Triethyloxonium Tetrafluoroborate1

R3O+ X-
(1; R = Et, X = BF4)

[368-39-8]  · C6H15BF4O  · Triethyloxonium Tetrafluoroborate  · (MW 190.02) (2; R = Et, X = PF6)

[17950-40-2]  · C6H15F6OP  · Triethyloxonium Hexafluorophosphate  · (MW 248.18) (3; R = Et, X = SbCl6)

[3264-67-3]  · C6H15Cl6OSb  · Triethyloxonium Hexachloroantimonate  · (MW 437.66) (4; R = Et, X = SbF6)

[19554-80-4]  · C6H15F6OSb  · Triethyloxonium Hexafluoroantimonate  · (MW 338.96) (5; R = Me, X = BF4)

[420-37-1]  · C3H9BF4O  · Trimethyloxonium Tetrafluoroborate  · (MW 147.93) (6; R = Me, X = PF6)

[12116-05-1]  · C3H9F6OP  · Trimethyloxonium Hexafluorophosphate  · (MW 206.09) (7; R = Me, X = SbCl6)

[54075-76-2]  · C3H9Cl6OSb  · Trimethyloxonium Hexachloroantimonate  · (MW 395.57) (8; R = Me, X = SbF6)

[29830-67-9]  · C3H9F6OSb  · Trimethyloxonium Hexafluoroantimonate  · (MW 296.87) (9; R = Me, X = 2,4,6-trinitrobenzenesulfonate)

[13700-00-0]  · C9H11N3O10S  · Trimethyloxonium 2,4,6-Trinitrobenzenesulfonate  · (MW 353.30)

(powerful alkylating agent for the ethylation of sensitive or weakly nucleophilic functional groups;1 analogous methylations are achieved using trimethyloxonium salts2)

Alternate Name: Meerwein's salt.

Physical Data: (1) mp 92 °C; 116 °C; (2) 142-144 °C; (3) 130-133 °C; 135-137 °C; (4) 139 °C; (5) 141-143 °C; 179-180 °C; (6) 262 °C; (7) 156-158 °C; 159 °C; (8) 239-241 °C (9) 120-130 °C with effervescence and resolidifying; 181-183 °C.

Solubility: (1)-(8) sol liquid sulfur dioxide; (1) and (5) sol dichloromethane, chloroform, nitromethane; insol diethyl ether; (2) and (6) sol dichloromethane, arsenic trifluoride; (3) and (7) sol nitromethane; slightly sol dichloromethane, chloroform, tetrachloromethane, 1,2-dichloroethane; (9) sol nitromethane. A scale of solvent nucleophilicity has been based on the solvolysis of (2).3

Form Supplied in: (2), (3), (5), and (7) are commercially available as colorless solids; (1) is supplied as a 1 M solution in dichloromethane or as a colorless solid stabilized with diethyl ether.

Preparative Methods: there is an established method for (1).4 (5) is reported2 being contaminated by ethyldimethyloxonium salt. Convenient syntheses of pure (5) have been published;5,6 similar methods yield (2) and (6).7 Syntheses of (3) and (7),1a,8 (4) and (8),8 and (9)9 are available.

Handling, Storage, and Precautions: solvent-free (1) is hygroscopic; it is conveniently stored in diethyl ether at -20 °C in a tightly stoppered bottle.10 Contrary to recommendations,4 the use of a dry box or an inert atmosphere is not required for most applications of (1). A sample of the oxonium salt-ether slurry may be transferred to the tared reaction flask, and after removal of the ether on a rotary evaporator the resulting solid may be used without further purification.10 When alkylation requires the absence of acid, (1) should be handled in a dry box and before use it should be washed with anhydrous diethyl ether and dried by a stream of nitrogen.

Solvent-free (5) is less hygroscopic2 than (1) and is stored in a tightly sealed bottle at -20 °C. It may be handled in the air for short periods of time. (9) is nonhygroscopic, may be stored at rt, but is more laborious to prepare than (5).

Trialkyloxonium salts have irritant and corrosive properties and are toxic due to their alkylating ability. In comparison with other alkylating agents, the dangers are minimized by the fact that oxonium salts are water-soluble, nonvolatile, crystalline solids, which are rapidly solvolyzed in aqueous solution. Nevertheless, skin and eyes should strictly be protected from contact with the solid agents or their solutions. Use in a fume hood.

