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

757

Second-order kinetics have been confirmed in the photocyanation of 3,4-dimethoxy-1- nitrobenzene with potassium cyanide24 (equation 22), that has lead to the assignment of the SN23ArŁ mechanism; this shows an interesting contrast to the SN1ArŁ photocyanation of 2-nitrofuran to give 2-cyanofuran as judged by the quantum yield independent of the cyanide concentration25.

NO2	NO2	NO2
	hν, KCN	+
	H2 O −t-BuOH
		(22)
OMe	CN	OH
OMe	OMe	OMe
	(88%)	(<1%)

The protonation of the triplet Ł state of 3-bromonitrobenzene is shown to be responsible for the acid-catalysed promotion of halogen exchange which follows a SN23ArŁ mechanism26 (equation 23). Cationic micellar effects on the nucleophilic aromatic substitution of nitroaryl ethers by bromide and hydroxide ions have also been studied27. The quantum efficiency is dependent on the chain length of the micelle. The involvement of counter ion exchanges at the surface of ionic micelles is proposed to influence the composition of the Stern-layer.

Br	Cl
LiCl	(23)
hν
NO2	NO2

(φ = 0.02)

The effect of an o-methyl substituent on the photosubstitution of o- and p-nitroanisole by hydroxide ions28 (equations 24 26) can be ascribed to both electronic and steric effects that determine the reactivity and selectivity.

	MeO	NO2	hν	HO	NO2 + MeO	OH
			OH −
	(φ = 0.111)				(φ = 0.020)	(φ = 0.091)
						(24)
	Me				Me
	MeO	NO2	hν	HO	NO2	(25)
			OH−

(φ = 0.005)

(φ = 0.005)

758	Tong-Ing Ho and Yuan L. Chow
MeO	NO2	hν		HO	NO2
		OH −
	Me			Me
	(φ = 0.145)			(φ = 0.056)	(26)
	+			MeO	OH
				Me
				(φ = 0.089)

Triplet exciplexes have been proposed to explain photolysis of 2-nitrodibenzo[b,e]- (1,4)dioxin in the presence of primary amines29 (equation 27). In polar solvents the exciplex dissociates to the solvated radical ions from which the diphenyl ethers formed; in apolar solvents only the nitrophenoxazine is obtained. In contrast, 1-nitrodibenzo [b,e] (1,4) dioxin is photostable in the presence of amines.

O	NO2	OH RHN
	hν	O	NO2
	RNH2
O	(t-BuOH/H2O)

OH RHN

+O

(24%, R = n-C3 H7) (24%, R = PhCH2 )

(56%, R = n-C3 H7) (53%, R = PhCH2 )

R	(27)
N	NO2

+

O

NO2

(7%, R = n-C3 H7)

The nucleophilic aromatic substitutions of 2-fluoro-4-nitroanisole with amines have been shown to be useful as biochemical photoprobes30. Nitrophenyl ethers such as 4- nitroveratrole and 3- or 4-nitroanisole have also been explored as possible photoaffinity labels31.

2. Intramolecular reactions (photo-Smiles rearrangements)

Excited-state intramolecular nucleophilic aromatic substitutions are known as photo-Smiles rearrangements. Ealier, these were reported for 2,4-dinitrophenyl ethers and s-triazinyl ethers32. The exploratory33 and mechanistic34 studies on photo-Smiles rearrangements of p-(nitrophenoxy)-ω-anilinoalkanes were carried out (equation 28).

	16. Photochemistry of nitro and nitroso compounds				759
O2 N	O(CH2 )nNHPh	hν	O2 N	N(Ph)(CH2 )nOH
	R			R	(28)

R = H, n = 2−5

R = OMe, n = 2− 4

Directive effects of the nitro group in photo-Smiles rearrangements have been systematically studied using a series ˇ-(nitrophenoxy)ethylamines 17, 19 and 22 as models35; the meta isomer 17 is photolysed to give the N,O-inverted product 18 cleanly in 75% (equation 29). However, photolysis of ortho (19) and para (22) isomers gave various by-products (such as 21, 24 and 25) in addition to the photo-Smiles rearrangement compounds 20 and 23 (equations 30 and 31). Predictably, both 19 and 22 are thermally rearranged in aqueous basic solution to give 20 and 23 cleanly. Apparently, excited nitrophenylic ether moieties preferentially undergo intramolecular nucleophilic attack by the amine group, wherein the m-nitro group can facilitate the collapse of the transition state to the product. Interestingly, the chain length has definite effects on the photochemical pattern of the nitro arene (acceptor) and amine (donor) moieties36 of the chain terminal. While nitrophenyl ethers with n D 2 6 (see equation 28) undergo photo-Smiles rearrangement, the higher (n 8) homologues show an intramolecular photoredox reaction (equation 32). Interestingly compound 26 with n D 7 exhibits neither photo-Smiles rearrangement nor intramolecular photoredox reactions. The photochemistry of N-(ω-(4- nitro 1-naphthoxyl)alkyl) anilines 27 shows exactly the same chain-length control on the product pattern to give 28 for n 6, and 29 31 and aniline for n 8.

