Triphenylmethyl fluoroborate

Tri- butylpyrylium can be obtained by dehydrogenating the corresponding 1,5-diketone with triphenylmethyl fluoroborate, It was shown by Farcasiu that 1,5-diketones can also he dehydrogenated and dehydrated to pyrylium salts by triphenylmethyl hexachloroantimonate generated in situ from chlorotriphenyl-methane and antimony pentachloride. Even pentaphenylpyrylium may thus be prepared at room temperature. ... 

In the case of t-butyl substituted a,/3-unsaturated ketones, however, no reaction with ketones occurred in the presence of triphenylmethyl fluoroborate instead, a 1,2-methyl shift in the unsaturated ketone accompanied by cyclization afforded a crystalline dihydro-furylium salt. ... 

When 2-methoxy-l,6-methano[10]annulene 32 was subjected to a cyclopropana-tion with diazomethane and cuprous chloride as catalyst reaction occurred preferentially at the 5,6-, 6,7- and/or 1,10-bonds and the adducts spontaneously underwent disrotatory opening yielding the corresponding methoxybicyclo[5.4.1]dodecapen-taenes. Hydride abstraction with triphenylmethyl fluoroborate was performed on the mixture and the ions 33 and 34 so produced were treated with dilute aqueous potassium hydroxide. The annulenones 13 and 14 were then separated by chromatography. 

The TT-pentadienyliron tricarbonyl cation is an important grouping. It can, for example, be generated by treatment of cyclohexadieneiron tricarbonyl with triphenylmethyl fluoroborate. This salt is a powerful hydride abstractor (36) and the reaction gives rise to 7r-cyclohexa-dienyliron tricarbonyl cation (37) ... 

Triphenylmethyl fluoroborate is prepared by dissolving 27 g. (0.104 mole) of triphenylmethanol ( purum, Fluka A G) in 260 ml. of propionic anhydride by warming on a steam bath. With an acetone-dry ice bath the solution is cooled to 10° and maintained between 10° and 20° while 31 ml. of 43% w/w fluoroboric acid is added portionwise with swirling. The yellow solid is collected, washed well with dry ether, and dried in a desiccator under vacuum to yield 34 g. (90-99%). The product is very hygroscopic, taking up water with hydrolysis. It is desirable to prepare this reagent immediately before use. 

It was originally reported (92, 174) that the cycloheptatriene complex [Fe(CO)3(C7H8)] with triphenylmethyl fluoroborate gave a tropylium complex [Fe(CO)3(C7H7)] BF4 but subsequent work (59) has shown that an adduct [Fe(CO)3(CvH8CPh3)] BF4 (XIV R = CPh3) is formed in 96%... 

Triphenylmethyl fluoroborate will also abstract a hydride ion from tri-carbonyleyclohexa-l,3-dieneiron to give the cyclohexadienyl complex [Fe(CO)3(C H7)] BF4 (5,9, 76), analogous to the cycloheptadienyl complex [Fe(CO)3(C7H9)l BF4 described above. 

C-3 (or C-4) from a 1,3-dioxolane actually, this reaction (see Section HI, 14) was observed for 2,2-disubstituted 1,3-dioxolanes, and it may be assumed that, if there is any competition between removal of a proton from C-2 and from C-3 (or C-4), the rate of the first reaction is far greater than that of the second. The possibility of dioxolanylium ions reacting with nucleophilic reagents was first demonstrated by Hanes-sian and Staub,152 and the scope of these reactions was independently investigated by Pedersen and coworkers.153-157 Hanessian and Staub152 treated methyl 5-0-benzoyl-2,3-0-benzylidene-j3-D-ribofuranoside (125) with triphenylmethyl fluoroborate in acetonitrile, and obtained, after hydrolysis, a 1 1 mixture of the two benzoates 127 and 128 (yield 83%), probably through the benzoxonium ion 126. The same reaction... 

Jacobsen and Pedersen153 confirmed the generation of benzoxonium ion 126 by treatment of compound 125 with triphenylmethyl fluoroborate in acetonitrile, but they treated it with tetraethylammonium bromide, obtaining a 3 2 mixture of the a and /3 anomers of methyl 2,5-di-0-benzoyl-3-bromo-3-deoxy-D-xylofuranoside (134) in a yield of 74%. This mixture could be anomerized to give the pure )8 anomer (overall yield 60%) by treatment with hydrogen bromide followed by the action of methanol. 

The Lewis acid triphenylmethyl fluoroborate (trityl fluoroborate) has been used to effect net hydride abstraction from 1,3-ditelluroles to give 1,3-ditellurolylium ions (Scheme 9) (82TL1531). There is some question as to whether the loss of hydride occurs by a single two-electron process or by two separate one-electron oxidations coupled with a proton loss. 

If a benzylic cation is bonded to more than one phenyl group, the stabilizing effects are additive. An extreme example is the triphenylmethyl cation. This cation is exceptionally stable, with three phenyl groups to stabilize the positive charge. In fact, triphenylmethyl fluoroborate can be stored for years as a stable ionic solid. 

The generation of the parent benzocyclopropenylium and cyclopropa[6]naphthalenylium ions via hydride abstraction from benzocyclopropene (3a) and 1 //-cyclopropa[(j]naphthalene (3b)," respectively, has been attempted with triphenylmethyl fluoroborate, but only small quantities of the corresponding aromatic aldehydes 4 were isolated after quenching the mixture with water. Aldehyde formation is ascribed to attack of water on the intermediate cycloproparenyl cation. Benzocyclopropene is ca. 5 times less reactive towards triphenylmethyl fluoroborate than cycloheptatriene. 

Scheme 1 Oxodicarboxylate (4) was converted to trans-decalone (6) whose enolacetate (7) on epoxydation followed by acid treatment yields oxoacetate (9), which was transformated to another oxoacetate (10). It was subjected to standard organic reactions to obtain compound (15). Decarboxylation followed by treatment with base and then triphenylmethyl fluoroborate afforded glutinosone (1) in 30% yield...

R Br, R I, R2N03 R2B 4 an< R2< 4 Successive treatment of benzene-1,2-dithiol with 2,5-dimethoxytetrahydrofuran, triphenylmethyl fluoroborate, and tri-ethylamine results in bis(benzo-l,3-dithiafulvenyl) (223).22 The tetraselena-... 

From the acetal 34, the fluoroborate salt of the ion 35 can be obtained by use of triphenylmethyl fluoroborate to split out hydride... 

The acetal 37 of (l,2,4/3)-cyclopentanetetrol predictably reacts, by hydride abstraction, with triphenylmethyl fluoroborate to give the fluoroborate salt of the ion 39. The ion 39 can rearrange reversibly to give the ion 40, and, subsequendy, the ion 41 this rearrangement. 

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