Hydrogen atom transfer from initiator

Tertiary amines with an a-hydrogen are among the most effective electron donors other electron donors include alcohols, amides, amino acids, and ethers. A third process, direct hydrogen atom transfer from RH to the ketone, is not common hut does occur with some photoinitiators. The overall result is the same as the electron-transfer process. Although two radicals are produced by photolysis of the photoinitiator, only one of the radicals is typically active in initiation—the aroyl and amine radicals in Eqs. 3-48 and 3-49, respectively. The other radical may or may not initiate polymerization, hut is active in termination. The decrease in photoinitiator concentration during polymerization is referred to as photo-bleaching. 

The HCo(CO)4 complex is therefore inferred to be involved in initial hydrogen transfer to carbon monoxide. This step was initially proposed to comprise rate-determining hydrogen atom transfer from HCo(CO)4 to free CO, affording a formyl radical, HtO subsequent reaction with further HCo(CO)4 would lead to the observed products (35). However, kinetic observations (the zero-order dependence on CO partial pressure) were later made which are inconsistent with such a process (36). 

The reaction can proceed from the initial complex 1 by two pathways. 21 and 41 form intermediates 22 and 42 via the late transition states 2TS1 and 4TS1, which describe a hydrogen-atom transfer from the nitrogen to the metal atom. We were unable to... 

The FVP of angular [3]phenylene and bis(2-ethynylphenyl)ethyne has been found to produce benz[g/z]fluoranthene and chrysene,54 respectively. A non-chain stepwise radical mechanism which is initiated by hydrogen-atom transfer from the donor to azulene has been proposed55 for the mechanism of the uncatalysed transfer-... 

Both of the above reaction pathways are suppressed if 443-D and perdeuteriated solvents are used simultaneously in cyclization reaction 265. Mainly oligomer and less than 5% of alkane 450 were then produced. This has been explained by different reactivities of radical 445 formed directly from 444 or formed by abstraction of hydrogen atom from 443 through complex 453. The more energetic radicals formed by hydrogen atom transfer from 443, add intermolecularly to an alkene and initiate preferentially oligomerization512, while... 

Scheffer et al. provided another unimolecular asymmetric transformation involving the Norrish type II reaction, a well-known excited state process of ketones that is initiated by an intramolecular hydrogen atom transfer from carbon to oxygen through a six-membered transition state (Scheme 5). [19a] An adamantyl ketone derivative 27 was found to crystallize from ethanol in very large prisms in the chiral space group P2 2 2. Upon irradiation of these crystals to approximately 10% conversion, the chiral cyclobutanol derivatives 28 were afforded as the major products in 80% ee. 

A limitation of the aforementioned methods is that they are unsuitable for the use of primary alkyl iodides. Under Et3B/02 initiation conditions, the desired radical is intended to be generated by iodine atom transfer to ethyl radicals, which is not favorable in the case of primary iodides. Thus ethyl radical addition competes with the desired radical when using triethylborane initiation along with primary iodides. In addition, generating radicals by hydrogen atom transfer from ethers or acetals has limited applicability. Because of the expanded synthetic potential of primary alkyl iodides as... 

Scheffer et al. provided another unimolecular asymmetric transformation involving the Norrish type 2 reaction, a well-known excited state process of ketones that is initiated by an intramolecular hydrogen atom transfer from carbon to... 

Fhal and Renaud have examined the alkylation of a radical generated from the a-iodoimide 333 with a variety of Lewis acids, as shown in Sch. 42 [70]. The stereogenic step in this process would be hydrogen atom transfer from tin to a Lewis acid-com-plexed radical generated from 333. Initial screening was performed for the reaction of allyltributylstannane and imide 333, which was conducted by precomplexation of the imide with the Lewis acid and then addition of the stannane in the presence of AIBN under irradiation at 10 °C. The Lewis acid prepared from BINOL was ineffective whereas that prepared from the bis-sulfonamide 337 was slightly superior to that from the TADDOL ligand 339. 

Hydrogen atom transfer from the hydride form of the catalyst to monomer (eq 15) is relatively unexplored in comparison with the initial reaction in the catalytic cycle, hydrogen atom abstraction from the growing radical. This is despite the fact that the two reactions are essentially the microscopic reverse, because the substituents on the organic fragment are relatively removed from the metal center. Early investigations of CCT were frustrated by the fact that concentrations of LCoH were below detection lim-... 

Ab initio calculations performed by Bigot and Roux have led to an alternative mechanism involving two photochemical steps and the intermediacy of an azirine, as shown in Scheme 40 . Hydrogen atom transfer from the enamine nitrogen to the nitrile carbon of 173 would generate diradical intermediate 177, which would then collapse to the postulated azirine 178. In a second photochemical step, the azirine would then undergo carbon-carbon bond homolysis to 179 followed by reclosure to the imidazole 176. In support of this proposed second photochemical step, structurally similar azirines have been shown to undergo facile carbon-carbon bond photolysis . This mechanism does not, however, explain the initial formation of a thermally unstable intermediate with the observed IR stretch, since azirines of this type are thermally stable at room temperature . ... 

This complex could be formed either by hydrogen atom transfer from inactivator to flavin, or by an electron transfer from an initially formed inhibitor carbanion to flavin. Collapse of this radical pair followed by protonation could explain formation of the adduct via a partially reduced flavin reacting with a partially oxidized inactivator. 

This reaction proceeds by an initial hydrogen atom transfer from cyclohexane to Cr(Vl). The cyclohexyl radical is rapidly trapped by the chromium species via one of three pathways ... 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

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