Page 172 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

P1: GOL Final Pages
Encyclopedia of Physical Science and Technology EN007F-314 July 6, 2001 16:59

328 Heterocyclic Chemistry

TABLE VII Chemical Shifts in Proton Reso- possible orientations (conformers) by rotation about sin-
nance Spectra of Various Heterocycles gle bonds. Thus, cyclohexane exists predominantly in the
Chemical shift at given position a chair form, although boat and twist forms of cyclohexane
are also known.
Compound 2 3 4
Aziridine 1.48 — —
Oxirane 2.54 — — chair boat twist
Thiirane 2.27 — — conformations of cyclohexane
Azetidine 3.58 2.32 —
With saturated heterocycles, conformational analy-
Pyrrole 6.62 6.05 —
sis is subject to additional considerations, such as
Furan 7.40 6.30 —
the inversion of a lone pair on a heteroatom as for
Thiophene 7.19 7.04 —
N-substitutedpiperidines,theeffectsofhydrogenbonding
Pyridine 8.50 7.06 7.46
toaheteroatom,anddipole–dipoleinteractionsinvolvinga
Pyridazine — 9.17 7.68
heteroatom (anomeric effects).
a
Parts per million on delta scale.
N
R
N
A somewhat similar system is 2,3-dihydroxyindole;
R
◦
here, the 2-hydroxy form is the sole tautomer at 20 C, but inversion at nitrogen of N-substituted piperidines
the equilibrium shifts progressively toward the 3-hydroxy
◦
form, which is favored by 95:5 at 100 C. The geometries of the four-membered heterocyclic
rings oxetane and thietane differ substantially from that
O H OH
3 3 of cyclobutane, the carbocyclic equivalent. Eclipsing
2 OH 2
1 H 1 O H
N N
H H
1 H 1 S
tautomerism of 2,3-dihydroxyindole 2 2
H H H H
Whereas 2-aminothiophene exists in the amino form
cyclobutane thietane
rather than the imino form, benzannelation can completely
alter the tautomeric preference; thus, 1-aminobenzo[c] 1,2-interactions are relieved by the puckered structure of
cyclobutane. Similar eclipsing interactions are not signif-
S S
icant between the sulfur and adjacent hydrogen atoms of
thietane; consequently, this molecule is effectively planar.
NH
NH 2
Like piperidine, tetrahydropyran adopts a chair confor-
tautomerism of 2-aminothiophene
mation. Hydroxylated and alkoxylated tetrahydrofurans
thiophene exists preferentially in the imine form, the are important because such ring systems occur in vari-
amino form possessing appreciably less resonance energy. ous sugars and RNA. 2-Alkoxytetrahydropyrans, unlike
alkoxycyclohexanes, exist preferentially with the alkoxy
substituent in the axial position. In methoxylcyclohexane,
S S
for example, unfavorable 1,3-diaxial interactions make the
equatorial position for the alkoxy substituent favored.
NH 2 NH
tautomerism of 1-aminobenzo [c] thiophene
O O OMe
Many other tautomeric equilibria of heterocycles have
been studied; theoretical calculations on the positions of OMe
these equilibria often agree with experimental values. conformations of 2-methoxytetrahydropyran
However, for the corresponding tetrahydropyrans, the
D. Conformation
axial lone pair on the ring oxygen atom can donate elec-
Whereas many fully unsaturated rings are very nearly tron density into the C OMe antibonding (σ ) orbital
∗
planar, their saturated or partially saturated analogs if the 2-alkoxy substituent is in the axial position. Ac-
containing tetrahedrallycoordinatedatomsarenecessarily cordingly, since such donation of electron density low-
three-dimensional.Thisfrequentlygivesrisetoapreferred ers the molecular energy, 2-alkoxy substituents exhibit a
orientation of the atoms in a molecule, differing from other strong preference for the axial position (referred to as the