2,4,6-Triphenylpyrylium Tetrafluoroborate1

[448-61-3]  · C23H17BF4O  · 2,4,6-Triphenylpyrylium Tetrafluoroborate  · (MW 396.21)

(converts primary amines into pyridinium salts which on reaction with nucleophiles NuH afford R-Nu)

Physical Data: mp 255-257 °C.

Solubility: insol H2O, EtOH, Et2O; sol CF3CO2H.

Form Supplied in: yellow solid.

Purification: recryst. from AcOH or (ClCH2)2.

Handling, Storage, and Precautions: may be stored indefinitely. Reports on possible mutagenic activity make it desirable to avoid direct contact with skin. Other 2,4,6-triphenylpyrylium salts are the commercially available sulfoacetate, and the hydrogen sulfate whose synthesis2 is analogous to that of (1) as described by eq 1 below.


2,4,6-Triphenylpyrylium salts may be obtained by a variety of methods.3 The strong green fluorescence of 1,3,5-triphenylpentane-1,5-dione (benzylidenediacetophenone) in H2SO4 was observed as early as 1896, but the cause was discovered later (H2SO4 acts as a dehydrogenating and dehydrating agent, yielding the fluorescent triphenylpyrylium cation). Michael condensation of chalcone (2) with acetophenone yields the above 1,5-dione, and this reacts with various hydride acceptors furnishing 2,4,6-triphenylpyrylium salts in excellent yields: with Iron(III) Chloride in Ac2O the tetrachloroferrate is obtained;4 with Tetrafluoroboric Acid or HClO4 in Ac2O the fluoroborate (eq 1) and the perchlorate, are obtained respectively.5 The same two salts may also be obtained using the corresponding triphenylmethyl salts as hydride acceptors,6 which yield triphenylmethane by hydride transfer.

A procedure which is included in Organic Syntheses is based on the above reaction,7 but the hydride acceptor is chalcone itself in acid medium (HClO4 or HBF4): its conjugate acid affords 1,3-diphenylpropan-1-one (3) by hydride transfer.8 From two moles of chalcone (2), one mole of acetophenone, and one mole of an ethereal solution of tetrafluoroboric acid in 1,2-dichloroethane the yield of 2,4,6-triphenylpyrylium tetrafluoroborate (1) was 55-60%.7

This optimal synthetic procedure is a two-component synthesis. Other two-component syntheses of (1) or salts with other anions include: the replacement of acetophenone in the above reaction by its equivalent synthon, phenylacetylene (Boron Trifluoride Etherate appears to be the best anion-forming reagent, leading9 to an 80% yield of 1); the reaction of phenylacetylene and 3-chloro-1,5-diphenylpentanedione in the presence of Tin(IV) Chloride;10 the benzoylation of dypnone (4) with benzoyl fluoride in the presence of boron trifluoride etherate (eq 2); the condensation of dypnone with benzaldehyde in acetic anhydride in the presence of FeCl3; and the condensation of dibenzoylmethane with acetophenone in acetic anhydride with sulfuric acid, when 80% yields are attained.

Finally, three-component syntheses have to be mentioned because they use simpler starting materials in one-step procedures: (i) condensation of two moles of acetophenone with one mole of benzaldehyde in the presence of boron trifluoride or boron trifluoride etherate;12 the chloroferrate is produced by the same condensation in the presence of acetic anhydride and FeCl3;4 (ii) as indicated earlier, phenylacetylene may replace acetophenone,9 and the hydride acceptor is chalcone which is formed in the condensation; (iii) a hydride acceptor becomes unnecessary when two moles of acetophenone are reacted with benzotrichloride in the presence of perchloric acid;13 instead of benzoylating dypnone, one may react two moles of acetophenone with Benzoic Anhydride in the presence of FeCl3; (iv) alternatively, one may react two moles of phenylacetylene with Benzoyl Chloride in the presence of SnCl4; (v) dibenzoylation of a-methylstyrene with benzoyl chloride and Aluminum Chloride affords triphenylpyrylium tetrachloroaluminate, which can be converted into other salts.14


Weak bases (e.g. sodium acetate in aqueous ethanol) convert (1) or its salts with other anions into the stable crystalline triphenylpyrylium pseudobase (5) (1,3,5-triphenyl-2-pentene-1,5-dione) (see eq 3).