NO2	NO2
hν			(29)
NaOH, H2	O
			OH
NH2		N
O
		H
(17)	(18)
NO2	NO2
O		NH
NH2			OH
hν
NaOH, H2 O
0˚C
(19)	(20)	14%	(30)
		NO2
			O
		+
			N
			H
		(21)	8%

760	Tong-Ing Ho and Yuan L. Chow
NO2		NO2
	hν
	NaOH, H2 O
	0˚C
O		HN
NH2		OH
(22)	(23) 11%

+

O2 N	O(CH2 )nNH	hν
		ON
		n ≥ 8
	(26)

NO2

+

NH

O

(24) 14%

NO2	(31)
	OH
	N
OH	H
	(25) 25%
	O(CH2 )n −1CHO

+ H2 N

(32)

C. Photorearrangements
1. Intramolecular redox reactions
a. o-Alkylnitrobenzenes. The photochemical			investigation	of 2,6-di-tert-butyl-1-
nitronaphthalene 32 was	continued by	Dopp	and Wong37 to	give binaphthylidene
				Ph
O2 N	O(CH2 )nNHPh	O2 N		N
				(CH2 )nOH

(27)

(28)

	16. Photochemistry of nitro and nitroso compounds			761
ON	O(CH2 )n−1CHO	O2 N	O(CH2 )n−1CHO

(29)	(30)
ON	O(CH2 )nNHPh

(31)

quinone 36 (equation 33) as the only isolable product.

		N	O
NO2		O
	hν
	π π
(32)		(33)
	− NO
O
	1. Dimerization	[intermediate]
	2. −2H
		(35)	(33)
(34)	O

O

(36)

762	Tong-Ing Ho and Yuan L. Chow

It is proposed that 32 reacts from its Ł excited state by the nitro-to-nitrite (33) inversion followed by nitrite homolysis, when the naphthoxy radical must diffuse away from the cages to obtain the dimerization intermediate 35. However, the source of oxidizing agents is not identified. In comparison, o-nitro-tert-butylbenzenes 37 are excited to undergo intramolecular H-atom transfer and cyclization to give indol-N-oxides 40 (equation 34)38. The discrepancy may arise from the nature of the excited state, e.g. that of 37 may react from its n Ł state.

		O−	Ο− +
		+
NO2	HO2 N	HO N	N
		CH2
	hν
	ππ		− H2O
R	R	R	R
(37)	(38)	(39)	(40)

R = H, 4- NHCOCH3 , 4- MeO, 4- Br, 4- CN, 4- NO2 , 4- CO2 H, 4- C6 H5, 5- C6 H5

(34)

Photolysis of 1,4-bis-(2-chloro-1,1-dimethylethyl)-2-nitrobenzene 41 in solution and in the solid state39 preferentially causes intramolecular hydrogen abstraction from the adjacent chloromethyl group instead of the methyl group, leading to the formation of indole 1-oxide 44 as the primary product. This is hydrolysed subsequently to afford the hydroxamic acid 46 and also the lactam 47 (equation 35). The molecular geometry and packing from the X-ray crystallographic structure of 41 have provided the rationale for the intramolecular hydrogen abstraction efficiency that, in turn, provides the basis of the structure reactivity relationship. Such correlation is extended to 1-t-butyl-3,5-dimethyl-2,4,6-trinitrobenzene, 1-t-butyl-3,4,5-trimethyl-2,6-dinitrobenzene and 1-t-butyl-4-acetyl-3,5-dimethyl-2,6-dinitrobenzene40.

