Hydrogen atom transfer steric effects

It was found that a variety of radical precursors could be added, and that the specific electronic and steric effects exerted on the resulting radical effected the diastereoselectivity of the hydrogen atom transfer. Increasing the size of R group appeared to increase the selectivity of the trap. For instance, reaction with t-butyl radical and tributyltin hydride gave the highest selectivity, >98 2 (70% yield), for the trans product (77). Reactions with electron-deficient radicals suffered from low yields and decreased selectivity. Results also indicate that reactions with tributyltin hydride produced higher selectivities but lower yields than those per-... 

The selectivity observed in most intramolecular functionalizations depends on the preference for a six-membered TS in the hydrogen atom transfer step. Appropriate molecules can be constructed in which steric or conformational effects dictate a preference for selective abstraction of a hydrogen that is more remote from the reactive radical. 

C now rises (from 14.9 with thiophenol) to 25.2 [20]. The steric enhancement of the isotope effect, to a magnitude giving strong evidence of tunnelling, is shown. Another possible case of steric contribution to the isotope effect in a hydrogen-atom transfer is reaction 10, where for various Ri values the tritium isotope effects are... 

The success of such reactions depends on the intramolecular hydrogen transfer being faster than hydrogen atom abstraction from the stannane reagent. In the example shown, hydrogen transfer is favored by the thermodynamic driving force of radical stabilization, by the intramolecular nature of the hydrogen transfer, and by the steric effects of the central quaternary carbon. This substitution pattern often favors intramolecular reactions as a result of conformational effects. 

In heterogeneous catalytic hydrogenations suprafacial (as) addition of hydrogen would be expected, as the transfer of hydrogen atoms from the catalyst surface to the reactant is usually assumed. However, in some Pt catalyzed reactions antarafacial (trans) addition of hydrogen is also observed. The ratio of diastereomeric products formed is determined by the chemisorption equilibrium of the surface intermediates and by the relative rates of hydrogen entrance to the different unsaturated carbon sites. Both effects are influenced by steric factors. 

In reactions with organic substrates, the 0 anion-radical is usually bad at electron transfer, but good at hydrogen atom abstraction. Because of the O spherical symmetry, its reactions are vulnerable by steric factors. Because of the O charged nature, its reactions with organic ions are of course susceptible to the effects of coulombic interaction also. Armstrong et al. (2004) showed that the predominant reactions with the anions of alanine and methylalanine proceed as follows ... 

Reaction of trialkoxyboranes with metal alcoholates, alcoholysis or hydride transfer reactions of tetrahydroborates with aldehydes or ketones all result in the formation of tetraalkoxobor-ates. Steric factors play an important role in these reactions. As a consequence, sec-alcohols react very slowly and tetra-r-alkoxoborates in general cannot be obtained by any of the reactions above. At elevated temperatures the tetraalkoxoborates revert to the trialkoxyborane and metal alkoxide.75 Thioalcoholysis of tetrahydroborates can also be effected but, in contrast to the situation in alcoholysis, the last hydrogen atom is more difficult to substitute, probably for steric reasons.119 Tetraalkoxoborates and tetramercaptoborates are readily hydrolyzed by water or moist air. 

Radicals are species with at least one unpaired electron, which, in contrast to organic anions and cations, react easily with themselves in bond-forming reactions. In the liquid phase, most of these reactions occur with diffusion-controlled rates. Radical-radical reactions can be slowed only if radicals are stabilized by electronic effects (stable radicals) or shielded by steric effects (persistent radicals). However, these effects are not strong enough to prevent diffusion-controlled recombination of, for example, benzyl radicals or tert-butyl radicals.1 Only in extreme cases are the radical or di-tert-butylmethyl radical recombination rates low.2 While the recombination rates of the triphenyl-methyl radical is reduced due to both steric and radical stabilizing effects, the steric effect alone slows the recombination of the di-/t>/-/-butyl methyl radical. Since neither of the radicals have C-H bonds (I to the radical centre, disproportionation reactions, in which the hydrogen atom is transferred, cannot occur. 

The above discussion is applied to most of the arylethenes. The importance of the single-bond rotation was argued from the view point of nonvertical excitation transfer from the low-energy sensitizer to the cw-stilbene [33-36,39, 112,113]. In the case of 8a, even for the trans Isomer the dihedral angle between the pyrenyl plane and the double bond is estimated to be about 30° due to the steric effect of the hydrogen atom at the peri position. In contrast, the dihedral... 

The most widely used method for the preparation of epoxides involves oxidation of an aUcene by a peracid, °° via a direct one-step transfer of an oxygen atom. More highly (alkyl) substituted alkenes react fastest showing that electronic effects are more important than steric effects in this reaction. Steric effects do, however, control the facial selectivity of epoxidation conversely hydrogen-bonding groups, such as OH and NH, can direct the reaction to the syn face. 

Note that the aldehyde approaches the alkene from the direction anti to the silicon atom. Therefore, when a chiral allylsilane or allylstannane with a substituent in the a-position is used, chirality transfer takes place, to generate the homoallylic alcohol with essentially no loss in enantiomeric purity. For example, reaction of the aldehyde 157 with the chiral allylsilane 158, using boron trifluoride etherate as the catalyst, gave predominantly the syn product 159 (1.151). The absolute stereochemistry can be determined by using a model in which the hydrogen atom on the a-carbon of the allylsilane eclipses the alkene (the so-called inside hydrogen effect ) in order to minimize steric interactions (1.152). 

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