Meerwein salt

Nucleophilic addition of methyl- or phenylphosphite n -butyl- esters to oxo-imine salts, generated by nitrone alkylation of triethyloxoniumtetrafluoro borate (Meerwein salt), leads to a-aminophosphinic acid esters (Scheme 2.201) (691). 

The unsymmetrical 3,6-dialkoxy-2,5-dihydropyrazines are prepared via the jV-carboxyanhy-drides of the particular amino acid (e.g., 7) and subsequent reaction with the methyl ester of glycine or alanine to give the corresponding cyclodipeptides (e.g., 8). These hexahydrodioxopy-razines are converted to the 2,5-dialkoxy-3,6-dihydropyrazines by alkylation using Meerwein salts, such as trimethyloxonium tetrafluoroborate or triethyloxonium tetrafluoroborate n lb. 

In some procedures, thioacetal cleavage occurs after alkylation at the sulfur atom with a reactive alkylating reagent such as methyl iodide, tnalkyloxonium tetrafluoroborate (Meerwein salt), or ethyl trifluoromethanesulfonate to give a tnalkylsulfonium salt. 

Bimetallic alkoxides ( Meerwein salts ) have been known for more than 70 years, since the report made by Meerwein and Bersin [1101] on the formation of alkoxosalts (analogs of hydroxosalts MM m(OH)n) on titration of alcohol or benzene solutions of acidic alkoxides by those of basic ones to the equivalence points determined using the acid-base indicators ... 

The classic study of Al(OR)3 was carried out by Tishchenko [1585] at the end of nineteenth century. His dissertation, entitled On the Action of Amalgamated Aluminum on Alcohols (Aluminum Alkoxides, Their Properties and Reactions), became a great resonance for the chemistry of alkoxides (see also Chapter 1). The synthetic approaches, that he developed are still in use the purity of the samples obtained by Tishchenko, taking into account the data he reported, was definitely not worse than that of those described at present. In 1929 Meerwein and Bersin [1101], performing the acidimetric titration of Al(OR)3 solutions by solutions of alkali alkoxides, discovered the existence of bimetallic alkoxides (Meerwein salts), which play an important role in the modern chemistry of metal alkoxides. 

The transient nature of carbocations arises from their extreme reactivity with nucleophiles. The use of low-nucleophilicity counterions, particularly tetrafluorobo-rates (B I, ), enabled Meerwein in the 1940s to prepare a series of oxonium and carboxonium ion salts, such as R30+BF4 and HC(OR)2+BF4, respectively.13 These Meerwein salts are effective alkylating agents, and they transfer alkyl cations in SN2-type reactions. However, simple alkyl cation salts (R 1 BF4 ) were not obtained in Meerwein s studies. The first acetyl tetrafluoroborate—that is, acetylium tetrafluor-oborate—was obtained by Seel14 in 1943 by reacting acetyl fluoride with boron trifluoride at low temperature [Eq. (3.1)]. 

Functionalized chromium carbene complexes can be prepared by the reaction of polyfunctional diorganozincs with photochemically generated Cr(CO)5.THF. The resulting intermediate ate complexes furnish under 1 atm of carbon monoxide an acyl complex which can be treated with Meerwein salt (Me30+ BF4" in dry CH2C12) at -30°C to give a chromium carbene complex (Scheme 9.3). 

Meerweins salt [lb]. Alternatively, the anionic intermediates 3 can be generated by reaction of acid chlorides with Na2[Cr(CO)5] (Scheme 1) [3]. 

Cyclopropanation reactions of nonheteroatom-stabilized carbenes have also been developed. The most versatile are the cationic iron carbenes that cyclopropanate alkenes with high stereospecificity under very mild reaction conditions. The cyclopropanation reagents are available from a number of iron complexes, for example, (9-alkylation of cyclopentadienyl dicarbonyliron alkyl or acyl complexes using Meerwein salts affords cationic Fischer carbenes. Cationic iron carbene intermediates can also be prepared by reaction of CpFe(CO)2 with aldehydes followed by treatment with TMS-chloride. Chiral intermolecular cyclopropanation using a chiral iron carbene having a complexed chromium tricarbonyl unit is observed (Scheme 61). 

Dialkylated quinoxalines are accessible by reaction of quinoxalines with reactive trialkylox-onium (Meerwein) salts. ... 

Alkylthio-selenazoles are alkylated on nitrogen using Meerwein salts to produce the corresponding 2-alkylthio-3-methyl-l,3-selenazolium salts. The alkylthio group in these salts can be replaced with selenium using NaHSe to afford 3-methyl-l,3-selenazole-2-selenone 70 (Scheme 5) <2003NJC1622>. 

Attempts to achieve the thermal ring closure of the Meerwein salt 137 to the benzo[c]phenanthridine system failed the major product was the 4-acyliso-quinolinium salt 138 (79T1861). The reaction between )V-benzylaminoacetal-dehyde dialkyl acetals (139) with methyl vinyl ketone in acidic solution, leading to benzo[h]quinolizine derivatives, was newly investigated. The... 

Benzomiophenium salts are produced by me reaction of me sulfur heterocycle with powerful alkylating reagents such as Meerwein salts benzomiophenium salts can themselves act as powerful alkylating agents... 

AT-Alkylation of oxazoles, or imidazoles carrying, for example, a phenylsulfonyl or acyl group on nitrogen, is more difficult, requiring methyl triflate or a Meerwein salt for smooth reaction. Subsequent simple alcoholysis of the imidazolium-... 

Yang and Woerpel [93] prepared a series of lactones which were then converted into dioxocarbenium ions by alkylation with Meerwein salts [91, 92]. 

Alkylation of oxazoles, or imidazoles carrying, for example, a phenylsul-fonyl group on nitrogen, is more difficult, requiring methyl triflate or a Meerwein salt for smooth reaction. Subsequent simple alcoholysis of the imida-zolium-sulfonamide releases the A -substituted imidazole. Moreover, since acylation of 4(5)-substituted imidazoles gives the sterically less crowded 1-acyl-4-substituted imidazoles, subsequent alkylation, then hydrolytic removal of the acyl group, produces 1,5-disubstituted imidazoles. ... 

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