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Molecular Propellers that Consist of Dehydrobenzo[14]annulene Blades
Author(s) -
Nobusue Shunpei,
Mukai Yuichi,
Fukumoto Yo,
Umeda Rui,
Tahara Kazukuni,
Sonoda Motohiro,
Tobe Yoshito
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201201061
Subject(s) - conformational isomerism , steric effects , annulene , chemistry , crystallography , basis set , stereochemistry , benzene , computational chemistry , density functional theory , molecule , organic chemistry
A new class of propel‐ ler‐shaped compound ( 4 ), which consisted of dehydrobenzo[14]annulene ([14]DBA) blades, as well as its naphtho homologues ( 5 and 6 ), have been prepared. Although NMR studies of compound 4 did not provide useful information regarding its conformation in solution, DFT calculations with different functionals and the 6‐31G* basis set all indicated that the D 3 ‐symmetric structure was energetically more favorable than the C 2 conformer. From X‐ray crystallographic analysis, it appeared that compound 4 adopted a propeller‐shaped‐, approximately D 3 ‐symmetric structure in the solid state, in which the [14]DBA blades were twisted substantially owing to steric repulsion between the neighboring benzene rings. On the contrary, in the case of compound 6 , although the DFT calculations with the B3LYP functional predicted that the D 3 ‐symmetric conformation was more stable, calculations with the M05 and M05‐2X functionals indicated that the C 2 conformer was more favorable because of π–π interactions between the naphthalene units of a pair of neighboring blades. Indeed, X‐ray analysis of compound 6 showed that it adopted an approximately C 2 ‐symmetric conformation. Moreover, on the basis of variable‐temperature 1 H NMR measurements, we found that compound 6 adopted a C 2 conformation and the barrier for interconversion between the C 2 – C 2 conformers was estimated to be 16.2 kcal mol −1 ; however, no indication of the presence of the D 3 isomer was obtained. The relatively small energy barriers to interconversion, despite the large overlapping of neighboring blades, was ascribed to the flexibility of the acetylene linkages, which could be deformed substantially in the transition state of the ring‐flip.