Benzyl intramolecular electron transfer

Chemoselective anodic methoxylation at a distinct carbon atom in the a-position to an amino group in a polypeptide was achieved by prior introduction of a silyl group as an electroauxiliary at this carbon atom [156]. Amide oxidation in A-acetylpyrrolidines substituted with electron-rich phenyl rings led to either methoxylation a to the nitrogen atom or in the benzylic position. Mechanistic studies indicate that both the amide and the phenyl oxidation compete, but intramolecular electron transfer leads to... 

In the solid state, the intramolecular electron transfer to a naked dication may partly take place from the proximal hexacyanoferrate(II) anions. The extent of electron transfer at the ground state becomes large with an increase in the half-redox potential. The low photosensitivity for Bzl-V2+ in the solid state appears to be caused by the fact that the electron transfer already takes place in the ground state and/or the bulky hexaferrate (II) ion is barely brought close to the positive pyridinium nitrogens attached to the benzyl group possessing a relatively bulky molecular size. 

Other reactions involving the cleavage of a carbon heteroatom bond include a promising method for the deprotection of benzyl ethers by irradiation in the presence of acceptors (Scheme 46) [240-241] and the liberation of alkyl radicals (capable of initiating a polymerization) from alkyltriarylborate salts [242-243], The PET induced decomposition of phenyldiazomethane leads to cis-stilbene the reaction however appears to involve addition of the radical cation to a neutral molecule prior to nitrogen loss [244]. The detachment of a halogen after intramolecular electron transfer from the a C-X bond to an electron-rich... 

In the toluene radical cation the electron hole is delocalized over the re-system and the deprotonation reaction is coupled with intramolecular electron transfer from the scissible bond to the aromatic ring, leading to the benzyl radical. In the 4-nitrobenzyl chloride radical anion, the unpaired electron resides in a re orbital that does not belong to the leaving group and C-Cl bond cleavage occurs simultaneously with an intramolecular electron transfer from this orbital to the a orbital of the scissible bond. 

Neta and Behar measured the rate constants for intramolecular electron transfer for an extended series of aryl (ArX) and benzyl halide (ArCH2X) radical anions bearing an electron withdrawing ring substituent (NO2, CN, COCH3), generated by pulse radiolysis in aqueous solution (Table 12), conditions under which spectroscopic evidence for formation of the intermediate radical anions was obtained [281-284]. 

The photochemical cleavage of the phosphorus-carbon bond in many benzylic phosphonic acids occurs through intramolecular electron transfer and gives monomeric metaphosphate together with substituted toluenes and, in many cases, 1,2-diarylethanes. The ultraviolet irradiation of (4-nitrobenzyl)phosphonic acid dianion in solution in aqueous ethanol yields l,2-bis(4-nitrophenyl)ethane, 4-nitrotoluene, H3PO4 and monoethyl phosphate (Scheme Irradiation of the same acid in the presence of dbu (other tertiary... 

Intramolecular protonation on the more hindered face of a steroid from a neighbouring hydroxyl group best explains a reversal of diastereoselectivity in the Birch reduction of styrene double bonds.266 The kinetics and product distribution of lithium metal reduction of benzaldehyde to benzyl alcohol in THF have been studied electron transfer from Li to PhCHO occurs in a slow step, but absorption of the PhCHO onto the metal surface is also crucial in determining the overall kinetics. The proposed mechanism successfully accounts for the formation of minor products, benzoin and... 

These two concepts can be combined by using benzyl trialkylsilanes as electron donors in intramolecular PET-reactions. Excitation of the iminium chromophore and electron transfer leads to arene radical cations which rapidly cleave the C-Si bond to give a 1,6-biradical which combines to give the indolizidine skeleton (Scheme 36) [140]. 

In several instances, Mannich-type cyclizations can be carried out expeditiously under photochemical conditions. The photochemistry of iminium ions is dominated by pathways in which the excited state im-inium ion serves as a one-electron acceptor. The photophysical and photochemical ramifications of such single-electron transfer (SET) processes as applied to excited state iminium ions have been expertly reviewed. In short, one-electron transfer to excited state iminium ions occurs rapidly from one of several electron donors electron rich alkenes, aromatic hydrocarbons, alcohols and ethers. Alternatively, an excited state donor, usually aromatic, can transfer an electron to a ground state iminium ion to afford the same reactive intermediates. Scheme 46 adumbrates the two pathways that have found most application in intramolecular cyclizations. Simple alkenes and aromatic hydrocarbons will typically suffer addition processes (pathway A). However, alkenic and aromatic systems with allylic or benzylic groups more electrofugal than hydrogen e.g. silicon, tin) commonly undergo elimination reactions (pathway B) to generate the reactive radical pair. 

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