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

797

H

H

hν

O

NH3 , MeOH

(103)

NO2

N

OH

Me

Me

(217)

(218) 47%, 99 de

(76%

EtOH

EtONa)

de = diastereomeric excess

H

H

R

R

X

hν

X

NOH

NH3 , MeOH

(104)

NO2

O

H

H

(220) R = Ph, X = CH2 (81%, 99 de)

(219) R = Ph, X = CH2

(221) R

=

CONHBu, X = CH2

(222) R

= CONHBu, X =

CH2 (62%, 99 de)

(223) R

=

CONHBu, X = O

(224) R

= CONHBu, X =

O (42%, 99 de)

Photolysis of nitro-steroids 225 yields the aci-nitronate at 254 nm131. This in turn gives various products, among them are ketone 226 and hydroxamic acid 227 (equation 105) which could be formed from the intermediate anions of the N-hydroxyoxaziridines, with a possible participation of gem-hydroxynitroso transient (or its anion; see Scheme 10). For comparison, N-butyl spiro-oxaziridine 228 in ethanol is photolysed at 254 nm (equation 106) to give N-butyl lactam 229 (50%) and the ketone 230 (25%). The former process is a well-known photoprocess of oxaziridine131.

R

		hν		+
		EtONa, EtOH		HO		N
		HO			H
HO			H			O
	H		O			OH
	NO2
	(225) R = C9 H19		(226) 22%		(227) 15%

(105)

Bu

N O O

O

Bu

N +

254 nm

EtOH

AcO

H

(228)

(229) 50%

(230) 25%

(106)

798		Tong-Ing Ho and Yuan L. Chow
	−O + O−			O−
	N			N
				O
	1		hν	R1			R1
	R
	R2			R2				R2
				(231)
O	O−			O		O−
N						Ν	O	1
R1			R1		R1			R
				R2				R2
	2					R2
	R

SCHEME 10

Aliphatic nitro compounds can be photolytically converted into oximes in acetone in the presence of triethylamine in moderate yields (30 74%)132, as shown by the examples in equation 107.

	hν	RCH2	OH
			N
RCH2 CH2 NO2
	Et3 N acetone			(107)
			H
(232)		(233)	R = Ph (44%)
		(234)	R = Indol-3-yl (41%)

C. Geminally Substituted Nitroalkanes

Both 2-nitro steroids 235 and 239 exist as the enols in ethanol, and are photolysed to give the corresponding ˛-diketones 237 and 238 (23% in 1:1 ratio)133 (equation 108) and 240 (equation 109), but different monoxime 236 and 241, respectively. On the contrary, 4-nitroketone 242 exists in the keto form, and is photolysed to give the ˛-oximino ketone 243 and its tautomer 244 without the diketones (equation 110).

		C8 H17
			O2 N
O2 N
			HO
			H
O	H	hν	(108)
	(235)	EtOH
O		HO	O
		+	+
HON		O	HO
	H	H	H
	(236) 12%	(237)	(238)

	16. Photochemistry of nitro and nitroso compounds		799
O2 N		HO
		hν
		EtOH
HO		O
	H	H
	Me Me	Me Me
	(239)	(240) 22%	(109)

HON

+

O

H

Me Me

(241) 12%

	hν
	EtOH
O	O
H	H
NO2	NOH
(242)	(243) 51%

(110)

+

O

H

H N

O

(244) 25%

It is proposed that ˛-hydroxyimino ketones are derived from a reaction pathway initiated from the hydrogen abstraction by the n Ł triplet-excited nitro group of the keto form, while ˛-diketones are formed from the nitro nitrite photorearrangement of the enol forms133.

Photochemistry of the ˛-nitroketones located in a steroidal ring has been studied134. The photoreaction of 245 (enol form) gives the corresponding cyclic N-hydroxy imide 247 (57 61%) (equation 111), whereas 250 (exclusively in the keto form) gives a cyclic imide which is formed from the ˛-oximinoketine 254 by light-promoted Beckmann rearrangement134 (equation 112). The mechanism of the formation of the N-hydroxy imide

800	Tong-Ing Ho and Yuan L. Chow

247 (see equation 111) can be visualized in analogy to that abserved in the nitronate rearrangement (e.g. Scheme 10); it is noteworthy that 245 reacts from its singlet excited enol (equation 114). Photoreactions of the seven-membered-ring ˛-nitrosteroidal ketone 255 in ethanol gave the corresponding ˛-hydroxyimino ketones in a low yield134 (equation 113).

