Photoinduced amination

An interesting challenge to Vanquickenborne s model of regiospecific labilization has been presented by Muir and co-workers,746 who studied the photoinduced amine aquation from complexes of the form tra .v-[RhL4Cl2]+ where L is a heterocyclic amine (pyrazine, pyrazole, pyridine, imidazole, and substituted analogs). Unlike the ammine and ethylenediamine analogs, where chloride loss dominates, both amine and chloride are stereoretentively labilized from these com-... 

Photoinduced amination. Addition of NH3 to a styrenic double bond as well as amination of the aromatic ring are possible upon photosensitization. 

The kinetics of formation and hydrolysis of /-C H OCl have been investigated (262). The chemistry of alkyl hypochlorites, /-C H OCl in particular, has been extensively explored (247). /-Butyl hypochlorite reacts with a variety of olefins via a photoinduced radical chain process to give good yields of aUyflc chlorides (263). Steroid alcohols can be oxidized and chlorinated with /-C H OCl to give good yields of ketosteroids and chlorosteroids (264) (see Steroids). /-Butyl hypochlorite is a more satisfactory reagent than HOCl for /V-chlorination of amines (265). Sulfides are oxidized in excellent yields to sulfoxides without concomitant formation of sulfones (266). 2-Amino-1, 4-quinones are rapidly chlorinated at room temperature chlorination occurs specifically at the position adjacent to the amino group (267). Anhydropenicillin is converted almost quantitatively to its 6-methoxy derivative by /-C H OCl in methanol (268). Reaction of unsaturated hydroperoxides with /-C H OCl provides monocyclic and bicycHc chloroalkyl 1,2-dioxolanes. 

Other radical cyclization approaches to the synthesis of piperidines include a CAN-mediated stereoselective cyclization of epoxypropyl cinnamyl amines <06TL705> and a cyclization of (-trimethylsilylmethylamine radical cation, generated via a photoinduced electron transfer reaction to a tethered -functionality <06JOC8481>. 

James TD, Sandanayake KRAS, Iguchi R et al (1995) Novel saccharide-photoinduced electron-transfer sensors based on the interaction of boronic acid and amine. J Am Chem... 

Appropriate combinations of boronic acid and fluorophores lead to a remarkable class of fluorescent sensors of saccharides (Shinkai et ah, 1997, 2000, 2001). The concept of PET (photoinduced electron transfer) sensors (see Section 10.2.2.5 and Figure 10.7) has been introduced successfully as follows a boronic acid moiety is combined intramolecularly with an aminomethylfluorophore consequently, PET from the amine to the fluorophore causes fluorescence quenching of the latter. In the presence of a bound saccharide, the interaction between boronic acid and amine is intensified, which inhibits the PET process (Figure 10.42). S-l is an outstanding example of a selective sensor for glucose based on this concept (see Box 10.4). 

The photooxidation of tertiary methylamines sensitized by electron acceptors such as 9,10-dicyanoanthracene in the presence of lithium perchlorate results in demethylation thus tropinone yields nortropinone257. Photoinduced cyanation of tertiary amines with oxygen, a sensitizer and trimethylsilyl cyanide results in a-cyano nitriles (equation 88)258. 

The importance of tertiary amines in the photochemically induced electron transfer reactions has also been addressed5. Direct irradiation of aromatic or aliphatic amines often leads to the scission of C—N, N—H or C—H bonds that lead to the subsequent chemical reactions by radical pathways6. In this section, photochemical reactions of amines reported since 1978 will be considered with emphasis on photoinduced electron transfer. Photochemical reactions of inorganic and organometallic compounds will not be included unless photochemistry of amine moieties is the primary interest. 

Photoinduced intramolecular interaction of t-S and tertiary amine moieties linked with a polymethylene chain has also been studied24. The photoexcitation of fraws-stilbene in which a tertiary amine is attached to the ortho position with a (CH2)i-3 linker leads to fluorescent exciplexes by intramolecular electron transfer, and results in no more than trans-cis isomerization. The failure to give adducts from the intramolecular exciplexes could arise from the unfavourable exciplex geometry to undergo the necessary bond formation. 

Grelier etal. (1997) synthesized a variety of UV stabilizers based upon hydroxy-phenylbenzotriazoles incorporating an isocyanate functionality. These were reacted with wood using microwave energy. The modified woods were exposed to UV irradiation and the photoinduced discolouration determined. It was found that the best protection was obtained when the UV stabilizer was used in conjunction with polyethylene glycol, or hindered amine light stabilizers. 

Photoinduced electron transfer promoted cyclization reactions of a-silyl-methyl amines have been described by two groups. The group of Pandey cyclized amines of type 135 obtaining pyrrolidines and piperidines 139 in high yields [148]. The cyclization of the a-silylated amine 140 leads to a 1 1 mixture of the isomers 141 and 142 [149]. The absence of diastereoselectivity in comparison to analogous 3-substituted-5-hexenyl radical carbocyclization stereochemistry [9] supports the notion that a reaction pathway via a free radical is unlikely in this photocyclization. The proposed mechanism involves delocalized a-silylmethyl amine radical cations as reactive intermediates. For stereochemical purposes, Pandey has investigated the cyclization reaction of 143, yielding... 

Highly efficient and stereoselective addition of tertiary amines to electron-deficient alkenes is used by Pete et al. for the synthesis of necine bases [26,27], The photoinduced electron transfer of tertiary amines like Af-methylpyrrolidine to aromatic ketone sensitizers yield regiospecifically only one of the possible radical species which then adds diastereospecifically to (5I )-5-menthyloxy-2-(5//)-furanone as an electron-poor alkene. For the synthesis of pyrrazolidine alkaloids in approximately 30% overall yield, the group uses a second PET step for the oxidative demethylation of the pyrrolidine. The resulting secondary amine react spontaneously to the lactam by intramolecular aminolysis of the lactone (Scheme 20) [26,27]. 

