Supplement F2: The Chemistry of Amino, Nitroso, Nitro and Related Groups.
Edited by Saul Patai Copyright 1996 John Wiley & Sons, Ltd.
ISBN: 0-471-95171-4
CHAPTER 16
Photochemistry of nitro and nitroso compounds
TONG-ING HO
Department of Chemistry, National Taiwan University, Roosevelt Road Section 4, Taipei, Taiwan (ROC)
Fax: 886-2-363-6359; e-mail: HALL@CHEM50.CH.NTU.EDU.TW
and
YUAN L. CHOW
Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6
Fax: (604)-291-3765; e-mail: YCHOW@SFU.CA
I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
748 |
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II. PHOTOCHEMISTRY OF AROMATIC NITRO COMPOUNDS . . . . . . . |
748 |
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A. Photoreduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
748 |
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B. Photosubstitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
753 |
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1. |
Intermolecular reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
753 |
2. |
Intramolecular reactions (photo-Smiles rearrangements) . . . . . . . . |
758 |
C. Photorearrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
760 |
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1. |
Intramolecular redox reactions . . . . . . . . . . . . . . . . . . . . . . . . . |
760 |
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a. o-Alkylnitrobenzenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
760 |
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b. o-Benzyl derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
764 |
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c. Nitrobenzenes with ortho CDX bonds and their derivatives . . . |
770 |
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d. Nitrobenzenes with ortho heteroatom substituents |
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and their derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
773 |
2. |
Nitro nitrite rearrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . |
776 |
D. Photoaddition and Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
776 |
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E. Photoisomerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
778 |
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F. Heterocyclic Compounds Containing Nitro Groups . . . . . . . . . . . . . |
780 |
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G. Photoretro-Aldol Type Reactions and Photodecarboxylation |
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to Generate Nitroaromatic Anions . . . . . . . . . . . . . . . . . . . . . . . . . |
782 |
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H. Photoredox Reactions in Aqueous Solutions . . . . . . . . . . . . . . . . . . |
785 |
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I. Photodissociation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
787 |
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J. Photonitration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
789 |
747
748 |
Tong-Ing Ho and Yuan L. Chow |
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III. PHOTOCHEMISTRY OF NITRO-OLEFINS . . . . . . . . . . . . . . . . . . . |
792 |
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IV. PHOTOCHEMISTRY OF ALIPHATIC NITRO COMPOUNDS . . . . . . . |
795 |
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A. Simple Nitroalkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
795 |
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B. aci-Nitronates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
795 |
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C. Geminally Substituted Nitroalkanes . . . . . . . . . . . . . . . . . . . . . . . |
798 |
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V. PHOTOCHEMISTRY OF C-NITROSO COMPOUNDS . . . . . . . . . . . . |
803 |
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A. Simple Nitrosoalkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
803 |
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B. Geminally Substituted Nitroalkanes . . . . . . . . . . . . . . . . . . . . . . . |
803 |
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C. Aromatic Nitroso Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . |
806 |
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D. Other C-Nitroso Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
807 |
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VI. PHOTOCHEMISTRY OF ALKYL NITRITES . . . . . . . . . . . . . . . . . . |
810 |
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VII. PHOTOCHEMISTRY OF N-NITRO AND N-NITROSO |
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COMPOUNDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
810 |
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A. Nitrosamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
810 |
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1. Photolysis mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
810 |
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2. |
Photo addition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
812 |
3. Sensitized nitrosamine photoreaction by dual proton |
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and energy transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
814 |
B. Nitramines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
816 |
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C. Nitrosamides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
816 |
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VIII. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
817 |
I. INTRODUCTION
Since the last review in the preceding volume published in 1982 by Chow on the photochemistry of nitro and nitroso compounds covering references up to 1979, there has accumulated significant amounts of data to require a follow-up review on this subject. This chapter is organized similarly to the last review, according to types of functional groups, i.e. the nitro and nitroso groups attached to carbon, oxygen and nitrogen. Both the synthetic and mechanistic research activities have expanded drastically in the last 15 years, and we focus our attentions more on the synthetic aspects unless it is required to do otherwise.