			Cl
CH2 Cl	CHCl	OH	N	OH
NO2
	N
			+	−
hν	+	O−		O
ππ
CH2 Cl	CH2 Cl		CH2 Cl
(41)	(42)		(43)
				(35)

16. Photochemistry of nitro and nitroso compounds			763
Cl		Cl
		OR
N	O −	N
+
− H2 O	ROH	OH
(43)	R = H, Me
CH2 Cl		CH2 Cl
(44)		(45)
			(35 continued)
	O	O
	N	NH
	OH
		+
	CH2 Cl	CH2 Cl
(46) 73% in MeOH		(47) 9% in MeOH

The light-induced yellowing of musk ambrette 48 is simulated41 by photolysis of 48 in 0.1 N methanolic sodium hydroxide solution to give the azobenzene 50 (through the intermediacy of azoxybenzene) and by-products 51 and 52, by intramolecular photocyclization (equation 36).

	OMe	MeO	O−	Me
	hν		N	NO2
	NO2		+
O2 N		O2 N	N
	Me		Me	OMe
	(48)		(49)

(36)

764		Tong-Ing Ho and Yuan L. Chow
	MeO		Me		OMe
(49) +		N	NO2 +
	O2 N	N		N	NO2
			OMe
		Me
				O	CH
		(50) 40%			(51) 4%

(36 continued)

	O
+	CH
O2 N	N
	Me
	(52) 6%

b. o-Benzyl derivatives. The photochemistry of o-nitrobenzyl derivatives carrying a well-placed heteroatom (see Scheme 3) has been reviewed thoroughly as a strategy in photoremovable protecting groups42. When alkyl o-nitrobenzyl ethers were irradiated, they were converted into o-nitrosobenzaldehyde releasing the alcohol intact as shown (Scheme 3); o-nitrobenzyl ethers have been used to protect hydroxyl groups during the chemical modifications of carbohydrates and their portions in nucleosides and oligoribo nucleotides. N-(2-nitrobenzyl)-1-naphthamide 53 is photolysed at 78 °C to

NO2				NO
CH2 Y	hν	YH	+	CHO
O −
N OH				NO
+
				H
CHY				C Y
				OH

Y = OR, NRR′

SCHEME 3

16. Photochemistry of nitro and nitroso compounds

765

give N-(˛-hydroxy-2-nitrosobenzyl)-1-naphthamide 54 (equation 37)43 as identifiable intermediate, which is decomposed slowly at room temperature to 1-naphthamide and 2-nitrosobenzaldehyde.

CONH	CH2	CONH	CHOH
		NO2	NO
		hν
(53)		(54)
		CONH2	CHO
			NO
		+

(37)

The derivatives of the o-benzylnitro group have been investigated by picosecond transient spectroscopy to examine the intermediates from the intramolecular hydrogen atom abstraction44. While the o-quinonoid intermediates are formed from both the singlet and triplet excited state o-nitrobenzyl ethers, observed transient absorption at 460 nm from excitation of o-nitrobenzyl p-cyanophenyl ether has been assigned to the biradical intermediate (see Scheme 4)45. Both 5-nitro-1,2,3,4-tetrahydro-1,4-methanonaphthalene 55 and 5-nitro-1,2,3,4-tetrahydro-1,4-ethanonaphthalens 56 incorporate structural contrasts that prohibit the contribution from the o-quinoid structure. Picosecond spectroscopy of the excited nitro-arenes gives46 transients with lifetimes of 770 ps and 410 ps that are assigned to the triplet excited state of 55 and 56, respectively. As photolysis of 56 affords nitrosoalcohol 57 (equation 38), the operation of the biradical route must provide the access to the product (Scheme 4).

hν	OH	(38)
H
NO2	NO
(56)	(57)

Recently, time-resolved resonance Raman spectroscopic studies47 of the excitation of 2- (20 ,40 -dinitrobenzyl)-pyridine 58 and 4-(20 ,40 -dinitrobenzyl)pyridine have shown that three transient intermediates are involved: they are aci-nitro acid 59, aci-nitro anion 60 and N H

766	Tong-Ing Ho and Yuan L. Chow

R H

H hν

+O

N

−O

RH OH

N O

R H

H ISC

+ O N

O−

S

−H

R

H

+ OH N

O− o- quinonoid

R H

O

N

OH

SCHEME 4

RH H

+ O N

O−

T

−H

R

H

+ OH N

O−

R

H

OH

N

O biradical

quinoid tautomer 61, the transient max is shown in parentheses (equation 39).

	NO2	T1		NO2
		ISC
	hν	S1
N	CH2	N	CH
	NO2			N
	(58)		HO	O
		(59) (370 nm)			(39)
				O−
			k1
	NO2			N
				O
		k2
N	CH	N	CH	+ H+
H	NO2			NO2
	(61) (565 nm)		(60) (490 nm)