Trialkyloxonium tetrafluoroborate

O-Alkylation of A-unsubstituted /3-lactams to give the corresponding 2-alkoxy-l- etines can be achieved by reaction of the azetidin-2-ones with hard electrophiles (trialkyloxonium tetrafluoroborates) followed by treatment with base (cf. Section 5.09.4.3.1) (67JHC619, 69LA(725)124). In contrast, reaction of the A-unsubstituted azetidin-2-ones (73) or their derived anions with a variety of softer electrophiles results in A-substitution, and some representative reactions are illustrated in Scheme 7. 

O-Alkylation of the readily available iV-unsubstituted azetidin-2-ones (/3-lactams) constitutes a versatile route to 2-alkoxy-l-azetines (cf. Section 5.09.3.2.3). Thus treatment of the /3-lactams (266) with trialkyloxonium tetrafluoroborates followed by basification affords the 2-alkoxy-l-azetines (267) in moderate yields (67JHC619,69LA(725)124). Similar treatment of the azetidine-2-thiones (268) (available from thiation of the corresponding /3-lactams with phosphorus pentasulfide) affords the analogous 2-ethylthio-1-azetines (269) (67JHC619), which are generally more stable than their 2-alkoxy analogues. 

Triazoles with alkyl, aryl, or acyl substituents on N(l) or N(4) can be readily quaternized with powerful quaternizing agents such as trialkyloxonium tetrafluoroborates. Quaternization takes place such that maximum separation of the substituents occurs. 

Alkylation of all three triazole annular nitrogens can be effected by trialkyloxonium tetrafluoroborates (Scheme 9) <72JOC2259>. 

O -Alkylations of azepin-2-ones, and benzazepin-2-ones, are most efficiently brought about by trialkyloxonium tetrafluoroborates (Meerwein s reagents) (B-69MI51600,73JOC1090, 81HCA373). S-Alkylation of the thiones is effected similarly. These oxonium salts have also proved useful for the alkylation of azepinedione precursors of azatropones and azatropolones (72JOC208) (see also Section 5.16.3.1.2). 5//-Dibenz[f>,e]azepine-6,11 -dione with triethyloxonium tetrafluoroborate O-ethylates at the amide carbonyl and not at the ketonic carbonyl as was first proposed (72AJC2421). 

In many cases, the main step in the syntheses of trialkyloxonium salts is the alkylation of a dialkyl ether with a reactive intermediate oxonium ion formed in situ. Thus, the most widely used method for preparing trialkyloxonium tetrafluoroborates by the reaction of epichlorohydrin and BF3 is based on the intermediacy of the inner oxonium salt 3291 95 97 [Eq. (4.19)]. 

The molecular constraint which precludes ligand exchange unambiguously ensures that the tricyclic compounds undergo nucleophilic attack exclusively at the pseudo-equatorial carbon atom, irrespective of the nature of the carbon atom situated in the apical ligand of the TBP. The compounds 1-4 were obtained by alkylation of the chalcogen atom of the corresponding bicyclic (thio)phosphate, 6 and 7, with trialkyloxonium tetrafluoroborate at -78 °c (2). 

Both types of thiatriazoline mentioned above and 4,5-dialkyliminothiatriazolines may be prepared in moderate to high yield by alkylation of 5-alkylamino-, 5-arylamino- and 5-sulfonylamino-thiatriazoles with trialkyloxonium tetrafluoroborates (equation 48). Thiatriazolines are formed exclusively with this reagent (78JCS(P1)1440). The thiatriazoline tetrafluoroborates are crystalline solids which may be stored at room temperature and serve as convenient sources for the thermally labile free bases (75). These may be liberated with aqueous NaHC03 (78JCS(P1)1440). 

The application of 300 MHz H-NMR to the polymerization of 3,3-dimethylthietane initiated with trialkyloxonium tetrafluoroborate (initiator concentration higher than 0.02 mole 1 ) permits polymeric sulfonium ions to be observed ... 

Triazoles with alkyl, aryl or acyl substituents on N-1 or N-4 can be quaternized. Because of the mesomeric distribution of the positive charge on triazolium compounds, representations such as (63) are convenient but the equivalent formula (64) may be used to denote the site at which quaternization has taken place (the mesomeric nature of (64) must be kept in mind of course). Trialkyloxonium tetrafluoroborates are powerful quaternizing reagents (70JOC2246). 

Other alkylating agents which have been used include diazomethane [2], trialkyl phosphates [11], alkoxypho.sphonium salts [12], dimethylformamide dialkyl acetals [13], trialkyl orthoformates [14], alkyl cyanoformates [15], dialkyl carbonates (less toxic) [16] and, where more powerful reagents arc needed, alkyl fluorosulfonates, trialkyloxonium tetrafluoroborates [17], alky] triflates and mesylates [18, 19]. 

