Triphenylmethyl perchlorate

Indenopyrans (45) which can be obtained after Schroth and Fischer from benzoxalenes (44) and lithium alkyls or aryls, are y-pyrans, and may be dehydrogenated by triphenylmethyl perchlorate to the indenopyrylium perchlorate (46). °... 

It is known that tropylium may be prepared from tropylidene via hydride abstraction by PhgC or MegC carbonium ions therefore, it is very likely that here too the dehydrogenation is a hydride transfer from the 1,5-dione to an acceptor. A similar dehydrogenation of chromanones to chromones, with triphenylmethyl perchlorate was reported. A study of the electrooxidation of 1,5-diones on a rotating platinum electrode showed that 1,5-diaryl-substituted diones afford pyrylium salts in these conditions and that the half-wave potentials correlate with yields in chemical dehydrogenations. 

Boron trifluoride etherate, is also a good catalyst for this hydride transfer to chalcone. Unlike triphenylmethyl perchlorate, however, chalcone is able to enter Michael additions with the 1,5-diketone followed by eliminations leading to unexpected products, e.g., 3-benzyl-2,4,6-triphenylpyrylium from 2-carbethoxy-l,3,5-tri-phenylpentane-l,5-dione and chalcone the benzyl group originates from chalcone, the elimination product being ethyl benzoylacetate. ... 

Recently it allowed the preparation of bispyrylium salts (where the two pyrylium nuclei with aryl groups in positions 2 and 6 are linked in position 4 by p-phenylene, w-phenylenej or 2,5-thienylene group ) by dehydrogenation with perchloric acid in acetic anhydride or with triphenylmethyl perchlorate in acetic acid. Directly linked dications were obtained similarly (cf. Section II,B,2,a ). 

A further extension of the MIMIRC reaction is seen in the synthesis of enantiomerically pure cyclohexanones. A successful diastereoselective MIMIRC reaction with 2-(rer/-butyldimethylsi-lyloxy)-4-phenyl-l,3-butadiene and an optically pure (Z)-y-alkoxy-substituted enone was performed using catalytic amounts (5 mol%) of triphenylmethyl perchlorate at — 78 ,C 360,408 (for a further example see Section 1.5.2.4.4.1.). 

Additions of silylated ketene acetals to lactones such as valerolactone in the presence of triphenylmethyl perchlorate in combination with either allyltrimethylsilane 82, trimethylsilyl cyanide 18, or triethylsilane 84b, to afford substituted cyclic ethers in high yields have already been discussed in Section 4.8. Aldehydes or ketones such as cyclohexanone condense in a modified Sakurai-cyclization with the silylated homoallylic alcohol 640 in the presence of TMSOTf 20, via 641, to give unsaturated cyclic spiro ethers 642 and HMDSO 7, whereas the 0,0-diethyllactone acetal 643 gives, with 640, the spiroacetal 644 and ethoxytrimethylsilane 13b [176-181]... 

The degree of dissociation is a = pic = 1 - qlc. We now consider the three special cases shown in Table 3. In order to ascertain which, if any, of the two approximations may be valid, we need to examine the magnitudes of K, Ky and c which are relevant to the systems under discussion. To determine a value of K which will be relevant to polymerizing systems, we need the dissociation constant of a carbonium salt with a large anion, in a solvent of e about 10, at 25 °C. The only relevant information is Longworth and Mason s value of K for triphenylmethyl perchlorate in ethylene dichloride [53], and values of K for two quaternary ammonium perchlorates [140] (see Table 4). 

Unoxidized carbon black reduced triphenylmethyl perchlorate to triphenylmethane and was then able to oxidize isopropanol ... 

The hrst X-ray crystal structure of a carbocation salt was reported in 1965. Triphenylmethyl perchlorate (27) has a planar central carbon. The three phenyl rings are each twisted 30°, so that overall the cation has a propellor shape. Disordered perchlorate anions sit above and below the central carbon, with a C—Cl separation of 4.09 A. 

