Sterically Hindered Phenyl Radicals

On interrupting the photolysis, IJj decayed with first-order kinetics and with rate constants from -30 to -90 °C that were independent of the concentrations of IBr, ditin and cyclopropane [3]. The product, also identified by EPR, was the 3,5-di-tert-butylneophyl radical, 2, that itself decayed with second-order kinetics. 

Reaction (28.3) was found to have a surprisingly low Arrhenius pre-exponential factor (log (Ah/s 1) = 5.3). l Br was synthesized in which the three tert-butyl groups had been essentially fully deuterated ( H content 99%) [4]. Under similar conditions in the EPR an even more persistent IJ, radical was obtained. This also decayed with first-order kinetics and yielded kf/kf 50 at -30 °C. It was thought probable that reaction (28.3) would provide one of the first clear and unequivocal examples of QMT in an H-atom transfer. This reaction and related reactions were therefore examined in considerable detail [4, 5]. 

The decay of IJj generated from InBr with Me3Sn (reaction (28.2)) and by the photolysis of 3 (reaction (28.5)) occurred with clean first-order kinetics and at 

2S Quantum Mechanical Tunneling of Hydrogen Atoms in Some Simple Chemical Systems 

for the classical rupture of a C-H/C-D bond, i.e., in the absence of significant QMT, the maximum possible DKIEs (kn/kj)) are, for example, 17, 53 and 260 at 243, 173 and 123 K, respectively. The experimentally measured DKIEs for the 1 — 2 isomerization were always much larger than these calculated values, viz., 80, 1400 and 13000 at 243, 173 and 123 K. Admittedly, in the case ofthe IJ, isomerization there will be a small additional contribution to the DKIE from secondary DKIEs but these are unlikely to be greater than 2 at 243 K and 6 at 123 K [4]. 

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The much less sterically hindered phenyl radicals 6J, 7J, and 8Jj were generated... 

Variable results were obtained when the enolates of more or less sterically hindered acetophenone derivatives were treated with diphenyliodonium salts in alcoholic solvents. Phenylation was observed together with side reactions such as polymerisation of free radical species. 9... 

It has been demonstrated that the nitroxyl radical, X, reacts with a secondary alkyl radical to form XI which, under high-temperature conditions (>120°C), regenerates the original diphenylamine molecule, Reaction (4.36). In essence, this group of stabilisers acts catalytically by scavenging alternately peroxy (ROO-) and alkyl radicals (R-). As stated earlier, sterically hindered phenols deactivate only two peroxy radicals per phenol molecule. Hence, under high-temperature conditions, aromatic amines are far superior to their phenolic counterparts. As shown in Table 4.3, the stoichiometric factor of the diphenylamines depends on the substituents in the para position [33]. The efficacy of the diphenylamine antioxidant is improved by alkylating the para positions. The stabilisation mechanism for phenyl-a-naphthylamines. Reaction sequence (4.37) [34], is described as follows ... 

Exceptional products of dimerization are formed by the sterically hindered radical-anions. For example, protonation of the dimeric dianions of l-Phenyl-2-t-butyl acetylene yielded the following hydrocarbons100)... 

Head-to-tail addition is favored for steric reasons because the propagating site preferentially attacks the less sterically hindered unsubstituted sp carbon of the alkene. Groups that stabilize radicals also favor head-to-tail addition. For example, when Z is a phenyl substituent, the benzene ring stabilizes the radical by electron delocalization, so the propagating site is the carbon that bears the phenyl substituent. 

From elementary organic chemistry, we know that the positions and hence reactivities of the electrons in unsaturated molecules are influenced by the nature, number, and spatial arrangement of the substituents on the double bond. As a result of these influences, the double bond reacts well with a free radical for compounds of the types CHj = CHY and CHj = CXY. These compounds constitute the so-called vinyl monomers where X and Y may be halogen, ally l, ester, phenyl, or other groups. It must, however, be noted that not all vinyl monomers produce high polymers. In symmetrically disubstituted double bonds (e.g., 1,2 disubstituted ethylenes) and sterically hindered compounds of the type CHj = CXY, polymerization, if it occurs at all, proceeds slowly. 

The stabilizers are used to perform specific functions. Thus, there are antioxidants, viz. sterically hindered phenols [e.g., pentaerythritol ester] and/or sterically hindered amines (hydro)-peroxide decomposers, viz. phosphite [e.g., tris(2,4-di-tert-butyl phenyl)-phosphite] radical scavengers such as thio-derivatives heat stabilizers [e.g., calcium-zinc type for PVCj light stabilizers and UV blockers [e.g. aluminum flakes or carbon black] for outdoor storage and applications, etc. 

On the basis of many spectroscopic measurements, the species was characterized to be an oxo-ferryl porphyrin K-cation radical (1), identical to compound I of peroxidases and catalases. Because of its high reactivity, introduction of sterically hindered groups at the ortho-positions of the phenyl rings are required to observe 1 as relatively stable species at low temperature. Alternatively, Nakamoto has reported the formation of compound I of Fe(OEP) and Fe(TPP) by laser irradiation to oxy forms of the complexes in oxygen matrices at 30 K [51, 52]. Because of the formation of compound I in the matrices, characterization of less sterically hindered porphyrin complexes can be made. 

The use of Et3B as a radical initiator makes it possible to carry out the addition of other alkyl radicals to nitrone (286) using alkyl iodides. Good yields have been obtained of products (288b-d) when an excess of the appropriate alkyl iodide was used (Scheme 2.110). It has been established that the yield of alkyl by-products (288a) tends to decrease with the increase of the reaction temperature. The stereochemical features of this reaction are explained by the alkyl radical addition taking place predominantly from the less hindered re-face of (286) to avoid steric interaction with the phenyl group (525). 

Yet it is much more stable than either. This must be because the central carbon, which bears most of the radical character, is sterically shielded by the twisted phenyl groups, making it very hard for the molecule to react. And when it does dimerize, we know that it does so through one of its least hindered carbon atoms. 

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