Benzyl cations methoxy

Benzyl groups having 4-methoxy (PMB) or 3,5-dimethoxy (DMB) substituents can be removed oxidatively by dichlorodicyanoquinone (DDQ).181 These reactions presumably proceed through a benzylic cation and the methoxy substituent is necessary to facilitate the oxidation. 

However, internal cycloaddition fails to give an adduct when an ethylenic moiety is linked to the ylid precursor. Even a styrene does not give any adduct and a diamine is obtained instead (note that in this instance zinc chloride was used instead of lithium fluoride). The same result is observed starting from the corresponding phenyl derivative, i.e., from /V-benzyl-/V-(methoxy methyl)trimethylsilylmethylamine. The proposed mechanism involves demethox-ylation under the action of the Lewis acid to form cation, K, which adds to the ylid to give the ethylenediamine framework.219... 

There is a parallel pattern for the photochemical ionisation of benzyl acetates 8.11 and 8.13 giving benzyl cations 8.12 and 8.14, in which a m-methoxy group is effective in promoting the reaction, but ap-methoxy group is not (in fact, the para isomer 8.13 cleaves homolytically rather than heterolytically). The explanation is... 

To avoid any loss of benzyl ester protection during acidolytic removal of the benzyloxy-carbonyl and ferf-butoxycarbonyl groups, electron-withdrawing substituents were used to destabilize the intermediate benzyl cation and thus to increase the acid stability. In addition to the very useful 4-nitrobenzyl esters (vide infra), the picolyl ester (see Section 2.2.1.2.2.3) as well as halo-P l or cyano-P°°l substituted benzyl esters have been reported, the latter being rarely used for a-carboxy protection. Conversely, an increase in sensitivity toward acids can be achieved by introduction of electron-releasing substituents, such as methoxy or methyl groups. Addition of scavengers to quench intermediate carbocations and to prevent electrophilic substitutions at sensitive amino acid side chains is beneficial in the deprotection of such esters. 

To our knowledge, 46 has never been observed in solution under stable conditions, even at low temperature. Pulse radiolysis " of benzyl chloride as well as flash photolysis ° of several derivatives in HHP have allowed the observation of the electronic absorption spectra of benzyl and its 4-methyl and 4-methoxy derivatives. The and NMR spectra of the 2,4,6-trimethylbenzyl cation and other more heavily substituted benzyl cations, however, have been studied at low temperature in superacid media. In the gas phase, cold benzyl radical has been probed by two-color, resonant two-photon ionization techniques, thus providing very accurate vibrational frequencies below 650 cm for the benzyl cation. Furthermore, the adiabatic ionization energy of benzyl radical and several isotopomers in the ground state were determined from their threshold photoionization spectra using resonant two-photon excitation and detection of electrons by pulsed field ionization. This information, combined with Af//° (CgH5CH2) from Ref. 212 leads to the value of Af//°m(46) reported in Table 9. 

The study of substituent effects on the stability of substituted benzyl cations is important as a source of thermodynamic information as well as a means to provide a conceptual link between the stabilities of a number of important carbocations. Mishima s study, in particular, shows that the stabilities in terms of standard Gibbs energy for 4-methoxy- and 4-nitro-substituted benzyl cations differ by some 24.9 kcal mol The former has a stability comparable to that of bicyclo[3.3.3]undecyl (manxyl) cation, while the latter is comparable to the c-pentyl cation. We present these two extreme cases in Table 4. The relative stabilities of other substituted benzyl cations are given in Refs. 50 and 51. 

The mass fragment peaks at m/z 249 and 165 represented the fragment ion (20) and the benzyl cation (2 2) respectively, revealing that one methoxy group existed in the same benzene nucleus together with the methylenedioxy group. The fact that... 

HCl in benzene gave (270) and the cis-halohydrin (271) in approximate ratio 2 9. Trichloroacetic acid converted (269) into (270) (63.5 %X and after hydrolysis into (272) (33.5%), and (273) (< 1%). The secondary trichloroacetate of (272) isolated from the reaction is probably a secondary product from the tertiary ester via an acyl shift. In aqueous sulphuric acid the product composition from (269) was (270) 17.5 %, (272) 35 %, and (273) 47.5 %. Analogously obtained product data from (268) and (274) are assembled in the Table these data indicate for (274) a high preference for syn-adducts (via an ion-pair intermediate consequent upon benzylic cation formation) and for (268) a high preference for anti-adducts. Epoxide (269) represents an intermediate situation the high syn-stereoselectivity in the reaction with trichloroacetic acid is, however, indicative of an ion-pair mechanism and also suggests that the conjugative electron release of methoxy is transmitted to the benzylic centre in spite of the built-in steric constraints. In aqueous acid the lack of stereoselectivity exhibited by (269) has been... 

Benzyl cation stability is strongly affected by the substituents on the benzene ring. A molecular orbital calculation estimating the stabilization has been done using STO-3G-level basis functions. The electron-donating p-amino and p-methoxy groups are found to stabilize a benzyl cation by 26 and 14 kcal/mol, respectively. 

Benzyl diphenylphosphinates (Ph2P(0)0CH2C6H4R) act as benzyl cation equivalents in the presence of (la). Allylsilanes, silyl enolates, and alkoxysilanes are efficiently benzylated with these phosphinates [149]. 2-(4-Methoxybenzyloxy)-3-nitropyridine is effective in the (la)-catalyzed protection of alcohols by a 4-methoxy-benzyl group [150]. 

Salts composed of an anion, RO—, and a cation, usually a metal, can be named by citing first the cation and then the RO anion (with its ending changed to -yl oxide), e.g., sodium benzyl oxide for CgH5CH20Na. However, when the radical has an abbreviated name, such as methoxy, the ending -oxy is changed to -oxide. For example, CHjONa is named sodium methoxide (not sodium methylate). 

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