It has been reported4,15 that substituted pyrylium salts reacted readily with ammonia or primary amines, yielding pyridines and N-substituted pyridinium salts, respectively. It has been observed16 that 2,4,6-triphenylpyrylium salts react with ammonia at room temperature to yield an ether-soluble crystalline adduct which dehydrates at the melting point, resolidifying to 2,4,6-triphenylpyridine.

N-Alkyl-2,4,6-triphenylpyridinium salts may be obtained either from primary amines and 2,4,6-triphenylpyrylium salts, or by quaternization of 2,4,6-triphenylpyridine with alkyl halides. Only the former reaction is relevant to the present context.

Nucleophilic Displacement of NH2 Groups from Primary Amines via N-substituted 2,4,6-Triphenylpyridinium Tetrafluoroborates.

Primary aromatic amines can be diazotized and subsequently converted into a variety of aromatic derivatives by nucleophiles which displace the diazonium group. For aliphatic primary amines, such a displacement may be achieved using pyrylium salts as the key reagents.

In 1926, Ziegler and Fries showed17a that 1-methyl-2,4,6-triphenylpyridinium chloride thermolyzed to 2,4,6-triphenylpyridine and presumably methyl chloride. In 1969, Susan and Balaban17b reported that 1-methyl-2,4,6-triphenylpyridinium iodide or chloride decomposed on melting, affording 2,4,6-triphenylpyridine and the corresponding methyl halides, which were isolated and identified; it was also observed that N-benzyl-2,4,6-triphenylpyridinium salts were easily solvolyzed on recrystallization from ethanol, yielding 2,4,6-triphenylpyridine. These early observations led to the development, by Katritzky and co-workers, of a synthetic method for converting primary amino groups into other functional groups by means of triphenylpyrylium and related cations.

The conversion of a primary amino group into a leaving group is achieved by reaction with 2,4,6-triphenylpyrylium tetrafluoroborate (1), which leads to an N-substituted 2,4,6-triphenylpyridinium tetrafluoroborate (6). The mechanism of this reaction has been studied using n-BuNH2 in conjunction with UV and 13C NMR spectroscopic methods;18 it was concluded that when there is sufficient amine present for each mole of pyrylium salt (either &egt;2 moles of primary amine, or one mole of tertiary amine, such as triethylamine, and &egt;1 mole of RNH2), the pyrylium salt is converted entirely to a vinylogous amide (7), which is then cyclodehydrated to the pyridinium cation (6) (eq 3). The protonations and deprotonations are assisted by the reaction solvent, which may be water or a lower alcohol (such as MeOH, EtOH, i-PrOH, t-BuOH), or dichloromethane with a small amount of acetic acid. With lower amine-to-pyrylium ratios than those mentioned above, a 1,5-diketone (5) (pyrylium pseudobase) is also formed from traces of water present or originating in the cyclodehydration. The rate-determining step is the cyclodehydration, which is accelerated when acetic acid is present.

As a result of these mechanistic studies, conditions were found for rapidly obtaining good yields of pyridinium salts (6) at room temperature in mildly acidic or basic media from primary amines whose NH2 group is bound to an aromatic, a primary aliphatic, or a secondary aliphatic carbon atom.

The displacement of 2,4,6-triphenylpyridine from 1-R-2,4,6-triphenylpyridinium salts (6) can be achieved with a variety of nucleophiles NuH, yielding RNu compounds as shown in Table 1.

Replacement of NH2 by Hydrogen.

Pyridinium salts (6) add H- from sodium borohydride, yielding 1,2-dihydro derivatives. When R is alkyl, benzyl, arylmethyl, or heteroarylmethyl, these 1,2-dihydropyridines split off 2,4,6-triphenylpyridine on heating at around 200 °C and afford RH compounds in &egt;75% yields. Other R groups yield complex mixtures of products, but pyridinium salts formed from RNH2 and 2,3,5,6-tetraphenylpyrylium salts react with Sodium Borohydride, yielding 1,4-dihydropyridines which on pyrolysis at 180-200 °C afford RH in good yields.

