Ethyl atom-transfer reaction

Triethylborane in combination with oxygen provides an efficient and useful system for iodine atom abstraction from alkyl iodide, and thus is a good initiator for iodine atom transfer reactions [13,33,34]. Indeed, the ethyl radical, issued from the reaction of triethylborane with molecular oxygen, can abstract an iodine atom from the radical precursor to produce a radical R that enters into the chain process (Scheme 13). The iodine exchange is fast and efficient when R is more stable than the ethyl radical. 

In contrast to the preceding atom-transfer reaction, the solvent-induced rate change for the reaction between l-ethyl-4-(methoxycarbonyl)pyridinyl and 4-(halomethyl)-nitrobenzenes is so large that a change in mechanism must be involved [215, 570]. In changing the solvent from 2-methyltetrahydrofuran to acetonitrile, the relative rate constant for 4-(bromomethyl)-nitrobenzene increases by a factor of up to 14800. This is of the order expected for a reaction in which an ion pair is created from a pair of neutral molecules [cf. for example, reaction (5-16)]. It has been confirmed therefore that, according to scheme (5-67), an electron-transfer process is involved [215, 570]. 

Naito has also described analogous tandem radical addition-cyclization processes under iodine atom-transfer reaction conditions [16,32], Treatment of 186 with z-PrI (30 eq.) and triethylborane (3x3 eq.) in toluene at 100 °C gave, after cleavage from the resin, the desired lactam product 190 in 69% yield (Scheme 46). Similar reactions involving cyclohexyl iodide, cyclopentyl iodide, and butyl iodide were also reported as well as the reaction with ethyl radical from triethylborane [16,32], The relative stereochemistry of the products was not discussed. 

Triethylborane in combination with oxygen provides an efficient and useful system for iodine atom abstraction from alkyl iodides and therefore is a good initiator for iodine atom transfer reactions.6 Indeed, the ethyl radical, issuing... 

The kinetics of the above reported chain transfer reactions seem to be also catalytically affected by the titanium compound present in the reaction system. In fact we have observed (Table III) that both the numbers of ethyl groups and aluminum atoms bound to the polymeric chains decrease with decreasing amount of titanium compound in the catalytic system. 

Transition State Models. The stoichiometry of aldehyde, dialkylzinc, and the DAIB auxiliary strongly affects reactivity (Scheme 9) (3). Ethylation of benzaldehyde does not occur in toluene at 0°C without added amino alcohol however, addition of 100 mol % of DAIB to diethylzinc does not cause the reaction either. Only the presence of a small amount (a few percent) of the amino alcohol accelerates the organometallic reaction efficiently to give the alkylation product in high yield. Dialkyl-zincs, upon reaction with DAIB, eliminate alkanes to generate alkylzinc alkoxides, which are unable to alkylate aldehydes. Instead, the alkylzinc alkoxides act as excellent catalysts or, more correctly, catalyst dimers (as shown below) for reaction between dialkylzincs and aldehydes. The unique dependence of the reactivity on the stoichiometry indicates that two zinc atoms per aldehyde are responsible for the alkyl transfer reaction. 

The simple addition reaction in Scheme 19 illustrates how the notation is used. Ester (1) can be dissected into synthons (2), (3) and (4). Synthons for radical precursors (pro-radicals) possess radical sites ( ) A reagent that is an appropriate radical precursor for the cyclohexyl radical, such as cyclohexyl iodide, is the actual equivalent of synthon (2). By nature, alkene acceptors have one site that reacts with a radical ( ) and one adjacent radical site ( ) that is created upon addition of a radical. Ethyl acrylate is a reagent that is equivalent to synthon (3). Atom or group donors are represented as sites that react with radicals ( ) Tributyltin hydride is a reagent equivalent of (4). In practice, such analysis will usually focus on carbon-carbon bond forming reactions and the atom transfer step may be omitted in the notation for simplicity. 

Neither R02 nor a02 will react with the pyridine ring or with the -OCH2CH3 units at measurable rates (Hendry et al., 1974 Wilkinson and Brummer, 1981). A limiting rate constant for R02 reaction with 0-CH2CH3 of 0.2 M 1 s-1 is based on values of koX for H-atom transfer by t-Bu02 from isopropyl acetate and t-butyl phenyl acetate (Hendry et al, 1974). The estimate assumes the CH groups in the 2 O-ethyl groups have a combined reactivity of (4 x 0.003) + (6 x 0.03) = 0.19 M 1 s 1. No data are available for R02 oxidation rates of the =P=S bond in model compounds. 

Toraya et al. [60-63] used B3LYP (with the 6-311G(d) basis set) for calculations on the H-atom transfer steps in diol dehydratase reaction. Both H-atom transfers, i.e., from the substrate and re-abstraction of a hydrogen atom from 5 -deoxyadenosine, were considered. The models used in these studies included the substrate, 1,2-propanediol, a potassium cation found in the active site, and an ethyl radical as a mimic of the dAdo radical (Fig. 19.1). The activation barrier for the abstraction of the pro-S hydrogen atom of substrate by dAdo was calculated to be 9.0 kcal mol while the activation barrier for the reverse reaction between product radical and 5 -deoxyadenosine was 15.7 kcal moUk In the absence of the potassium cation the forward activation barrier is 9.6 kcal moU indicating that coordination of the substrate by the potassium cation has a minimal energetic effect on the H-atom transfer step, but seems to hold the substrate and intermediates in... 

Metathetical reactions of methyl and ethyl radicals transfer of atoms other than hydrogen... 

Metathetical reactions of ethyl radicals transfer of hydrogen atoms C2HS + RH C2H6 + R... 

Figure 7.30 The experimental (shaded areas) and calculated decay rates via tunneling for normal and deuterated ethyl chloride. The reaction coordinate for HCl or DCl loss is primarily the transfer of the H (D)-atom from the terminal carbon to the Cl atom. Taken with permission from Booze et al. (1991).

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