General Reactivity.

The high alkylating potential of trialkyloxonium salts has been well known since their discovery.11 When either methylation or ethylation of weakly nucleophilic functional groups is desired, the use of the more reactive Me3O+ is preferred to that of Et3O+ salts.2b Ranking the common methylating agents in a sequence of decreasing methylating ability, Me2Cl+SbF6- > (MeO)2CH+BF4- > Me3O+BF4- (SbCl6-, PF6-) > MeOSO2CF3 > MeOSO2F > (MeO)2SO2 > MeI,12,13 demonstrates that Me3O+ salts possess high, but not the highest, reactivity (see also O-Methyldibenzofuranium Tetrafluoroborate). Since the low solubility of Me3O+ salts may be a problem, sometimes the less reactive (and more toxic) MeOSO2F (magic methyl)13 may be the reagent of choice for methylations. Trialkyloxonium SbF6-, SbCl6-, and PF6- salts are less sensitive to decomposition due to high anion stability than the corresponding tetrafluoroborates.1e Nevertheless, the latter are preferentially used depending on convenience and availability.

Alkylation of Anionic Nucleophiles.

The efficient transfer of alkyl groups from Me3O+ or Et3O+ salts to anionic nucleophiles has been applied to remove halide ions via alkylation and to replace them by suitable complex anions. Thus N-cyano- or N-acyl-N,N,N-trialkylammonium tetrafluoroborates have been obtained under anhydrous conditions via reaction of the corresponding halides with Et3O+BF4- (1).14 Since, besides the desired salts, only volatile alkyl halides and the corresponding ethers are formed, the method is superior to the treatment with Silver(I) Tetrafluoroborate, which requires separation from silver halide precipitates. Using Me3O+BF4- (5) the first stable b-phospha- and b-arsatrimethinecyanines (10+BF4-)15,16 have been prepared from the appropriate bromophosphane or -arsane precursors (10), the solutions of which show only low equilibrium concentrations of cyanine bromides (10+Br-), (eq 1); there are similar applications to transition metal-halogen complexes.17

Of the numerous alkylations of anionic substrates with trialkyloxonium salts,1c,e esterification of carboxylate anions is of special preparative interest. Although oxonium salts are readily hydrolyzed in aqueous solution, water may be a suitable solvent, when a high excess of Et3O+BF4- (1) or Me3O+BF4- (5) is used in the presence of NaHCO3 or Diisopropylethylamine.18,19 Under these conditions, the in situ generated carboxylate anions of N-acylamino acids,18 dipeptides (eq 2),18 or labile hemins19 are smoothly O-alkylated. ε-Amino, guanidino, carboxamido, and hydroxyl functions of alcohols or phenols may be present, but remain unaltered. Smooth esterifications of even sterically hindered carboxylic acids with equimolar amounts of (1) and (5) may conveniently be performed in dichloromethane using diisopropylethylamine as base.10 Similarly, ethyl (Z)-2-methyl-4-oxo-2-butenoate is prepared from the corresponding hydroxybutenolide via O-ethylation of a carboxylate anion intermediate (eq 3).20

In the absence of a suitable base, no esterification of acetamino acids is achieved using Me3O+BF4- (5), since under these conditions the carboxamide is more reactive than the carboxylic acid. The resulting imino ester salt may readily be converted into the corresponding N-ethylamino acid via reduction with NaBH4 (eq 4).21

Trialkyloxonium salts, like other strongly electrophilic agents, when treated with ambident anions yield considerable amounts of products derived from alkyl attack at the site of highest electron density.1d,e,22 Thus, in aprotic polar solvents, enolates of aliphatic ketones give mainly enol ethers via O-ethylation with Et3O+BF4- (1),23 while C-alkylation is predominant with the less reactive ethyl iodide (eq 5).22,23 The ambident anions of imines behave similarly: with (1), enamines (via N-alkylation) are preferentially formed instead of the usually obtained C-alkylation products.24 A discussion of the factors influencing the competition of C- and heteroatom-alkylation is available.22 Exclusive mono- or bis-O-alkylations have been reported to occur for the reactions of cyclic b-diketones with equimolar or excess amounts of (1) (eq 6).25

