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						787
R					R
C	OR				C	OR
H	hν	ISC	T1	H+	H
		S1		slow
NO					N+
2				HO	O
	R			R			R
	C	OR		C	OR		C+
	+						OR
	H	H2 O
		−(H3 O+)
		fast
N+	O−			N+		N
HO			HO OH			HO	OH
					COR		COR
					[O]
				NO			NO2

SCHEME 8

I. Photodissociation

A new type of photodissociation for p-nitrobenzyl 9,10-dimethoxyanthracene-2- sulphonate 164 has been reported to give 9,10-dimethoxy-anthracene-2-sulphonic acid 165, 9,10-dimethoxy-2-(p-nitrobenzyl)-anthracene 166 and p,p0 -dinitrobibenzyl101 (equation 81). It is suggested to occur from excited intramolecular electron transfer followed by radical ion decompositions and recombinations.

OMe
SO2 OCH2	NO2
	hν
	OMe
OMe	SO3 H
(164)
	OMe
	(165)

(81)

788	Tong-Ing Ho and Yuan L. Chow

OMe

CH2

NO2

(165)+

(81 continued)

OMe
(166)	O2 N	CH2−
+
		2

Topologically controlled intramolecular coulombic interactions have been applied to study the photochemical cleavage reactions of a series of 4-nitrophenyl ethers linked through a methylene chain to a tertiary amine (e.g. 167, 170, 172 and 173). The product distribution is controlled by the chain length102,103. The photoproduct pattern in aqueous basic media (pH D 12) is shown in equation 82, where the usual meta-substituted photoproduct 168 is the highest at n D 5 and decreased to ca 10% for n D 3 and to nil for n D 2. In contrast, 2-methoxy-4-nitrophenol, the p-substituted photoproduct, increases from a trace amount for n D 5 to quantitative yield for n D 2. The latter agrees with the clean photolyses of 173. As shown, the photocleavage of the p-alkyl ether linkage occurs preferentially for the substrate containing a short (two or three methylene units) link between donor and acceptor; this may arise from unusual stabilization of the intramolecular charge transfer state and constitute a new type of photocleavage reaction.

O2 N		O (CH2 )n	N
	R
		pH 12
	(167) n = 5, R = OMe		hν
	(170) n = 3, R = OMe
	(172) n = 2, R = OMe					(82)
	(173) n = 2, R = H
O2 N	O	(CH2 )n N	+	O	N	OH
				2
	R					R
	(168) n = 5, R = OH				(169)	R = OMe, H
	(171) n = 3, R = OH

New photochemical cleavage reactions of ortho-substituted CDC double bonds were reported by introducing a 2-nitrophenyl group to the double bond104. Photolysis of 1- (2-nitrophenyl)-1-alkenes 174 in methylene chloride solution without oxygen affords aryl

16. Photochemistry of nitro and nitroso compounds

789

and ˛,ˇ-unsaturated aldehyde in 30

80% yields (equation 83).

D

D

h

D

D

CH-(CH

CH)n-C6H4

-ONO2

2 !2

ArCR (

CH-CH)n

O

(83)

ArCR

CH Cl

(174) R D H, Me; n D 0,1

(175) R D H, Me; n D 0,1

Ar D Ph, p-ClC6H4, p-MeC6H4,

1-C10H7,

2-C10H7

Photolysis of 4- and 3-nitrophenyl acetates (176 ! 177; 178 ! 179) in neutral aqueous solution leads to the corresponding phenols with quantum yields 0.002 and 0.006105 (equation 84). A greater difference in the photoreactivity (quantum yields of 0.002 and 0.129, respectively) is shown between 2-methoxy-4-nitrophenyl acetate 180 and 2- methoxy-5-nitrophenyl acetate 182. The nitro substituent clearly exhibits a meta-activating effect in the hydrolysis of phenyl acetates.

