Stereochemistry of a spherand-type calixarene.
The stereochemistry of the spherand-type calixarene 2a is analyzed in terms of the configuration of the three 2,2'-dihydroxybiphenyl subunits (R or S) and the disposition of the methylene groups (crown or twist). X-ray crystallography indicates that the neutral 2a and its mono- or dianion (in form of the salts 2a(-) x C(5)H(5)NH(+) and 2a(2)(-) x 2Et(3)NH(+)) exist essentially in the same conformation (RRS/SSR-twist). This asymmetric RRS/SSR-twist form is the lowest energy conformer according to molecular mechanics calculations. Low-temperature (1)H NMR data indicate the presence of a major conformer of C(1) symmetry, in agreement with a RRS/SSR-twist form. The lowest energy topomerization pathway mutually exchanges two pairs of methylene protons and is ascribed to an enantiomerization process involving rotation around an Ar-Ar bond.
Agbaria
K
K
Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel.
Biali
S E
SE
Böhmer
V
V
Brenn
J
J
Cohen
S
S
Frings
M
M
Grynszpan
F
F
Harrowfield
J M
JM
Sobolev
A N
AN
Thondorf
I
I