Concurrent reactions, benzyl derivatives

Nucleophilic Substitution at Benzyl Derivatives. The sharp break from a stepwise to a concerted mechanism that is observed for nucleophilic substitution of azide ion at X-l-Y (Figs. 2.2 and 2.5) is blurred for nucleophilic substitution at the primary 4-methoxybenzyl derivatives (4-MeO,H)-3-Y. For example, the secondary substrate (4-MeO)-l-Cl reacts exclusively by a stepwise mechanism through the liberated carbocation intermediate (4-MeO)-T, which shows a moderately large selectivity toward azide ion ( az/ s = 100 in 50 50 (v/v) water/ trifluoroethanol). The removal of an a-Me group from (4-MeO)-l-Cl to give (4-MeO,H)-3-Cl increases the barrier to ionization of the substrate in the stepwise reaction relative to that for the concerted bimolecular substitution of azide ion. The result is that both of these mechanisms are observed concurrently for nucleophilic substitution of azide ion at (4-MeO,H)-3-Cl in water/acetone solvents. These concurrent stepwise and concerted nucleophilic substitution reactions of azide ion with (4-MeO,H)-3-Cl show that there is no sharp borderline between mechanisms for substitution at primary benzylic carbon, but instead a region of overlap where both mechanisms are observed. 

The Menschutkin reaction of benzyl tosylates [21]-OTs with dimethyl-anilines or pyridines in acetonitrile generally proceed by a second-order bimolecular Sn2 mechanism for most ring-substituted compounds the plot of obs vs. [Nu] passes through the origin within experimental uncertainty (Yoh et al., 1989). However, for the reactions of strong ED derivatives under the same conditions, it was found that there was a significant intercept (i.e. a first-order component) in the kobs vs. [Nu] plots represented by (39) the intercept is a constant of the benzyl substrate independent of the amine nucleophiles, indicating a concurrent reaction zeroth-order in amine (Kim et al., 1995, 1998). 

As the precursor for the B unit, the selectively benzylated -fucal derivative 75- could be obtained crystalline from L-fucal by applying a phase-transfer-catalyzed process T64). By a Ferrier reaction of di-O-acetyl-L-rhamnal and a subsequent retro enol ether formation trough hydride attack at C-3 (71.) the -amicetal 72 was obtained (64) this reaction was concurrently also es-cribed by others (72). 

Above 170 °C the amidrazone ylide (36) decomposes with loss of triethylamine and concurrent cyclization to give an 85% yield of 2-phenylbenzimidazole (Scheme 19) (B-75M140800). Poorer yields ( 40%) are obtained when N-benzyl-o-nitroaniline is pyrolyzed in the presence of iron oxalate. No doubt this last reaction is similar in many respects to the reactions shown in Scheme 2. Both 2-phenyl-imidazoles and -benzimidazoles (as well as other 2-substituted analogues) can be obtained as a result of thermal rearrangement of the 1-substituted isomers (Section 4.07.1.2.2), by radical substitution methods (Section 4.07.1.7) or via the 2-lithio derivatives (Sections 4.07.1.6, 4.07.3.7). 

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