Being formal analogs of enolates, lithium transition metal acylate complexes are readily O-alkylated with Et3O+BF4- (1) or Me3O+BF4- (5) to give alkoxycarbene complexes.26 The metal acylate complexes are conveniently prepared from commercially available metal carbonyl complexes via addition of a suitable nucleophile (e.g. alkyl-, aryl-, alkynyllithium, or lithium dialkylamide). Nucleophilic addition and subsequent alkylation may be performed in a one-pot reaction (eq 7).26,27 Alkoxycarbene complexes (especially those of chromium or tungsten) are valuable, versatile building blocks for numerous organic syntheses.28 The unique benzannulation depicted in eq 8 may serve as a representative example;28a the starting material of this reaction is available via addition of Phenyllithium to Hexacarbonylchromium and sequential methylation with (5).26

Ethylations of lithium (or sodium) 1-alkynyltrialkylborates with Et3O+BF4- (1) afford (Z)/(E)-alkenyldialkylboranes via an addition-rearrangement mechanism.29,30 The boranes may readily be hydrolyzed to the corresponding (Z)/(E)-alkenes (eq 9).29a Oxidation of the alkenyldialkylborane intermediates provides a valuable route to ketones free of complications associated with the production of (Z)/(E) mixtures (eq 9).29a The highly stereoselective formation of a cyclic (E)-alkenylborane is favored, when 3-methoxypropynyltriethylborate is alkylated with (1) (eq 10).30

Alkylations of Uncharged Nucleophiles.

Usually Me3O+ or Et3O+ salts are applied to alkylate uncharged molecules.1 The resulting cationic intermediates, though frequently subject to subsequent reactions with nucleophiles, have been isolated as stable salts in many cases.1a,c,e Thus trialkyloxonium salts are obtained from alicyclic (or acyclic dialkyl) ethers (eq 11)11b and alkoxycarbenium salts from dialkyl (or vinyl) ketones.1a The structures of some of these salts have been determined by using X-ray diffraction, e.g. the 1-ethoxyhomotropylium cation prepared from cyclooctatrienone according to eq 12 has been shown to be best regarded as a linear ethoxyoctatrienyl cation and not as a homoaromatic system.31 O-Alkyllactonium salts may be generated from lactones (eq 13)32,33 as well as trialkoxycarbenium salts from dialkyl carbonates.1a,32a However, most acyclic esters do not react with Me3O+ or Et3O+ salts.1a,c,e,33 A discussion on the basis of equilibrium constants and calculations of methyl affinities has been published.33 Even O-alkylation of N-methylenecarbamates leading to 1,1-dialkoxy-2-azapropenylium salts can only be achieved if the N-alkylation is sterically unfavorable, otherwise iminium salts are preferentially formed (eq 14).34

Reactions of Me3O+ or Et3O+ salts frequently afford isolable salts via elimination of a suitable leaving group from reactive, primary alkylation products, e.g. the alkylation of an alkoxy function generates an unstable trialkyloxonium intermediate, which readily loses a dialkyl ether to yield a stabilized carbocation. The formation of alkoxycarbenium salts from acetals (eq 15)32a or that of 1,3-dioxolan-2-ylium salts from 2-alkoxyethyl carboxylates (eq 16)35 may serve as examples. The latter reaction has also been used to prepare ethylene acetals and hence aldehydes from esters via hydride transfer to 1,3-dioxolan-2-ylium intermediates (eq 17).36

Alkylations with Me3O+ or Et3O+ salts are usually performed to increase the reactivity of numerous functional groups toward nucleophiles. This is achieved by generating heteroatom-stabilized cations (mostly carbenium ions) from hetero-p-systems or by improving the leaving group ability of saturated n-bases via alkylation. For preparative applications, O- and S-alkylations play the most important role.

Activation via O-Alkylation.