II.PHOTOCHEMISTRY OF AROMATIC NITRO COMPOUNDS A. Photoreduction
The photoreduction of aromatic nitro compounds to the amino compounds can be carried out on the surface of semiconductor particles such as titanium oxide1 with H-atom donors (equation 1). At a shorter duration of the photoinduced reduction of p- nitroacetophenone, the hydroxylamine intermediate can be obtained in about 30% yield. The reaction mechanism proposed is based on the photoexcitation of TiO2 to generate an electron and a positive hole (equations 2 and 3). Aliphatic nitro compounds such as 12-nitrododecanoic acid can be reduced to 12-amino dodecanoic acid in 90% yield by this method.
NO2 |
NH2 |
hν, TiO2 |
(1) |
EtOH |
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X |
X |
X = H, p-CHO, p-COCH3 , p-CN, p-CH3 , p-CH3 O, m-CH CH2 , m-COCH3 (80−90%)
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16. Photochemistry of nitro and nitroso compounds |
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749 |
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TiO2 (e, h+ ) |
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−H2 O |
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N O |
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2e −, 2H+ |
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−H2 O |
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The photoreduction of nitrobenzene derivatives by 10-methyl-9,10-dihydroacridine (AcrH2) occurs by a consecutive six-electron reduction process in the presence of perchloric acid to yield aniline derivatives and the 10-methyl acridinium ion2 (equation 4). In comparison, thermal reduction of these nitrobenzene by AcrH2 under comparable conditions yields hydroxylamine or aniline depending on the substituent3. The photochemical reduction proceeds by electron transfer from AcrH2 to the n, Ł triplet excited state of nitrobenzenes to give, after secondary processes, nitrosobenzene as the first product. Subsequently nitrosobenzene is reduced in an acid-catalysed thermal reduction by AcrH2 to hydroxylaminobenzene and in the subsequent photoreduction of the hydroxylaminobenzene to aniline (Scheme 1).
H |
H |
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+ PhNO2 + 3H+ |
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Me |
hν |
(AcrH2 ) |
(4) |
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3 |
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+ PhNH2 + 2H2 O |
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Me
(AcrH+ )
Intramolecular redox reactions for bichromophoric compounds containing nitro and amino (or amino acid) groups have also been examined. For example4, irreversible
750 |
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Tong-Ing Ho and Yuan L. Chow |
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PhNO |
hν |
1 PhNO |
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ISC |
3 PhNO |
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3 PhNO2 |
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hν |
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SCHEME 1
intramolecular electron transfer from the aliphatic amine moiety to the photoexcited nitrophenyl group in p-nitrophenylalanine 1 and p-nitrophenylethyl amine 2 is more efficient for the former giving p-aminobenzaldelhyde in 80%. The presence of an ˛-carboxyl group facilitates electron transfer (equation 5). The proposed reaction mechanism is summarized in Scheme 2, in which intramolecular electron transfer from the amino group to the excited-state nitrophenyl moiety initiates the process to afford the internal radical ion pair 3. Decarboxylation and electron reorganization gives imine 4, which is hydrolysed to p-nitrosophenyl acetaldehyde 5. In basic medium, 5 undergoes deprotonation to 6 ( max D 418) which slowly decomposes to p-aminobenzaldehyde.
NH2
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O2 N |
CH2 CH R |
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CHO |
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pH 10 |
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(1) |
R = CO2 H |
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(80%) |
(2) |
R = H |
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Both CIDNP and ESR techniques were used to study the mechanism for the photore-
duction of 4-cyano-1-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen
ž
abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO,
ž
ArNO2H and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed.
Laser flash photolysis was also applied to study the anion radicals of trans-isomers of 4-nitro, 4,40-dinitro- and 4-nitro-40-methoxystilbenes, that are generated by triplet state quenching with 1,4-diazabicyclo[2.2.2]octane (DABCO) in polar solvents at room temperature7. The study shows that electron transfer competes against the trans ! cis
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16. Photochemistry of nitro and nitroso compounds |
751 |
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CH |
CH |
CO − |
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CH2 CH |
NH |
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H2 N |
CHO |
SCHEME 2
isomerization and that the radical ions decay by back electron transfer to the ground state. Magnetic field effects have been observed for the intramolecular photoredox reaction of the bichromophoric compounds 8 and 9, that contain an electron donor and a nitro-aromatic moiety as excited electron acceptors8. Irradiation of 8 and 9 will, by intramolecular redox reactions, afford 10 and 11 respectively (equations 6 and 7). The nitroso-aromatic products are characterized as cage products derived from a triplet biradical intermediate which originates from the triplet state nitroarene. Therefore, when an external magnetic field (0.64 tesla) is applied the cage nitroso product decreases by 8 9%. Similar observations have been made for the compounds containing tertiary amine and nitro-aromatic moieties connected by an alkyl chain 139 (equation 8).
