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

solvent, such as toluene, n-heptane, cyclohexane, p-xylene and diethyl ether148, to give intense ESR signals of nitroxides. The latter are formed by hydrogen abstraction from the solvent and subsequent spin trapping (equations 126 128).

(128)

D. Other C-Nitroso Compounds

The photochemistry of di-tert-butyl nitroxide was studied149. When di-tert- butylnitroxide (DTBN) is excited at 254 nm to the Ł state in pentane solution, it is cleaved to tert-butyl radical and 2-methyl-2-nitrosopropane (with quantum yield of 0.21). The tert-butyl radical is scavenged by DTBN to give di-tert-butyl-tert-butoxyamine150 (equation 129).

DTBN with a quantum yield of 1.7. The products of the photoreaction are 2-methyl-2- nitrosopropane 285, isobutylene, tert-butyl chloride, di-tert-butyltrichloromethoxyamine 286 and di-tert-butylhydroxylammonium chloride 287 (equation 130).

Irradiation at the DTBN chloroform charge-transfer absorption yields151 285 ( D 1.01), 287 ( D 0.6), tert-butyl chloride ( D 0.06), isobutylene ( D 0.99) and di- tert-butyl (dichloromethoxy) amine 288 ( D 0.56) (equation 131). Also151, irradiation of DTBN at 300 nm in methylene chloride gives 2-methyl-2-nitrosopropane and di-tert- butyl-tert-butoxyamine ( D 0.014) products characteristic of the locally excited ( Ł ) state, and also 2-methyl-2-nitrosopropane ( D 0.11), 287 ( D 0.047), tert-butyl chloride ( D 0.004), isobutylene ( D 0.093) and di-tert-butyl (chloromethoxy) amine 289 ( D 0.05) (equation 132) from the DTBN CH2Cl2 charge-transfer state.

(131)

OCH2 Cl (132)

N

(289)

Mechanistically, the reaction in pentane with up to 35% methylene chloride is proposed to occur via ˛-cleavage (equations 133 and 134). Other reactions in chloroform, carbon tetrachloride and 65% methylene chloride are proposed to occur by electron transfer to the chlorocarbon with initial formation of di-tert-butyloxoammonium chloride 290 and

Laser flash photolysis at wavelengths within the charge-transfer absorption bands of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and carbon tetrachloride yields the oxoammonium chloride of TEMPO 291 ( max D 460 nm) and the trichloromethyl radical in an essentially instantaneous ( 18 ps) process152. The primary photochemical reaction is an electron transfer from TEMPO to carbon tetrachloride followed by immediate decomposition of the carbon tetrachloride anion radical to chloride and trichloromethyl radical (equation 140). The laser flash photolysis of TEMPO and of other nitroxides in a variety of halogenated solvents have confirmed the generality of these photoreactions152.

(140)

Photolysis of benzofurazan N-oxide 292 in chloroform generates the nitroxyl radical 393153 (equation 141), which is formed due to hydrogen abstraction from the solvent

by the lowest triplet state of 292. The photoreaction of 292 in triethylamine provides the diethylnitroxyl radical derived from triethylamine through oxygen transfer (equation 142). ESR spectroscopy154 indicated involvement of an exciplex155.

N

The stable nitroxyl radicals can be used for quenching the singlet excited naphthalene through an electron exchange mechanism156.

VI. PHOTOCHEMISTRY OF ALKYL NITRITES

The photolysis of methyl nitrite at low temperature in an argon matrix was studied157. The products include formaldehyde, and nitroxyl HNO which also reacts to form N2O and water. The 355-nm photodissociation of gaseous methyl nitrite has been studied by monitoring the nascent NO product using a two-photon laser-induced fluorescence technique158.

VII. PHOTOCHEMISTRY OF N-NITRO AND N-NITROSO COMPOUNDS

A. Nitrosamines

Since the last review in this series, a number of reports has been published to clarify the primary photoprocess and to show the application of aminium radical reactions in syntheses.

1. Photolysis mechanisms

In the gas phase, N-nitrosodimethylamine (NND) is photolysed at the S0 S1 (n, Ł ) transition band at 363.5 nm, to cause N N bond scission with of unity159,160; the recombination of the two radicals is equally efficient to give NND leaving no

subsequent secondary reaction to give N-methyl methyleneimine160 (equation 143); similar photoreactions have been observed on irradiation with a low-pressure mercury lamp in cyclohexane to give slowly the timer of the imine174. In low-temperature insert matrices, such irradiation give hydrogen-bonded complex 294 which can be detected by IR spectroscopy161 (equation 143). These reactions have been reviewed162. The fast singlet

excited-state dissociation is probably the reason why no fluorescence has been detected from nitrosamines.