With Et3O+BF4-, 2,3-disubstituted cyclopropenones afford 1-ethoxycyclopropenylium salts,37,38 which readily react with nucleophiles;38 the synthesis of cyclic acetals or dithioacetals from 2,3-diphenylcyclopropenone may serve as an example (eq 18).39 O-Alkyllactonium salts, generated according to eq 13, are smoothly converted into cyclic orthoesters with sodium alkanolates32 or into thionolactones via sulfhydrolysis with sodium hydrosulfide (eq 19).40 Besides these kinetically controlled additions to the carbenium center, many reactions of lactonium salts with nucleophiles have been reported to occur with ring opening.1a,c,32a The highly regio- and stereoselective formation of methyl (E)-4,8-alkadienoates via the in situ methylation of g-alkenyl-g-butyrolactones in the presence of allylsilanes illustrates this mode of reaction (eq 20).41

The rather intractable carboxamide or lactam groups are transformed into highly reactive, versatile intermediates by O-alkylations with Me3O+ or Et3O+ salts.1c,e,42 Depending on the structure of the substrates and the workup, the resulting alkoxyaminocarbenium salts1,42,43 (eq 21)32a are obtained or their subsequent transformation products: imino esters1c,44 (eq 22)44a, amide acetals1c,42,43 (eq 23),43a or ketene O,N-acetals43,45 (eq 24)43a, which are usually submitted to further conversions.1a,c,e,42-45

The important bis-lactim ether method for the enantioselective synthesis of, for example, nonproteinogenic amino acids from the corresponding 2,5-diketopiperazines,46 is based on the amide O-alkylation (eq 25)46a as well as the smooth hydrolysis of secondary1c,21,47 or tertiary48 carboxamides (eq 26)48 and lactams.49

Activation via S-Alkylation.

Although several reagents are available for S-alkylations of acyclic or cyclic thioethers, Me3O+ or Et3O+ salts are frequently preferred since they provide sulfonium salts with nonnucleophilic anions.1c,e,50 Deprotonation of suitable sulfonium derivatives yields sulfonium ylides as intermediates or isolable species,51 which are of high preparative value. The stereoselective epoxy-annulation of ketosulfonium salts via sulfonium ylides serves as an example52 (eq 27).52a Sulfonium salts are usually generated to provide leaving groups, e.g. for nucleophilic displacements53 (eq 28)53a or for the formation of reactive iron carbene complexes54 (eq 29).54a

The rather impracticable hydrolysis of cyclic thioacetals may smoothly be performed via S-alkylation50 with excess Me3O+BF4- in the presence of water (eq 30).55 Similarly, a thioamide is easily hydrolyzed after S-alkylation with Et3O+BF4- (eq 31);56 analogous activations of thiono functions have been reported.57

The example depicted in eq 31 shows the striking chemoselectivity of ethylations with Et3O+BF4- in favor of a thioamide group, which is exclusively attacked at the sulfur atom.56 The carboxamide function present, which has a similar reactivity,47b in this case is sterically shielded.

Activation via N-Alkylation.

Alkylations of amines or imines leading to ammonium58 or iminium salts59 usually do not require powerful alkylating agents; nevertheless, Me3O+ or Et3O+ salts have also been applied.1,58,59 Thus a dibenzo[b,g][1,5]thiazocine derivative with Me3O+BF4- is exclusively methylated at the amino group60 (eq 32).60a However, the corresponding sulfoxide yields the O-methylated product, in which significant N-S interaction with the hypervalent sulfur atom has been shown to be present60b (eq 33).60a,b

Besides numerous N-alkylations of alicyclic or aromatic five- and six-membered N-heterocycles,1,61 reactions of preparative interest of Me3O+ or Et3O+ salts have been carried out with nitriles. The resulting nitrilium salts1c,62 are versatile and reactive intermediates;63 (see also O-Methyldibenzofuranium Tetrafluoroborate). Their hydrolysis yields secondary carboxamides, and reactions with alcohols or amines result in the formation of imino esters or amidines.63

N-Ethylnitrilium salts are conveniently converted into aldehydes by reduction with trialkylsilanes and sequential hydrolysis of the resulting imines (eq 34).64 The same salts yield secondary amines via imino esters formed in situ, and subsequent reduction with Sodium Borohydride (eq 35).65 Since neither the nitrilium salts nor the intermediates have to be isolated, the transformations are performed as one-pot procedures starting from suitable nitriles.64,65 N-Methylnitrilium salts have also been used for the smooth acylation of pyrrole and indole derivatives (eq 36).66

Related Reagents.

Dimethoxycarbenium Tetrafluoroborate; Dimethyliodonium Hexafluoroantimonate; O-Methyldibenzofuranium Tetrafluoroborate; Methyl Trifluoromethanesulfonate.

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Hartwig Perst

Philipps-Universität Marburg, Germany

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