Hydrogen atom transfers

Copyright 2001 John Wiley Sons, Inc. ISBNs 0-471-35833-9 (Hardback) 0-471-22041-8 (Electronic) 

Orbital Interaction Theory of Organic Chemistry, Second Edition. Arvi Rauk 

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In a photochemical experiment, irradiation of benzene leads to Sj, which connects to the ground-state surface via the conical intersection shown. Benzene, the much more stable species, is expected to be recovered preferentially, but the prebenzvalene structure which hansfomis to benzvalene is also fomied. Another possible route from the prebenzvalene, along a different coordinate, will lead to fulvene [90, p.357] after a hydrogen-atom transfer from... 

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. 

Fig. 41. Empirical correlation between 0-0 distance, barrier height and hydrogen-atom transfer distance in OH-O fragment.

The bicyclic product is formed by coupling of the two radical sites, while the alkene results from an intramolecular hydrogen-atom transfer. These reactions can be sensitized by aromatic ketones and quenched by typical triplet quenchers and are therefore believed to proceed via triplet excited states. 

Photolysis of bicyclo[2.2.2]octan-2-one (A) gives B in good yield. When A labeled as shown is used, the aldehyde group carries deuterium to the extent of 51.7%. Write a mechanism to account for the overall transformation. Calculate the isotope effeet for the step in which hydrogen-atom transfer occurs. What mechanistic conclusion do you... 

FIGURE 24.21 A mechanism for the methylmalonyl-CoA mntase reaction. In the first step, Co is rednced to Co dne to homolytic cleavage of the Co —C bond in cobalamin. Hydrogen atom transfer from methylmalonyl-CoA yields a methylmalonyl-CoA radical that can undergo rearrangement to form a snccinyl-CoA radical. Transfer of an H atom regenerates the coenzyme and yields snccinyl-CoA. 

Vitamin E actually consists of a family of compounds, the most active of which is a-tocopherol. The mechanism of the vitamin s action is not completely certain, but it seems likely that it might undergo hydrogen atom transfer reactions with free radicals to give a stable radical (see also Chapter 17, Problem 7). 

A photoinduced hydrogen atom transfer in cw-l-(2-pyrrolyl)-2-(2-quino-line)ethene was reported (94JA3171).The rate eonstant A (5 —> 4) increases with increasing temperature from 2.1 10 s at 15.8°C to 7.7 10 s at 39.5°C, giving an aetivation energy of 9.4 keal/mol. 

It is important to emphasize that the hydroxy dithioketal cyclization can be conducted under mild reaction conditions and can be successfully applied to a variety of substrates.15 However, the utility of this method for the synthesis of didehydrooxocane-contain-ing natural products requires the diastereoselective, reductive removal of the ethylthio group. Gratifyingly, treatment of 13 with triphenyltin hydride and a catalytic amount of the radical initiator, azobisisobutyronitrile (AIBN), accomplishes a homolytic cleavage of the C-S bond and furnishes didehydrooxocane 14 in diastereo-merically pure form (95 % yield), after hydrogen atom transfer. 

Thus, for radicals 19, there is a strong preference for 1,5-hydrogen atom transfer (Table 1.5).111 Although 1,6-transfer is also observed, the preference for 1,5-hydrogen atom transfer over 1,6-transfer is substantial even where the latter pathway would afford a resonance stabilized benzylie radical.111112 No sign of 1,2-, 1,3-, 1,4-, or 1,7-transfer is seen in these cases. Similar requirements for a co-lincar transition state for homolytic substitution on sulfur and oxygen have been postulated. S,6I)... 

Other postulated mechanisms for spontaneous initiation include electron transfer followed by proton transfer to give two monoradicals, hydrogen atom transfer between a charge-transfer complex and solvent,110 and formation of a di radical from a charge-transfer complex, JJ[Pg.111]

Cases of addition-abstraction" polymerization have also been reported where propagation occurs by a mechanism involving sequential addition and intramolecular 1,5-hydrogen atom transfer steps (Section 4.4.3.4). 

There are no proven examples of 1,2-hydrogen atom shifts this can be understood in terms of the stereoelectronie requirements on the process. The same limitations are not imposed on heavier atoms (e.g. chlorine). The postulate309 that ethyl branches in reduced PVC are all derived from cbloroethyl branches formed by sequential 1,5-intramolecular hydrogen atom transfers as described for PE (Section 4,4.3.1) has been questioned.,6,6 It has been shown that many of these ethyl branches are derived from dichloroethyl groups. The latter are formed by sequential 1,2-chlorine atom shifts which follow a head addition (Section 4.3.1.2). 

In contrast to phenols (Section 5,3.4), phenothiazine (39) is reported to be an excellent scavenger of both carbon-centered and oxygen-centered radicals by hydrogen atom transfer and is also used to stabilize monomers in storage.198... 

Benzene may react by addition as shown in Scheme 6.12 (this pathway is also open to other aromatic solvents). The cyclohexadienyl radical is a poor initiating species and may terminate a second chain by hydrogen atom transfer. According to this process, benzene is a retarder rather than a transfer agent. 

In the case of S, it has been proposed that reinitiation may occur by hydrogen-atom transfer to monomer (Scheme 6.13).I2,6S... 

The vinylsilanes (e.g. 40, 41) do not readily homopolymerize. Forsyth et al.Mj explored the mechanism of grafting these monomers using dodecane as a model for PE. Their work suggests that multiple monomer units are attached through a sequence of addition and intramolecular hydrogen atom transfer steps by a mechanism analogous to that shown in Scheme 7.33 on page 394. 

To avoid these stability problems, it is necessary to minimize the proportion of chains that terminate by radical-radical reaction. One way of achieving this is to conduct the polymerization in the presence of an appropriate chain transfer agent. For example, if polymerization is performed in the presence of a H-donor chain transfer agent, conditions can be chosen such that most chains terminate by hydrogen-atom transfer. Bagby et al.iA examined the thermal stability of PMMA formed with dodecanethiol. These polymer chains will then possess, more... 

AA sec acrylic acid abstraction sec hydrogen atom transfer abstraction v,v addition and micleophilicity 35 by aikoxy radicals 34-5, 124-5, 392 by alkoxycarbonyloxy radicals 103,127-8 by alkyl radicals 34 5, 113, 116 by f-amyloxy radicals 124 by arenethiyl radicals 132 by aryl radicals 35, 118 by benzovloxy radicals 35, 53, 120, 126 wilh MM a" 53, 120 by /-butovy radicals 35, 53, 55, 124 solvent effects 54, 55. 123 with alkenes 122 3 with ally I acrylates 122 wilh AMS 120, 123 wilh BMA 53, 123 with isopropenvl acetate 121 with MA 120 with MAN 121 with MMA 53, 55, 120.419 with VAc 121 with vinyl ethers 123... 

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