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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

II. PHOTOCHEMISTRY OF AROMATIC NITRO COMPOUNDS . . . . . . .

748

A. Photoreduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

748

B. Photosubstitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

753

1.

Intermolecular reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

753

2.

Intramolecular reactions (photo-Smiles rearrangements) . . . . . . . .

758

C. Photorearrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

760

1.

Intramolecular redox reactions . . . . . . . . . . . . . . . . . . . . . . . . .

760

 

a. o-Alkylnitrobenzenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

760

 

b. o-Benzyl derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

764

 

c. Nitrobenzenes with ortho CDX bonds and their derivatives . . .

770

 

d. Nitrobenzenes with ortho heteroatom substituents

 

 

and their derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

773

2.

Nitro nitrite rearrangement . . . . . . . . . . . . . . . . . . . . . . . . . . .

776

D. Photoaddition and Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . .

776

E. Photoisomerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

778

F. Heterocyclic Compounds Containing Nitro Groups . . . . . . . . . . . . .

780

G. Photoretro-Aldol Type Reactions and Photodecarboxylation

 

to Generate Nitroaromatic Anions . . . . . . . . . . . . . . . . . . . . . . . . .

782

H. Photoredox Reactions in Aqueous Solutions . . . . . . . . . . . . . . . . . .

785

I. Photodissociation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

787

J. Photonitration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

789

747

748

Tong-Ing Ho and Yuan L. Chow

 

III. PHOTOCHEMISTRY OF NITRO-OLEFINS . . . . . . . . . . . . . . . . . . .

792

IV. PHOTOCHEMISTRY OF ALIPHATIC NITRO COMPOUNDS . . . . . . .

795

A. Simple Nitroalkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

795

B. aci-Nitronates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

795

C. Geminally Substituted Nitroalkanes . . . . . . . . . . . . . . . . . . . . . . .

798

V. PHOTOCHEMISTRY OF C-NITROSO COMPOUNDS . . . . . . . . . . . .

803

A. Simple Nitrosoalkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

803

B. Geminally Substituted Nitroalkanes . . . . . . . . . . . . . . . . . . . . . . .

803

C. Aromatic Nitroso Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . .

806

D. Other C-Nitroso Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . .

807

VI. PHOTOCHEMISTRY OF ALKYL NITRITES . . . . . . . . . . . . . . . . . .

810

VII. PHOTOCHEMISTRY OF N-NITRO AND N-NITROSO

 

COMPOUNDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

810

A. Nitrosamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

810

1. Photolysis mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

810

2.

Photo addition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

812

3. Sensitized nitrosamine photoreaction by dual proton

 

 

and energy transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

814

B. Nitramines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

816

C. Nitrosamides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

816

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

X

X

X = H, p-CHO, p-COCH3 , p-CN, p-CH3 , p-CH3 O, m-CH CH2 , m-COCH3 (8090%)

 

16. Photochemistry of nitro and nitroso compounds

 

749

TiO2

 

hν

TiO2 (e, h+ )

 

 

 

 

 

(2)

 

 

 

 

 

 

OΗ

 

+

O

2e

 

 

 

 

 

 

 

 

 

 

 

 

R

 

 

N

 

 

 

 

R

N

 

 

 

 

O

2H+

OH

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

H2 O

 

 

 

 

 

R

 

 

N O

2e

 

R

N

OH

(3)

 

 

2H+

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

H

 

 

 

 

 

 

 

 

2e , 2H+

 

 

 

 

 

 

 

 

 

 

 

H2 O

 

 

 

R

 

 

 

NH2

 

 

 

 

 

 

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

 

3

 

+ PhNO2 + 3H+

 

N

 

 

Me

hν

(AcrH2 )

(4)

3

 

+ PhNH2 + 2H2 O

 

+

 

N

Me

(AcrH+ )

Intramolecular redox reactions for bichromophoric compounds containing nitro and amino (or amino acid) groups have also been examined. For example4, irreversible

750

 

 

 

 

 

 

 

Tong-Ing Ho and Yuan L. Chow

 

 

 

PhNO

hν

1 PhNO

*

ISC

3 PhNO

*

 

 

 

 

PhNO

 

 

 

 

 

 

 

 

 

 

2

 

 

 

 