752 |
Tong-Ing Ho and Yuan L. Chow |
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O2 N |
O(CH2 )12 |
O |
P(Ph)2 |
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ON |
O(CH2 )12 |
O |
P(Ph)2 |
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O2 N |
O(CH2 )12 N |
S |
hν |
ON |
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(9)
+
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O2 N |
O(CH2 )n O |
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n = 2 12 |
(13) |
hν |
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ON |
O(CH2 )nO |
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(14) |
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O(CH2 )n O |
(6)
O(CH2 )11 CHO
(7)
(11)
H
N
S
(12)
NHMe
(8)
X
(15) X = NHCH3 (16) X = N(CH3 )CHO
16. Photochemistry of nitro and nitroso compounds |
753 |
Time-resolved fluorescence studies were also carried out on a series of zinc(II) complexes of meso-tetraphenylporphyrins covalently linked to 1,3-dinitrobenzene and 1,3,5-trinitrobenzene as acceptors to study the photoinduced electron transfer process, which is the initial process for the photosynthesis10.
B. Photosubstitution
1. Intermolecular reactions
Nucleophilic substitution is the widely accepted reaction route for the photosubstitution of aromatic nitro compounds. There are three possible mechanisms11,12, namely
(i) direct displacement (SN2ArŁ ) (equation 9), (ii) electron transfer from the nucleophile to the excited aromatic substrate (SR N1ArŁ) (equation 10) and (iii) electron transfer from the excited aromatic compound to an appropriate electron acceptor, followed by attack of the nucleophile on the resultant aromatic radical cation (SRC N1ArŁ) (equation 11). Substituent effects are important criteria for probing the reaction mechanisms. While the SR N1ArŁ mechanism, which requires no substituent activation, is insensitive to substituent effects, both the SN2ArŁ and the SRC N1ArŁ mechanisms show strong and opposite substituent effects.
ArXŁ C Y ! [ArXY] ! ArY C X |
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! [Ar]ž |
! [ArY] ž ! ArY C e |
(10) |
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ArXŁ ! [ArX]Cž |
! ArY or Ar( H)XY |
(11) |
When o-, m- and p-nitroanisole with 14C-labelled at the methoxy group were irradiated under identical conditions in methanol in the presence of sodium methoxide, only m-nitroanisole underwent methoxy exchange, with the limiting quantum yield ( D 0.08) (equation 12)11. Both the meta activation and labelled isotope experiments support a complex intermediate and indicate an SN23ArŁ mechanism (direct substitution in the triplet state) for this reaction (equation 12) and for 4-nitroveratroles (equation 13). Further evidence from quenching and lifetime experiments also support a direct displacement SN2ArŁ mechanism for the photosubstitution reaction of nitroaryl ethers with hydroxide ions13.
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754 |
Tong-Ing Ho and Yuan L. Chow |
Regioselectivity in nucleophilic aromatic substitution reactions is also of interest. 4- Nitroanisole reacts with n-hexylamine and ethyl glycinate, to give regioselective methoxy and nitro group photosubstitution (equation 14), respectively14. Mechanistic evidence15 indicated that the latter is produced through a SN23ArŁ reaction pathway whereas the former arises from a radical ion pair via electron transfer from the amine to the 4-nitroanisole triplet excited state (SR N1ArŁ mechanism). The regioselectivity for the photosubstitution of 4-nitroveratrole with amines (equation 15) is dependent on the ionization potential of the amine used. Both laser flash16 and steady-state photolyses17 have shown that amines with high ionization potential follow the SN2ArŁ pathway, but amines with low ionization potential follow the SR N1ArŁ mechanism.
