooling.
The diffraction analysis of a single crystal with asynchrotron X-ray source was conducted at –178 °C at
the beamline PF-AR NE3A at the KEK Photon Factoryusing a diffractometer equipped with a Dectris PILATUS
2M-F PAD detector.

Mehν

dioxane
80 °C, 18 h86%
O
Bpin =BO

2. PPh 3, o-DCB
180 °C, 48 hMeScheme 1: Synthetic route to [8]CPhen3,9.

Figure 1: Crystal structures of [8]CPhen3,9. Naphthyleneunits identical with those of [8]CaNAP [2] are colored in
red and blue. A minor structural deviation of one
phenanthrenylene panel was found as a disorderedstructure, but the major structure with 83% occupancy
was adopted as the representative structure for discussion.
References[1] P. Sarkar, Z. Sun, T. Tokuhira, M. Kotani, S. Sato,and H. Isobe, ACS Cent. Sci. 2, 740 (2016).
[2] Z. Sun, P. Sarkar, T. Suenaga, S. Sato and H. Isobe,Angew. Chem. Int. Ed. 54, 12800 (2015).
* isobe@m.tohoku.ac.jp