In solution photochemistry in the presence of acids, the primary process is also the same except that both NND and the aminyl radical are protonated; the recombination of the aminium radical and NO to give 295A is too slow to compete with bond scissions174 (Scheme 11). The failure of oxygen to quench nitrosamine photoreactions in either solution (see below) or gas phases under various conditions must also mean a very short lifetime of singlet excited nitrosamines, in agreement with the fast dissociation159,160.

The molecular structure of nitrosamines is well described using NND and N- nitrosopiperidine (NNP) as the model. Their singlet and triplet excited stated are well defined163. The triplet state can be generated by excitation at the S0 T1 (n, Ł ) transition at 450 nm, but not from the singlet excited state owing to its fast photodissociation. The triplet state does not show any chemical reactivity. A resonance stabilized nitrosamine in acidic solution is associated with a proton (or proton donor)164; this species is photolysed at the 342-nm band (n, Ł transition) to give aminium and nitric oxide radicals by a chain mechanism in methanol in either the presence or absence of an olefin at room temperature163. Summarizing all available evidence, the primary photoreaction of a nitrosamine (NND) is shown in Scheme 11. It is most interesting to note that while the recombination of Me2Nž and ž NO is extremely fast, that of an aminium radical (e.g. Me2NHC ž) with ž NO to give 295A is slow, as shown by the lifetime of the piperidinium

radical (>100 microsecond) in water164. A competing reaction for the latter pair is the slow elimination according to equation 143. Also, estimations from flash photolysis show that these rates are slower than the hydrogen abstraction of Me2NHC from methanol163, that is <10 4 M 1 s 1. Supported by simple INDO closed-shell calculation, this suggests that the N-protonated species 295A possesses much higher energy than the O-protonated

ation at 313 nm under these conditions gives a new species showing absorption at 391,

375 and 362 nm, which reverted to 295 partially on warming to 30 °C. The absorption peaks suggest that the new species must be 295A, which is photolabile at 150 °C, presumably undergoing an elimination similar to that shown in equation 143. Thus, while in the photolysis in the presence of an acid at room temperature the n, Ł excitation with a single photon causes the direct formation of an aminium radical and subsequent reactions, that at < 150 °C must be a biphotonic process to give chemical changes.

2. Photoaddition

Owing to the high electrophilic reactivity of aminium radicals, photolysis of nitrosamines under nitrogen in an acidic solution in the presence of an olefin results in the addition of a dialkylamino group and nitric oxide across the double bond. These C-nitroso compounds may form the dimers and are generally isolated as the tautomerized oxime if it is possible. Under oxygen, the photoreaction is not quenched but diverted to the formation of the corresponding nitrates instead of C-nitroso compounds; this arises from oxidation of nitric oxide to nitrogen trioxide during the photolysis. The reaction pattern has been described previously174, and applications of this photoaddition under oxidative and non-oxidative conditions to a variety of olefins have since been reported165 168. Some examples with subsequent modifications are shown in equations 144 and 145 to demonstrate its versatility.

NMe2

H

(297)

NMe2

R R′

Me2 N NO, H+

(145)

(299)(300) R, R′ = NOH

(301)R = ONO2 , R′ = H

(302)R = OH, R′ = H

The non-oxidative photoaddition of NND to cis,trans-cyclodecadiene168 (296) and trans,trans,trans-cyclododecatriene166,168 (299) give the expected oximes 297 and 300

in 85 and 76% yields, respectively, the former as a mixture of syn- and anti-oximes, but the latter is the syn-isomer. Under oxygen, the oxidative addition gives the expected nitrate isomers 298 and 301 in high yields and small amounts of the corresponding alcohols and ketones in both cases. Upon LAH reduction, the former gives ˛- and ˇ-alcohols168 in 65 and 13% (equation 144), respectively, while the latter 301 gives the open-chain aminoalcohol 303 as the major product in addition to minor yields of 302166 (equation 145). The LAH promoted cleavage of 301 has been explained with a reasonable mechanism169.

