Chemistry Reference
In-Depth Information
On-Bu
On-Bu
Ph
Ph
Si
S
n-BuO
Ph
Si
Ph
S
n-BuO
Si
On-Bu
Ph
S
Ph
On-Bu
132
133
Sakurai and coworkers prepared
aza
-buckybowls, triazasumanene 138 and its
derivative (Scheme
43
)[
153
]. Similar to the synthesis of 129, Pd-catalyzed
cyclotrimerization of enantiopure (1
S
,4
R
)-134 exclusively gave
syn
-135, which
was hydrolyzed, followed by condensation, to furnish bowl-shaped lactam 136.
A three-step operation, including thiation reaction utilizing Lawesson's reagent,
deprotection, and finally methylation, allowed 136 to generate thioimidate 137.
Three dihydropyridine rings were aromatized to afford (
C
)-(
)-138 by treatment of
a mixture of an oxidant Ph
3
CBF
4
and a base 2,6-di-
tert
-butyl-4-methylpyridine
(DTBMP). Similarly, the dextrorotatory enantiomer (
A
)-(+)-138 was prepared from
(1
R
,4
S
)-134. The structure of sulfone 139, an oxidized derivative of 138, was
analyzed by X-ray crystallography.
3.1.2 Structures and Properties
The bowl inversion dynamics of sumanene (2) was studied by variable temperature
NMR experiments, and its value was found to be 19.6 kcal/mol [
134
]. Chemical
exchange NMR spectroscopy (EXSY) of 117 (E
D) also allows the determina-
tion of the bowl inversion barrier (ca. 19.7-20.4 kcal/mol), and the values depend
on the solvent used [
137
]. These results show that 2 is much more rigid than
corannulene. Unlike corannulene derivatives, the correlations between structures
and bowl-to-bowl inversion barriers (
¼
G
{
inv
) for substituted sumanenes have not
been compressively studied. Based on the results currently available, substituents
and their positions of the sumannenyl core affect
ʔ
G
{
inv
(Table
7
). For example,
hexa-alkylated sumanenes 118 show slightly lower bowl
ʔ
inversion barriers
G
{
inv
for (
C
)-129
was found to have a higher inversion barrier (21.6 kcal/mol) [
145
]. Triketone 131
(23.3 kcal/mol) [
149
] has the highest inversion barrier of the simple carbon
sumanene family, but the value is still much smaller than that for triazasumanene
139. A chiral HPLC was used to investigate the racemization rate for a solution of
enantiopure 139 in tetralin at 488 K, and the result of this experiment indicated the
bowl inversion energy (
(18-19 kcal/mol) [
137
] than the parent sumanene, whereas
ʔ
G
{
inv
) to be 42.2 kcal/mol.
Based on the crystallographic analysis, sumanene (2) and trimethylsumanene
129 have similar bowl depths (1.11
ʔ
) and comparable POAV pyramidalization
angles at the hub six carbons with a value around 8.8
[
152
]. Hexafluorosumanene
123 (bowl depth 1.24
Å
, POAV angle 9.6
) is more curved than 2 and 129. The
Å