Replacement of NH2 by Carbon Nucleophiles.

When R in (6) is benzyl, nucleophilic attack by sodium derivatives of Ethyl Cyanoacetate, ethyl phenylacetate or Diethyl Malonate in refluxing dioxane or 1,2-dimethoxyethane affords the corresponding monobenzylated esters in &egt;70% yields. With other R groups, more sterically hindered pyridinium salts have to be used, such as (8) (eq 4). However, (6) with any type of R group reacts smoothly with nitronate anions, and affords C-alkylated nitro compounds in good yields (unlike other agents which are O-alkylating).

Replacement of NH2 by Nitrogen or Phosphorus Nucleophiles.

Pyridinium salts (6) alkylate potassium phthalimide or sodium derivatives of succinimide, N-ethylbenzenesulfonamide or N-phenylbenzenesulfonamide; on hydrolysis of the resulting benzenesulfonamides, secondary amines RNHRŽ are obtained. Tertiary amines PhCH2NRŽ2 can be obtained from (6) with piperidine or morpholine without any interference from quaternary salts; similar selectivity is obtained when the phenyl group in PhCH2 is replaced by an a-, b-, or g-pyridyl group. Quaternary salts may be obtained, however, by transferring the R group of (6) to pyridine or 2- or 4-picoline (&egt;70% yields). At 130 °C in DMF (6) reacts with Sodium Azide, forming alkyl or benzyl azides in high yields. Triphenylphosphine reacts with (6) to yield phosphonium salts when R is benzyl or 2- or 4-picolyl.

Replacement of NH2 by Oxygen, Sulfur, or Selenium Nucleophiles.

On heating at ca. 200 °C a powdered mixture of (6) with the sodium salt of a carboxylic acid (RŽ = Me, Ph, Pr) in the presence of 2,4,6-triphenylpyridine, esters RŽCO2R are obtained in >60% yields. Pyridinium salts whose relief of steric strain is higher, e.g. when using (8) instead of (6), require lower temperatures, e.g. 100 °C with RŽ = Me. From such esters, alcohols may be obtained by hydrolysis. Starting from RNH2 and 2,4,6-triphenylpyrylium trifluoroacetate, crystalline triphenylpyridinium trifluoroacetates are obtained; on gentle pyrolysis around 160 °C, these salts yield 2,4,6-triphenylpyridine and ROCOCF3 esters, which are easily hydrolyzable to alcohols ROH. Such conversions of Alk-NH2 into Alk-OH proceed much more cleanly and efficiently than via diazotization.

Two pathways exist for obtaining aldehydes RCHO from RCH2NH2 via reaction with (1): oxidation of the pyridinium salt (6) (R = ArCH2) by Potassium Dichromate in the presence of Bu4N+ BF4- in refluxing 1,2-dichloroethane; and pyrolysis of (6) with sodium 1-oxido-4,6-diphenyl-2-pyridone. Yields are moderate.

Nitrate esters, RONO2, can be obtained in good yields by pyrolyzing under vacuum 2,4,6-triphenylpyridinium nitrates prepared from RNH2 and 2,4,6-triphenylpyrylium nitrate.

Phenoxides (sodium phenoxide or a-naphthoxide) react in refluxing dioxane with (6) (R = benzyl, 3- or 4-picolyl) in the presence of Bu4N+ BF4-, affording the corresponding alkyl aryl ethers.

The pyridinium salts formed from 2,4,6-triphenylpyrylium thiocyanate and primary amines afford thiocyanates RSCN on heating with triphenylpyridine; isothiocyanates RNCS are byproducts formed in low yield. An analogous reaction allows the preparation of selenocyanates Alk-SeCN. When using a pyridinium salt with high steric requirements such as (8), even aryl thiocyanates may be thus obtained from ArNH2.