The oxidative behaviour of the acridinium carbocations 61 was also explored by the group of Lacour in the photoinduced electron transfer reaction [160]. In the amount of 2 mol%, the achiral hindered acridinium salt 61 catalyzed the aerobic photooxidation of the primary benzylic amine to benzylimine in the yield of 74% (Scheme 63). 

G. Pandey, Synlett 1992, 546-552 . .Synthetic Perspectives of Photoinduced Electron Transfer Generated Amine Radical Cations". 

Another aspect of supramolecular photochemistry concerns the use of covalently linked bichromophoric and multichromophoric molecules. In the example of Figure 8.19 a two-step sequential, photoinduced electron transfer is observed in the trichromophoric species D-D-A. Local excitation of the cyanonaphthalene acceptor results in charge separation first to the amine and then to the methoxyaniline donor in times of a few tens of ns... 

Enhancement of fluorescence due to the complexation of metal ions with fluoroionophores has been used as a well-precedented technique to analyze for the presence of metal ions [189-191], A number of studies have reported chelating fluorophores whose emission spectra change upon the addition of metal ions [192-198]. One remarkable result of this emission intensity enhancement is shown in Scheme 23, where the chelation of zinc chloride to 9,10-bis(((2-(dimethylamino)ethyl)methylamino)methyl)anthracene drastically enhances the observed fluorescence by a factor greater than 1000-fold [199], In the absence of Zn2+, the singlet excited state of anthracene moiety is strongly quenched by intramolecular photoinduced electron transfer from the amine to the anthracene moiety. The complex formation of Zn2+ with the amine moiety may result in the largely positive shift of the one-electron oxidation potential. Thus, intramolecular photoinduced electron transfer is strongly suppressed by the complexation of the amine moiety with Zn2+,... 

Photoinduced electron-transfer reaction of aromatic compounds with amines is one of the most fundamental reactions in the electron-donor-acceptor systems, which was recently reviewed by Lewis [35], Because of the low oxidation potentials of the amines, the photoinduced one-electron transfer from the amines to the excited singlet states of aromatic hydrocarbons ( Aril ) readily occurs to give the radical cations of amines and the radical anions of aromatic compounds even in the less polar solvents. 

The fluorescence quenching experiments of aromatic hydrocarbons by tertiary amines, including /V,/V-dialkylanilincs, in less polar solvents show the typical exciplex emissions [382-384], but products are not obtained or inefficiently produced. On the other hand, in polar solvents such as acetonitrile or methanol, the photoinduced electron transfer from the amines to Aril efficiently occurs to give the addition products. Interestingly, some primary and secondary aliphatic and aromatic amines caused the photoinduced electron transfer even in nonpolar solvents. 

The 1,4-photoaddition of aliphatic amines with benzene via photoinduced electron transfer was first reported by Bryce-Smith more than 30 years ago [375-378], In the photoreaction of triethylamine with benzene, the proton transfer from the radical cation of triethylamine to the radical anion of benzene is proposed as a probable pathway (Scheme 113). In the case of tertiary amines, the photoaddition is accelerated by the addition of methanol or acetic acid as a proton source. Similar photoaddition of diethyl ether to benzene takes place assisted by trifluoroacetic acid, where methanol is not affective [379], In these photoreactions, a-hydrogen next to the heteroatom moves to the radical anion of benzene as a proton, followed by radical ccoupling to give 1,4-addition products. Similar photoaddition of amines to the benzene ring has been reported by Ohashi et al. [380,381],... 

The radical anions of electron-deficient aromatic compounds and aromatic hydrocarbons, which are generated by photoinduced electron transfer, can be proto-nated by protic solvents or by the radical cations of amines to produce their neutral radicals [Eq. (7)]. Dispropotionation of the radicals yields the reduction products. The radical anions of 1,1-diphenylethene in the presence of 1,4-dimethoxynaphthalene as an electron donor is also protonated by protic solvents to give Markownikoff adducts [402,403] (Scheme 118). 

Photoinduced carboxylation of aromatic hydrocarbons via their radical anions in the presence of tertiary amines has been reported by Tazuke and Ozawa [404], Similar photofixation of carbon dioxide on styrene has been reported Toki et al. [405], In these photoreactions, the anionic part of the radical anions are trapped by electrophiles such as proton and carbon dioxide (Scheme 119). 

The photochemical reaction of C6o with triethylamine leads in deoxy-genated toluene to the formation of 29 (Scheme 5). The reaction is initiated by photoinduced electron transfer between 1C60 and the amine (ground-state CT-complex). As a model compound for intramolecular electron transfer compound... 

The radical anion Cw, can also be easily obtained by photoinduced electron transfer from various strong electron donors such as tertiary amines, fer-rocenes, tetrathiafulvalenes, thiophenes, etc. In homogeneous systems back-electron transfer to the reactant pair plays a dominant role resulting in a extremely short lifetime of Qo. In these cases no net formation of Qo is observed. These problems were circumvented by Fukuzumi et al. by using NADH analogues as electron donors [154,155], In these cases selective one-electron reduction of C6o to Qo takes place by the irradiation of C6o with a Xe lamp (X > 540 nm) in a deaerated benzonitrile solution upon the addition of 1-benzyl-1,4-dihydronicoti-namide (BNAH) or the corresponding dimer [(BNA)2] (Scheme 15) [154], The formation of C60 is confirmed by the observation of the absorption band at 1080 nm in the near infrared (NIR) spectrum assigned to the fullerene radical cation. 

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