Benzyl radical substituent effects

A good correlation with ordinary Hammett a values was based on 16 well-behaved substituents, and p-SOMe conformed well to this. Various other substituents showed deviations which were attributed to enhanced + R effects. These included p-SPh and this was explained in terms of 7t(pd) bonding, which was thus taken to play no part in the effect of p-SOMe on the methyl hyperfine splitting. More recently several 4-substituted benzyl radicals of the type RSO C6H4CH2 (n — 0,1 or 2 R = Me, Ph, Tol, COMe or OMe) have been examined by ESR spectroscopy249. The ability to delocalize spin density onto the substituent decreases in general as n increases and the effect of R depends on the oxidation state of sulfur. These authors have devised a new scale of substituent effects (sigma dot... 

Arnold s o -scaIe (Arnold, 1986 Dust and Arnold, 1983 Wayner and Arnold, 1984), which reflects the spin-density distribution in meta- and para-substituted benzylic radicals and leads to parameters similar to those derived from other sources (Jackson, 1986), provide a good example. Arnold s scheme is based on the hypothesis that hyperfine coupling constants of the a-hydrogen in substituted benzylic radicals give information regarding the effect of substituents on the distribution of spin throughout the radical and that this is connected with the stability of the benzylic radical. 

Benzylic radicals offer themselves to a similar analysis. Some barriers to rotation have been determined (Conradi et ai, 1979). The barrier to rotation of 9.8 + 0.8 kcal mol for the a-cyano-a-methoxybenzyl radical [21] (Korth et al., 1985) could not be interpreted rigorously in terms of a captodative effect because estimates had to be made for the effect of a single captor or donor substituent on the rotational barrier. Within these limitations the barrier does not reflect more than an additive substituent effect. 

TABLE 1. Substituent effect for abstraction of benzylic hydrogen by radicals... 

The syntheses of iron isonitrile complexes and the reactions of these complexes are reviewed. Nucleophilic reagents polymerize iron isonitrile complexes, displace the isonitrile ligand from the complex, or are alkylated by the complexes. Nitration, sulfonation, alkylation, and bromina-tion of the aromatic rings in a benzyl isonitrile complex are very rapid and the substituent is introduced mainly in the para position. The cyano group in cyanopentakis(benzyl isonitrile)-iron(ll) bromide exhibits a weak "trans" effect-With formaldehyde in sulfuric acid, benzyl isonitrile complexes yield polymeric compositions. One such composition contains an ethane linkage, suggesting dimerization of the transitory benzyl radicals. Measurements of the conductivities of benzyl isonitrile iron complexes indicate a wide range of A f (1.26 e.v.) and o-o (1023 ohm-1 cm.—1) but no definite relationship between the reactivities of these complexes and their conductivities. 

It is well known that kH is similar for all alkyl-substituted radicals but rate constants for reaction of tin hydride with carbonyl-substituted radicals are not known. Substituents can effect the rate constant for hydrogen transfer. For example, the benzyl radical is about 50 times less reactive than a primary alkyl radical. 

A study of the photo-sensitized ring-opening reactions of the radical cations (76) of arylcyclopropanes (75) with methanol, water, and cyanide nucleophiles suggests a three-electron 5k2 mechanism (Scheme 11).185 The isolated products are methyl propyl ethers, derived from nucleophilic attack of methanol on the radical cation (76). They were detected by UV-VIS spectroscopy and shown to react with nucleophiles by transient kinetic methods. The benzyl radical (77) reacts with the DCB radical anion to afford monoaromatic ether (78) by oxidation and protonation or the disubstituted ether (79) by addition of DCB. Regio- and stereo-selectivity of the substitution were complete regiochemistry and rate constant were profoundly effected by the electronic nature of the aryl substituents.186 Elsewhere, a combined ab initio and CIDNP study... 

The photochemically generated cyclopentane-1,3-diyl diradials (87) were part of a study of spin delocalization through the EPR Z)-paramctcr. These biradicals were a model system for cumyl and benzyl radicals and experimental data were combined with MO calculations to map the electronic effects on D by varying the aromatic substituent (Ar = heterocycle).218 This parameter was also measured for a related series of... 

Merenyi R, Janousek Z, Viehe HG (1986) Studies on the captodative effect. Entropy/enthalpy compensation as solvent effect in radical forming reactions. A relative radical stabilisation scale. In Viehe HG (ed) Substituent effects in radical chemistry. Reidel, Dordrecht, pp 301-324 Merga G, Schuchmann H-P, Rao BSM, von Sonntag C (1996) The oxidation of benzyl radicals by Fe(CN)63. J Chem Soc Perkin Trans 2 551-556... 

Substituent effects have been reported previously and work on this topic continues. The half wave potentials of a series of substituted benzyl chlorides and bromides gave excellent correlations with Hammett o- substituent constants48. The positive p values from these Hammett LFERs (p = 5.0 and 2.8, respectively for chlorides and bromides) suggest that the potential-determining electrochemical process involves the formation of radical anion intermediates. 

As a next step in this analysis we investigated 18) a series of 1,2-diphenyl tetraalkyl-ethanes 27 which generate resonance stabilized tertiary benzyl radicals 28 at elevated temperatures (Fig. 3). Having worked out a method for analysis of the steric effect we hoped to succeed also in quantitatively separating it from the resonance effect of substituents. It is immediately recognized from Fig. 3 and the related correlation Eq. (9 and 10) that thermolysis occurs at much lower temperatures (100 200 °C) and with much lower activation enthalpies than in the aliphatic Cq—Cq series 11. 

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