Pinacols oxidative cleavage

One-pot Synthesis of Arylboronic Acids and Aryltrifluorobo-rates. Hartwig also reported the synthesis of arylboronic acids and aryltrifluoroborates in a one-pot sequence by Ir-catalyzed borylation of arenes followed by the oxidative cleavage of the boronate ester with NaI04 and the displacement of pinacol by KHF2, respectively (eq 4). These two-step sequences give products that are more reactive and selective in the subsequent chemistry, such as Suzuki coupling, than the initially formed pinacol-boronates. 

The initial step of the coupling reaction is the binding of the carbonyl substrate to the titanium surface, and the transfer of an electron to the carbonyl group. The carbonyl group is reduced to a radical species 3, and the titanium is oxidized. Two such ketyl radicals can dimerize to form a pinacolate-like intermediate 4, that is coordinated to titanium. Cleavage of the C—O bonds leads to formation of an alkene 2 and a titanium oxide 5 ... 

The solvent isotope effect suggests that no O-H cleavage is involved in the slow step and the effect of O-methylation indicates that a cyclic complex is involved. The induction factor is probably obscured by the reaction of Mn(III) and Mn02 with pinacol itself. The typical glycol-cleavage mechanism advocated for oxidations by Pb(IV) and I(VII) (p. 349) may well operate, viz. 

The acidity dependences of V(V) oxidations are significant. That of pinacol , which undergoes 100% C-C cleavage, is a+bh ). The first (acid-independent) term is rare in V(V) oxidations and implies that V02 is the active oxidant the second term implies, on the basis of the Zucker-Hammett hypothesis, that the transition state has the structure (J5), the mechanism being... 

Evidence concerning the relative extents of C-C and C-H fission is less well defined for Ce(IV) and Mn(III) as compared with V(V). Pinacol is cleaved to acetone in all cases, but while Mn(IlI) pyrophosphate [like V(V)] oxidises pinacol much faster than butane-2 3-diol, the rate ratio with Ce(IV) is only approximately 3 and the production of acetaldehyde from butane-2 3-diol by Ce(IV) oxidation demonstrates C-C cleavage . It is probable, therefore, that Mn(III) oxidises the disecondary glycol by C-H fission. 

Thermal activation. Cleavage of benzpinacols can also be achieved by thermal reactions, in which powerful electron acceptors such as DDQ ( ed = 0.6 V versus SCE) effectively oxidize electron-rich pinacols. For example, the blue-green color of the EDA complex formed upon mixing of tetraanisylpinacol and DDQ in dichloromethane bleaches within minutes (in... 

Other nucleophiles, like cyanide, trifluoroethoxide and triethylamine, have been investigated in DMSO as solvent and Af-methylacridone was consistently formed as the final product, presumably via oxidation of a Af-methylacridone radical anion (42). Although this radical was never observed in EPR experiments, homolytic cleavage of a pinacol dianion is assumed to be likely (Scheme 29). 

This C-C bond cleavage is avoided when the oxidation is performed in the presence of chloride ions. In this case, pinacol-type rearrangement products involving a C-S bond cleavage are obtained, as in Eq. (72) [125]. A chlorosulfonium intermediate is believed to participate in this type of reaction. 

Kimura, M., Kobayashi, K., Yamamoto, Y., Sawaki, Y. Electrooxidative pinacol-type rearrangement of f3-hydroxy sulfides. Efficient C-S cleavage mediated by chloride ion oxidation. Tetrahedron 1996, 52,4303-4310. 

Oxidation of 12 with excess Collins Reagent in methylene chloride yields the sensitive keto aldehyde 13. A pinacol cyclization of the keto aldehyde 13. to the tricyclic intermediate 14 is accomplisehd by reaction with finely powdered metallic Titanium inTHF. 2, 13 Oxidation to the ketol without appreciable glycol cleavage can be readily accomplished via an oxysulfonium intermediate. 14, 15 "phe ketol can then be converted to the MEM ether derivative by treatment with MEM chloride and diisopropylethylamine in methylene chloride at reflux to give the MEM ether derivative 15. ... 

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