Trityl cation hydride abstraction with

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 most deshielded material within this category (8) was reported in 2002 by Piers and coworkers. They used a novel, highly fluorinated borate anion, the solvent benzene, and hydride abstraction with trityl to produce the species Bu3Sn(benzene)+ with a Sn chemical shift of 5 434. By this time, authors were referring to such species as solvated stannyl cations. 

The products are stable complexes they are 18-electron and, as they are coordinatively saturated, do not undergo p-hydride elimination. A second step is required to remove the iron and generate synthetically useful, iron-free products. This can be done in mundane ways by treatment with a strong acid or a strong electrophile (e.g. bromine) to achieve electrophilic cleavage of the C-Fe bond. Some other methods are also possible. A Fp-alkene complex can be regenerated by hydride abstraction with the trityl cation (the H must be trans to Fe) or by acid treatment if there is a p-alkoxy group. Final decomplexation can be achieved by treatment with iodide (Schemes 6.119 and 6.120). 

The resulting macrocyclic ligand was then metallated with nickel(II) acetate. Hydride abstraction by the strongly electrophilic trityl cation and proton elimination resulted in the formation of carbon-carbon double bonds (T.J. Truex, 1972). 

Ph3C BF4, CH2CI2, 5-30 min, 80-95% yield. " The mechanism of this cleavage has been determined to involve complex formation by the trityl cation with the sulfur, followed by hydrolysis, rather than by hydride abstraction. ... 

The advantage of this procedure is that one can make useful comparisons between sets of measurements obtained under a variety of conditions. An informative example of the method is presented in the next section. In practice, it is often convenient to add trityl cations to a solution of an unknown as a marker one must, however, beware of offering to the trityl cation a substrate from which it can abstract a hydride or other ion, with formation of a more stable cation [8]. 

Alkoxy substituted thiopyrylium compounds are obtained by hydride abstraction from the thiin precursor with trityl salts, or by elimination of halogen from a thiin. By contrast, alkylthio substituents may be generated in creating the cationic species as shown in equation (115), though of course only 2- and 4-substituents are available in this way (76CC899). Amino substituted compounds may be prepared by substitution of the methylthio groups, but the nature of the product is strongly dependent on the nature of the amine (see Section 2.25.3.4). 

Treatment of (100) with trityl cation leads to hydride abstraction and formation of the 67r-electron aromatic cation (24), as previously described in Section 2.28.2.4. 

Trityl (triphenylmethylium) and tropylium cations are two commercially available carbenium ions. They react with alkenes by either direct addition or by hydride abstraction tropylium ions are usually less reactive than trityl. As shown in Eq. (39), trityl ions initiate styrene polymerization by direct addition. 

The hydride in silanes ArSiR2H can be abstracted by the trityl cation, leaving principally a silicenium ion. This highly electrophilic species interacts with the anion associated with the trityl cation either by coordination (e.g., CIO4 ) or by halide abstraction (e.g., BF4 ) [3]. 

The completion of the substitution process to produce a substituted arene free of the metal has not been worked out in general. Direct abstraction of the endo hydride of the r] -cyclohexadienyl intermediate with trityl cation is not effective, but an interesting thermal rearrangement has been observed at 130-150 °C to create an isomeric cyclohexadienyl ligand that bears an exo (easily abstracted) hydrogen substituent (equation... 

When initiation is more complex, the elementary reactions can sometimes be studied separately. This is the case for initiation of the polymerization of 1,3-diox-olane with trityl salts. In the first reaction, hydride transfer takes place and then the newly formed cation reacts again with monomer. This second process is considerably slower than the former one. The first hydride abstraction was studied by (disappearance of the trityl cation absorption at X = 430 nm, Cmax =" 3-6 10 ) and by polarography (observation of (C6H5)3C giving a reversible one-electron wave for the trityl ion reduction with Eyz 0.51 V). 

The reaction of 4,7-di- -butyl-l,3-benzodioxole with trityl cation results in hydride abstraction from the 2-position to give the dioxolium cation which can be conveniently trapped by alcohols to give the 2-alkoxy derivatives <84IZV1632>. 

Hydride abstraction from unsymmetrically substituted (cyclohexadiene) Fe(CO)3 derivatives may lead to mixtures of salts. The l-OMe(IV), for example, yields V (R = OMe) with some VI the former is hydrolyzed by water to the neutral separable complex 7 from which the salt can be regenerated by reagents such as trimethyloxonium fluoroborate. The 2-OMe (diene) yields VI with a little of the 3-OMe cation . The transition state of the reaction may resemble the cationic product. The l-C02Me (diene) also reacts in the V position d through initial attachment of the trityl to the carbonyl oxygen. Considerable experimental information is available on the course of abstractions ., ... 

Protonation of dienol complexes (244) with strong acids (e.g. HBF4, HPFe, or HCIO4) generates the cisoid ( -pentadienyl)iron cations (248) (Scheme 69). This reaction is believed to proceed via initial formation of the transoid pentadienyl cation, followed by isomerization to the more stable cisoid form of the cation. Computational methods on the unsubstituted cationic complex (248, R = Nu = H) suggest that the cisoid form is 9.2 kcal mol more stable. Protonation of an alkene that is adjacent to a diene complex can also lead to the formation of ( -pentadienyl)iron cations. Additionally, hydride abstraction using the trityl cation also produces (jj -pentadienyl)iron cations the rich chemistry of these species will be described later (Section 7.1). 

In a new and significant development, it was found that chlorocyclohexadienyl manganese tricarbonyl complexes undergo palladium-catalyzed coupling reactions with a variety of carbon nucleophiles to afford products from which hydride may be abstracted with trityl cation, thus generating new [( -arene)Mn(CO)3]+ complexes containing substituents that cannot be introduced in any other manner. This is particularly true of electron-withdrawing substituents. Scheme 16 illustrates the chemistry involved. 

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