Dithiocarbonates are formed in good yield from (6) (R = Alk) and potassium ethyl xanthate in refluxing benzene; the more reactive salts (6) (R = PhCH2) react similarly both in ethanol and in benzene. Thiols result by hydrolyzing the thiouronium salts formed from (6) and thiourea in refluxing chlorobenzene. When (6) (R = 4-picolyl) was refluxed in ethanol with sodium p-toluenesulfinate, the corresponding sulfone was formed in high yield.

Replacement of NH2 by Halide Anions.

Primary amines RNH2 react with 2,4,6-triphenylpyrylium fluoride in refluxing benzene-ethanol so that the resulting water is continuously removed as azeotrope. On heating the pyridinium fluorides so obtained (after careful drying) above their melting points, i.e. at 80-120 °C, the corresponding fluorides RF are formed in high yields. A reaction analogous to the Schiemann procedure, namely pyrolysis of (6) with KF and triphenylpyridine, results in low yields of RF contaminated with RH.

To obtain chlorides, (6) can be heated at 200-240 °C in a KCl-NaCl-ZnCl2 eutectic, when the RCl distills out in average yields around 50%. Alternatively, one can start from 2,4,6-triphenylpyrylium chloride and RNH2, removing water by azeotropic distillation and heating the resultant pyridinium chloride at 100-130 °C.

Starting from RNH2 and 2,4,6-triphenylpyrylium bromide or iodide, one readily obtains the corresponding crystalline halides, which on pyrolysis afford RBr and RI, respectively. For bromides, triphenylpyridine as flux increases the yield and reduces fusion and decomposition temperatures. In the case of iodides, even ArNH2 groups may afford ArI, but in this case the Sandmeyer reaction is equally effective.

In addition to the nucleophilic deaminating substitutions described above, mediated by (1), other reactions of (6) are possible with particular N-substituents R. Thus with R = PhCH2CH2, elimination yielding styrene occurs on pyrolysis.34 The N+,N- pyridinium betaine (6) with R = ArCON- affords isocyanates RNCO on heating, as an alternative to the Curtius rearrangement.35 The analogous betaine formed from (1) and H2N-N=CR-NHRŽ splits off triphenylpyridine on heating and rearranges to unsymmetrical carbodiimides.36

A synthesis of nitriles RCN from aldehydes RCHO is based on converting the aldehyde into its phenylhydrazone. Subsequent reaction with (1) in refluxing formamide yields triphenylpyridine and the nitrile. Alternatively, 1-amino-4,6-diphenyl-2-pyridone is reacted with the aldehyde, leading to an aldimine whose pyrolysis affords the nitrile in high yield along with 4,6-diphenyl-2-pyridone.37

Conversion into Other (Hetero)aromatic Systems.

In addition to the use of 2,4,6-triphenylpyrylium salts for mediating the nucleophilic displacement of amino groups, they are versatile intermediates for obtaining other interesting compounds such as 2,4,6-triphenylnitrobenzene,38 2,4,6-triphenylphenol and -phenoxyl,39 -phosphabenzenes,40 and -thiopyrylium salts.41

Whereas (1) reacts with hydrazine to form a diazepine derivative (9), the pseudobase (5) is converted by hydrazine into a monohydrazone (10) which cyclizes to a pyrazoline derivative (11) (eq 5).42 Both the pyrylium salt and the pseudobase react with monosubstituted hydrazines (R = Ph, Me, SO2Ar, CO2H) in ethanol, affording monohydrazones which cyclize to pyrazoline derivatives (11). Strong acids split off acetophenone from these pyrazolines and afford pyrazoles (12) under aromatization. However, if the monohydrazone (10) is refluxed in acetic acid, it undergoes a different cyclization, leading to a pyridinium salt (13); the corresponding N+,N- pyridinium betaine (14) may be a source of nitrenes RN. Similar reactions occur when (1) or the pseudobase (5) are treated with hydroxylamine in ethanol: a crystalline isoxazoline results, and on treatment with mineral acids an analogous C-C bond splitting occurs leading to 3,5-diphenylisoxazole and PhAc. If the reaction with hydroxylamine is carried out in acetic acid, 2,4,6-triphenylpyridinium N-oxide results.42,43

Summary of Reactions.