Reductive coupling of aldehydes 36 in the presence of triethylsilyl hydride leading to ethers 37 is found to be mediated by EGA Ethers 37 are obtained in a CH Clj— —LiClO4—BU4NCIO4—(Pt) system in 80-90% yield which is comparable to those of the triphenylmethyl perchlorate catalyzed reaction... 

Thiopyran-3(6//)-ones (133 R=H or Me) are converted into 3-hydroxythiopyrylium perchlorates (134 R = H or Me) by triphenylmethyl perchlorate. Deprotonation of these salts (134) by tertiary base did not liberate the thiopyrylium-3-olates (132) but instead gave dimeric products (135 and 136) in high yield. Evidence for the transient formation of the mesomeric betaines (132) is provided by the appearance of a greenish-yellow coloration which rapidly fades. The parent betaine (132 R = H) gave exclusively the endo dimer (136 R = H) (70%). In addition to the endo dimer (136 R = Me) (75%), the 5-methyl derivative (132 R = Me) also gave a small amount of the exo dimer (135 R = Me) (6%). The dimerization (132 136) appears to be rapid and irreversible Attempts to trap the... 

Chromenes, e.g. (157), which have a 2-hydrogen atom are converted by strong acids into the pyrylium salt (158) and the stereoisomeric chromans (159) (the latter pair in equal amounts), but a more efficient conversion of a 2iT-chromene (and also of a 4H-pyran) into the pyrylium salt, e.g. (160), is effected by addition of triphenylmethyl perchlorate in an inert solvent (67AC(R)1045). 

Acid-catalyzed addition of water and alcohols to 4/f-chromenes gives the expected products as predicted by Markovnikov s rule (56JCS4785) an anti-Markovnikov addition of methanol followed by the reintroduction of a double bond in the alternative position gives an overall effect of substitution of hydrogen by methoxy and this is effected by treating methyl 2if-chromene-3-carboxylate (166) with triphenylmethyl perchlorate and addition of methanol to the resulting benzopyrylium salt (167) (72CR(C)(274)650). 

From benzofurans. Russian workers have reported several examples of the isomerization of 2,3-dihydrobenzo[6]furans into chromans. Triphenylmethyl perchlorate in acidic media converts the 2-ethyl derivative into 2-methylchroman, whilst the same chroman results when the 2,3-dimethylbenzofuran is treated with aluminosilicates at high temperature (72DOK(204)879, 75CHE278). 

Similarly, chromanones are converted into benzopyrylium salts in this case though the oxidant is triphenylmethyl perchlorate. Hydrolysis affords the chromone (60CB1466, 60CI(L)1192>. 

The conversion of 4if-pyrans to pyrylium salts has also been accomplished. Thus, the ester (678) yields the 2,6-dimethoxycarbonylpyrylium salts (679 R = H or Me) on treatment with triphenylmethyl perchlorate in liquid sulfur dioxide (Scheme 272) (73ACS1385) and the amide (680) is formed in an analogous manner (74ACS(B)517). 

Whilst the isoflavene (681) forms a di(isofiavenyl) ether (682) with perchloric acid, it is quantitatively converted into isoflavylium perchlorate (683) on treatment with triphenylmethyl perchlorate (Scheme 273) (82JCS(Pl)ll93). 

The direct interaction of benzyl carbonyl compounds 38 and 48 with aromatic aldehydes in the presence of PPA or AC2O + HC104(86KGS125 87TH1), or with acetals or acylals of aliphatic or aromatic aldehydes in the presence of triphenylmethyl perchlorate (73KGS881 75ZOR1962), gives rise to benzo[c]pyrylium salts 62 in yields of 15-70 %. 

Secondary trityl ethers can best be prepared by reaction with triphenylmethyl perchlorate, but these derivatives tend to be rather labile.2... 

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