Atom transfer processes

Pulsed source techniques have been used to study thermal energy ion-molecule reactions. For most of the proton and H atom transfer reactions studied k thermal) /k 10.5 volts /cm.) is approximately unity in apparent agreement with predictions from the simple ion-induced dipole model. However, the rate constants calculated on this basis are considerably higher than the experimental rate constants indicating reaction channels other than the atom transfer process. Thus, in some cases at least, the relationship of k thermal) to k 10.5 volts/cm.) may be determined by the variation of the relative importance of the atom transfer process with ion energy rather than by the interaction potential between the ion and the neutral. For most of the condensation ion-molecule reactions studied k thermal) is considerably greater than k 10.5 volts/cm.). 

Since it thus appears that reactions other than the atom transfer process are occurring, one must consider the possibility that the low k (thermal)/ (10.5 volts/cm.) ratios may result from a variation of the relative importance of the atom transfer reaction channel with ion energy. Similarly, in some of the cases where (thermal) = (10.5 volts/cm.) the relative importance of the atom transfer process may also change with ion energy. Thus the value of k(thermal)// (10.5 volts/cm.) does not necessarily provide conclusive evidence for the interaction potential between the ion and the neutral molecules. 

The Homer - Emmons reagent (52) is effective in the one carbon homologation of ketones possessing acidic a-hydrogen atoms <96SL875> and electron-deficient alkenes add to 2-phenylseleno-l,3-dithiane in a photo-initiated heteroatom stabilised radical atom transfer process, giving products of considerable synthetic potential <96TL2743>. 

In deuterated solvent the rate of exchange was found to be lower than in aqueous media the ratio k H20) k D20) was found to be dependent on the acid concentration. Sullivan et al. have suggested a hydrogen atom transfer process and a water bridging process... 

The effects of deviations from the Born-Oppenheimer approximation (BOA) due to the interaction of the electron in the sub-barrier region with the local vibrations of the donor or the acceptor were considered for electron transfer processes in Ref. 68. It was shown that these effects are of importance for long-distance electron transfer since in this case the time when the electron is in the sub-barrier region may be long as compared to the period of the local vibration.68 A similar approach has been used in Ref. 65 to treat non-adiabatic effects in the sub-barrier region in atom transfer processes. However, nonadiabatic effects in the classically attainable region may also be of importance in atom transfer processes. In the harmonic approximation, when these effects are taken into account exactly, they manifest themselves in the noncoincidence of the... 

The first metal-catalyzed nitrogen atom-transfer process was reported by Kwart and Khan, who demonstrated that copper powder promoted the decomposition of benzenesulfonyl azide when heated in cyclohexene.280 Evans has demonstrated that Cu(i) and Cu(n) triflate and perchlorate salts are efficient catalysts for the aziridination of olefins employing TsN=IPh as the nitrene precursor.281 Subsequent to this finding, intensive effort has focused on the identification of... 

Bromides are less reactive than the corresponding iodides in atom transfer processes. However, activated bromides such as diethyl bromomalonate [36] and bromomalonitrile [53] react with olefins under Et3B/02 initiation. Kha-rasch type reactions of bromotrichloromethane with alkenes are also initiated by Et3B/02 [41]. On the other hand, a remarkable Lewis acid effect was reported by Porter. Atom-transfer reactions of an a-bromooxazolidinone amide with alkenes are strongly favored in the presence of Lewis acids such as Sc(0Tf)3 or Yb(0Tf)3, this reaction was successively applied to the... 

Some of the issues involved in trying te-un-derstand atom transfer processes are illustrated by a discussion of the observations on the reduction of perchlorate ion. No examples are known of oxygen removal from C10 under mild conditions by... 

Addition of 0- to double bonds and to aromatic systems was found to be quite slow. Simic et al. (1973) found that O- reacts with unsaturated aliphatic alcohols, especially by H-atom abstraction. As compared to O, HO reacts more rapidly (by two to three times) with the same compounds. In the case of 1,4-benzoquinone, the reaction with O consists of the hydrogen double abstraction and leads to the 2,3-dehydrobenzoquinone anion-radical (Davico et al. 1999, references therein). Christensen et al. (1973) found that 0- reacts with toluene in aqueous solution to form benzyl radical through an H-atom transfer process from the methyl group. Generally, the O anion-radical is a very strong H-atom abstractor, which can withdraw a proton even from organic dianions (Vieira et al. 1997). 

In fact, the C-H bond activation by the zirconium or tantalum hydride on 2,2-dimethylbutane can occur in three different positions (Scheme 3.5) from which only isobutane and isopentane can be obtained via a P-alkyl transfer process the formation of neopentane from these various metal-alkyl structures necessarily requires a one-carbon-atom transfer process like an a-alkyl transfer or carbene deinsertion. This one-carbon-atom process does not preclude the formation of isopentane but neopentane is largely preferred in the case of tantalum hydride. 

As a result of an interesting coincidence, the second bond dissociation energy of H2O2, D(H—O2 ) = 47 Kcal/mole happens to be of a similar magnitude as D(PH—H). This coincidence is the reason that in 4a,4b-dihydrophenanthrenes the H atom transfer process... 

