Pinacol coupling reactions with carbonyl compounds

Manganese. Li and Chan [34] and Rieke [35] have independently reported that manganese reagents can accomplish the pinacol coupling of aromatic carbonyl compounds. In the Li/Chan study, reaction of an array of aromatic aldehydes proceeded in good to excellent yield in the presence of Mn/Ac0H/H20, albeit with poor diastereoselectivity (Eq. 3.15). Under these conditions, aliphatic aldehydes are reduced to the corresponding alcohol, and ketones (aromatic or aliphatic) do not react. 

Pinacol coupling reactions can lead to either the syn- or anti-diols. The stereochemical course of the reaction depends on the reducing agents and, of course, on the structure of the carbonyl compounds. Recent studies employing reagents that form the C-C bond according pathways B and C have met with success. 

The coupling of carbonyl compounds " to give 1,2-diols, known as the pinacol coupling, has been carried out in aqueous media. Clerici and Porta extensively studied the aqueous pinacol coupling reactions mediated by Ti(III). Schwartz reported a stereoselective pinacol coupling with a cyclopentadienetitanium complex. Pinacol-type couplings were also developed by using a Zn-Cu couple, Mn, °... 

The ready availability of arylboronates by an aromatic C-H borylation provides a synthetic link to the well-established palladium-catalyzed cross-coupling reactions, rhodium-catalyzed 1,4-addition to a,p-unsaturated carbonyl compounds, and other bond forming reactions using arylboronic esters (Scheme 2.12). Borylation of 1,3-dichlorobenzene with pinacolborane is followed directly by a cross-coupling reaction with methyl p-bromobenzoate for the synthesis of a biaryl product in 91% yield [60]. Pinacol esters of arylboronic acids react much slower than the free acids [62], but both derivatives achieve high isolated yields and comparable enantioselectivities (91% ee) in asymmetric 1,4-addition to N-benzyl crotonamides [63]. Borylation of arenes followed by oxidation of the C-B bond is synthetically equivalent to an aromatic C-H oxidation to phenols [64]. Oxidation of the resulting arylboronates with Oxone in a 1 1 acetone-water solution is completed within 10 min at room temperature. 

The stereoselective 1,4-addition of lithium diorganocuprates (R2CuLi) to unsaturated carbonyl acceptors is a valuable synthetic tool for creating a new C—C bond.181 As early as in 1972, House and Umen noted that the reactivity of diorganocuprates directly correlates with the reduction potentials of a series of a,/ -unsaturated carbonyl compounds.182 Moreover, the ESR detection of 9-fluorenone anion radical in the reaction with Me2CuLi, coupled with the observation of pinacols as byproducts in equation (40) provides the experimental evidence for an electron-transfer mechanism of the reaction between carbonyl acceptors and organocuprates.183... 

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