N
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
4.6 Heteroaromatics
111
Estimation of 13 C Chemical Shifts of Multiply Substituted Pyridines
(δ in ppm)
The 13 C chemical shifts in multiply substituted pyridines can be estimated using the
substituent effects in the monosubstituted parent compound.
Example: Estimation of the chemical shifts for 2-amino-5-methylpyridine
Larger discrepancies between estimated and experimental values are to be expected
if the substituents are ortho to each other and if strongly electron-donating and -accepting groups occur simultaneously. Also, tautomerization and zwitterion formation have large effects on 13 C chemical shifts.
N
NH 2
H 3 C
C-2 base value
149.8
C-3 base value
123.7
2-NH 2
8.4
2-NH 2
-15.1
5-CH 3
-2.3
5-CH 3
-0.9
estimated
155.9
estimated
107.7
exp
156.9
exp
108.4
C-4 base value
135.9
C-5 base value
123.7
2-NH 2
1.8
2-NH 2
-9.7
5-CH 3
0.0
5-CH 3
8.9
estimated
137.7
estimated
122.9
exp
138.6
exp
122.5
C-6 base value
149.8
2-NH 2
-1.6
5-CH 3
1.3
estimated
149.5
exp
147.6
6
2
5
3
4