Triphenylcarbenium salts

Heterocyclic Fused Tropylium Salts from Cycloheptatrienes AND Triphenylcarbenium Salts (Scheme 52)... 

Resulting poly(a-hydroxyacids) are important biomaterials used as resorbable sutures and prostheses [196]. The mechanism of polymerization is not well established. Polymerization may be initiated with Lewis acids (SbF3, ZnCl2, SnCl4) however, other typical cationic initiators (e.g, triethyloxonium or triphenylcarbenium salts) fail to initiate polymerization [197]. Thus, it is not clear whether polymerization proceeds by typical cationic mechanism or rather involves the coordination mechanism. The chain transfer to polymer resulting in transesterification was postulated [198,199] and confirmed later by detailed, 3C NMR studies of lactide copolymers [200]. 

Triphenylcarbenium salts (PhgC X") selectively oxidize secondary t-butyl or triph-enylmethyl (trityl) ethers derived from alcohols. The oxidation proceeds via initial hydride abstraction followed by loss of the group on oxygen. The secondary-overprimary selectivity results from preferential formation of an oxocarbenium ion intermediate at the secondary center (R2 C-OTr is formed faster than RH C-OTr). 

Olah et al. (1992) studied the reaction of triphenylcarbenium tetrafluoroborate (10 mmol) with diphenyldiazomethane in dry dichloromethane hoping to detect the 1,1,2,2,2-pentaphenylethyl cation (9.25). The reaction yielded, however, tetraphenyl-ethene (79%) and a small amount (< 0.2%) pentaphenylethane. Using perdeuterated triphenylcarbenium salt or C-enriched diphenyldiazomethane, the authors demonstrate, by analysis of the labeled products, that the results are consistent with the mechanism (9-17), i.e., with the 1,1,2,2,2-pentaphenylethyl cation as steady-state intermediate, which is expected to undergo 1,2-phenyl migration via a phenonium ion and subsequent phenyl group scrambling. 

Triphenylcarbenium salts (PhsC ) readily abstract a hydride from V cyclo-hexadiene iron tricarbonyl complexes to form cationic tj -cyclohexadienyl salts, the hydride being removed from one of the methylene groups adjacent to the V-diene unit. When the cyclohexadiene ligand is monosubstituted, or has two or more different functionaUties attached, a mixture of regioisomers may potentially result. For example, hydride abstraction from 38 results in a 1.5 1 ratio of the two possible regioisomers 39 and 40. 

A brief survey of the applications of triphenylcarbenium salts in organometallic chemistry has been published, and there are many examples that serve to demonstrate the utility of these salts in synthesis. 

Hydride abstraction of complex 5 using triphenylcarbenium tetrafluoroborate provides the iron complex salt 6a [81]. Similar sequences afford the corresponding 3-methoxy-substituted and 2-methoxy-substituted complex salts 6b and 6c [82,83]. 

Hydride ion abstraction from 177-2-benzothiopyrans by triphenylcarbenium fluoroborate produces the highly colored 2-benzothiopyrylium salts 303 (Equation 43) <1998JOC4626>. 

Benzoylisothiochromene is oxidized to the benzo [c]thiopyrylium salt by triphenylcarbenium fluoroborate in almost quantitative yield (Equation 98) <1994J(P1)3129>. 

Thiopyran, derived from ethyl vinyl sulfide, is converted into thiopyrylium fluoroborate by reaction with triphenylcarbenium fluoroborate in 54% overall yield (Scheme 235) <2001EJ02477>. 3-Benzoylisothiochromene is oxidized to the 2-benzothiopyrylium salt in a similar manner <1994J(P1)3129>. 

Isoseleno- and isotellurochromenes 85 can be readily converted to the corresponding 2-benzoseleno- and 2-benzo-telluropyrylium tetrafluoroborates 86 by treatment with triphenylcarbenium tetrafluoroborate (Equation 35) <2002J(P1)606>. An analogous reaction has also been used for the conversion of tellurochromenes 87 to the corresponding 1-benzotelluropyrylium salts 88 (Equation 36) <1999J(P1)1665>. When the ring substituent was methyl or isopropyl, the desired benzotelluropyrylium salts formed but could not be isolated due to elimination of a proton from the substituent. 

