Richter
Iminium and Pyridinium Photochemistry
7/13/05
Group Meeting
Essentials of Imminium Photochemistry:
Mariano, P. S. Tetrahedron. 1983, 39, 3845.
1. Exists in the structure with a full double bond and positive charge on
nitrogen. Isoelectronic with simple olefins.
X
N+
R
Y
R'
2. Static/planar structure with high rotational barriers. Can be lowered if
conjugated to systems capable of stabilizing positive charges. Therefore
little isomerization observed.
3. Only one photochemical transition possible: p - p*. Occurs in the UV and
visible regions (similar to olefins).
4. Pyridiniums and phenyl substituted iminiums have long-lived singlet
excited states.
5. HOMO and LUMO p-orbitals much lower than the corresponding olefins.
p*
0
Leader in the Field: Patrick Mariano
Born: 1942
BS: Fairleigh Dickinson University (1964)
Ph.D.: University of Wisconsin (H. Zimmerman - 1969)
Postdoc: Yale (H. Wasserman - 1970)
University of Texas (Austin): 1971 - 1980
University of Maryland: 1980 - 1999
University of New Mexico: 2000 - present.
Disclaimers:
1. Most reactions were pulled either from reviews or journals like
Tetrahedron Letters where reporting yields are not required therefore
many yields are not given.
2. Most reactions had little mechanistic analysis associated with the
report.
p*
p
p
H2C=CH2 H2C=N+H2
6. Ground state reactions observed: nucleophilic attack or deprotonation
adjacent to the nitrogen atom.
7. Excited state reactions observed: cis-trans isomerization (C=C not C=N),
electrocyclizations, and cycloadditions. Note: Due to the increased
electron density on carbon, nucleophilic additions should not be possible.
8. Low LUMO allows easy single-electron reduction to form a-amino
radicals.
9. Great receptors in SET processes. Unique reactivity available for such
excited states (depending on ionization energies and electron affinities:
A* + B
A•–B•+
Cyclizations and Dimerizations:
5. Mariano, P. S. Tetrahedron. 1983, 39, 3845. (Sole Example)
1. Mariano, P. S. Tetrahedron. 1983, 39, 3845.
Ph
H2SO4
N
hn
Ph
N+
ClO4– H
hn
N
OMe
N
OMe
2. Mariano, P. S. J. Org. Chem. 1977, 42, 1122.
ClO4–
hn
N+
N
CN
H
CN
hn
O
7/13/05
Group Meeting
Iminium and Pyridinium Photochemistry
Richter
N
MeCN, HCl
O
N
X
O
N+
ClO4– H
O
hn
RN
hn
RN
EWG
hn
hn
Cl–
N+
NH2
R
H2N
H2O, HCl
N
N
R
H
hn
N+
N+
N+
Cl–
4. Katritzky, A. R. J. Chem. Soc. Perkin Trans. 1. 1979, 2535.
8. Mariano, P. S. Tetrahedron. 1983, 39, 3845.
O
PhN
hn
40%
NPh
+
N
Ph
45–55%
7. Beavers, L. E. J. Org. Chem. 1957, 22, 1740.
15–70%
N+
hn
EWG
NH2
R
RN
N+
O-
X
6. Taylor, E. C. J. Am. Chem. Soc. 1961, 83, 4484
Taylor, E. C. J. Am. Chem. Soc. 1963, 85, 776.
3. Bradsher, C. K. Tetrahedron Lett. 1966, 3341.; Lyle, R. E. J. Org.
Chem. 1973, 38, 2351.; Katritzky, A. R. Heterocycles. 1987, 26, 2963.
Park, Y.-T. J. Heterocyclic Chem. 1991, 28, 1083.
X
HN
R
R
RN
O
CO2R'
hn
O
RN
N+
R
NR
RN+
NR
Additions of Alcochols and Ethers:
Hydrolyses:
1. Mariano, P. S. Tetrahedron. 1983, 39, 3845
Katritzky, A. R. Heterocycles. 1987, 26, 2963.
1. Mariano, P. S. Tetrahedron. 1983, 83, 3845.
RO2C
CO2R
RO2C
hn
N+
R
CO2R
MeOH
N+
R
OH
N
Me
hn
R
R
R
R
N
Me
Reductions:
