Patai S., Rappoport Z. 1996 The chemistry of functional groups. The chemistry of amino, nitroso, nitro and related groups. Part 2 / 0747 821

16. Photochemistry of nitro and nitroso compounds

777

yields67 (equations 58 and 59).

							CHO
								+	Ar
								N
	+	ArNO2						O−	(58)
	Ar = C6 H5					(119) 73%		(Ar = C6 H5			)
	Ar = 2-Thiophenyl					(120) 78%		(Ar = 2-Thiophenyl)
	Ar = 2-Furanyl					(121) 80%		(Ar = 2-Furanyl)
	+	I					NO2
				S
		(122)
CHO							CHO		(59)
	+					I		+
		S							S
	N					+		N
	O							O
	−							−
(123)	55%					(120)		27%

Irradiation of 2-nitro-5-iodothiophene (122) in the presence of benzene or indene gives, instead of addition, the coupling products68 124 and 125 (equations 60 and 61). The latter photoreaction also gave a minor amount of the secondary product 126 through reduction of the C I bond69 in 125.

						NO2
I		NO2 +		hν	S	(60)
	S
	(122)				(124)
		+
I	S	NO2
	(122)	hν				(61)
		CH3 CN
				+
		S	I			S
		(125)			(126)

778	Tong-Ing Ho and Yuan L. Chow

Interestingly, the photoreaction of indene with nitroarenes gives good yields of coupling products as shown in equation 62 (66% from 2-nitrothiophene and 52% from 2-nitrofurane). Nitrobenzene gives the reduction product. Strangely there is no reaction for p-nitrotoluene with indene.

	ArNO2	+	hν	Ar
			CH3 CN
				(62)
	Ar =	,	,
		S	O

E. Photoisomerization

The effects of nitro substituents on the cis trans isomerization of stilbenes has been reviewed70 (equation 63). The trans-to-cis isomerization occurs from a triplet excited state, whereas the reverse cis-to-trans isomerization occurs through a main route which bypasses the triplet state. A nitro substituent usually causes a significant enhancement of the quantum yield of the intersystem crossing. Nitro substituent effects on the photoisomerization of trans-styrylnaphthalene71 (equation 64), trans-azobenzenes72 and 4-nitrodiphenylazomethines73 (equation 65) have been studied for their mechanisms.

	O2 N		O2 N
		H	hν	H
			hν		(63)
	C	C		C
			C
	H	Ph	H	Ph
	O2 N		O2 N
		H	hν
			hν
	C	C		C C	(64)
	H			H	H
	X		X		X
			hν
	C N		hν	C N	(65)
	H		H

X

X = NO2 or H

The presence of a nitro substituent can enhance the intramolecular charge transfer in the excited state dramatically, so that the normal trans-to-cis isomerization of 1-[2(4-nitrophenyl)ethenyl]pyrene in cyclohexane is completely suppressed74 in polar solvents such as acetonitrile (equation 66).

16. Photochemistry of nitro and nitroso compounds			779
O2 N		H	H
		C C	Py
		hν (hexane)
	H
C	C	O2 N
H	Py

(66)

Py =

Upon irradiation (>400 nm), 1-(9-anthryl)-2-nitroethylene 127 undergoes 4 C 2 and 6 C 6 as well as isomerization reactions75 (equation 67).

H			O2 N
	NO2			H
	hν			+	H
					NO2
	H		H
					H
(127)			(128) 3%
					H
					H
					O2 N
					(129) 56% (4π + 2π)
					(67)
		H			H
H
+			+		H
	H
			NO2		NO2
	H
O2 N		H			H
					H
	H				H

(130) 17% (6π + 6π)

(131) 12% (6π + 6π)

780	Tong-Ing Ho and Yuan L. Chow

F. Heterocyclic Compounds Containing Nitro Groups

Nanosecond laser flash photolysis was applied to study excited-state 2-nitrothiophene in polar and non-polar solvents76: the transient absorption at 545 š 5 nm was assigned to its lowest triplet state. The rate constants of the interaction of this triplet excited state, with a number of substrates such as cyanide and hydroxide ions, have been determined77. Similarly, the transient absorption at 490š 5 nm was assigned to the lowest triplet excited state of 5-nitro-2-furoic acid78, and that at 500 š 5 nm to that of N-(n-butyl)-5-nitro-2- furamide79.

