Phenylmethyl radical

Fig. 3. The five unexcited canonical structures contributing to the normal state of the phenylmethyl radical.

Exercise 6-11 Draw valence-bond structures for the phenylmethyl radical, C6H5CH2-,... 

Exercise 14-6 a. Write the initiation and propagation steps involved in the radical bromination of methylbenzene (toluene) with bromine. Write the low-energy valence-bond structures of the intermediate phenylmethyl radical. 

Different behavior is observed for the stable allyl radical, bis-(9-fluorenyl)-phenylmethyl radical (225) [139] (Scheme 19). Photolysis of this radical in poor H atom donating solvents is suggested to lead to the intermediate (226) which subsequently reacts with molecular oxygen yielding a variety of cleavage and oxygenation products. The excited radical lifetime in this case is less than 30 ps. 

The novel diradicaloid hydrocarbon 23, which may be prepared50 in a similar manner as 21, may exist as two triply bridged but electronically independent tri-phenylmethyl radicals (23a). Alternatively, interaction between the radical centers... 

One-electron oxidation of an olefin, arene, or a bibenzyl group can lead to C—H or C—C bond cleavage to produce an allyl or benzyl radical [40, 41]. This area has been pioneered by Arnold [41], The PET reaction of 1,1,2,2-tetraphenylethane and methyl-3,3-diphenylethyl ether have been reported by Arnold and coworkers [41] to provide heterolytic C—C bond cleavage through an intermediate tetra-phenylethane cation-radical. The cation-radical intermediate fragments to di-phenylmethyl radical and diphenylmethyl carbocation. 

The stabilization of the benzhydryl ylid is expected to be quite high since the related fluorenylid (2) can be isolated as a salt free solid. The high yield of 32 was attributed to hydrogen atom abstraction from the cyclohexene by the diphenylcarbene and subsequent coupling of di-phenylmethyl radicals 46>. 

Phenylmethyl radical The re.sonance-.sliibilized ben/.ylic radical CftllsCllv. 

Intermolecular single-electron transfers between radicals and ions have been extensively studied, mainly with aromatic radical-ions and ketyls (Kaiser and Kevan, 1968 Szwarc, 1968). The electron exchange between the tri-phenylmethyl radical and its carbanion causes broadening of the esr lines, which allows the relevant bimolecular rate constants to be determined, the activation energies being of the order of one kcal mol . ... 

Okamoto and his colleagues60) described the interesting polymerization of tri-phenylmethyl methacrylate. The bulkiness of this group affects the reactivity and the mode of placement of this monomer. The anionic polymerization yields a highly isotactic polymer, whether the reaction proceeds in toluene or in THF. In fact, even radical polymerization of this monomer yields polymers of relatively high isotacticity. Anionic polymerization of triphenylmethyl methacrylate initiated by optically active initiators e.g. PhN(CH2Ph)Li, or the sparteine-BuLi complex, produces an optically active polymer 60). Its optical activity is attributed to the chirality of the helix structure maintained in solution. 

Here the radical 1 acts as a strong terminator to prevent the formation of oligomers and polymers. On the other hand, it is expected that the substituted diphenylmethyl radicals which are less stable than 1 serve as both initiators and primary radical terminators. In fact, it was reported [84] that the apparent polymerization reactivities decreased in the following order diphenylmethyl, phenylmethyl, and triphenylmethyl radicals, which were derived from the initiator systems consisting of arylmethyl halides and silver. 

Although the resonance or delocalization stabilization of tri-phenylmethyl will be at its maximum only when the radical is completely planar, such a structure involves repulsion by adjacent ortho hydrogen atoms ... 

Radicals in which the odd electron is on a nitrogen next to an aromatic ring are stabilized by resonance analogous to that of tri-phenylmethyl. In the case of Wurster s salts, the nitrogen analogs of semiquinones, there are two equivalent resonance structures in the acid form, but in the less stable basic form one of the structures requires separation of charge. Evidence for the unpaired electron has been obtained by measurement of the paramagnetism.144... 

The oxidation of secondary alcohols (66) to (67) is possible by indirect electrooxidation utilizing thioanisole as an organic redox catalyst in a PhCN-2,6-lutidine-Et4NOTs-(C/Pt) system at 1.5 V vs. SCE (Scheme 25) [81] and is also performed in the presence of 2,2,2-trifluoroethanol [82]. It is suggested that the initially formed cation radical sulfide species derived from the direct discharge of the sulfide provides phenylmethyl-alkoxysulfonium ions, which are transformed to (67) and thioanisole. 

