Secondary amines photoreactions

The photoamination of aromatic hydrocarbons and arylalkenes by ammonia and primary aliphatic amines occurs via photoinduced electron transfer in the presence of m-dicyanobenzene (Scheme 15) [56-58]. In this photoreaction, secondary amines are less reactive than ammonia and primary amines, and the high concentration of the aminated products retards the photoreaction. This is due to the fact that the oxidation potentials of the aminated products produced by the photoreaction are usually much lower than those of the starting ammonia and primary amines. The secondary amines quench the reactive species much faster than the primary amines. Yasuda extended this photoreaction to intramolecular cyclization reactions [59],... 

The photoreactions of aliphatic amines with aromatic hydrocarbons have also been reported by several groups. With tertiary amines, deprotonation occurs from the radical cations of amines at the a-carbon to generate carbon radicals which react with the radical anion of aromatic hydrocarbons. With secondary amines, deprotonation from the radical cations of amines occurs both at the a-carbon and at the nitrogen atom, so that the reaction becomes complicated [64-65]. 

Lewis and his coworkers reported the intramolecular photoaddition of aliphatic secondary amines to arylalkenes to give 5- and 6- membered cyclic amines (Scheme 17) [66-67]. Ohashi reported that the other type of photoaddition reaction takes place when 1,4-dicyanobenzene and triethylamine are used as substrates. This photoreaction can be explained in terms of the deamination of the initially produced diethylaminoethylated compound (Scheme 18) [68]. 

The photoreactions of anthracene [4] and stilbene [5] with amines have been reported by Yang in 1973 and Lewis in 1977, respectively. In 1974, Hixson found the photoreaction of 1,2-diphenylcyclopropane with primary and secondary amines, which gave the addition products [6], The electron-transfer quenching of the excited singlet of substrates with amines results in the formation of the exciplex or radical ion pair. These reactive species undergo proton transfer from a-C-H or N-H of amines... 

Information about the relative kinetic acidities of aminium radicals has also come from a number of studies probing product distributions of SET-promoted photochemical reactions. One of the earhest investigation in this area was conducted by Cohen and co-workers, in which product distributions of SET-induced photoreactions of unsymmetrically substituted tertiary amines were used to gain information about how alkyl substituents affect the rates of aminium radical a-CH deprotonation. Irradiation of solutions of 4-benzoylbenzoic acid containing a variety of tertiary amines leads to formation of dealkylation products (Scheme 9). In these processes, the initially formed aminium radical 10 transfer a proton to the 4-benzoylbenzoic acid anion radical to generate an a-amino radical, which then donates an electron to another 4-benzoylbenzoic acid molecule. Hydrolysis of iminium ions 11, formed in this manner, then gives carbonyl and secondary amine products. The results of these studies qualitatively indicate that dealkylation occurs preferentially at the less alkyl-substituted a-carbon. 

The different polarization patterns for the reactant amine and for the vinyl-amine demonstrate the involvement of two different types of intermediates in the photoreaction of carbonyl compounds with amines. The important question is whether these intermediates are formed independently, or whether the neutral radicals are secondary intermediates formed by deprotonation of the radical ions. 

Tsujimoto et al., 1977). Pyridines and 4-cyanopyridine react with triethyl-amine, but in these cases secondary photolysis reactions do not play a dominant role, and the amines [75a,b] (Bryce-Smith et al., 1977) and [76] (Caronna et ah, 1978) are produced (Scheme 12). The cyano group is obviously effective in making the reaction regio-selective. 2,3-Dicyanopyridines suffer decyana-tion upon irradiation in the presence of tertiary amines in acetonitrile solution (Tada et al., 1980). 2-Cyanopyridine and 4-cyanopyridine also undergo photoreactions with alkenes (e.g. cyclopentene) to give adducts in which the cyano group is replaced by the alkene (Bernard et al., 1981). 

Two competing processes have been reported for the photoreaction of 2-cyanopyridine in the presence of methanol and ethanol. The products formed are the ethers 382 by attack on the pyridine ring and the hydroxyalkyl derivatives 383 by reaction at the cyano group h Reductive decyanation occurs on irradiation of the pyrazines 384 in the presence of secondary or tertiary amines. It is likely that a SET process is involved. SET processes are also involved in the reactions of 9-cyanophenanthrene in the presence of amines. Using non-polar solvents affords the diethylaminyl radical exclusively and the products 385 (principal product), 386 and 387 are obtained. In acetonitrile, however, the... 

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