The photolysis of 2-methyl-5-nitro-1H-imidazoles 132 in a water-containing solvent80 gives oxime 133, which is in turn hydrolysed to 134 followed by a dehydrative cyclization to the 1,2,4-oxadiazole 135. Light-induced hydrolysis of 135 gives 136 (equation 68).

R

R

O2 N

N

Me

O

N

Me

hν

N

N

HON

(132)

(133)

H2 O

O

O

RNH

N

−H2 O

RNH

N

(68)

N

Me

N

Me

HO

OH

O

(135)

(134)

hν

H2 O

O

NH2

RNH

O

(136)

The solvent effects on the relative stabilities of 4-nitroimidazole and 5-nitroimidazole exhibit interesting patterns81. In the excited state the 5-nitro isomer is more stable than the 4-nitro isomer in aprotic solvents, while the stability order is reversed in the ground state.

The photodimerization of 3-methyl-4-nitro-5-styrylisoxazoles 137 has been studied in the solid state82. Truxillic acids 139 are prepared by the oxidation of the photodimers 138 (equation 69).

16. Photochemistry of nitro and nitroso compounds						781
			Is		CO2 H
Ar		hν	Ar
					Ar
		Solid State
			Ar	KMnO4
Is					Ar
			Is		CO2 H
(137)			(138)	(139)		(69)
Me		NO2
Is =	;		Ar = C6 H5, 2-ClC6 H4 ,	4-ClC6 H4 , 2,4-Cl2 C6 H3
N
	O

Nitroimidazoles (such as metronidazole and misonidazole) can enhance the porphyrin sensitized Type I photooxidations83; that is, electron transfer from the sensitizer to the oxygen molecule is facilitated to give more of the superoxide ion (Scheme 6). The Type II mechanism operates by energy transfer from the sensitizer to afford the singlet oxygen84.

Type I

Sens C h ! 1Sens

1Sens C S ! 1(Sens ž /SCž )

1(Sens ž /SCž ) ! Sens ž C SCž Sens ž C 3O2 ! Sens C O2 ž SCž C O2 ž ! products

Type II

Sens C h ! 1Sens

1Sens C 3O2 ! 3Sens C 1O2 3Sens C3 O2 ! Sens C1 O2

1O2 C S ! products

(Sens D Sensitizer, S D substrate) SCHEME 6

Whereas several 1-aryl-4-nitroimidazoles are found to be good sensitizers for superoxide ion formations85 (Type I photooxidation), only 1-phenyl-2-methyl-4-nitroimidazole 140 is a photosensitizer for singlet oxygen, i.e. by energy transfer of type II photooxidation (equation 70).

O2 N

	N
	Me	Ph	Me		Ph		CH2
	N
				hν
	+	C	C			C C	(70)
					HOO
				O2
		Me	Me		Me		Ph
				(95%)

(140)

782	Tong-Ing Ho and Yuan L. Chow

G. Photoretro-Aldol Type Reactions and Photodecarboxylation to Generate Nitroaromatic Anions

Photolysis of p- or m-nitro phenylethyl alcohols 141 and 142 in aqueous solution causes retro-aldol type reactions, where the quantum efficiencies are pH dependent86 89. In these compounds the nitro group is placed so that no intramolecular hydrogen abstraction can occur and the excited state must follow an alternative route. The m- or p-position of the nitro group controls the product pattern as shown in equation 71; while the p-nitro group promotes biphenyl formation, the m-nitro group gives predominantly m-nitrotoluene at pH D 14. Photolysis of 144 at pH 7 14 does not change the product pattern significantly. Photolysis of 141 at pH D 12 changes the 142/143 ratio to 20/80. However, the expected formaldehyde product was not isolated.

