Ketones photoinduced electron-transfer

Intensive studies concerning the photoreductive cyclization of distinct ketones and aldehydes are made by Cossy et al. [170], They describe how bicyclic tertiary cycloalkanols 173 and 174 can be prepared from, s-un-saturated ketones 172 in good yields, initiated by photoinduced electron transfer from triethylamine in acetonitrile or by photoionization in pure hexamethyl-phosphoric triamide (HMPA) [171, 172], The reaction is stereo-, chemo- and... 

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]. 

In addition to the former example, Pandey et al. achieved efficient a-aryla-tion of ketones by the reaction of silyl enol ethers with arene radical cations generated by photoinduced electron transfer from 1,4-dicyanonaphthalene. Using this strategy various five-, six-, seven-, and eight-membered benzannulated compounds are accessible in yields in the range 60-70% [39],... 

Photoinduced electron transfer (PET Scheme 6.2) is a mild and versatile method to generate radical ion pairs in solution," exploiting the substantially enhanced oxidizing or reducing power of acceptors or donors upon photoexcitation. The excited state can be quenched by electron transfer (Eq. 7) before (aromatic hydrocarbons) or after intersystem crossing to the triplet state (ketones, quinones). The resulting radical ion pairs have limited lifetimes they readily undergo intersystem ... 

In 1986, Belotti, Pete and Portella reported that intramolecular ketone/olefin coupling could be achieved via photoinduced electron transfer (irradiation of an aliphatic ketone in HMPA)78. Several examples of this chemistry are highlighted below (Scheme 26). Intramolecular additions to C=C and allenes were also reported with yields in the range 70-80% however, additions to C=N were unsuccessful. 

Studies of the same system in ethanol [22] showed that this solvent favoured the formation of the CIP (A, = 690 nm), evolving already 25 ps after photolysis. For this to occur, the amine and the ketone must be cosolvated in the ground state. Indeed, decreasing stretching frequencies of the benzophenone carbonyl by addition of amine indicates ground state complex formation prior to photoinduced electron transfer (cf. Scheme 2). However, 50 to 300 ps after the laser flash, a new... 

The synthetic value of the photoinduced electron-transfer reactions of ketones has been expanded (Scheme 8.62) by generating a five-membered carbocyclic framework, as shown in the preparation of compound 226. 

Photoinduced electron transfer from amines to the excited ketones generates the radical cations of amines and the radical anions of ketones [140]. The radical anions of ketones are usually reduced to the corresponding alcohols. However, in some cases the coupling reaction between the radical anions and... 

Picosecond and nanosecond transient spectroscopic studies to elucidate the nature of various intermediates formed in the photoinduced electron-transfer reaction of amines have been most extensive for the ketone-amine systems. These studies have been described in detail in a recent review by Yoon et al. [10]. Some aspects of these studies are briefly described here. In the earlier studies of Cohen and coworkers, the photochemical reactions of benzophenone with aliphatic amines were probed by fluorescence quenching, determination of product quantum yields, and nanosecond laser-flash photolysis [143-147]. They proposed that the reactions of amines with... 

Cossy and coworkers have reported the stereoselective synthesis of several bicyclic pentanols in good yields, by the intramolecular addition of the ketyl radicals of S,e-unsaturated ketones, generated via photoinduced electron transfer, using triethyl-amine (TEA) or hexamethylphosphonic triamide (HMPA), as donors in acetonitrile... 

A similar electron transfer mechanism has been proposed for photosensitized electron transfer catalysis of the Mukaiyama-aldol reaction of aldehydes and ketones with enol silanes [301], Photoinduced electron transfer from enol silanes to a monocationic -bridged porphyrin [302, 303] leads to the production of a... 

Intramolecular photoinduced electron transfer in (58) leads to decarboxylation to give hydroxylactams in good yield (Griesbeck et al.), and irradiation of either the dextro or /eavo-rotatory crystals from the self-assembly of acridine and diphenyl acetic acid molecules, is reported to induce a decarboxylating condensation to give (+) or (—)-(59) with ees in the 35% region (Koshima et al,). The formation of ketones from the photolysis of nitrites is a low-yield process in... 

Topics that have formed the subjects of reviews this year include contemporary issues in electron transport research, dynamics of bimolecular photoelectron transfer reactions, photophysical properties of functionalised fullerene derivatives, carbon-carbon bond formation via radical ions, photoinduced electron transfer processes in ketone, aldehyde, and ester synthesis, photochemical reactions between arenenitriles and benzylic donors, photo-oxidation of conjugated dienes, photoredox reactions of aromatic nitro compounds, electron transfer-mediated photochemistry of some unsaturated nitrogen-containing compounds, reactions of 02( Ag), carbon dioxide activation by aza-macrocyclic complexes, and photochromism of chalcone derivatives. ... 

Cossy, J., Belotti, D., Generation of Ketyl Radical Anions by Photoinduced Electron Transfer (PET) between Ketones and Amines. Synthetic Applications, Tetrahedron 2006, 62, 6459 6470. 

The photocycloaddition of L-ascorbic acid derivatives (e.g., 93) with 4-chlorobenzaldehyde (94) and benzyl methyl ketone led to preferential attack on the less hindered a-face of the enone with approximately 2 1 regioselectivity (33% de for 96) (see Scheme 22) [147]. When the substrate was changed to benzophenone, the regioselectivity was reversed, even though the facial selectivity remained the same (35% de). This was proposed to be the result of a mechanistic switchover, from a 1,4 diradical process for benzophenone to a photoinduced electron transfer process for the other substrates. 

Alkylating agents. These selenides, activated through photoinduced electron transfer, react with enol silyl ethers, forming a-alkylated ketones. 

APPLICATIONS OF PHOTOINDUCED ELECTRON TRANSFER PROCESSES TO KETONE, ALDEHYDE, AND ESTER DERIVATIVES IN ORGANIC SYNTHESIS... 

A typical example of ketone-olefin photoreaction involving a photoinduced electron transfer is represented by the system comprised of biacetyl and 1,1-diethoxyethene, where one simple isomeric oxetane 40 is obtained (Scheme 20). This can be explained by polarity reversal of the reactivity of the ketone through PET. ... 

Scheme 8.24 Photoinduced electron transfer from ketone 56 to the iridium complex occurs coti-certedly with protonation by the chiral phosphate. In the process, the ketyl radical becomes significantly more basic than phosphoric acid, and rapid radical cyciization from the neutral ketyl as an H-bonded adduct to the chiral phosphate occurs. Asymmetric induction results, due to the rapid radical cyciization. Hantsch dihydropyridine esters provide the required proton and hydrogen atoms...

Akaba, R., Niimura, Y, Fukushima, T., Kawai, Y, Tajima, T., Kuragami, X, Negishi, A., Kamata, M., Sakuragi, H., and Tokumaru, K., Photoinduced electron transfer carbon-carbon bond cleavage of radical cations of carbonyl compounds in solution. 2,4,6-TriphenylpyryHum salt-sensitized oxygenation of aralkyl ketones and aldehydes, /. Am. Chem. Soc., 114, 4460,1992. 

Photoinduced Electron Transfer Reactions of Oxiranes and Epoxy Ketones... 

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