2,4,6‐Trimethylpyrylium Tetrafluoroborate

(1; X− = BF4−) [773-01-3] C8H11BF4O (MW 210.00) InChI = 1S/C8H11O.BF4/c1-6-4-7(2)9-8(3)5-6;2-1(3,4)5/h4-5H,1-3H3;/q+1;-1 InChIKey = PYOPRIFMELCENN-UHFFFAOYSA-N (2; X− = ClO4−) [940-93-2] C8H11ClO5 (MW 222.64) InChI = 1S/C8H11O.ClHO4/c1-6-4-7(2)9-8(3)5-6;2-1(3,4)5/h4-5H,1-3H3;(H,2,3,4,5)/q+1;/p-1 InChIKey = SPTGLTSDLLDPOO-UHFFFAOYSA-M (3; X− = CF3OSO2−) [40927-60-4] C9H11F3O4S (MW 272.27) InChI = 1S/C8H11O.CHF3O3S/c1-6-4-7(2)9-8(3)5-6;2-1(3,4)8(5,6)7/h4-5H,1-3H3;(H,5,6,7)/q+1;/p-1 InChIKey = FCPRZFWDVUXXNH-UHFFFAOYSA-M (4; X− = HO2CCH2SO3−) [80731-12-0] C10H14O6S (MW 262.31) InChI = 1S/C8H11O.C2H4O5S/c1-6-4-7(2)9-8(3)5-6;3-2(4)1-8(5,6)7/h4-5H,1-3H3;1H2,(H,3,4)(H,5,6,7)/q+1;/p-1 InChIKey = SCGNGCBBNHDIPT-UHFFFAOYSA-M (synthon reacting with various nucleophiles) Physical Data: (1) mp 244 °C (dec). (2) mp 244 °C (dec). (3) mp 119–120 °C. (4) mp 115–116 °C. Solubility: (1) sol H2O, AcOH, CF3CO2H. (2) sol CF3CO2H. (3) sol H2O, CHCl3, MeOH, EtOH, (ClCH2)2. (4) sol H2O, MeOH, EtOH, AcOH; insol Et2O, Me2CO. Form Supplied in: white solids. Preparative Methods: 2,4,6-trimethylpyrylium salts were the first pyrylium salts devoid of hydroxy or alkoxy substituents to be obtained, by Baeyer and Piccard in 1911.3 Until then, protonated or alkylated γ-pyrones were the only known pyrylium salts. At that time, pyrones were puzzling nitrogen-free bases; the discussion about which of their two oxygen atoms was attacked lasted a long time and eventually led to the clarification of the concepts of aromaticity and aromatic sextet. Pertinent references may be found in reviews.1 A low-yield synthesis3 was based on the reaction of 2,6-dimethyl-4-pyrone with methylmagnesium iodide, followed by treatment with acid (perchloric acid was then favored). The preferred syntheses of 2,4,6-trimethylpyrylium perchlorate 2 are based either on the diacetylation of Isobutene (eq 1),4 or on the acetylation of mesityl oxide (eq 2),5 using Acetic Anhydride and 60–70% perchloric acid. In the former synthesis, instead of the gaseous isobutene, one uses t-butanol which is dehydrated in situ.6 (1) (2) In both methods, isomesityl oxide is the actual reaction intermediate. The reaction mechanism is shown in eq 3. (3) Since organic perchlorates may explode on impact when dry, other anions have lately been preferred, namely tetrafluoroborate or trifluoromethanesulfonate (triflate), obtained from t-butanol with Ac2O and the corresponding acid, analogously to eq 1. When using 48% fluoroboric acid, the yield of fluoroborate 1 is 50%,7 and when using anhydrous triflic acid, the yield of triflate (3) is 42%.8 2,4,6-Trimethylpyrylium sulfoacetate 4 can be prepared inexpensively on a large scale (in kg amounts) according to published procedures9 starting either from isobutene or from t-butanol (eq 4). Excess acetic anhydride is no longer required for reacting with the water from HClO4 or HBF4 solutions. The sulfoacetate 4 crystallizes from the reaction mixture on addition of acetone, in which it is insoluble. It has a high solubility in water. (4) Purification: recrystallize from: 1, EtOH, MeOH; 2, H2O, AcOH; (3), dioxane–AcOH (7∶1) or CHCl3–CCl4 (2∶1); 4, MeOH–Me2CO. Handling, Storage, and Precautions: all salts may be stored indefinitely. Reports on possible mutagenic activity make it desirable to avoid direct contact with skin. The perchlorate 2 is explosive; therefore it should be prepared only in the required amount for further use. The dry perchlorate should be handled with great care, and never be heated, crushed, rubbed, or pushed through a narrow opening. It may be kept in a cork-stoppered flask when moistened with water or dry THF. Use in a fume hood.