	NO2		NO2
		hν
		R1		R1
	R	2	2	(84)
			R
(176)	R1 = H, R2 = OCOCH3		(177)	R1 = H, R2 = OH
(178)	R1	= OCOCH3 , R2 = H	(179)	R1 = OH, R2 = H
(180)	R1	= OMe, R2 = OCOCH3	(181)	R1 = OMe, R2 = OH
(182)	R1	= OCOCH3 , R2 = OMe	(183)	R1 = OH, R2 = OMe

A new triplet diradical is detected by ESR from the photolysis of 2-nitrobiphenyl106 (equation 85). The spectrum shows a temperature dependence which implies that the observed triplet state is a ground state.

	O
NO2	N OH
	(85)
	hν

J. Photonitration

The nitration reagents (NO2Y) for electrophilic aromatic nitration span a wide range and contain anions Y such as nitric acid (Y D OH ), acetyl nitrate (Y D OAc ), dinitrogen pentoxide (Y D NO3 ), nitryl chloride (Y D Cl ), N-nitropyridinium (Y D pyridine) and tetranitromethane [Y D C(NO2)3 ]. All reagents contain electron-deficient species which can serve as effective electron acceptors and form electron donor acceptor (EDA) complexes with electron-rich donors including aromatic hydrocarbons107 (ArH, equation 86). Excitation of the EDA complexes by irradiation of the charge-transfer (CT) absorption band results in full electron transfer (equation 87) to form radical ion

790	Tong-Ing Ho and Yuan L. Chow

pairs. Subsequent fragmentation to 184 (equation 88) and radical recombination gives the nitration products (equation 89). This photoinduced inner-sphere electron transfer provides a new method of photonitration107 and is a topic of current interest108. The EDA complexes of tetranitromethane (the electron acceptor) with arenes can be photolysed to cause the nitration of the arenes such as anisole109, anthracene110, naphthalene111, fluorene112, benzene113, dibenzofuran114 and others. The photonitration of naphthalene with tetranitromethane is summarized in Scheme 9108.

ArH C NO2Y	KEDA	[ArH, NO2Y]	(86)
[ArH, NO2Y]	hCT	[ArHCž , NO2Y ž ]	(87)
	fast
[ArHCž , NO2Y ž ] ! [ArHCž , NO2]Y			(88)
		(184)
184 ! ArNO2 C HY			(89)

When the naphthalene and tetranitromethane charge-transfer complex is photolysed in dichloromethane or acetonitrile at a low temperature, the nitro-trinitromethyl adducts 185, 186, 187 and hydroxy-trinitromethyl adduct 188 together accounted for 85 95% of the product mixture; the remaining products are 1- and 2-nitronaphthalene. The adduct 188 is a secondary product formed by hydrolysis of the corresponding nitrite during photolysis. Adducts 185, 186 and 187 are all unstable and easily undergo elimination to give mainly 1-nitronaphthalene, with 2-nitronaphthalene as minor product. In Scheme 9, the formation of the radical ion pair is followed by fast fragmentation of the tetranitromethane radical anion to give a ‘triad’. The initial chemical process is assumed to come from the trinitromethanide attack on naphthalene cation radical followed by the radical recombination (see equation 90).

			4		H	NO2
+
			NO2
+ (O2 N)3 C−
			2
		H	C(NO2 )3		H C(NO2 )3
	ONO		NO2	(185 and 186)
hydrolysis
H			H			H
OH			ONO			NO2
H C(NO2 )3	H	C(NO2 )3		H C(NO2 )3
(188)				(187)			(90)
In the nitration of arenes with	N2O4,	the	red-coloured	transient	arises		from
the metastable precursor complex [ArH, NOC ]NO3 which is				formed	in	the	prior

16. Photochemistry of nitro and nitroso compounds				791
+ C(NO2 )4	ArH	C(NO2 )4
	CT Complex
ArH
hν		H	C(NO2 )3
ArH+ (NO2 )3 C− NO2			+ NO2
‘triad’

NO2
H	C(NO2 )3	H	C(NO2 )3
−HC(NO2 )3
+
NO2	,		,
H	NO2	O2 N	H
(185)		(186)
H C(NO2 )3		H	C(NO2 )3
	H		H
	NO2 +		OH
(187)		(188)
	−H2 O

C(NO2 )3

SCHEME 9

792	Tong-Ing Ho and Yuan L. Chow

disproportionation of nitrogen dioxide induced by the aromatic donor115 (equation 91). Irradiation at this change-transfer absorption band at a low temperature results directly in aromatic nitration, which has been shown with 1,3,5-trimethylbenzene, toluene and others.

