Photochemically Induced Electron Transfer

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. 

A. Photochemically-induced Electron Transfer Involving Organosilicon... 

The majority of publications on intramolecular long-range electron transfer in bridge molecules deals with dark secondary processes of radiation induced or photochemically induced electron transfer. Intramolecular electron transfer arises in this case from electron implantation to the donor fragments of the molecule. 

J. S. Bader and D. Chandler, Computer simulation of photochemically induced electron transfer, Chem. Phys. Lett., 157 (1989) 501—4. 

Cyclopentanols 214 and 217 are synthesized by intramolecular ketyl radical addition to tethered olefin by a photochemically induced electron-transfer activation of 8,co-unsaturated ketones 213 and 216, respectively, using either hexamethyl-phosphoric triamide (HMPA) or triethylamine in CH3CN [98] as solvent (see Scheme 8.59). 

Jonas, M., Blechert, S., and Steckhan, E. (2001) Photochemically induced electron transfer (PET) catalyzed radical cyclization A practical method for inducing structural changes in peptides by formation of cyclic amino acid derivatives. Journal of Organic Chemistry, 66, 6896-6904. 

Bellelli A, Branori M, Brzezinski P, Wilson MT. Photochemically induced electron transfer. Methods 2001 24 139-52. 

The photosensitized electron transfer reaction forms the reduced lipophilic electron acceptor BNA which is ejected into the continuous organic phase and thus separated from the oxidized product. In order to monitor the entire phase transfer of the reduced acceptor, BNA, a secondary electron acceptor, p-dlmethyl-amlnoazobenzene (dye),was solubilized in the continuous oil phase. The photochemically induced electron transfer reaction in this system results in the reduction of the dye (0 = 1.3 x 10 3). Exclusion of the sensitizer or EDTA or the primary electron acceptor, BNA, from the system resulted in no detectable reaction. Substitution of the primary acceptor with a water soluble derivative, N-propylsulfonate nicotinamide, similarly results in no reduction of the dye. These results indicate that to accomplish the cycle formulated in Figure 6A the amphiphilic nature of the primary electron acceptor and its phase transfer ability in the reduced form are necessary requirements. 

Electron transfer to or from a conjugated tr-system can also induce pericyclic reactions leading to skeletal rearrangements. A typical example is the Diels-Alder cycloaddition occurring after radical-cation formation from either the diene or the dienophile [295-297], The radical cation formation is in most cases achieved via photochemically induced electron transfer to an acceptor. The main structural aspect is that the cycloaddition product (s Scheme 9) contains a smaller n-system which is less efficient in charge stabilization than the starting material. Also, the original radical cations can enter uncontrollable polymerization reactions next to the desired cycloaddition, which feature limits the preparative scope of radical-type cycloaddition. 

Coloured ion-pair associates have been observed between [Mo(CN)s] and the metal ions Fe, Cu, U02, and VO. The long-wavelength absorption band is attributed to an intervalence CT transition, and from the solvent dependence of this band the interionic distances have been estimated. A Mo e.s.r. signal has been observed in frozen aqueous glycerol solutions at 93 K and has been ascribed to the photochemically induced electron transfer ... 

Tandem fragmentation-cyclization of bicyclic ketones connected to unsaturated side chains have been used to construct hi-, tri-, and spirocyclic ketones [334], Cossy et al. have shown that depending upon the substitution pattern, ketyl radical anions obtained from photochemically induced electron transfer from amines to cyclopropylketones lead either to the formation of 3-substituted cycloalkanones or to ring expanded products (Scheme 78) [335],... 

Photochemically induced electron transfer reactions of carbyne complexes were also reported by McElwee-White and co-workers [Eq. (58)]... 

The first fluorescence and fluorescence quantum yields from the excited state of the radical anion of 1,4-benzoquinone have been reported by Cook et al., and electron transfer from aromatic donors to the S state of chloranil giving the singlet radical ion pair has been observed on the fs/ps time scale (Hubig et al.). A series of porphyrin quinones having variable acceptor strengths of the quinone moiety has been synthesised by Dieks et al. and may be considered to be well-suited as biomimetic model compounds for studying photochemically-induced electron transfer in photosynthesis. 

Photochemically-induced electron transfer from N-methylphenothiazine in anionic micelles to cupric ions occurs in the Stern layer. This process is complete within the 15 ns duration of the 347 nm ruby laser excitation. The primary photochemical event is formation of the Tj state of N-methylphenothiazine and both this and the subsequent electron transfer occur with unit quantum yield completely suppressing competitive photochemical pathways. Added Co and Ni deactivate the triplet state. A related system in which Eu " replaced Cu was examined, and electron transfer was again observed. In this case back reaction occurs in the ground state and intermicellar events may be kinetically distinguished from the much faster intramicellar events. ... 

The photochemical dimerization of p-methoxystyrene does not occur in non-polar solvents or in the absence of electron acceptors. This observation has been rationalized in terms of a reaction path in which the first step is photochemically induced electron transfer to give the styrene cation radical. This species reacts with a ground-state partner to give the cyclobutane cation radical which is then neutralized. 

X. A. Photochemically-Induced Electron Transfer Involving Organosilicon Compounds... 

Computer simulations on a photochemically induced electron transfer problem in aqueous solution provide a similar decay of the energy autocorrelation function. The question arises what is the nature of the very fast decay (less than 0.2ps) of the correlation function in the different systems In real systems one contribution to an instantaneous... 

Cossy, J., Aclinou, R, Bellosta, V., Furet, N., Baranne-Lafont, J., Sparfel, D., and Souchaud, C., Radical anion ring opening reactions via photochemically induced electron transfer. Tetrahedron... 

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