2

 

2

 

 

 

 

 

2

 

 

AcrH2 + H+

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3 PhNO2

*

 

 

 

 

 

 

(AcrH2 +

PhNO2

)

 

 

 

 

AcrH2 + PhNO2 + H+

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

AcrH+ +H2 O

 

 

AcrH2 + H+

AcrH+

 

(AcrH +

PhNO

)

 

 

 

 

 

PhNO

 

 

 

 

PhNHOH

 

 

 

 

 

 

 

 

 

 

2

2

 

 

 

 

 

 

 

 

 

 

 

 

fast

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

hν

+ H+ A crH+

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A crH2

 

 

 

 

 

 

 

 

 

 

 

 

PhNH2

 

 

 

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

 

 

hν

 

O2 N

CH2 CH R

 

H2 N

CHO

 

 

 

pH 10

(5)

 

 

 

 

(1)

R = CO2 H

 

 

(80%)

(2)

R = H

 

 

 

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

 

16. Photochemistry of nitro and nitroso compounds

751

 

 

NH2

 

 

 

 

 

 

+

 

 

 

 

 

 

 

 

 

NH2

O N

CH

CH

CO

e

 

O N

CH

 

CH CO

 

 

 

2

2

 

2

 

2

 

2

2

 

(1)

 

 

 

 

 

 

(3)

 

 

 

 

 

 

CO2

 

 

 

O

 

 

 

+ H+

 

H2 O

 

 

 

 

N

CH2 CH

NH

 

O N

 

 

CH2 CHO

 

 

 

O

 

 

 

 

 

 

 

 

 

 

 

(4)

 

 

 

 

OH

(5)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

OH

 

O

 

 

 

O N

 

CHCHO

 

N

CHOHCHO

 

 

 

 

 

 

 

 

 

H

 

 

 

 

 

(6) 418 nm

 

 

 

 

(7)

 

 

OH

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

O2 N

O(CH2 )12

O

P(Ph)2

 

(8)

 

 

 

 

hν

O

 

 

 

ON

O(CH2 )12

O

P(Ph)2

 

(10)

 

 

O2 N

O(CH2 )12 N

S

hν

ON

 

(9)

+

 

 

NMe2

O2 N

O(CH2 )n O

 

n = 2 12

(13)

hν

 

 

 

ON

O(CH2 )nO

 

 

(14)

 

+ O2 N

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

(9)

h , e

X

Y

 

ArX ! [ArX] ž

! [Ar]ž

! [ArY] ž ! ArY C e

(10)

e

Y

 

 

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.

NO2

 

NO2

 

 

hν

(12)

 

MeONa, MeOH

 

 

 

O14 CH3

 

OCH3

 

 

 

 

(φ = 0.08)

NO2

 

NO2

 

 

hν

 

 

 

 

MeONa, MeOH

 

(13)

O14 CH3

 

OCH3

OCH3

 

OCH3

 

 

 

 

(φ = 0.21)

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

 

CH3 (CH2 )5 NH2

 

 

 

 

 

 

hν

 

NO2

NO2

 

EtO2 CCH2 NH2

(14)

 

OMe

 

hν

 

 

 

NHCH2 COOEt

OMe

 

OMe

OMe

 

NH(CH2 )5CH3

+ CH3 (CH2 )5NH2

 

hν

 

 

 

NO2

 

NO2

N , hν

(15)

H

 

 

 

 

 

N

 

 

 

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.

OCH3

OCH3

 

 

 

hν

 

 

(16)

 

 

CN

 

 

 

 

 

NO2

CN

 

OCH3

 

OCH3

 

 

hν

 

 

 

 

RNH2

 

 

 

R = Me, C3 H7, iso-C4 H9

 

 

NO2

 

NHR

 

hν, R2 NH, R = Me, Et, (CH2 )4 ,

(17)

 

 

 

 

 

(CH2 )5

 

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

 

 

OMe

 

OMe

NHC6 H13

 

F

 

NHC6 H13

F

 

+ n-C6 H13 NH2

hν

+

(18)

 

 

 

NO2

 

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%)

Соседние файлы в папке Patai S., Rappoport Z. 1996 The chemistry of functional groups. The chemistry of amino, nitroso, nitro and related groups. Part 2