A requirement for an a/m-orientation of the hydridic p-C—H and C—metal bonds as in [10] is indicated by the reaction of threo-3-deuterio-2-(trimethylstannyl)butane with triphenylcarbenium tetrafluoroborate in methylene chloride at 24° which yields a mixture of 3-deuterio-l -butene, /ra v-2-deuterio-2-butene, and undeuteriated c/.v-2-butene as the major product (Hannon and Traylor, 1981). Comparison of the product distributions for the protio- and deuterio-stannanes yields primary and secondary isotope effects of 3.7 and 1.1 respectively. These reactions appear to avoid the complications of adduct formation between the triarylcarbenium salt and the hydride donor, but the preferential formation of the cw-2-butenes is not fully explained. The requirement for the anti-orientation is also shown by the relatively low hydride-donating properties of tris[(triphenylstannyl)methyl-methane (Ducharme et ai, 1984a) which adopts a C3-conformation with the P-C—H gauche to all three C—Sn bonds. In contrast, 1,3,5-triphenyl-2,4,6-trithia-1,3,5-tristannyladamantane, in which anti-orientations with respect to the bridgehead C—H bond are locked, shows high reactivity (Ducharme et al., 1984b). 

The treatment of 2/f-l,3-ditelluroles in acetonitrile at 30° with an equimolar amount of triphenylcarbenium tetrafluoroborate produced substances, the NMR spectra of which suggested that they were 1,2-ditellurolium salts. ... 

Methyl rheniumpentacarbonyl has been subjected to three tetrafluoro-boranation studies, all by Beck and co-workers (20,162,163). The first study (162) involved treating the tetrafluoroborate derivative (formed by reaction of [Re(CH3)(CO)5] with triphenylcarbenium tetrafluoroborate) with a variety of cr and v donors, L, forming the salts [Re(L)(CO)s] BF4 [Eq. (51)]. The second study involved the formation of binuclear rhenium compounds (20) [Eq. (52)]. The third study used the tetrafluoroboranation reactions as a route to a polynuclear rhenium compound (163) [Eq. (53)]. 

Some other salts, e.g. (CfiH5)3C SbF (formed in situ), resulted predominantly in H transfer or proton elimination 19). These facts have been rationalized (Adv. Polymer Sci. 37, 1980 Sect. 3.2.1) when we discussed factors influencing the relative proportions of initiation by direct addition. These factors include first of all the ability of the second THF molecule to add to the oxonium ion formed by reaction between THF and the initiating carbenium ion (preformed or transient). Steric hindrance, like that observed in oxonium ions formed by triphenylcarbenium ions and THF, is... 

The salts are stable both in the solid phase and dissolved in dry acetonitrile. Absorption and fluorescence spectra of these cation salts in acetonitrile exhibit identical spectral properties as the same cations generated in acidified organic solvents or aqueous acidic solution [7-15]. The photooxidation of the triphenylcarbenium ion has also been studied via its tetrafluoroborate salt [31]. 

The synthesis of 7-oxygenated carbazoles via the iron-mediated route requires the 2-methoxy-substituted iron complex salt 52 (Scheme 14). Complexation of the methoxycyclohexadienes 47 and 48 with pentacarbonyliron in the presence of catalytic amounts of l-(p-anisyl)-4-phenyl-l-azabuta-1,3-diene affords a mixture of the regioisomeric 1 -methoxy- and 2-methoxy-ri -cyclohexadieneiron complexes 49 and 50 [77, 78, 84]. Hydride abstraction by triphenylcarbenium tetrafluoroborate provides a mixture of regioisomeric ri -cyclohexadienyUron complexes 51 and 52. Separation of the 1-methoxy- and 2-methoxy-substituted complex salts 51 and 52 can be achieved by selective hydrolysis of 51 to the cyclohexadieno-ne—tricarbonyliron complex 53 [74]. 

Together with triphenylcarbenium and silver salts, protic adds are used in hydride abstraction reactions. It has been shown that... 

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