Pinacol cleavage

Reduction of aromatic and polyethylenic ketones —pinacols Cleavage of carbon-halogen bonds ... 

Perrier, S., Sankararaman, S., and Kochi, J.K., Photoinduced electron transfer in pinacol cleavage with quinones via highly labile cation radicals. Direct comparison of charge-transfer excitation and photosensitization, /. Chem. Soc., Perkin Trans. 2, 825, 1993. 

Boronic esters are easily prepared from a diol and the boronic acid with removal of water, either chemically or azeotropically. (See Chapter 2 on the protection of diols.) Sterically hindered boronic esters, such as those of pinacol, can be prepared in the presence of water. Boronic esters of simple unhindered diols are quite sensitive to water and hydrolyze readily. On the other hand, very hindered esters, such as the pinacol and pinanediol derivatives, are exceedingly difficult to hydrolyze and often require rather harsh conditions to achieve cleavage. 

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 ... 

Other complex sulphones, e.g., 69 with several labile hydrogens, were studied54. Thus disulphonyl pinacols can be reduced (by four-electron cleavage) into pinacols in protic solution or decomposed by means of EGBs into benzil and methyl phenyl sulphone (R = Ph). With R = alkyl, other modes of cleavage by EGBs were found5 5- 57, especially the scission of the central C—C bond when 50% of the /i-ketosulphone could be isolated. 

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. 

Indeed, the (200-fs) laser excitation of the EDA complexes of various benz-pinacols with methyl viologen (MV2+) confirms the formation of all the transient species in equation (59). A careful kinetic analysis of the decay rates of pinacol cation radical and reduced methyl viologen leads to the conclusion that the ultrafast C—C bond cleavage (kc c = 1010 to 1011 s- ) of the various pinacol cation radicals competes effectively with the back electron transfer in the reactive ion pair. 

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). 

Scheme 7 Cleavage of Pinacol Esters of Boronic Acids by Exchange with Diethanolamine...

Several bicyclo[2.2.1]heptane derivatives with a 1,4-dicarbonyl moiety undergo C—C bond cleavage via intramolecular pinacol coupling, promoted by samar-ium(II) iodide.190... 

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