Crossed-pinacol coupling

Although yields and diastereoselectivities are moderate, a cross-pinacol coupling reaction between a 1,2-diketone and an aldehyde is accomplished with Sml2 in the presence of HMPA. ... 

Enantioselective synthesis of /3-amino alcohols by Sml2-mediated cross-pinacol coupling of the planar chiral iV-sulfonyl(ferrocenylidene)amine with ferrocene carboxaldehyde is achieved by facile reduction of the ferrocenyli-dene amine with Sml2, followed by enantioselective addition to the aldehyde (Equation (75)). ... 

Cross pinacol coupling of aldehydes. Pedersen et al.2 have reported the first general method for stereoselective pinacol coupling of aldehydes by use of this vanadium(II) reagent. Aryl aldehydes couple in the presence of 1 to pinacols in >90% yield and with high diastereoselectivity (dllmeso = 12-100 1). Aliphatic aldehydes under similar conditions couple only slowly, but depending on the struc-... 

A crossed-pinacol coupling was reported using Et2Zn and with a BINOL catalyst gave good enantioselectivity. A combination of Mg and MeaSiCl was also used to a crossed-pinacol. ... 

Cross pinacol coupling. The VCI3-THF complex gives [V2Cl3(thf)6]2(Zn2Cl6) on treatment with zinc dust. Coupling of aldehydes furnishes 5yn-1,2-diols. 

Ri = R3, R2 = R4 izz Homo-pinacol coupling Ri R2 R3 R4 i=> Cross-pinacol coupling... 

Scheme 2.29 Titanium-catalyzed cross-pinacol coupling reaction in the presence of the bis-biphenol ligand 41...

Scheme 2.30 Cross-pinacol coupling reaction catalyzed by the hetero dinuclear dihemisalen complex 42...

Pinacol coupling. Yb(0) metal can effect pinacol reduction of aromatic, but not aliphatic ketones.1 However, it can effect cross-coupling of benzophenone with aliphatic ketones, often in high yield (equation I). 

Piezochromism, polysilanes, 3, 609 PIMS, see Photoionization mass spectrometry Pinacolboranes, cross-coupling reactions, 9, 170 Pinacolboronates, synthesis, 9, 167 Pinacol coupling... 

Indium mediated Barbier-type cross coupling between carbonyl compounds and allyl halides proceed efficiently under solvent-free conditions. No apparent competing pinacol-coupling or homo-coupling of the allyl halide was observed. The reactions were found to be mediated also by zinc, tin, bismut and copper [45]. 

The mixed coupling of aliphatic compounds is also well established. Through the use of an excess (usually 10-fold) of one of the reactants cross-coupling of aliphatic carbonyls is highly effective. This protocol was first developed by (Torey and then applied to reductive pinacolic couplings by McMurry. Li et al. applied this method to the synthesis of the alkaloid isoharringtonine (83 equation... 

Shikonin. Torii has accomplished an efficient intermolecular pinacol coupling reaction, en route to the total synthesis of shikonin, a compound with antiinflammatory, antibacterial, and antitumor activity (Eq. 3.11) [29], Because the aromatic aldehyde possessed a substituent capable of chelating to vanadium, Torii anticipated, based on precedent, that selective cross-coupling would be possible. The pinacol reaction proceeded in 73% yield and with good diastereoselectivity (5.5 1). 

From a synthetic standpoint the intermolecular pinacolic coupling reaction is limited because only homocoupling reactions are generally practical. Cross-coupling reactions mediated by SmF are restricted to specialized, matched partners [35]. Thus a-dicarbonyl compounds can be heterocoupled with aldehydes, providing facile entry to 2,3-dihydroxy ketones. Although selectivities vary, in some cases the diastereoselectivity of the process can be quite high (Eq. 25). 

Pinacol coupling. SmBr2 prepan agent for cross-coupling of carbonyl co... 

If both carbonyl substrates have sufficiently different redox potentials, a reasonable yield of crossed product is obtained without resorting to using one substrate in excess. This result was unexpected given the relative ease that a bisaromatic ketone should undergo ketyl formation followed by self pinacol coupling and was one of the first results that indicated more than one mechanism may be operative in McMurry couplings. In this case, the diaryl ketone may be reduced selectively to a dianion, which then undergoes nucleophilic addition to the saturated ketone. 

---