Benzylic substrates, and

Poor to modest yields of trinitromethyl compounds are reported for the reaction of silver nitroform with substituted benzyl iodide and bromide substrates. Compounds like (36), (37), and (38) have been synthesized via this route these compounds have much more favourable oxygen balances than TNT and are probably powerful explosives." The authors noted that considerable amounts of unstable red oils accompanied these products. The latter are attributed to O-alkylation, a side-reaction favoured by an SnI transition state and typical of reactions involving benzylic substrates and silver salts. Further research showed that while silver nitroform favours 0-alkylation, the sodium, potassium and lithium salts favour C-alkylation." The synthesis and chemistry of 1,1,1-trinitromethyl compounds has been extensively reviewed. The alkylation of nitronate salts has been the subject of an excellent review by Nielsen." ... 

This reaction is only plausible if phosphide complex 73 is the most nucleophilic species present in the mixture. Therefore, the base must not be too strong to deprotonate 72, otherwise the generated phosphide anion would quiekly react with the benzyl halide to give racemic 74. Sodium trimethylsilanoate appeared to be a good choice. When benzyl bromide was used as the electrophile in the presence of 5% of 73, the tertiary phosphine 74 was obtained in quantitative yield and with 77% ee. Interestingly, with benzyl chloride the reaction proceeds equally well but is extremely slow it took one month to reach completion compared to 3 h with benzyl bromide. These remarkable results were extended to a variety of benzylic substrates and secondary phosphines (Table 6.12). In these reactions instead of using the rather sensitive complexes 73 or analogues, the air-stable precursor Pt((7, 7 )-Me-DuPhos)(Q)(Ph)] was employed. 

Alkenes, allylic or benzylic substrates and alcohols are also converted to... 

Rates that are independent of aromatic substrate concentration have been found for reaction of benzyl chloride catalyzed by TiCl4 or SbFj in nitromethane. This can be interpreted as resulting from rate-determining formation of the electrophile, presumably a benzyl cation. The reaction of benzyl chloride and toluene shows a second-order dependence on titanium tetrachloride concentration under conditions where there is a large excess of hydrocarbon. ... 

A study of alkylations with a group of substituted benzyl halides and a range of Friedel-Crafts catalysts has provided insight into the trends in selectivity and orientation that accompany changes in both the alkyl group and the catalysts. There is a marked increase in substrate selectivity on going from / -nitrobenzyl chloride to /i-methoxybenzyl chloride. For example, with titanium tetrachloride as the catalyst, Aitoi Abenz increases from 2.5 to 97. This increase in substrate selectivity is accompanied by an increasing preference for para substitution. With /i-nitrobenzyl chloride, the ortho para ratio is 2 1 (the... 

The mechanism is postulated to involve the initial formation of a Schiff base 17 from the condensation of the anilinic amine 16 with the carbonyl-containing substrate. This is followed by a Claisen condensation between the benzylic carbonyl and the activated a-methylene of the imine. ... 

Parenthetically we might also note that primary allylic and benzylic substrates are particularly reactive in S 2 reactions as well as in SN-1 reactions. 

Unsaturation at the p Carbon. The SnI rates are increased when there is a double bond in the P position, so that allylic and benzylic substrates react rapidly (Table 10.5). The reason is that allylic (p. 221) and benzylic (p. 222) cations are stabilized by resonance. As shown in Table 10.5, a second and a third phenyl group increase the rate still more, because these carbocations are more stable yet. It should be remembered that allylic rearrangements are possible with allylic systems. 

Iron phthalocyanine is an efficient catalyst for intermolecular amination of saturated C-H bonds. With 1 mol% iron phthalocyanine and 1.5 equiv. PhlNTs, amination of benzylic, tertiary, and ally lie C-H bond have been achieved in good yields (Scheme 31). With cyclohexene as substrate, the allylic C-H bond amination product was obtained in 75% yield, and the aziridination product was found in minor amount (17% yield) [79]. 

The degradation of acenaphthene is initiated by benzylic monooxygenation, and the pathway was determined using [l- C]acenaphthene by the isolation of intermediate metabolites (Selifonov et al. 1998). Importantly, the method proved applicable even when only limited biotransformation of the substrates had taken place by partial oxidation. 

A similar polymer-stabilized colloidal system is described by James and coworkers [66]. Rhodium colloids are obtained by reducing RhCls, 3H2O with ethanol in the presence of PVP. The monophasic hydrogenation of various substrates such as benzyl acetone and 4-propylphenol and benzene derivatives was performed under mild conditions (25 °C and 1 bar H2). The nanoparticles are poorly characterized and benzyl acetone is reduced with 50 TTO in 43 h. 

SN1 Very strong effect reaction favored by polar solvents Weak effect reaction favored by good nucleophile/weak base Strong effect reaction favored by good leaving group Strong effect reaction favored by 3°, allylic, and benzylic substrates... 

It is clear from Table 44 that the E2/SN2 ratios observed for reactions of crown ether-separated KOt-Bu ion pairs will greatly depend on the type of substrate and solvent. Di Biase and Gokel (1978) have recently reported many examples of the use of this reagent either as a nucleophile [for example, in its reaction with benzyl chloride and in the reaction with isatoic anhydride (34)] or as a strong base [for example, in the basic oxidation of fluorene to 2-carboxybiphenyl (35)]. 

Figure 16. Optimized structures of the endo (lOa-anf and exo (IQb-anti) r -silyll allyl complexes with 4-(dimethylamino)-styrene as a substrate. Highlighted in (a) is the steric contact between the hydrogen of the benzylic carbon of the substrate and a hydrogen of the mesityl substituent 12 63 A) for the endo-anti intermediate, lOa-on/i. Highlighted in (b) is the steric contact between on hydrogen of the methyl group of the substrate and a hydrogen of the mesityl suhstituent 12 47 A) for the exo-anti silyl-allyl intermediate, Wb-anti. Hydrogen atoms are not shown except those whose interactions are highlighted.

The secondary a-tritium KIEs in these reactions, on the other hand, are small and normal. Benzyl substrates have looser SN2 transition states than methyl substrates (vide infra) and thus, the benzyl substrate reactions would be expected to have slightly larger (normal) KIEs rather than the inverse KIEs... 

The authors concluded that the transition states for the Menshutkin reactions of the benzyl substrates were early (reactant-like) with nitrogen-alpha carbon bond formation lagging behind alpha carbon-oxygen bond rupture. The transition states for the Menshutkin reactions with the methyl and ethyl substrates, on the other hand, are tight (product-like) with nitrogen-alpha carbon bond formation greater than alpha carbon-oxygen bond rupture. 

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