Dications benzylic

The propensity of S-S dications to undergo dealkylation was found to decrease in the order of methyl > ethyl > benzyl. This order of reactivity parallels the increase in the stability of the corresponding carbocations.94 Dealkylation of dication 77 affords thiosulfonium salt 78 in quantitative yield.95 Kinetic studies suggest SN1 mechanism of dealkylation. In addition, reaction of sulfoxide 79 with a substituent chiral at the a-carbon results in racemic amide 80 after hydrolysis. 

Substituted benzylic mono- and dications (88 and 89) were investigated by JH and 13C NMR spectroscopy and IGLO-DFT calculations.102... 

The results suggest that chinoid type structures are the predominant resonance contributors for 88. The IGLO/DZ//3-21G calculated 13C NMR chemical shifts of benzylic monocations 88 correlate reasonably well with the experimentally obtained data. The 13C NMR chemical shifts of the carbocation centers (CH2 carbon) are calculated 10.6-12.5 ppm too deshielded. Similar results were obtained for benzylic dications 89. NMR chemical shifts of arenium ions derived from various classes of polycyclic aromatic hydrocarbons have been calculated using GIAO-DFT methods.103... 

P-silyl substituted benzyl cations, 151-152 cyclobutadienyl dication, 150... 

Hydride transfer reactions from [Cp2MoH2] were discussed above in studies by Ito et al. [38], where this molybdenum dihydride was used in conjunction with acids for stoichiometric ionic hydrogenations of ketones. Tyler and coworkers have extensively developed the chemistry of related molybdenocene complexes in aqueous solution [52-54]. The dimeric bis-hydroxide bridged dication dissolves in water to produce the monomeric complex shown in Eq. (32) [53]. In D20 solution at 80 °C, this bimetallic complex catalyzes the H/D exchange of the a-protons of alcohols such as benzyl alcohol and ethanol [52, 54]. 

Some benzylic mono- and di-cations have been studied by the NMR/DFT/IGLO technique. Of the stable dications, the trimethyl species (76 R = Me) was found to be the major resonance contributor to the structure of (75), and the same was found to be true for the trimethoxy derivative. In the related monocations, for (77) the major resonance contributor was (78), and this was also the case for the pentamethyl and 2,5-dimethyl-4-r-butyl compounds. The dication (79) and the trication (80),... 

The 1,4-dibenzylium dication (111) could not be formed in the superacidic media. The ionization of the 2,3,5,6-tetramethyl-l,4-bis(chloromethyl)benzene (109) in SbF5/S02ClF at -78°C gave only the monocation Lewis acid-base complex (110). Thus, the observed NMR chemical shifts of the positively charged benzylic carbon Cl and the CH2CI carbon are deshielded as expected for the monocation/ Lewis acid-base complex (5 C 170.2 and 82.9, respectively). 

It was also of interest to generate the related symmetric primary benzylic trication. However, the ionization of the 2,4,6-tris(chloromethyl)mesitylene (112) in excess SbF5/S02ClF at -78°C gave apparently only the dienylic allylic dication Lewis acid-base complex (113). The unionized chloromethyl carbon displayed a relatively deshielded NMR absorption, 35.3, indicative of a weak Lewis acid-base interaction. The terminal methylene carbons of the dienylic system showed a 8 C of 197.7 and the terminal carbons of the allylic system displayed a 8 C of 194.8, quite similar to that of the previously described dienylic allylic dications, 107 and 108. 

Hexakis(benzylthio)benzene acts as a rather efficient electron donor. The ESR spectrum obtained with in situ electrolysis indicates a pattern corresponding to the twelve equivalent protons in the benzyl positions. However, cyclovoltammet-ric measurements reveal that the radical cation exists in a complicated equilibrium with the dication and the parent neutral precursor53. Radical ions generated from C60 have been the subject of several publications and discussions within the last years. Several authors have postulated that C6o + can be observed in solid C60. However it could be demonstrated that this signal has to be ascribed to C6a peroxide or its decomposition products54. 