Y			Y				X
X	CH2 CH2 OH	hν	X	CH3	+
		pH = 14				Y		CH2
(141)	X = NO2 , Y = H		(142)	2%				2
(144)	X = H, Y = NO2		(145)	>95%			(143)	98%
							(146)	<5%

(71)

The photo-retro-aldol type reaction is general for appropriately substituted nitroaromatic compounds with a p- or m-nitrobenzyl carbanion moiety as the photolabile leaving group. Its mechanism is shown in equation 72, in which the reactive triplet nitrophenylethyl alcohol undergoes retro-aldol cleavage assisted by solvent water (or hydroxide ion) as the deprotonating base in the primary photoprocess. The photogenerated carbanion subsequently reacts to give the observed products (ArCH2R and ArCHR CHRAr) in de-aerated solution. The photogenerated nitrobenzyl anion can transfer one electron to a suitable electron acceptor (including the original nitrobenzyl derivatives) to generate the anion radical, which can be observed by electron paramagnetic resonance (EPR) spectroscopy87,88,90. The dimerization products (143,146) are derived from the nitrobenzyl radical. The yield of 146 is small owing to the fact that the m-nitrobenzyl anion is reluctant to give up its electron to form an m-nitrobenzyl free radical. The EPR experiment consistently shows very weak signals for the m-nitrobenzyl system. In aerated solution, the photogenerated carbanion reacts with oxygen by similar electron transfer to give hydroperoxides as the products (equation 73).

OH

3

H

hν

R

O

−

+ R2 CHO

S0

S1

ArCHR

(72)

ks t

ArCH

CHR

ArCH2 R + ArCHRCHRAr

OH

hν

+

KI

ArCH2 CH-Ph

PhCHO

ArCH2 OOH

ArCH2 OH

(73)

O2

Ar = 3- and 4-NO2 C6 H4

16. Photochemistry of nitro and nitroso compounds

783

Ketals 147, 148 and 149 cleanly photolysed to give the expected products 150 153 by an analogous photo-retro-aldol process in aqueous solution (pH 7) (equation 74).

O

O

O

hν

R

COCH2 CH2 OH

Y

CH2

C

R

X

(147) R = Ph, X = H, Y = NO2

(150) R = Ph

(148) R = Ph, X = NO2 , Y = H

(151) R = Me

(74)

(149) R = Me, X = H, Y = NO2

+ Y

CH3

+ Y

CH2

2

X

X

(152)

(153)

The product ratio

for 152/153

is

similar to those

observed in compounds

141

and 144. The isolation of 150 and 151 implicates the formation of dioxocarbocation intermediates (154, 155) that can be trapped by water to give hemiorthoesters and ultimately the ester products (equation 75). The photogenerated p-nitrobenzyl anion also can be detected by the EPR spectrum of the corresponding radical anion through electron transfer88.

147, 148, 149	hν	−		H2 O		O
		ArCH2 + O + O			O
					HO	R
			R			(75)

(154) R = Me

(155) R = Ph

150, 151

The photodecarboxylation of nitrophenyl acetate in aqueous media was also investigated recently89 92, especially with respect to the kinetic and spectral properties of the photogenerated p-nitrobenzyl carbanion; its triplet state ( max ca 290 nm) was identified to have a lifetime of 90 nanoseconds at pH > 5.0. The proposed reaction mechanism following 266-nm laser excitation of p-nitrophenyl acetate is summarized in Scheme 792.

