Pyridinium benzoate

Treatment of pyridinium benzoate with carbonyl dibromide in toluene gives benzoyl bromide [1919] ... 

A stirred solution of methyl 2-(benzylamino)benzoate (12.6 g, 52 mmol) and pyridine (4.2 g, 53 mmol) in Et20 (500 inL) was slowly treated with bromoacetyl bromide (10.6 g, 52.5 mmol). Stirring was continued for 2 h, the precipitated pyridinium bromide was filtered off and the filtrate was evaporated under reduced pressure. The resulting syrupy methyl 2-[benzyl(bromoacetyl)aimno]benzoate was dissolved in MeOH (1 L) and the solution was saturated with NH, at 20 C and left for 8 h. The solution was evaporated in vacuo and the residue was crystallized (CH2C12/Et20) yield ll.Og (79%) mp 189-190 C. 

This is an Sn2 process, since inversion is found at R. Another good leaving group is NTS2 ditosylamines react quite well with acetate ion in dipolar aprotic solvents RNTs2 4- OAc — ROAc. Ordinary primary amines have been converted to acetates and benzoates by the Katritzky pyrylium-pyridinium method (p. 447). Quaternary ammonium salts can be cleaved by heating with AcO in an aprotic solvent. Oxonium ions can also be used as substrates RsO -f R COO —> R COOR R2O. 

Trimethylsilyl iodide 17, which can be generated in situ by reaction of trimethyl-silyl chloride (TCS) 14 with Nal in acetonitrile [1], converts alcohols 11, in high yields at room temperature, into their iodides 773a, HI, and hexamethyldisiloxane (HMDSO) 7 [1-8, 12]. Likewise esters such as benzyl benzoate are cleaved by Me3SiCl 14/NaI in acetonitrile under reflux [Ij. Reactions of alcohols 11 with trimethylsilyl bromide 16 in chloroform or, for in situ synthesis of 16 from liBr and TCS 14 in acetonitrile and with HMDS 2 and pyridinium bromide perbromide, proceed only on heating in acetonitrile or chloroform to give the bromides 773 b in nearly quantitative yield [3, 8, 12] (Scheme 6.1). 

Adogen has been shown to be an excellent phase-transfer catalyst for the per-carbonate oxidation of alcohols to the corresponding carbonyl compounds [1]. Generally, unsaturated alcohols are oxidized more readily than the saturated alcohols. The reaction is more effective when a catalytic amount of potassium dichromate is also added to the reaction mixture [ 1 ] comparable results have been obtained by the addition of catalytic amounts of pyridinium dichromate [2], The course of the corresponding oxidation of a-substituted benzylic alcohols is controlled by the nature of the a-substituent and the organic solvent. In addition to the expected ketones, cleavage of the a-substituent can occur with the formation of benzaldehyde, benzoic acid and benzoate esters. The cleavage products predominate when acetonitrile is used as the solvent [3]. 

Pyridinium dichromate-Chlorotri-methylsilane, 327 Tricarbonyl(methyl benzoate)-chromium, 19... 

In order to arrive at ketone 2, benzoate 3 was reduced with lithium aluminium hydride, and the resulting alcohol oxidised with pyridinium... 

The first step in the mechanism involves the reduction of Cu(II) to Cu(I) by ascorbyl-6-hexadeeanoate giving dehydroascorbic acid and a weak acid HY benzoic acid). In fact this stage of the process has no importance since Cu(I) benzoate may directly be used to initiate the polymerization by reducing the pyridinium salt. The strong Bronsted acid formed attacks the monomer and initiates the polymerization. Notably, lower polymer yields were obtained by using pyridium salt rather than iodonium salt. 

Our final example is a base-labile 4-(phenylsulfonyl)methyl-l,3-dioxolane protecting group for aldehydes and ketones.4 Protection is carried out by the reaction of diol 17,1 (obtained by dihydroxylation of ally phenyl sulfone) with a carbonyl compound in the presence of pyridinium p-toluene sulfonate [Scheme 2.17], Cleavage is accomplished by treatment with DBU. /erf-Butyldimethylsilyl ethers, p-toluenesulfonate esters, tetrahydropyranyl ethers, carboxylic esters and benzoates are well tolerated. A disadvantage to the use of 17.1 is the introduc-... 

Attack of pyridine on the bis-dibromocarbene adduct (474) results in formation of a pyridinium benzocyclobutene (475). Krohnke reaction of the latter with p-nitro-sodimethylaniline affords the benzocyclobutenone (476), which is opened by sodium hydroxide to the arylacetic acid (477 R = H, n = 1). Direct oxidation of (475) with alkaline permanganate gives a mixture of the phthalate (478) and benzoate (477 R = Br, = 0) the latter probably being formed via (476). °... 

The reaction of benzoyl chloride with ethanol and pyridine to form ethyl benzoate and pyridinium chloride can proceed either by nucleophilic attack of the ethanol or pyridine in the first step. With nucleophilic attack by pyridine, an acyl-pyri-dinium species forms, so this kind of reaction amounts to nucleophilic catalysis by this added base. If ethanol attacks first, the pyridine only acts to scavenge the HCl formed in the reaction. Write these two possible mechani.sms, and derive rate laws that distinguish these possibilities. 

Preparation.—Full details have now appeared of a useful addition to the limited list of methods for the conversion of primary amines into alcohols or their derivatives. Reaction of the amine with 2,4,6-triphenylpyrilium tetrafluoro-borate and pyrolysis of the resulting pyridinium salt (1) in the presence of sodium acetate or benzoate gives the corresponding alcohol as an acetate or benzoate ester in good yield (Scheme 1). 

Benzoylimidazole (35) reacted with alcohols (or amines) to yield benzoates (or ben-zamides) in the presence of 2 equiv. pyridinium chloride (Scheme 11). The proposed mechanism (Scheme 12) involved a combination of acid and nucleophilic catalysis. Thus, protonation of benzoylimidazole (35) by pyridinium chloride (36) yielded pyridine (38) as a by-product, which reacted with the protonated intermediate (37) to give... 

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