HO

NMe2

(303)

The oxidative photoaddition of NNP to 3-butenyl chloride and bromide in the presence of perchloric acid gave 2-nitrato-5-azoniaspiro [4,5] decane perchlorate (306) in 38 and 46% yield, respectively165 (equation 146). The yield of the salt is obviously much higher, but it is difficult to extract from aqueous solution. The oxidative photoaddition to 3- butenol and its acetate gives 72 80% of the expected product 305, while that to various benzoate esters gives about 26 33% and that to p-toluenesulphonate gives no product.

ONO2

(304) X = Cl, Br

NNP

(305)

(146)

Na2 CO3

ONO2

+

N ClO4 −

(306)

Several examples of aromatic hydrocarbon sensitized additions of NNP to the same arenes were demonstrated to occur if an acid is present; this is in contrast to the failure of benzophenone to sensitize the photoreactions. Irradiation of anthracene in the presence of NNP and hydrochloric acid gives 308 in 70% yield and a small amount of 309 derived from the acid-catalysed elimination of piperidinium ion and addition of ethanol165 (equation 147). Anthracene possesses Es D 76.3 kcal mol 1, f D 0.27 ands D 5 ns, and can sensitize NNP (Es D 75 kcal mol 1) readily to its singlet excited

The crucial requirement of excited-state proton transfer (ESPT) is suggested by the failure of 1-naphthyl methyl ether to undergo self-nitrosation under similar photolysis conditions. The ESPT is further established by quenching of the photonitrosation as well as 1-naphthol fluorescence by general bases, such as water and triethylamine, with comparable quenching rate constants and quantum yield. ESPT shows the significance in relation to the requirement of acid in photolysis of nitrosamines; and acid association is a photolabile species.

Further studies of the self-nitrosation of 1-naphthol with NND reveal a high degree of stereospecificity in ESPT and the implication of at least two exciplexes in the excited state171. In dioxane, 1-NpOH and NND form ground state complexes showing max at 380 450 nm with the association constant Ka D 7. Excitation of GSC (e.g. at 370 nm) shows weak exciplex fluorescence peaking at 480 nm, but gives no self-nitrosation product 312. On the contrary, excitation of 1-NpOH at 300 nm in the presence of NND causes the self-nitrosation according to equations 149 and 151. Fluorescence of 1-NpOH is quenched by NND without showing new exciplex emission, although an exciplex 317 must be assumed to rationalize ESPT and energy transfer (Scheme 12), and to connect

with the mechanism in equation 149. The exciplex 317 from the dynamic diffusioncontrolled process and excited GSC do not interconvert to each other. The latter does not undergo ESPT, most probably owing to its geometry. It is deduced that exciplex 317 has a favourable geometry for ESPT.

B. Nitramines

Nitramines are known to photodissociate from their , Ł state to give aminyl and nitric oxide radicals; in the presence of an acid the aminyl radicals are protonated to give aminium radicals, which can initiate addition to olefins. As a synthetic reaction, photolysis of nitramines in the presence of acids can be conveniently run under oxygen to give oxidative addition similar to those shown in equation 145; indeed N-nitrodimethylamine is photolysed with triene 299 under such conditions to give a mixture of 301 and 302, similar to results observed in the oxidative nitrosamine photoaddition169. To simplify the isolation, the crude products are reduced with LAH to form the open-chain amino alcohol 303. Some other oxidative photoadditions of N-nitro dimethylamine to other olefins are reported. As the photoreaction has to use a Corex filter and product yields are no better than those shown by nitrosamines, further investigations were scarcely carried out.

C. Nitrosamides

Many intramolecular photoadditions of N-nitrosamides were published recently172. Nitrosamides are photolysed to give amidyl and nitric oxide radicals, but thermalized to undergo the diazo ester rearrangement. In contrast to intermolecular reactivities, alkenyl amidyl radicals preferentially add intramolecularly to the inside double bond rather than abstract a C-5 hydrogen atom162. An interesting entry to ˇ-lactam synthesis is the photolysis of 318 to give 319 in 59% (equation 153). Nitrosamide 320 is photolysed in CBrCl3, which also act as a radical trapping agent, to give the bromoamide 321 in 89% (equation 154).

Owing to their importance in toxicology and as carcinogens, nitroamides must be considered seriously. The study of several N-nitroso-N-acetyl amino acids has been reported