				O
				NO2
	O			OH
	NO2
				NO2
				(111)
			(246)
(245)			hν
	O	OH		O
	OH	N	O	NOH
	N		+
	+
	O
	(247) 57%	(248) 12%		(249) 19%
	O			O
	hν
	NO2			NH
				O
(250)			(251) 12%
				(112)
	OEt		O	O
+	+		+
	O			NOH
Me
(252) 11%	(253) 8%			(254) 5%

16. Photochemistry of nitro and nitroso compounds

801

C8 H17

		hν	(113)
O	H	O	H
	NO2		HON
	(255)		(256) 28%
	1	OH
	OH
	NO2	NO2
	hν

	H +		(114)
O	OH		O
	N		OH
	O	hν	N
			O

Photolysis of six-membered steroidal ˛-nitro enones135 257 in protic solvents results in an unexpected ˛-cleavage of the carbonyl group to give 3-alkoxy-2-nitro-2,3- secocholest-4-en-3-one 258 (equation 115) while irradiation of 259 gives the parent cholest-l-en-3-one 260 which is obtained by exchange of the nitro group with a hydrogen atom (equation 116).

C8 H17			O2 N
		hν	O
O2 N
		EtOH or MeOH

RO

H

O

(258) R = Et, Me (25−30%)

(257)

(115)

802	Tong-Ing Ho and Yuan L. Chow

C8 H17

hν	(116)
EtOH
O	O
H	H

NO2

(260) 37%

(259)

In ethanol 2,4-dinitro-5˛-cholestan-3-one, 261, exists entirely as the enol, which is irradiated to give a mixture of diosphenols, 262, and its isomer, 263, in 55% yield (equation 117)136. Similarly, photolysis of an equilibrium mixture of 264 and 265 gives 266 in 48% yield (equation 118).

C8 H17

O2 N

O

H

(261)

NO2

O

HO

H

(262)

C8 H17

O2 N

O

H

NO2 (264)

O

O2 N

HO

H

NO2

(100% in EtOH)

(117)

hν

HO

+

O

H

(263)

O2 N

HO

H (118)

hν NO2

(265)

HO H

(266)

16. Photochemistry of nitro and nitroso compounds

803

The mechanism for this efficient removal of the two nitro groups to give an ˛-diketone can be written in various ways with reference to existing proposals for allied compounds. It is noteworthy that it should specifically involve the homolysis of the C N bond of the 4-nitro group, which is assumed to occur in the initial stage upon excitation.

V.PHOTOCHEMISTRY OF C-NITROSO COMPOUNDS

A.Simple Nitrosoalkanes

Both the molecular potential energy137 and the photodissociation138 of nitrosomethane were studied by ab initio SCF-CI techniques. There are geometry changes induced by the excitation and n Ł transition of nitrosomethane; the single excited-state surface has an energy barrier along the dissociation coordinate138. The first triplet state is not a dissociate state.

The photoelectron spectra of nitrosomethane, 2-methyl-2-nitrosopropane and perhalogeno nitrosomethanes has been re-examined and re-assigned on the basis of ab initio SCF-CI calculations139. Photoionization quantum yields140 have been measured for 2-methyl-2-nitrosopropane at wavelengths 147, 123, 105 and 107 nm. The results show that photoionization at energies up to 1.5 eV above threshold is of low probability. The data have been compared with those of recent photoelectron spectroscopy.

Since the primary photochemical process for nitrosoalkane involves the homolytic dissociation of the C N bond to generate free radicals141, recent studies on the photochemistry of nitrosoalkanes pay more attention to radical reactions and to the methods of detection, such as spin trapping studies coupled with ESR techniques142.

B. Geminally Substituted Nitroalkanes

The diastereomeric 2-chloro-2-nitroso-p-menthane and 3-chloro-3-nitroso-p-menthane (267) epimerize during photolysis143 (equation 119) and can concurrently give the nitroxide 269 as detected by ESR spectrometry, which confirms the mechanism for the photolysis of geminally substituted nitroso compounds (equations 120 and 121).

