Arylmethyl chlorides

Complex (16), which has a similar structure to Co11 salen, catalyzes the electrocarboxylation of arylmethyl chlorides.274 The enhancement of the catalytic life of (16) as compared to Co-salen may be due to the absence of imino bond in its ligand. The catalytic reduction of halogenated compounds has also been attempted at poly[Mn(salen)]-coated electrodes (M = Ni,253 Co275), which might have potential use for determination of organohalide pollutants.275... 

The reactions of sodium dimethyl and diisopropyl phosphite with 4-nitrobenzyl chloride, 9-chlorofluorene, and diphenylchloromethane provided information that supported the proposed reaction mechanism. The RaPO anion acts towards an arylmethyl chloride as a base and abstracts a proton to form a carbanion, which can then participate in single-electron transfer processes to produce carbon-centred radicals. ... 

Electrophilic substitution of ArFt gives benzylic (arylmethyl) chlorides (see Section 26-4A, Exercise 22-21). 

Arylmethyl chlorides or bromides are quite reactive compounds that are readily available or easily prepared, and as a result they are useful intermediates for the synthesis of other side-chain derivatives. Thus phenylmethyl chloride can be hydrolyzed to phenyl methanol, converted to phenylethanenitrile with alkali-metal cyanides, and oxidized to benzenecarbaldehyde (benzaldehyde) ... 

TABLE II. Values of /J — /S for Various Reactions of Arylmethyl Chlorides... 

The Sjsfl reaction of an arylmethyl chloride ArCH2Cl, where Ar is a E group, provides a good example. The rate-determining step in such a reaction is the ionization of the chloride. 

FIGURE 5.15. Effect of a polar solvent on the ionization of an arylmethyl chloride, ArCH2Cl... 

FIGURE 5.19. Plot of — logio against flor for the solvolysis of arylmethyl chlorides in moist formic acid at 25°. Data from Ref. 2. 

FIGURE 5.21. Plot of ao of arylmethyl chlorides with iodide in dry acetone. The rate constants are from Ref. 4. 

Suppose now that we carry out the solvolysis of our arylmethyl chlorides in solvents of increasing nucleophilicity, i.e., tendency to combine covalently with positively charged carbon. The transition states should then show a... 

FIGURE 5.22. Plot of logio against for solvolysis of arylmethyl chlorides in formic acid/water/dioxane. Rate constants are from Refs. 1 and 2. 

A very nice example is provided by the arylmethyl chlorides ArCH2Cl, where ArH is an even AH. As we have seen, both the Sj l and Sj 2 reactions are accelerated by an increase in the E activity of Ar (Figs. 5.19 and 5.21), but the acceleration is greater in the case of the S yl reaction. Therefore while benzyl chloride hydrolyzes by a mixed mechanism in aqueous ethanol, the corresponding hydrolyses of more reactive arylmethyl chlorides are Sjyl. 

Streitwieser,A.,Jr., Perrin,C Polarographic reduction of substituted benzyl chlorides and polycyclic arylmethyl chlorides. J. Am. Chem. Soc. 86, 4938 (1964). 

A Bischler-Napieralski intramolecular cyclization involving aryl acetyl chlorides in phosphoryl chloride affords 4-arylmethyl-4H-pyrrolo[l,2-fl][l,4]benzo-diazepines 236 (Scheme 50 (1995EJM593)). 

Benzylic electrophiles bearing electron-withdrawing groups at the arene do not always yield the expected products of nucleophilic substitution on treatment with a nucleophile. One important side reaction is the dimerization of these compounds to yield 1,2-diarylethenes (stilbenes). This dimerization does not require such highly activated systems as the example sketched in Scheme 4.28, but can even occur with, for example, 2- or 4-nitrobenzyl chloride [120, 121]. The latter compounds are converted into the corresponding stilbenes by treatment with KOH in ethanol [120]. Di-arylmethyl halides behave similarly and can yield tetraarylethenes on treatment with a base. These reactions presumably proceed via the mechanism sketched in Scheme 4.27, in which the amphiphilic character of the nitro group plays a decisive role (metalated nitroalkanes or 4-nitrobenzyl derivatives can act as nucleophiles and as electrophiles). 

Reaction of succinylhydrazide derived from (A)-amino-2-methoymethylpyrrolidine (SAMP) with benzylmagne-sium chlorides allows us to prepare a number of enantiopure 5-arylmethyl-pyrrolidones and -pyrrolidines.265 An enantioselective desymmetrization of anhydrides was reported by Fu. Arylmagnesium chlorides react in toluene in the presence of 1 equiv. of (—)-sparteine with 3-substituted glutaric anhydrides, giving aryl ketones with 87-92% ee (Scheme 90).266... 

Also, l-(alkoxycarbonyl)-3-(arylmethyl)thioureas 6 are converted to the corresponding acylcarbodiimides 7 using carbonyl chloride and EtjNj... 

Arylmethyl cleavage reactions have also been studied in less polar solvents like saturated hydrocarbons (hexane, cyclohexane), ethers (diethyl ether, tetrahy-drofiiran, dioxane), and methylene chloride. In these solvents, ion pair chemistry is less likely and valuable information can be obtained about the photogenerated radical pairs. 

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. 

Although steric effects and substituent effects leading to carbonium ion stabilization are of greatest importance in governing the mechanism and relative rate of nucleophilic substitution processes, there are other substituent effects that are recognized and of importance. We have mentioned earlier in this chapter that arylmethyl and allylic cations are stabilized by electron delocalization. It is therefore easy to understand why substitution reactions of the ionization type proceed more rapidly in such systems than in simple alkyl systems. It has also been observed that nucleophilic substitutions of the direct displacement type also take place more readily, but the reason for this is not apparent. Allyl chloride is 33 times more reactive than ethyl chloride toward iodide ion in acetone, and benzyl chloride is 93... 

Oxidation of an arylmethyl group to an aldehyde by treatment with chromyl chloride ... 

Let us first examine the reactions of the compounds considered in the previous section (p. 255), i.e., arylmethyl derivatives ArCH2X where ArH is an even AH. Let us consider their Sjy2 reactions with the nucleophile X this could, for example, correspond to chloride exchange between an... 

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