Arylmethyl formation

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. 

An El report deals with the tautomerism of unsubstituted and the differentiation of substituted pyrimidin-4(3//)- and -4(l//)-ones (90OMS115). Another publication on 2-alkyl (Et, Pr) or 2-arylmethyl-substituted derivatives shows that the substituent decisively influences their fragmentation and that, especially in the case of 2-arylmethyl substituents, an intramolecular cyclization in the formation of the ion again becomes very... 

Heating 5-(arylmethyloxy)-2-phenyl-4-(trifluoroniethyl)oxazoles 126 effects thermal rearrangement to 4-(arylmethyl)-2-phenyl-4-(trifluoromethyl)-5(4//)-oxa-zolones In some cases, the formation of 2-(arylmethyl)-2-phenyl-4-... 

Another example of the simultaneous formation of both the pyrrolo and furo compounds has been observed. Nearly identical yields of 102c and 103b were isolated <2005JME5329>. Nonetheless, other 6-substituted derivatives of pyrrolo[2,3- pyrimidines have been prepared without complication, for example, 102d <2001JHC349> and a variety of arylmethyl compounds, illustrated by 102e <2003BMC5155>. 

Treatment of symmetrically substituted bis(arylmethylthio)alkynes with either IC1 or Br2 and in the strict absence of water promotes cyclization to 1//-2-benzothiopyrans (Scheme 129). Initial formation of a cyclic halonium species induces the electrophilic ring closure. In the presence of nucleophiles, ipso attack is favored and a spirocyclohexa-dienone results. The alkynes are accessible from arylmethyl thiocyanates and sodium acetylide <1998JOC4626, 2003EJ047>. 

Homolytic photodissociation of a benzyl-anilino C—N bond has also been observed in a series of iV-(arylmethyl)anilines. The main products identified for 27 are aniline, triphenylmethane and 9-phenylfluorene (28) (equation 4)126. The quantum yields for the formation of Pt C are high (0.6-0.8, 248-nm excitation) and independent of solvent. On the basis of the results of laser flash photolysis and ESR studies, the formation of 28 occurs via the intermediate 29 as a result of electrocyclization of Pt C (Scheme 5). In contrast, the dimerization of the benzyl and diphenylmethyl radicals, leading to the formation of 1,2-diphenylethane and 1,1,2,2-tetraphenylethane, respectively, are efficient in the cases of 30 and 31 (equations 5 and 6)127. In addition, products resulting from the coupling of the photodissociated benzyl and aniline radicals are also observed for 31, presumably due to the less sterically hindered PhCH2 radical when compared with Ph2CH and Ph3C radicals. 

Photolysis of benzene solutions of l-methoxycarbonyl-2-naphthylmethyl 2,6-di-methyl substituted phenyl ethers induces C-O cleavage with formation of 2,4-cyclohexadienone intermediates which are subsequently photo-rearranged into meta substituted phenols. In methanol, 9-anthrylmethoxy-pyrid-2-one or l-pyrenylmethoxypyrid-2-one undergo photoheterolysis to give the C-O heterolysis products l-hydroxypyrid-2-one and the arylmethyl methyl ether, together with 2-pyridone, aryl-substituted methanol and aryl aldehyde derived from homolysis of the N-O bond. Evidence shows that an intramolecular exciplex plays a crucial role in C-O bond heterolysis. 

Alonsono and co-workers [146] have used substituted 1-naphthyl and phe-nylphosphonium chlorides as precursors for the generation of the corresponding arylmethyl radicals and cations in both nanosecond LFP and product studies. For instance, the salt 101 has a quantum yield for cation formation of 0.71 in methanol and the sole product observed was the corresponding methyl ether. No transient radical was observed in this solvent. In contrast, in 5% acetonitrile in dioxane, the radical was observed but now the cation was absent. No fluorescence was observed in either solvent suggesting that Si is very reactive. Redox potentials indicate that the conversion of the radical/radical ion pair to the cation/triphenyl-phosphine pair would be exothermic by some 25 kcal/mol. Therefore, both heterolytic cleavage from Si or homolytic cleavage followed by electron transfer were suggested as possible pathways for cation formation. 

Reduction of 2-arylidenecycloalkanones Formation of rran5-2-arylmethyl-cycloalkanols is the result of a directed hydroalumination of the immediate allylic alcohols. 

The discussion so far has treated only those reaction intermediates that can be formed from a parent unsubstituted PAH. We also wish to present the treatment for cases in which a delocalized it intermediate is formed from methyl-substituted derivatives. For example, in considering the formation of an arylmethyl cation, thermocyclic arguments (5, 6) have to be implemented to estimate the difference in Ec>(0 between the arylmethyl cation and the fragments before the union, that is, the neutral even PAH and the CH2 + ion. The localized positive charge of the CH2+ ion and the condition qr = 0 in the neutral even PAH leads to the following ... 

