The Stereochemistry of Overcrowded Homemerous Bistrucyclic Aromatic Enes With Alkylidene Bridges
Tapuhi, Yitzhak; Suissa, Michal Rachel; Cohen, Shmuel; Biedermann, Paul Ulrich; Levy, Amalia; Agranat, Israel
Original version
Tapuhi, Y., Suissa, M. R., Cohen, S., Biedermann, P. U., Levy, A., & Agranat, I. (2000). The stereochemistry of overcrowded homomerous bistricyclic aromatic enes with alkylidene bridges. Journal of the Chemical Society, Perkin Transactions 2, (1), 93-100. http://dx.doi.org/10.1039/A906696IAbstract
The objective of the research was to study the e
ff
ects of alkylidene bridges on the conformations and the
conformational behaviour of overcrowded homomerous bistricyclic aromatic ethenes (
1
). The isopropylidene-
bridged bistricyclic ethene
2
and
3
were synthesized by a reductive “dimerization” of
7
, using TiCl
4
–Zn–pyridine–THF.
The methylene-bridged bistricyclic ethenes
4
–
6
were synthesized by LiAlH
4
–AlCl
3
–Et
2
O reductions of the
corresponding bianthrones. The structures of
2
–
6
were established by
1
H- and
13
C-NMR spectroscopy and in the
cases of
2
and
3
, also by X-ray analysis. Compounds
2
and
3
adopted
C
i
-
anti
-folded conformations with 53.0
and 28.8
folding dihedrals between pairs of benzene rings of tricyclic moieties. The central C
9
C
9
bond in
2
was
essentially planar. A short C
9
C
10
distance of 2.81 Å in
2
indicated an intramolecular overcrowding e
ff
ect in
the highly folded bistricyclic ethene. Semiempirical PM3 and AM1 calculations of the
anti
-folded,
syn
-folded,
twisted and orthogonally twisted conformations of
2
and
4
indicated that
anti
-folded
2
and
4
were the most stable
conformations with folding dihedrals of 48.7
and 45.0
, respectively at AM1. A DNMR spectroscopic study of
E
,
Z
-isomerizations and conformational inversions gave
∆
G
c
‡
(
E
Z
)
=
99.6 kJ mol
1
(CDBr
3
) and
∆
G
c
#
(inversion)
=
97.9 kJ mol
1
(hexachlorobutadiene) in
5
and
∆
G
c
‡
(inversion) > 108 kJ mol
1
(benzophenone) in
3
. These high
energy barriers were interpreted in terms of less overcrowded fjord regions in the
anti
-folded ground-state
conformations.