Trimethylpyrylium Perchlorate

Besides acetophenone, this reaction was also applied to p-chloro- andp-methoxyacetophenone, and even to an aliphatic ketone, acetone (although the yield was stated to be only half as large as that obtained from mesityl oxide, i.e., less than 30%, Dorofeenko and co-workers reported a 45% yield of 2,4,6-trimethylpyrylium perchlorate from acetone, acetic anhydride, and perchloric acid), and is the standard method for preparing pyrylium salts with identical substituents in positions 2 and 4. The acylating agent may be an anhydride in the presence of anhydrous or hydrated ferric chloride, or of boron fluoride, or the acid chloride with ferric chloride.Schneider and co-workers ... 

Caution 2,4,6-Trimethylpyrylium perchlorate is explosive. Operations should he conducted behind a shield, and directions should be followed closely (see Note 1 of Method I and Section 4 before carrying out these preparations). 

The preparation of 2,4,6-trimethylpyrylium perchlorate may be carried out on a much smaller scale, such as one-tenth, with only a small lowering of the yield. 

Since the reaction is quite exothermic, the mixture must be well stirred to avoid developing any local hot spots which could lead to explosions. Although no difficulties were encountered in either the submitters or checkers laboratories, it is well to keep in mind that 2,4,6-trimethylpyrylium perchlorate is potentially hazardous hence due precaution should be exercised at all times. 

Although a small sample of 2,4,6-trimethylpyrylium perchlorate may with care be recrystallized from acetic acid to give white crystals, m.p. 245-247° dec., it is recommended that this not be done with larger quantities. The 2,4,6-trimethylpyrylium perchlorate is of satisfactory purity for use in the 4,6,8-trimethyl-azulene preparation without further purification. 

Trimethylpyrylium perchlorate has been prepared from 2,6-dimethylpyrone and methylmagnesium halides 6 from mesityl oxide and sulfoacetic acid 6 from mesityl oxide (or less satisfactorily from acetone) and a mixture of acetic anhydride and perchloric acid 7 from mesityl oxide, acetyl chloride, and aluminum chloride 8 and from <-butyl chloride, acetyl chloride,... 

The 2,4,6-trimethylpyrylium perchlorate, obtained and stored as described by Balaban and Nenitzescus or by Hafner and Kaiser,3 is used directly. 

The impact sensitivity of 2,4,6-trimethylpyrylium perchlorate was examined by Dr. T. E. Stevens at the Redstone Arsenal... 

Trimethylpyrylium perchlorate is a very versatile and useful starting material. Thus its reaction with cyclopentadienyl-sodium has made 4,6,8-trimethylazulene 12 easily available for general studies of the properties of azulenes 18 and for the synthesis of related compounds.14 In addition, pyrylium salts are readily converted to a variety of pyridine derivatives 9 15 as well as to derivatives of nitrobenzene16 and phenol.9 17,18 It is clear that its value as a starting material is such that it is receiving wide use. 

The regioselectivity of the addition of organometallics to 2,4,6-trisub-stituted pyrylium ions 166 (R2 = R4 = H) is mainly determined by the structure of the entering group R. Thus 2,4,6-trimethylpyrylium perchlorate 177 was found to react with MeMgl, MeLi, or MeNa exclusively to give 2,2,4,6-tetramethyl-2Z/-pyran (176),212,220 237 238,246 whereas i-PrMgX, /-PrLi,... 

Irradiation of 2,4,6-trimethylpyrylium perchlorate in water gives a rearranged pyrylium salt (4) which is cleaved to the 5-oxohexenal (3). Replacement of the 4-methyl by A-t-butyl alters the course of the photolysis to give the cyclopentene (5) as the main product, together with small amounts of several others (72CC1240,73JA2406). 

When 2,4,6-trimethylpyrylium perchlorate is heated with [0-2H]acetic acid, the 3- and 5-hydrogens are deuterated. This substitution is unlikely to occur by direct electrophilic displacement in the cyclic compound but may proceed through an acyclic dienone formed by prior addition of acetate ion at C-2 (69RRC247). 

In contrast to the preferential attack on ring atoms at C-2 by nucleophiles (Section 2.23.2.3.2), a substituent at C-4 is more susceptible (than if it were at C-2) to attack by an electrophile such as a carbonyl compound. Thus, 2,4,6-trimethylpyrylium perchlorate (107) yields the coloured 4-styryl derivative (109) as the only product of reaction with a benzaldehyde (67JOU1809, 71T3503) but a 2-methyl group in cations such as 4,6-diphenyl-2-methylpyrylium (108) also reacts reaction at a second methyl group in (107) requires drastic conditions (72BSF3173). 

It is usual to produce 2,4,6-trisubstituted pyrylium salts by this method, otherwise isolation of the products is difficult. Although yields are only fair, considerable variation in the substituents is possible and some satisfactory examples are known. The detailed preparation of 2,4,6-trimethylpyrylium perchlorate also includes a brief survey of methods and merits of the procedure (73OSC(5)ll06). 

It was assumed (59LA(625)74) that the reaction proceeds through acylation of the non-conjugated methylene ketone and the Russian work certainly supports this view. The observation that 4-methylpent-4-en-2-one gives a vastly improved yield of 2,4,6-trimethylpyrylium perchlorate compared with 4-methylpent-3-en-2-one on reaction with acetic anhydride and perchloric acid also favours this suggestion and led to a proposed mechanism (Scheme 249) (61JCS3573). 

Azulene, 4,6,8-trimethyl-, 44, 94 Azulenes from cyclopentadienes and 2,4,6-trimethylpyrylium perchlorate, 44, 98... 

A novel method for estimating ring current in aromatic molecules has recently been provided (79TL437). This involves measurement of the H NMR a- and y-methyl chemical shifts in compounds (258), readily prepared from ArNHfe and 2,4,6-trimethylpyrylium perchlorate. Quantity D defined by equation (18) is a qualitative measure of ring current... 

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