A superior route to these compounds using the condensation procedures involves the use of alkoxycyclopropenyl cations (equation 73) . These can be prepared readily by alkylation of cyclopropenones with trialkyloxonium tetrafluoroborates . This process can be extended to a large number of active methylene compounds if the conditions of the reaction are controlled carefully and the tertiary non-nucleophilic base diisopropylethy-lamine is used Otherwise, a large number of side reactions are observed. 

Several examples of intramolecular epoxide formation have been reported. - In general, a keto sulfide is reacted with a trialkyloxonium tetrafluoroborate to yield a sulfonium salt, which cyclizes upon treatment with base. A particularly clever variation involves the in situ generation of an allylic ylide (Scheme 2). ... 

The action of n-butyllithium in THF at low temperature leads to the anion. After reaction with an electrophilic compound, e hydrolysis can generally be carried out in polar solvents (acetone, alcohols, acetonitrile) in the presence of mercury(II) chloride or oxide and water.In other cases, NBS, - chloramine T, cerium(IV) ammonium nitrate, n-tributyltin hydride, trialkyloxonium tetrafluorobor-ate, thallium nitrate or photochemistry can be used. Desulfurization by Raney nickel gives hydro-carbons. ... 

In reactions of chiral compounds requiring the use of a base, the proton sponge practically causes no racemisation and favours the retention of high optical purity. An example is the conversion of optically active alcohols into ethers under the action of trialkyloxonium tetrafluoroborates (equation 19)218-220. 

R3NCH2SR]+ salts with trialkyloxonium tetrafluoroborates. The reaction has been generalized to give many products.49... 

The diazines react with alkyl halides to give mono-quaternary salts, though somewhat less readily than comparable pyridines. Dialkylation cannot be achieved with simple alkyl halides, however the more reactive trialkyloxonium tetrafluoroborates do convert aU three systems into di-quatemary salts. ... 

The alkylation of aromatic 1,2,4-triazines has been studied by various groups.251 -259 Alkylation with iodomethane affords the red 1-methyl-1,2,4-triazinium salts 4c-j predominantly and in a few cases yields the colorless 2-methyl-l,2,4-triazinium salts 5. The best results are obtained when nitromethane is used as the solvent for the alkylation. Similar results are obtained when trialkyloxonium tetrafluoroborates or dialkyl sulfates are used as alkylation agents. The formation of either the 1- or the 2-isomers depends on the substituents bound to the 1,2,4-triazine ring. In cases with a bulky substituent in the 3-position, alkylation occurs exclusively at Nl, in other cases mixtures of N1 and N2 alkylated triazinium salts are obtained. Introduction of a more bulky substituent, such as isopropyl or aryl substituents, in the 6-posi-tion hinders quaternization at Nl. 

Recently, cationic iV-sulfinyl amines and sulfur diimides have been reported. These species are readily prepared by alkylation of a IV-sulfinyl compound or a sulfur diimide with a trialkyloxonium tetrafluoroborate [Eq. (8)].5... 

Zur Alkylierung werden sowohl Alkalimetall- und Erdalkalimetall-Salze als auch Silber-, Quecksilber- und Kupfer-Salze herangezogen. Als Alkylierungsmittel werden Dialkylsulfa-te, Trialkyloxonium-tetrafluoroborate oder langkettige Alkylchloride benutzt. 

Isothiazoles are quatemized by iodoalkanes, dialkyl sulfate, trialkyloxonium tetrafluoroborate or diazomethane. 

Diem et al. reported a mild alkoxylation reaction using trialkyloxonium tetrafluoroborate (Meerwein s salt) [7]. In this reaction. Proton Sponge (1) was found to be an effective base, as well as Hiinig s base (diisopropylethylamine) (Table 8.1). The reaction of (R)-(+)-l-phenylethyl alcohol with Meerwein s trimethyloxonium tetrafluoroborate (2.1 equiv.) and Proton Sponge (1) (2.1 equiv.) in dichloromethane at room temperature gave methyl ether in 69% yield without loss of optical purity (Table 8.1, run 1). (5)-(—)-2-Methyl-l-butanol was also methylated in 57% yield (run 3). 

Phthalides are also versatile precursors for the preparation of l-alkoxybenzo[c]furans. (7-Alkylation of (94) with trialkyloxonium tetrafluoroborate or diethoxymethylium tetrafluoroborate with... 

Preparation.— The classical Williamson route to ethers from alcohols and alkyl halides can lead to racemization of chiral centres carrying hydrogen and situated a-or /8- to the alcohol OH groups. A new synthesis of methyl and ethyl ethers that avoids such problems uses trialkyloxonium tetrafluoroborates as the alkylating agents [equation (14)] in the presence of a non-nucleophilic base to remove the acid... 

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