A subtle approach198 to the stepwise removal of one member of a family of protecting groups is illustrated by the reductive cleavage of the (1-naphthyldiphenylmethyl) group from 3 -0-(p-anisyldi-phenylmethyl)-5 -0-(l-naphthyldiphenylmethyl)thymidine on treatment with the anthracene radical-anion in oxolane. 3 -0-(p-Anisyldi-phenylmethyl)thymidine was isolated in 87% yield, and the yield of thymidine was only 3%. It is noteworthy that the relative reactivity of 5 -0-(l-naphthyldiphenylmethyl) and 5 -0-(p-anisyldiphenylmethyl) derivatives of thymidine towards radical anions is the reverse of that towards acid. 

Dimerization of the sodium naphthalenide radical anion would result in a loss of aromatic stabilization, but this is not true for 1, which can form a C-C bond and a resonance-stabilized bis-phenylmethyl dianion, 2 (Section 26-4C). 

Using time-resolved laser flash photolysis techniques, transient carbocations in the photolysis of benzyl halides have been widely observed. A variety of phenylmethyl halides157-160, substituted diphenylmethyl halides154,155,161-163 and substituted triphenyl-methyl halides162,164 has been successfully used as precursor under various reaction conditions. The photogeneration of the transient cations is often accompanied by that of the corresponding transient radicals. 

Recently the ultrafast radical-clock technique (kr = 5xl012 -1013 s 1) has been developed (Newcomb et al, 2000 and references therein). Two probes, trans, trans-2-methoxy-3-phenylmethyl cyclopropane and methyl cubane were used to study the... 

Since 2,4,6-triphenylphenol can be easily oxidized to a stable radical, we were interested in the behaviour of various substituted 2,4,6-triphenylbenzenes. When Schromm 140) oxidized 2,4,6-triphenyl-aniline by shaking a solution in benzene with lead dioxide, no radical was detected either by ESR spectroscopy or by trapping with tri-phenylmethyl. A red color (A. = 520 nm) possibly due to the ArNH radical was, however, observed.,40>. The addition of 70% perchloric acid changes the color to deep blue-violet. The compound was isolated when a benzene solution of 2,4,6-... 

Therefore the free electron is delocalized to a greater extent than in tri-phenylmethyl, and the radical is more stable. 

Michl s views were supported in a recent study of photodissociation of A-(tri-phenylmethyl)anilines by Siskos et al. [112]. These compounds were shown to undergo efficient homolysis of the C-N bond, producing trityl radical with quantum yields of 0.6-0.8, which were independent of solvent polarity. The trityl radical produced either 9-phenylfluorene, triphenylmethane, or trityl cation upon the absorption of another photon, and a detailed mechanistic scheme was proposed for these transformations. Later these workers demonstrated that an increase in the number of phenyl groups in the molecular series PhCH2NHPh, Ph2CHNHPh, and PhaCNHPh caused the fluorescence quantum yield to decrease and the yield of the C-N bond homolysis to increase drastically [134]. An analogous study was conducted on the photodissociation of methoxy-substituted benzyl, benzhydryl. 

Chemical reactivity at a saturated carbon atom attached to a benzene ring is greatly enhanced over that of ordinary alkyl carbons. Delocalization stabilizes both reaction transition states and intermediates. After reading this section once, go Kick and take a good look at the sets of resonance forms for the phenylmethyl (benzyl) radical, cation, and anion. There are four forms for each one. leading to increased stability and ease of formation. 

Benzylic resonance Resonance stabilization of the phenylmethyl (benzyl) radical, C6H5CH2 (also cation and anion). 

The monomers with relatively small N-substituents such as N-methyl methacrylamide afford polymers rich in syndiotacti-city. The bulkier N-isopropyl, N-t-butyl, and N-phenyl derivatives result in polymers with a slightly reduced syndio-tacticity. However, radical polymerization of bulky N-[di (4-butylphenyl)phenylmethyl] methacrylamide (DBuTrMAM, 67) and N-triphenylmethylmethacrylamide (TrMAM, 68) produces a nearly complete isotactic polymer due to helix formation of the polymer chain. When the radical polymerization of these monomers was carried out in the presence of optically active menthol, single-handed helical, optically active polymers were produced. Because poly(TrMAM) was insoluble in solvents, its CD spectra were measured in the solid state to support the chiral structure. 

Miranda MA, Font-Sanchis E, Perez-Piieto G, Scaiano JC (2002) Flash photolysis of (E)-l,2-bis(l-chloro-l-phenylmethyl)cyclopropane. Generation of 1,5-diphenylpentadienyl radical and 1,5-diphenylpentadienylium cation. J Org Chem 67 1162-1166... 

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