		[ArH, NO+]NO3 −		hν		−
ArH + NO2 (N2 O4 )					[ArH+ , NO ]NO3
						(91)
				ArNO2 + H+ + NO3		−
III. PHOTOCHEMISTRY OF NITRO-OLEFINS
Photolysis of 4-nitro-2,5-cyclohexadienyl acetates			in methanol gives 4-hydroxy-2,5-

cyclohexadienyl acetates stereospecifically116 although the mechanism (equation 92) involves the scission of the C N bond (and therefore, the possible loss of chirality) to form the cyclohexadienyl radical and nitrogen dioxide pair in a solvent cage 190. A recombination at the oxygen site (NO2) gives the corresponding nitrite 191, which is then further photolysed to give the alcohol 192 via the alkoxy radical. The clean retention of stereochemistry in nitrite 191 implies that the radical pair 190 in the cage maintains a tight relation on the same face.

Me	NO2	Me	NO2	Me	ONO
	Y		Y		Y
	hν
	MeOH
H	OAc	H	OAc	H	OAc
(189) Y = F, Cl, Me		(190)		(191)

Me	O	Me	OH
		Y
		MeOH	+ CH2 OH
H	OAc	H	OAc
		(192)

(92)

The crystalline state of 193 was irradiated with sunlight at 5 °C (equation 93) to afford the cyclobutanes 194 and 195 in a 3:1 ratio117. Compound 195 obviously arose from the dimerization of the cis-isomer of 193. The disordered crystal structure of 193 permits isomerization of 193 to the cis-isomer which photolytically reacted with 193 to give 195. Interestingly, the crystalline state of compound 196 and 198 was photolysed to 197 and 198, respectively (equations 94 and 95), but ˇ-nitro-p-methylstyrene was photostable.

	16. Photochemistry of nitro and nitroso compounds							793
				Ph NO2		Ph
Ph	NO2	hν		+
							NO2	(93)
				NO2 Ph		NO2	Ph
(193)				(194)			(195)
CH3 O						Ar	NO2
				hν
CH3 O				NO2				(94)
						NO2 Ar
	(196)					(197)
Cl	Cl			Ar		Ar
				hν
		NO2						(95)
				NO2	NO2
	(198)			(199)

Irradiation of 1-methyl-2-nitrocyclohexene 200 in benzene in the presence of methyl acrylate showed a dual pathway to give both isoxazoline 201 (54%) and the C-nitroso dimer 202 (22%)118 (equation 96). The isoxazoline 201 arose from an excited-state intramolecular cyclization and scission to give a nitrile N-oxide which is trapped by the acrylate. Concurrently, the photoinduced nitro nitrite inversion also occurs competitively to give the C-nitroso compound which is isolated as the dimer 202.

	O−
NO2	+ N	MeCO(CH2 )4
	O
	Me
	Me
	hν	N	CO2 Me
			O

(200)					+		CH2		CHCO2 Me
	hν						−
		[MeCO(CH2 )4 C
					N		O]

(201) 54%

(96)

ONO		O	O	O
			Me	Me
	Me	Me	+	+
			N	N
	hν or ∆		−O	O−
		NO
			(202) 20%

794	Tong-Ing Ho and Yuan L. Chow

The functionalization of an unactivated but strategically located carbon can be initiated by intramolecular alkoxy radical hydrogen abstraction that can be induced by nitrite photolysis. Thus photolysis of 6ˇ-nitrocholest-4-ene 203 in methanol under nitrogen causes the nitro-to-nitrite conversion in the first step, followed by the secondary nitrite photochemical transformation to afford cholest-4-en-6-one 204 (7%) cholest-4-en-6ˇ-ol 205 (11%), and compounds 206 (24%) and 207 (7%)119 (equation 97). While a number of products is obtained, it is significant that ˇ-nitro configuration is stereospecifically retained in the nitrite intermediate, as can be judged from the ˇ-alcoholic configuration. A small amount of leakage to 4ˇ-nitrite (and 4ˇ-OH product 207) indicates a possibility of, but not the necessity of, the dissociative mechanism proposed in the nitro nitrite conversion in equation 92, although it must be mentioned that a C N bond homolysis is generally accepted in photoexcitation of nitroalkanes (see Section IV.A).

