Chemistry Reference
In-Depth Information
Me
2
dab confi guration to defi ne the absolute conformation of the HT forms in solu-
tion for the fi rst time.
78
The 2,2
-bipiperidine ligand (bip) is analogous to Me
2
dab, but is much more
sterically rigid. This rigidity has two components: fi rst, the chelate ring is part of a
three-ring system, decreasing its fl uxional character with respect to the Me
2
dab
ligand; second, the CH groups projecting toward the G coordination sites are
unable to rotate away from the G bases during G base rotation around the Pt-N7
bond. The bip ligand was able to decrease the dynamic motion roughly one billion
times relative to (NH
3
)
2
and ca. 100 times relative to Me
2
dab in
cis
- A
2
PtG
2
complexes.
76 - 78,101,104,106,113
Most of our work involved the
C
2
- symmetrical isomers of Me
2
dab and bip, in
which the chiral N, C, C and N centres in the chelate-ring had either
S,R,R,S
or
R,S,S,R
confi gurations.
′
5.1.5 Retro Models with Untethered Guanine Bases
The use of Me
2
dab and bip ligands allowed the simultaneous observation by
1
H
NMR spectroscopy of all possible conformers (two HT and one HH conformer,
Figure 5.6) that can be formed in
cis
- A
2
PtG
2
complexes with a
C
2
- symmetrical
carrier ligand. Moreover, the conformers were present at equilibrium in different
amounts depending upon the chiralities of the carrier ligands; thus these ligands
were named chirality-controlling chelates (CCC). The relative amounts were the
same for bip and Me
2
dab compounds with a given chirality and they differ only in
dynamics.
Free
5'-GMP
HHd
HTd
HTu
HHu
8.7
8.4
8.1
ppm
9.0
Figure 5.6
H8 region of the
1
H NMR spectrum for (
R
,
S
,
S
,
R
)-Me
2
dab-Pt(5
′
-GMP)
2
(Ref. 53
for related work)