The described superelectrophilic activation and fluorene-cyclization is thought to involve a lowered energy of the LUMO and concomitant delocalization of positive charge into the aryl ring(s).32b Calculations at the 4-31G//STO-3G level on a model system (Figure 2) have shown that the amount of positive charge in the phenyl ring increases upon formation of the dication (67) when compared to the monocation (66) and the benzyl cation (calculations are based on fully planar structures). It is well known... 

Triflic acid catalyzes the arylation of Af-methyl-l,2,3,6-tetrahydropyridine 227 to give 4-phenylpiperidine 229 (Scheme 59) <2001TL5821>. The reaction is believed to proceed by initial formation of a 1,4-dication 228, which forms in preference to the 1,3-dication 231. When yV-methyl-5-phenyl-l,2,3,6-tetrahydropyridine 230 is arylated under the same conditions, 3,3-diphenylpiperidine 232 is formed as the sole product showing the stabilization of the 1,3-dication intermediate 231 by the tertiary 5-position. Intramolecular arylation of 2- and 6-benzyl-substituted 1,2,3,6-tetrahydropyridines can also be catalyzed by triflic acid <20050L4309>. 

The cationic hexamethylbenzene ruthenium complex 254 can be depro-tonated by treatment with t-BuOK to give the ruthenium complexes 255 and 256 in which the r 4-o-xylylene ligand is coordinated via its exocyclic diene (Scheme 23). Protonation of 255 gives the dication 15c, whereas protonation of 256 produces the fluxional tj3-benzyl derivative 257 which is stabilized by an agostic C—H bond (156,157). 

In the solid state, the intramolecular electron transfer to a naked dication may partly take place from the proximal hexacyanoferrate(II) anions. The extent of electron transfer at the ground state becomes large with an increase in the half-redox potential. The low photosensitivity for Bzl-V2+ in the solid state appears to be caused by the fact that the electron transfer already takes place in the ground state and/or the bulky hexaferrate (II) ion is barely brought close to the positive pyridinium nitrogens attached to the benzyl group possessing a relatively bulky molecular size. 

The overall reaction is currently regarded as an apparent hydride transfer (for review, see ) because a) hydrogen is transferred directly between the reactants in most cases, without exchange with the solvent, 2) the base-catalyzed disproportionation of benzaldehyde to benzyl alcohol and benzoic acid (Cannizzaro reaction) i ems to be a hydride transfer (3) stereospecificity can be observed in model reactions. In addition, Verhoeven and coworkers have analyzed the thermodynamics of the photoinduced reaction between l-benzyl-3-carbamido-l,4-dihydropyridine and l,l-dimethyl-4,4 -bipyridylium dication, and have concluded that a thermal 1 e reaction between a 1-alkyl-1,4-dihydronicotinamide and a carbonyl compound is unlikely. 

In the reaction of sulfoxide (144) with acetic anhydride the available evidence points to the formaticHi of the disulfide dication intermediate (145) and subsequent rate-determining loss of a benzylic proton to give (14(>a pathway d Scheme 35). On the basis of experiments conducted on related substrates the alternative mechanism involving the formation of the quinidomethide intermediate (146b) was ruled out. 

In a study of Troger s base derivatives 68 [R = H, Me RR = (CH2)3] in acid media (see Section II,B,3), Greenberg et al. found the dication of the dimethyl derivative (186, R = Me) to have nonidentical benzylic methylene groups and nonequivalent methyl NMR resonances. These results are consistent with the open iminium ion structure 69 (R = Me), and contrast with the results with Troger s base itself (68, R = H) and its spiro (cyclobutyl) derivatives 68 [RR = (CH2)3], in which the protonated closed structure 186 [RR = (CH2)3M] is favored (84JOC1127). 

For the tweezer complex with the GI viologen, the overall picture is the same. However, the complex with the G2 viologen guest behaves very differently. The benzyl loss intermediate is not formed at all. Instead, the tweezer is lost generating the bare G2 dication at m/z 1005 which can then undergo further fragmentation by benzyl cation loss. 

Cyclization. Cyanoacetic esters undergo O, A-diprotonation in TfOH. If the ester bears a benzyl orphenylethyl group at the a-position, the derived dication would undergo an intramolecular Eriedel-Crafts reaction. ... 

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