Triplet carbenes have a singly occupied p orbital, as is the case for radicals, and hence react like those radicals. Hydrogen atom transfer reactions are fundamental reaction pathways of triplet carbenes. The reaction of a triplet carbene with a hydrocarbon is quite analogous to the free radical hydrogen atom transfer process (Scheme 9.6). 

Mikami has carried out a number of investigations aimed at elucidating mechanistic aspects of this Si-atom transfer process. In particular, when the aldol addition reaction was conducted with a 1 1 mixture of enoxysilanes 60 and 62, differentiated by the nature of the 0-alkyl and 0-silyl moieties, only the adducts of intramolecular silyl-group transfer 63 and 64 are obtained (Scheme 8B2.6). This observation in addition to results obtained with substituted enol silanes have led Mikami to postulate a silatropic ene-like mechanism involving a cyclic, closed transition-state structure organized around the silyl group (Scheme 8B2.6). 

Christensen and co-authors (1973) found that O reacts with toluene in aqueous solution to form benzyl radical through an H-atom-transfer process from the methyl group. In general, the O- anion radical is a very strong hydrogen atom abstractor, which can withdraw a proton even from organic dianions (Vieira et al. 1997). 

Table 4-1 compares two different reactions, namely, anode oxidation and oxidation with cerium ammonium nitrate (which are bona fide electron-transfer processes) and bromination by /V-bromosuccinimide in the presence of azobis(iso-butyro)nitrile (which is bona fide hydrogen-atom-transfer process). Both electron-transfer and hydrogen-atom-transfer processes have the benzylic radical as a common intermediate, but positional selectivity is stronger for electron-transfer processes. Another important point is the preference of the 2-positioned methyl group over the 1-positioned group, in terms of selectivity. For 1,2,3-tetramethylbenzene, such a preference reaches values from 16 to 55, and it is over 200 for 5-methoxy-1,2,3-tctramcthylbcnzcnc. 

Subsequently, the following experiments were carried out to find out whether this radical reduction occurred through hydrolysis of an organometallic intermediate or from a hydrogen-atom transfer process. 

Susuki and Tsuji reported the first Kharasch addition/carbonylation sequences to synthesize halogenated acid chlorides from olefins, carbon tetrachloride, and carbon monoxide catalyzed by [CpFe(CO)2]2 [101]. Its activity is comparable to or better than that of the corresponding molybdenum complex (see Part 1, Sect. 7). Davis and coworkers determined later that the reaction does not involve homolysis of the dimer to a metal-centered radical, which reduces the organic halide, but that radical generation occurs from the dimeric catalyst after initial dissociation of a CO ligand and subsequent SET [102]. The reaction proceeds otherwise as a typical metal-catalyzed atom transfer process (cf. Part 1, Fig. 37, Part 2, Fig. 7). 

An alternative equation, also derived by Marcus18, is from Johnston s19 BEBO calculation of activation energies. It is specifically useful for these H atom transfer processes. This is Eq. (5), with the terms as above. 

The mechanism of these transformations seems to be substrate-dependent and only the cycloisomerization of aryl and primary iodides was thought to proceed as shown in Scheme 31. The stereoselectivity of the isomerization of 110 to 111 is better accommodated with the intermediacy of l-methyl-5-hexenyl radical59. Later, it was proposed that the isomerization of 6 to 109 also proceeds via a radical-mediated atom transfer process initiated by homolytic fragmentation of an ate-complex intermediate 112 (Scheme 32)60. 

When metallo-enzymes effect the oxidation or rednction of organic snbstrates or simple molecules such as H2O, N2 or O2, they often function as multielectron donors or acceptors with two or more metals at the active The electronic conpUng between the metals is often accompanied by uniqne spectroscopic features such as electron spin spin coupling. The metal metal electronic coupling may facilitate the multi-electron-transfer reactions with the snbstrates. In simpler molecular systems, two electron-transfer processes most often reqnire snbstrate binding , as in an inner-sphere, gronp (or atom ) transfer process. ... 

Figure 14). The Keggin structure can therefore accommodate as many as 24 additional electrons in this fashion. Indeed, an additional eight electrons (32 in all ) can be accommodated in nonbonding molecular orbitals, two electrons per trimeric unit. Each bears a terminal exchangeable water ligand (thereby keeping the anion charge low), and the browns have been shown to participate in atom-transfer processes, for example, equation (5).

Miyabe et al. developed a tandem addition/cycUzation reaction featuring an unprecedented addition of alkoxycarbonyl-stabihzed radicals on oxime ethers [117], and leading to the diastereoselective formation of /1-amino-y-lactone derivatives [118,119]. The reaction proceeds smoothly in the absence of toxic tin hydride and heavy metals via a route involving a triethylborane-mediated iodine atom-transfer process (Scheme 37). Decisive points for the success of this reaction are (1) the differentiation of the two electrophilic radical acceptors (the acrylate and the aldoxime ether moieties) towards the nucleophilic alkyl radical and (2) the high reactivity of triethylborane as a trapping reagent toward a key intermediate aminyl radical 125. The presence of the bulky substituent R proved to be important not only for the... 

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