C8 H17

NO2

(203)

MeOH

hν

+

ONO

ONO

O

(204)

	HON		HON
+	+		+
OH		OH	OH
(205)	(206)		(207)

(97)

16. Photochemistry of nitro and nitroso compounds

795

IV. PHOTOCHEMISTRY OF ALIPHATIC NITRO COMPOUNDS

A. Simple Nitroalkanes

The primary photochemical reaction for nitromethane in the gas phase is well supported by experiments to be the dissociation of the C N bond (equation 98). The picosecond laser-induced fluorescence technique has shown that the ground state NO2 radical is formed in <5 ps with a quantum yield of 0.7 in 264-nm photolysis of nitromethane at low pressure120. The quantum yield of NO2 varies little with wave-

length,	but	the	small yields of		the excited state NO2 radical increase significantly
at 238	nm.	In	a crossed laser		molecular beam study of nitromethane, it was found

that excitation of nitromethane at 266 nm did not yield dissociation products under collision-free conditions121.

h
CH3NO2 ! CH3ž C NO2ž	98

Two independent and complementary techniques, product emission spectroscopy and

molecular beam			photofragment translational energy spectroscopy, have	been	applied
to confirm		the	C N cleavage as the primary process at 193 nm	in the	( Ł )
excitation	122
		. The majority of the NO2 radical produced is in the vibrationally excited

2B2 state, and unimolecular dissociation to NO C O is revealed by molecular beam studies. Several products (OH, HONO and NO2) were detected under one-photon and collision-free photoexcitation (222, 249 and 308 nm) of 2-nitropropane123. The collision-free photolysis at 282 nm for nitroethane, 1-nitropropane, 2-nitropropane and tert-nitrobutane has indicated that the OH radical is formed in the primary process124. The participation of a five-membered ring intermediate in the process is supported by relative yield data and by the observation that CH3CD2NO2 yields OH exclusively and no OD. No OH formation from nitromethane is observed. In marked contrast to the nitromethane photodissociation, no evidence is found for simple C N bond fission for nitromethyl radical (žCH2NO2) which was studied using a fast beam photofragment translation spectrometer125.

Nitromethane was photolysed in solid argon at 14 K to give syn- and anti-CH3ONO126 as identified by IR absorptions. On prolonged photolysis, nitromethanol, CO, NO, HNCO and the hydrogen-bonded complexes H2CO Ð Ð Ð HNO and H2O HNCO were detected by infrared absorption. When the enhanced role of cage recombinations is taken into account, the proposed mechanism in argon matrix is compatible with that determined from gas-phase studies of the photolysis of nitromethane. When nitromethane was exposed to ionizing irradiation in a solid martix and studied by ESR, the primary process was electron ejection127. This is frequently followed by specific electron capture, so radical species are trapped in the rigid matrix. In dilute solutions of CD3OD such a captive yields nitromethane radical anions, and in that of CFCl3 nitromethane radical cations. In marked contrast, the exposure of nitromethane liquid to gamma rays at 77 K gives mainly CH3 and NO2 radicals.

B. aci-Nitronates
Further studies on the	photochemistry of aci-nitronate anion have revealed that
the reaction occurs from	the Ł triplet excited states causing an oxygen migration

to give hydroxamic acids128,129. The photorearrangement gives regiospecific products with the retention of the configuration at the migratory terminus in high yields (equations 99 102).

796

(208)

NO2

(210)

Tong-Ing Ho and Yuan L. Chow

		O
NO2	hν	NHOH
	MeNH2	(99)

(209)91%

OH

O N OEt

OEt

hν

(100)

EtONa EtOH

(211) 75%

NO2

OH

N O

hν

MeOH MeONa

AcO

(212)

(213) 78%

(101)

+

(214) 17%

		hν
MeOH			MeNH2		(102)
				N
					O
NO2
				OH
(215)		(216)			95%

Correlative studies revealed that the faster the rate of nitronate formation, the higher the yields of the hydroxamic acids130 (equations 103 and 104).