The photodecarboxylation of p-(nitrophenyl) glyoxylic acid 156, which was studied by time-resolved and steady-state methods at room temperature93, leads to p-nitrosobenzoic acid and carbon dioxide in good yields with D 0.28 in aqueous solution at pH 2 12 and excitation at 313, 280 or 254 nm (equation 76). An intermediate ( max D 350, 2 ms) observed by nanosecond laser flash photolysis is assigned to the aci-form of the nitroketene

784

Tong-Ing Ho and Yuan L. Chow

O N

CH CO

−

O N

CH CO −

2

2

2

2

2

2

S0

S1(τ1 ≤5 ps, φ = 0.6)

CO

+

2

− ca 100 ps

O

O N

CH

O N

CH

C

2

2

2

2

O −

T1 (τ

ca 90 ns)

pH > 3

T1

−

CH

CH

2

2

pH < 3

N + −

N+

CH2

CH3

−

−

Ο

Ο

Ο

Ο

p-nitrobenzyl anion

N +

NO

H+

−

2

HO

Ο

aci-form

O2 N

CH2 CH2

NO2

SCHEME 7

derived from the excited-state decarboxylation and rearrangement.

O2 N	COCOOH	hν	ON	COOH + CO2

(156)

(76)

Evidence for the trapping of a non-Kekule intermediate in m-nitro participation was obtained in a photo-retro-aldol type reaction94. Photolysis of 157 in aqueous acid solution

16. Photochemistry of nitro and nitroso compounds

785

(H2SO4, H0 D 1.65) leads to the formation of 3-(N-30-nitrobenzyl-N-hydroxyamino) benzyl alcohol 160 in a high yield (equation 77).

	CH2 CH OH		CH2		CH2 OH
	Ph	hν
NO2		−O	+ N	HO	N
			OH		OH
	(157)		(158)
			H+(−H2 O)		− H2 O
	CH2 OH		CH2 OH		CH2 OH
		158
CH2	N		N		NO
	OH		OH		(159)
			+

NO2

(160) 80%

(77)

The intermediacy of nitrobenzyl carbanions in such photolysis is general, and also has been found in the photooxygenation of a series of nitrobenzyl derivatives including 2-methoxy-(m- and p-nitrobenzyl) ethanols95.

H. Photoredox Reactions in Aqueous Solutions

The photoredox reaction of o-nitrobenzyl compounds is mediated by intramolecular hydrogen abstraction and subsequent redox oxygen transfer following excitation in various solvents, and even in the solid state, due to the proximity of the two functional groups. Such reactions do not appear to involve catalysis by either external acids or bases. Similar photoredox reactions of m- and p-nitrobenzyl derivatives also take place, but only in aqueous solution and are subject to catalysis96,97. Irradiation of p-nitrobenzyl alcohol gives p-nitrosobenzaldehyde as the only product, in a reaction which is strongly catalysed by hydroxide ion97 (equation 78). m-Nitrobenzyl alcohol gives m-nitrobenzaldehyde as the major product by hydrogen ion catalysis98 (equation 78); the azoxy compound 161 is also obtained in this case.

786	Tong-Ing Ho and Yuan L. Chow
	Y			Y
X	CH2 OH	hν	X		CHO
		pH >11
				O−	(78)
		+		N
					N
				+
			OHC		CHO
				(161)

X, Y = H or NO2

This new type of photoredox reaction of p- and m-nitro-substituted aromatic derivatives is not observed in organic solvents, and is99,100 extended to m-nitrobenzyl derivatives 162 containing alcohol, alkyl ether, ester or amine functions; these compounds undergo photooxidation to produce m-nitrobenzaldehyde (or m-nitroacetophenone) as the major isolable product100 (equation 79).

XR1		COR3
C R2	hν
R3	H +
NO2	NO2	(79)
(162)

XR1 = OH , OMe , OEt , OCOMe , NH2 , OCH2 Ph , OPh , H

R2 = H , CH2 OH , Me , CH2 CO2 Et , CH2 OPh , OMe , COCH3

R3 = H , Me , CH2 OH

A general reaction mechanism for m-nitrobenzyl derivative is proposed (Scheme 8) which involves a non-Kekule intermediate100. The mechanism for the p-nitrobenzyl alcohol involves the highly polarized intermediate 163, which is consistent with the observed strong solvent effect and base catalysis of the reaction (equation 80).

CH2 OH							CHOH		CHO		CHO
	hν	S1	ISC	T1						−H2 O
					[OH−]
NO2					HO		N+	HO	N		NO
							O−		OH
							(163)

(80)