Me
hn
EtOH, HCl
Me
N
N
Me
+
MeOH, HCl
N
Me
N
hn
N+
Me
N
N
N+
Me
CO2Et
Me
N
Me
CONH2
hn
NH4OH
ascorbic acid
N
Bn
N
N
N
H
2. Matsuo, T. Tetrahedron Lett. 1975, 16, 1389.
N+
O
Me
15-65%
THF
N+
MeOH
EtO2C
Me
CONH2
N
Me
hn
Me
Bn
N
MeOH, HCl
N+
hn
Me
R
N
H
Me
CO2Et
+
R
N
1. Mariano, P. S. Tetrahedron. 1983, 83, 3845.
EtO2C
+
hn
CHO
HO
H
EtOH
CN
H2O
CONH2
N+
R
N+
Me
CONH2
hn
H2O, DEA
O
R
OH
3. Mariano, P. S. Tetrahedron. 1983, 39, 3845.
N
N
hn
hn
N
R
R
Me
N+
N
OO
H2O, O2
R
2. Mariano, P. S. Tetrahedron. 1983, 39, 3845.
N
7/13/05
Group Meeting
Iminium and Pyridinium Photochemistry
Richter
Et2N
N
R
10%
7/13/05
Group Meeting
Iminium and Pyridinium Photochemistry
Richter
Cyclizations with Allylic Silanes:
Pyridinium Photoadditions:
1. Mariano, P. S. J. Org. Chem. 1984, 49, 213.
1. Mariano, P. S. Acc. Chem. Res. 1983, 16, 130
Mariano, P. S. Tetrahedron. 1983, 39, 3845.
Ph
TMS
N+
Me
OMe
N
MeOH
Me
Me
Ph
hn
Me
hn
Me
Me
N+
2. Mariano, P. S. J. Org. Chem. 1987, 52, 704.
MeO
R
+
N
MeO
hn
hn
N
MeO
N+
MeOH
(±)-xylopinine
OMe
OMe
OH
2. Burger, U. Tetrahedron. 2000, 56, 43111.
OMe
OAc
Cl–
O
N+
3. Mariano, P. S. J. Org. Chem. 1987, 52, 1478
Mariano, P. S. Tetrahedron Lett. 1985, 26, 5867.
OAc
AcO
ClO4–
CO2Et
N+
MeO
hn
N
MeO
CO2Et
erythrina alkaloid skeleton
O
tBuO2C
OAc
hn
O
MeOH
O
NH2
HO
HO
OH
N
73%
tBuO2C
cephalotaxin skeleton
NH2
OH
OH
HO
– Protected Pentoses
(J. Org. Chem. 2002, 67, 3525).
Me
NMe2
N
OH
OH
– Allosamizoline
(Tetrahedron Lett. 2001, 42, 4755)
EtO2C
TMS
MeS
HO
4. Mariano, P. S. J. Org. Chem. 1984, 49, 228.
N+
85%
3. Syntheses utilizing reaction (Mariano)
– Mannostatin A
– Trehazolamine
(J. Org. Chem. 1998, 63, 6072)
(J. Org. Chem. 2005, 70, 5618)
60%
TMS
CO2Et
Makes 2 diastereomers
which are separable by
crystallization.
MeO
MeOH
O
65%
70%
OMe
MeO
NMe
H2O, HO–
Me
TMS
80+%
OMe
MeO
ClO4–
NHR
MeOH
O
OHC
HO
OH
Me
OBn O
O
NHAc
7/13/05
Group Meeting
Iminium and Pyridinium Photochemistry
Richter
Oxidations:
3. Mariano, P. S. Tetrahedron. 1983, 39, 3845.
R
1. Mariano, P. S. Tetrahedron. 1983, 39, 3845.
O
hn
O
hn
NH2
H2O
N+
NH2
O
Me
N
N
hn
ROH, HCl
N
hn
CO2Me
N+
Me
Me
N
N
20%
–
27%
O
Me
MeOH, O2
N+
R
5%
10%
9%
4. Streith, J. Heterocycles. 1977, 6, 1513
Streith, J. Heterocycles. 1977, 6, 2021.
N
-O
+
40%
40%
35%
2. Sugimori, A. Chem. Lett. 1992, 715.
CO2Me
N
R
Me
Et
iPr
N
OR
N
+
RCO2H, PhH
Me
R
OMe
hn
N+
N-
N
N
CO2Et
45–95%
Me
CO2Et
42%
H
N
Me
H
R
N
O
N
EtO2C
5. Katritzky, A. R. Tetrahedron Lett. 1982, 23, 1241.
Miscellaneous Reactions:
Ph
1. Mariano, P. S. Tetrahedron Lett. 1982, 23, 919.
Ph
hn
hn
ClO4–
N+
Me
N
MeOH
Me
Me
Me
20–80%
Ph
ClO4–
R
Ar
N
H2O
Ph
HN
N
N
85%
N
Ar
6. Undheim, K. J. Chem. Soc. Perkin Trans. 1. 1979, 1150.
O-
Ph
hn
Tol.
N
N+
OMe
2. Mariano, P. S. Acc. Chem. Res. 1983, 16, 130.
N+
Ph
Ph
R
N+
S
R
hn
H2O
O
N
R
2–5%
S
R
Iminium and Pyridinium Photochemistry
Richter
7. Streith, J. Chem. Ber. 1987, 120, 355.
hn
N+
N-
CO2Et
NCO2Et
N
"good yield"
hn
N+
NC
+
NCO2Et
PhH
NC
PhH
CN
N
H
CHO
+
PhH
O-
N
H
"poor yield"
8. Katritzky, A. R. Heterocycles. 1976, 5, 71.
ON+
R
CN
NC
OH
hn
N+
+
NC
"poor yield"
hn
O2
O
O
N
R
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Group Meeting