Cl		NO
	hν
NO	hν	Cl
(267)		(268)
		(119)
		Cl
	R2 NO	R =
	(269)
Cl
hν	R2 CCl + NO	(120)
R2 C
NO

804	Tong-Ing Ho and Yuan L. Chow
		Cl
	R2 CCl + R2 C			(R2 CCl)2 NO	(121)
		NO

Photolysis of the blue solid (C)-10-bromo-2-chloro-2-nitrosocamphane (270) with red light produces two nitroxide radicals 271 and 272 and 10-bromo camphor 273, 10-bromo- 2-chloro-2-nitro camphane 274 in addition to some minor products (equation 122). A complex reaction mechanism has been proposed144.

Me	Me			O	Me	Me
				CH2 Br	Cl	CH2 Br
				N
		hν
			Me	Cl
BrCH2	Cl
	NO		Me
(+)-(270)				(271)
			Me	Me		Me
	Br	O		Me
			CH2 Br
	CH2					CH2 Br
		N		O
+				+
Me
Me	(272)				(273) 40%
				Me		Me
				+
				BrCH2		Cl
						NO2
					(274) 25%
						(122)
Solid-state	photochemistry	of	( )-2-chloro-2-nitrosocamphane		275	was studied145

by irradiation of the blue-crystal with red light to invert the configuration at C(2) (equation 123). This also causes a photochemically initiated Beckmann rearrangement to form chloroxime 276 to give nitroxide radical 278 (equation 124). The intermediate chloro oxime 276 is proposed to arise from the n Ł excitation and is believed to be the common intermediate for the photo-epimerization and Beckmann rearrangement. Extended

16. Photochemistry of nitro and nitroso compounds

805

irradiation produces two additional nitroxides 279 and 280, and camphor oxime 281, camphor 282, 2-chloro-2-nitro camphane 283 as well as 2-chloro-2-nitratocamphane 284 (equation 125).

Me Me

Me Me

		hν
		Me
	NO				N
Me					O
				Cl
	Cl
(−)-(275)			(276)
Me	Me
				Me		Me
							Me
				−
			+ C	N
		Cl Beckmann
Me	O			C
	N			H2
(276)		C lO	−
	Me Me				Cl	Me		Me
	N	Me			N			Me
ClO C			O	C
	C				C
	H2				H2
								OCl −
					O Me			Me
					N			Me
			O	C
					C
					H2

Me

Me

Cl

Me

NO

(+)-(277)

(123)

(124)

(278)

806		Tong-Ing Ho and Yuan L. Chow
	Me	Me		Me	Me	Me
		Me		Me
275	hν			+
		O
	Me	N		Me	N	Me
		Cl	Me
		Cl			O
		(279) 7.5%			(280) 7.5%
	Me	Me		Me	Me	Me
				Me
	+	+		+
	Me	OH		Me	Me	Cl
		N		O
						NO2
	(281) 15%			(282) 40%	(283) 15%
					Me	Me

+

Me

(125)

Me	Cl
	ONO2
(284) 15%

C. Aromatic Nitroso Compounds

Nitrosobenzene was studied by NMR and UV absorption spectra at low temperature146. Nitrosobenzene crystallizes as its dimer in the cis- and trans-azodioxy forms, but in dilute solution at room temperature it exists only in the monomeric form. At low temperature ( 60 °C), the dilute solutions of the dimers could be obtained because the thermal equilibrium favours the dimer. The only photochemistry observed at < 60 °C is a very efficient photodissociation of dimer to monomer, that takes place with a quantum yield close to unity even at 170 °C. The rotational state distribution of NO produced by dissociation of nitrosobenzene at 225-nm excitation was studied by resonance-enhanced multiphoton ionization. The possible coupling between the parent bending vibration and the fragment rotation was explored.

The homolysis of the C NO bond and nitroxide formation147 have been studied using a series of sterically hindered aromatic nitroso compounds such as pentamethyl nitrosobenzene, 2,3,5,6-tetramethylnitrosobenzene, 2,4,6-trimethylnitrosobenzene and