Trityl tetrafluoroborate, oxidation

Oxidation of alcohols. Trimethylsilyl ethers of secondary alcohols are oxidized in CH2CI2 to ketones in 95-1007o yield by trityl tetrafluoroborate. Oxidation of ethers of primary alcohols is too slow to be useful. Consequently selective oxidation of ethers of secondary alcohols is possible. Oxidation of bissilyl ethers is not clean, but use of bistrityl ethers or bis-r-butyl ethers is a useful method. The latter ethers are useful for selective oxidation of some 1,3-diols, because only... 

Trityl tetrafluoroborate oxidizes secondary alcohols to ketones primary, secondary diols are selectively oxidized at the secondary position by this reagent. /Lt-Oxo-bis(chlorotriphenylbismuth) is a mild reagent for the oxidation of alcohols, and is particularly suitable for the oxidation of the hydroxy-group in allylic alcohols. Dichlorodicyanobenzoquinone (DDQ) with periodic acid in HCl-benzene has also been used for the latter purpose. ... 

The agents used for oxidative aromatization of various thiopyrans were air-dioxygen (81KFZ38 83KGS1689 91KGS181), trityl tetrafluoroborate (81CC1143 85CL1119), 3,4-di-(/-butyl)-l,2-benzoquinone (DTBQ)... 

Bis(ethylenediamine)gold(III) chloride reacts with /3-diketonates in aqueous base via Schiff base condensation to form complexes of gold(III) with a 14-membered macrocyclic tetraaza ligand such as (292).1711-1713 The X-ray structure showed the cation to be nearly planar. Delocalization of -electrons within the six-membered /3-diketonate rings was indicated by the observed pattern of C—C and C—N distances. Open-chain tetraaza ligand complexes in which condensation of only one /3-diketonate has occurred, can be isolated as intermediates in this reaction. They may be used for further condensation with a different /3-diketonate. Oxidation of this complex with trityl tetrafluoroborate introduces a double bond in position C2C3.1712,1714... 

Oxidation of the 1,2,4-dithiazoline (36) to the cation (34) with trityl tetrafluoroborate (74AP828) is an example of electrophilic attack at ring hydrogen. Nucleophilic attack with... 

Regioselective oxidation of 2- and 4-alkyl-l,4-hutanediols. 2,2-Disubstituted 1,4-butanediols (1) are oxidized by this combination -.electively to the 7-butyrolactones 2. Cobalt(II) alkanoates and trityl tetrafluoroborate show comparable selectivity. 

In the early 1970s, Barton et al. published the results of their woik on the oxidation of acetals and ethers by hy de transfer. o They observed that substituted benzyl ethers and benzyloxy carbonates, on brief exposure to trityl tetrafluoroborate in dichloromethane at 0 C followed by aqueous work-up, afforded go( yields of the parent alcohols together with the corresponding bennldehydes. Undw the same conditions, the tetrahydropyranyl ether of cholesterol was also efficiently deprotected. A mechanism was proposed which involved an initial hydrogen abstraction, followed by quenching of the resulting stabilized cation by water (Scheme 6). 

Ten years later, a Japanese group led by Oikawa developed a mechanistically related method for the selective debenzylation of substituted benzyl ethers based on the reagent 2,3-dichloro-S,6-dicyanobenzo-quinone (DDQ). In contrast to the trityl tetrafluoroborate reaction, the oxidation proceeds at room temperature in the presence of water. Furthermore, under these convenient and essentially neutral conditions, many functional groups, including other common protecting groups, such as isopropylidine, methoxy-methyl, benzyloxymethyl, tetrahydropyranyl, acetyl, r-butyldimethylsilyl, benzyl, benzoyl and tosyl, are unaffected. As a result of the hig levels of selectivity which can be achieved, this method for the depro-... 

Scheme IS depicts a high yield, general method for specific ortho alkylation of polycyclic aromatic hydrocarbons. In this example, biphenyl is subjected to reductive methylation followed by oxidative rearrangement with trityl tetrafluoroborate to give 2-methylbiphenyl. In unsymmetrical substrates the regioselectivity is poor phenanthrene gives a 3 2 mixture of 4-methyl- and 1-methyl-phenanthrene.

Lactonization Reaction of 1 with trityl tetrafluoroborate not only effects oxidative debenzylation as expected (4, 567), but also cyclization to 2, d/-aplysis-tatin, a brominated sesquiterpene. Attempted hydrogenolysis of the benzyl ether resulted only in hydrogenation of the double bond. 

Oxidation of ols and diols (7, 414-415). Even unprotected scc-alcohols can be oxidized to ketones by trityl tetrafluoroborate (2 equiv.) in CHaCla at 25 (10-13 hours). Typical yields are in the range 65-857o- Primary, secondary diols can be oxidized selectively to hydroxy ketones by the reagent (2.1-2.2 equiv.) under the same conditions (yields 55-807o). 

Under catalysis by a ruthenium complex, N-methylmorpholine 7V-oxide rapidly oxidizes most alcohols to the corresponding aldehyde or ketone in high yield at room temperature. Homoallylic alcohols are exceptional, undergoing conversion at a slow rate, if at all. Preferential oxidation of primary, secondary diols at the secondary centre leading to keto-alcohols has been achieved by treatment of the bis-trityl derivative with trityl tetrafluoroborate. ... 

Diols (65) can be oxidized regioselectively to butyrolactones (66) when = = Ph using benzoyl peroxide and nickel bromide or trityl tetrafluorobor-ate. ° When the substituents are simple alkyl groups, the oxidation is around 90% selective. A further use of horse liver alcohol dehydrogenase is in the oxidation of cyclic or acyclic me o-diols (e.g. 67), which results in the formation of optically pure lactones (e.g. 68). Yields are typically 65%, and the reactions can be run on up to 2 g quantities. 

Acid-catalyzed dealkoxylation is particularly suitable for the preparation of highly reactive, cationic iron(IV) carbene complexes, which can be used for the cyclopropanation of alkenes [438] (Figure 3.11). Several reagents can be used to catalyze alkoxide abstraction these include tetrafluoroboric acid [457-459], trifluoroacetic acid [443,460], gaseous hydrogen chloride [452,461], trityl salts [434], or trimethylsilyl triflate [24,104,434,441,442,460], In the case of oxidizing acids (e.g. trityl salts) hydride abstraction can compete efficiently with alkoxide abstraction and lead to the formation of alkoxycarbene complexes [178,462] (see Section 2.1.7). 

The same approach allows preparation of various pyrylium carboranes from the corresponding 4/f-pyran carboranes 174a,b and 175b by the action of acetyl perchlorate,245 perchloric acid,244 and triarylamine radical cation salts,244,245 as well as electrochemically.243 The oxidation of condensed 4H-pyran 345 with trityl perchlorate, 2,3,5,6-tetra-substituted 4f/-pyrans 431 and 153 with tropylium tetrafluoroborate or 153 with heterocyclic salt 393 led to useful preparations of pyrylium salts 394,330 395a,359 and 395b,360 respectively. 

The agents used for oxidative aromatization of 2/f-thiopyrans were trityl perchlorate,42-155,156 tetrafluoroborate,267 and iodide.267 Thus 3,5-diphen-yl-2//-thiopyran (222) was found to aromatize either on its own to thiopyrylium salts 396 or by S-methylation to l-methyl-3,5-diphenylthiabenzene (398) via intermediate 397,267 as shown in Scheme 18. 

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