The primary amino group in alkyl or aralkyl amines is a very poor leaving group. By reaction with (1) (or other pyrylium salts with higher steric requirements), better leaving groups are obtained. Only ditosylates RNTs2 are comparable, but their versatility is far smaller.

The relative reactivities increase in the order: (6) < (8) < RN2+, and within each of these three classes in the order: R = aryl < alkyl < benzyl. The mechanisms of substitutions or fragmentations differ and may be radical, SN1, or SN2.

Advantages of deaminations proceeding via pyridinium salts are the preparation of alkyl and benzyl fluorides, the C-alkylation of nitronate anions, the lack of rearrangement in reactions involving neopentyl derivatives,44 and the accessibility of natural amino compounds. Some of the reactions mediated by (6) result in cyclizations, e.g. formation of epoxides; N-vinylpyridinium salts, which cannot be obtained by quaternization, may also be prepared.45

Optical and Electrochemical Properties.

A recent review,46 dedicated to (1) as a photoinduced electron-transfer (PET) agent, outlines its advantages over cyanoaromatic photosensitizers: enhancement of radical-ion formation and reduction of back-electron transfer (both due to the absence of net charge separation associated with the electron-transfer step); efficiency as a photosensitizer in both the singlet and triplet excited states, and the possibility to favor one of these states by appropriate choice of concentration and temperature; the unique ability to study photooxygenations of organic radical cations with molecular oxygen without interference from singlet oxygen or superoxide radical anions (both of which are formed when other photosensitizers are used). Of the PET reactions sensitized by (1), the above review cites: [2p; + 2p] or [4p + 2p] cycloadditions or cross-cycloadditions; (Z/E) isomerizations; sigmatropic rearrangements; oxygenation of alkenes yielding 1,2-dioxetanes (derived from the monomeric or dimeric alkene), epoxides, or ring-opened oxidized products; fragmentations of C-C, C-O, C-S, or C-N bonds (in the last case, dinitrogen is often extruded from azo derivatives).

The ability of pyrylium salts, including (1), to act as laser solutes has been reviewed.47 The electrochemistry of pyrylium salts, including (1), has also recently been reviewed.48

Related Reagents.

2,4,6-Trimethylpyrylium Tetrafluoroborate.