OMe |
NH(CH2 )5 CH3 |
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OMe |
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OMe |
NO2
16. Photochemistry of nitro and nitroso compounds |
755 |
The theory of ‘merging resonance stabilization’ was proposed to explain the difference in the regioselective displacement of 1-methoxy-4-nitronaphthalene by cyanide and methylamine18 (equation 16). The replacement of a nitro group by an electron-withdrawing cyano group must contribute to stabilize the transition states. The proposal’s usefulness has become limited since the replacement of the methoxy group by methylamin is shown to occur under non-photolytic conditions. Time-resolved transient spectroscopy was applied to study the mechanism of the nucleophilic substitution for 1- methoxy-4-nitronaphthalene with amines19. These studies indicated that primary amines cause the replacement of the nitro group whereas secondary amines displace the methoxy substituent (equation 17). The spectroscopic evidence shows the existence of the anion radical of 1-methoxy-1,4-nitronaphthalene in the secondary amine reactions, that give higher yields than the primary amine reactions. It was concluded that the reaction with secondary amines is an electron-transfer process (SR N13ArŁ ), and that with primary amines is simply an SN23ArŁ process.
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R = Me, C3 H7, iso-C4 H9 |
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NR2 |
NO2
Dual mechanistic pathways are often implied for divergent products in nucleophilic aromatic photosubstitutions. For example, the photoreaction of 2-fluoro-4-nitroanisole with n-hexylamine gives rise to higher yield from the fluoride than from methoxy substitutions20 (equation 18); the former major process is ascribed to an SN23ArŁ mechanism occurring from the Ł triplet excited state, whereas the latter minor process has an SR N13ArŁ mechanism involving the n Ł triplet excited state.
756 |
Tong-Ing Ho and Yuan L. Chow |
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OMe |
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OMe |
NHC6 H13 |
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NO2 |
NO2 |
The pH dependence of the regioselectivity for the nucleophilic photosubstitution of 3,4-dimethoxy-1-nitrobenzene by n-butylamine gives21 2-methoxy-5-nitro-N-butylaniline as the major product at pH D 11 (equation 19). At pH D 12, the ratio of the major product to 2-methoxy-4-nitro-N-butylaniline increases to 12:1; the increased selectivity is caused by hydroxide ion, which can either promote exciplex formation or act as a base catalyst in deprotonation steps following the -complex formation22.
NO2 |
|
NO2 |
NO2 |
|
NO2 |
|
|
hν |
+ |
+ |
(19) |
|
|
BuNH2 |
|
|
|
|
OMe |
pH = 11 |
NHBu |
OMe |
OH |
|
|
||||
OMe |
|
OMe |
NHBu |
|
OMe |
|
|
(51%) |
(16%) |
|
(11%) |
Mechanistic studies also indicate that 4-nitroveratrole (equation 20) and 4,5- dinitroveratrole (equation 21) undergo both singlet and triplet nucleophilic aromatic substitution with ethyl glycinate23. An electron transfer process competes against the nucleophilic aromatic photosubstitution for singlet excited 4-nitroveratrole, causing a decreased product yield in equation 20.
OMe |
|
|
|
|
|
OMe |
|
|
OMe |
|
|
|
|
|
NHCH2 CO2 C2 H5 |
||
+ |
H2 NCH2 CO2 C2 H5 |
|
hν |
|
|
|
|
|
S |
21A r* |
|
|
(20) |
||||
|
|
|
|
|||||
|
|
N |
|
|
|
|
|
|
NO2 |
|
|
|
|
|
NO2 |
|
|
|
|
|
|
|
|
(48%) |
|
|
OMe |
|
|
|
|
|
|
|
OMe |
|
OMe |
|
|
|
|
|
NHCH2 CO2 C2 H5 |
|
|
+ H2 NCH2 CO2 C2 H5 |
|
hν |
|
|
|||
|
|
23 A r* |
(21) |
|||||
|
|
|
|
|
S |
|||
O2 N |
|
|
|
|
N |
O2 N |
|
|
|
|
|
|
|
|
|||
NO2 |
|
|
|
|
|
|
|
NO2 |
|
|
|
|
|
|
|
|
(73%) |