Thus, the PMO Fw and PMO-w methods correlate the formation of an arylmethyl cation with the following ... 

By correlating the formation of an arylmethyl anion with the difference between its tt energy and that of the fragments, we obtain equation 16 for the anionic case This equation is formally identical to the result given for the cationic situation in equation 13. 

Yrs are two-centre two-electron repulsion energies. For aromatic ions, the onsite electron pair densities, i.e., the diagonal elements of the spinless second order density matrix are lower than those of any classical structure. For radicals, however, the pair densities are increased relative to those in the corresponding classical structures. Thus the resonance energy of the ion exceeds that of the radical by 21 Ec I [33]. FIMO and PMO do not differentiate between the formation of an arylmethyl radical and its carbocation. Empirically, radicals are better described by these methods this has been related to the constant Coulson charge order Q = 1 for arylmethyl radicals as opposed to the variable rt-charge order 1 on arylmethyl cations [16,39]. The actual PPP values of Ec have been correlated to an excellent accuracy to their PMO-o) counterparts [16]. 

The formation of dialkyl (a-aminoalkyl)phosphonothioates (33) through the Kabachnik-Medved -Fields interaction of dialkyl hydrogenphosphonothioates and carbonyl compounds in the presence of anmionia was established in the early days of the study of that reaction, as was the formation of dialkyl [a-(arylamino)arylmethyl]phos-phonothioates (34) by the addition of (RO)2P(S)H to anils (Scheme... 

The photoinduced anti-Markovnikov addition of methanol to 1,1-diphenylethene reported by Arnold and co-workers in 1973 provides the first example of the addition of a nucleophile to an arylalkene radical cation. There are now a number of studies that demonstrate the generality of nucleophilic addition of alcohols, amines, and anions such as cyanide to aryl- and diaryl-alkene radical cations. Product studies and mechanistic work have established that addition occurs at the 3-position of I-aryl or 1,1 -diarylalkene radical cations to give arylmethyl or diaryl-methyl radical-derived products as shown in Scheme I for the addition of methanol to 1,1-diphenylethene. For neutral nucleophiles, such as alcohols and amines, radical formation requires prior deprotonation of the 1,3-distonic radical cation formed in the initial addition reaction. The final product usually results from reduction of the radical by the sensitizer radical anion to give an anion that is then protonated, although other radical... 

Short syntheses of enantiomerically pure narciclasine (68) and lycoricidine based on the intramolecular acid-catalyzed arene-epoxide coupling have been described (288,289). Bromohydroxylation of a protected aminocyclohexenol afforded the corresponding bromohydrin as a mixture of two trans stereoisomers, which was subsequently transformed to link an arylmethyl moiety in basic medium with the concomitant formation of the epoxide ring, thereby setting the stage for the... 

Whereas secondary amines are suitable catalysts for activation of a,(3-unsaturated aldehydes, more difficulties are usually encountered with sterically demanding substrates, such as a,(5-unsaturated ketones. Primary amines can be useful catalysts in such cases. Yoshida et al. [52] reported an amino acid-catalyzed sulfa-Michael addition of arylmethyl mercaptans to cyclic enones. The proposed mechanism invokes the formation of an imine intermediate. However, even with the best screened catalyst, 5-trityl L-cysteine, the reaction proceeded with modest levels of enantioselectivity (8-58% ee). 

Ethyl format el aryl magnesium bromide Arylmethyl thioethers s. 18, 664... 

CuOTf-catalyzed C-S bond formation allows the formation of arylmethyl- and diarylsulfones. The coupling between aryl iodides and sulfinic acid salts proceeds in DMSO in the presence of A(A7-dimethylethylenediamine to give the sulfones with variable yields (eq 124). ... 

The tra 5-relation of the substituents in synthetic ( )-dehydrodiiso-eugenol was elucidated by oxidative degradation and NMR (23). In order to explain the lack of formation of cw-arylmethyl-dihydrobenzo-furans, even by the action of H202-peroxidase on (Z)-isoeugenol, relieve of steric compression of adjacent cw-methyl and quinonemethide moieties by rotation about the 7,8-bond prior to cyclization of the... 

Figure 3. Correlation of the relative rates of formation of arylmethyl anions with calculated SCF energy differences...

The correlation of the rates of formation of arylmethyl free-radicals by molecular oibital theory will be discussed. Different levels of sophistication among pi-electron methods lead to conflicting conclusions concerning the degree of possible electron localization and the importance of non-bonded interactions with the principal radical site. The arylmethyl radical systems have been generated both by hydrogen abstraction... 

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