1. (a) Katritzky, A. R. T 1980, 36, 679. (b) Katritzky, A. R.; Marson, C. M. AG(E) 1984, 23, 420. (c) Katritzky, A. R.; Musumarra G. CSR 1984, 13, 47. (d) Katritzky, A. R.; Sakizadeh, K.; Musumarra, G. H 1985, 23, 1765.
2. Dinculescu, A.; Balaban, A. T. OPP 1984, 16, 407.
3. (a) Balaban, A. T.; Dinculescu, A.; Dorofeenko, G. N.; Fischer, G. W.; Koblik, A. V.; Mezheritskii, V. V.; Schroth, W. Pyrylium Salts. Synthesis, Reactions, and Physical Properties; Academic: New York, 1982. (b) Balaban, A. T.; Schroth, W.; Fischer, G. Adv. Het. Chem. 1969, 10, 241. (c) Schroth, W.; Balaban, A. T. MOC 1992, E7b, 767.
4. (a) Dilthey, W. JPR 1916, 94, 53. (b) Dilthey, W. JPR 1917, 95, 107.
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6. Simalty-Siemiatycki, M.; Fugnitto, R. BSF 1965, 1944.
7. Dimroth, K.; Reichardt, C.; Vogel, K. OSC 1973, 5, 1135.
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9. Bos, H. J. T.; Arens, J. F. RTC 1963, 82, 845.
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17. (a) Ziegler, K.; Fries, F. A. CB 1926, 59, 242. (b) Susan, A. B.; Balaban, A. T. RRC 1969, 14, 111.
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19. (a) Katritzky, A. R.; Lewis, J.; Nie, P. L. JCS(P1) 1979, 442. (b) Katritzky, A. R.; Horvath, K.; Plau, B. JCS(P1) 1980, 2554.
20. Katritzky, A. R.; de Ville, G. Z.; Patel, R. C. TL 1980, 21, 1723.
21. (a) Katritzky, A. R.; Kashmiri, M.; Wittmann, D. K. T 1984, 40, 1501. (b) Katritzky, A. R.; Chen, J. L.; Marson, C. M.; Maia, A.; Kashmiri, A. T 1986, 42, 101.
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24. Katritzky, A. R.; Gruntz, U.; Kenny, D. H.; Rezende, M. C.; Sheikh, H. JCS(P1) 1979, 430.
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26. Katritzky, A. R.; Saba, A.; Patel, R. C. JCS(P1) 1981, 1492.
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29. (a) Katritzky, A. R.; Thind, S. S. JCS(P1) 1980, 865. (b) Katritzky, A. R.; Gruntz, U.; Mongelli, N.; Rezende, M. C. CC 1978, 133.
30. (a) Katritzky, A. R.; Chermprapai, A.; Patel, R. C. CC 1979, 238. (b) Katritzky, A. R.; Chermprapai, A. JCS(P1) 1980, 2901.
31. (a) Katritzky, A. R.; Gruntz, U.; Ikizler, A. A.; Kenny, D. H.; Leddy, B. P. JCS(P1) 1979, 436. (b) Katritzky, A. R.; Horvath, K.; Plau, B. S 1979, 437.
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33. (a) Eweiss, N. F.; Katritzky, A. R.; Nie, P. L.; Ramsden, C. A., S 1977, 634. (b) Katritzky, A. R.; Eweiss, N. F.; Nie, P. L. JCS(P1) 1979, 433.
34. (a) Katritzky, A. R.; El-Mowafy, A. M. CC 1981, 96. (b) Katritzky, A. R.; El-Mowafy, A. M. JOC 1982, 47, 3506.
35. Katritzky, A. R.; Lloyd, J. M. JCS(P1) 1982, 2347.
36. Katritzky, A. R.; Lewis, J.; Nie, P. L. JCS(P1) 1979, 446.
37. (a) Katritzky, A. R.; Nie, P. L.; Dondoni, A.; Tassi, D. JCS(P1) 1979, 1961. (b) Katritzky, A. R.; Molina-Buendia, P. JCS(P1) 1979, 1957.
38. Dimroth, K.; Berndt, A.; Reichardt, C. OSC 1973, 5, 1128.
39. Dimroth, K.; Berndt, A.; Perst, H.; Reichardt, C. OSC 1973, 5, 1130.
40. (a) Märkl, G. AG(E) 1966, 5, 846. (b) Märkl, G.; Lieb., F.; Merz, A. AG(E) 1967, 6, 87.
41. (a) Wizinger, R.; Ulrich, P. HCA 1956, 39, 207. (b) Reynolds, G. A. S 1975, 638.
42. (a) Balaban, A. T. T 1968, 24, 5059. (b) Balaban, A. T. T 1970, 26, 739. (c) Buchardt, O.; Pedersen, C. P.; Svanholm, U.; Duffield, A. M.; Balaban, A. T. ACS 1969, 23, 3125.
43. Snieckus, V.; Kan, G. CC 1970, 1208.
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45. Katritzky, A. R.; Bapat, J. B.; Claramunt-Elguero, R. M.; Yates, F. S.; Dinculescu, A.; Balaban, A. T.; Chiraleu, F. JCR(S) 1978, 395; JCR(M) 1978, 4783.
46. Miranda, M. A.; Garcia, H. CRV 1994, 94, 1063.
47. Rulliere, C.; Declemy, A.; Balaban, A. T. Can. J. Phys. 1985, 63, 191.
48. Farcasiu, D.; Balaban, A. T.; Bologa, U. L. H 1994, 37, 1165.

Alexandru T. Balaban

Polytechnic University, Bucharest, Romania, and Texas A & M University, Galveston, TX, USA

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