Pinacol coupling, samarium iodide

Another well-known transformation of carbonyl derivatives is their conversion to pinacols (1,2-diols) via an initial one-electron reduction with highly active metals (such as sodium, magnesium, aluminum, samarium iodide, cerium(III)/ I2, yttrium, low-valent titanium reagents (McMurry coupling), etc.), amines, and electron-rich olefins and aromatics as one-electron donors (D).43 Ketyl formation is rapidly followed by dimerization44 (equation 22). 

Addition of HMPA to Sml2 in THE changes the reaction course of the benzaldehyde dimerization. Although samarium(ii) iodide promotes pinacol coupling of benzaldehyde, use of 2.8equiv. of HMPA leads to the formation of, in addition to the pinacol (10% yield), a dimer (60% yield) that results from the connection of a carbonyl carbon and a phenyl para-c3.rbon (Equation (31)). ... 

The pinacol coupling reactions of keto amides with samarium(ll) can be extended to an asymmetric version by substrate control. Treatment of asymmetric keto amide 13 with samarium(ll) iodide, HMPA, and /-butyl alcohol in THF gives one of the r//-diastereomers in 79% yield exclusively (Equation (37)). ... 

Pinacol coupling of 2-substituted ferrocene carboxaldehydes with samarium(ll) iodide in THF also affords dl-coupling products selectively. 

Intramolecular pinacol coupling of 2,2 -biaryldicarbaldehyde with samarium(ll) iodide shows that axial chirality transfer to central chirality proceeds in a stereospecific manner. ... 

Samarium(II) iodide mediates diastereo- and enantio-selective pinacol coupling of chiral a-ketoamides.166... 

Samarium iodide promotes the pinacol homo-coupling of aldehydes to give 1,2-diols, but typically with little difference in the yields of the syn- and anti-isomers. Addition of Lewis acids, however, improves the selectivity, apparently by complex-ing both reactants in the transition state. Chiral aldehydes such 2-phenylpropanal can give syn/anti ratios >50, even without additives, and in some cases they give exclusively one product with appropriate choice of conditions. 

The 1 1 complexes of samarium(II) iodide and tetraglyme derivatives promote inter-molecular pinacol coupling to give vicinal diols. When two such glyme chelates are attached (top and bottom) to ferrocene, high yields and diastereoselectivities are obtained the glyme chelation can hold two samarium ions apart by a distance appropriate for inclusion of two reactant aldehyde molecules in between. ... 

Keto amides couple with samarium(ll) iodide to give //-isomers of pinacol dimers exclusively (Equation (36)). 

A monoprotected pinacol can be obtained in 81% yield by intramolecular coupling of 2,2 -biaryldicarbaldehyde mono-dibenzyloxy acetal, using samarium(ii) iodide in THF in the presence of BF3-OEt2 (Equation (47)). As in the case of homocoupling of 2,2 -biaryldicarbaldehyde, the /ra r-isomer is produced selectively. Addition of the Lewis acid is important to obtain high yields, otherwise the yield drops to 37%. ... 

Reductive coupling of carbonyls to alkenes Titanium(IV) chloride-Zinc, 310 of carbonyls to pinacols Titanium(III) chloride, 302 Titanium(IV) chloride-Zinc, 310 of other substrates Samarium(II) iodide, 270 Reductive cyclization 2-(Phenylseleno)acrylonitrile, 244 Tributylgermane, 313 Tributyltin hydride, 316 Triphenyltin hydride, 335 Trityl perchlorate, 339 Reductive hydrolysis (see Hydrolysis) Reductive silylation Chlorotrimethylsilane-Zinc, 82... 

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