Coupling reactions samarium

An interesting novel coupling reaction of allenes with carbonyl compounds mediated by a lanthanide metal species was reported recently [80], The samarium(II) iodide-mediated reaction of various ketones or aldehydes 153 with methoxyallene (56) afforded exclusively y-addition products 4-hydroxy-l-enol ethers 154 in moderate to good yields with low cis/trans selectivity (Scheme 14.39). 

Coupling reactions of aldehydes or ketones to 1,2-diols proceed with low-valent metals such as magnesium, zinc, and aluminum.Because it is not easy to control the stereoselectivity (diastereoselectivity and/or enantioselectivity) of the reactions with such main group metals, low-valent species of early transition metals are frequently employed with electron-donating ligands. The representative reagents are low-valent titanium and samarium species. 

Some additives accelerate the pinacol coupling reactions. Addition of Me3SiCl to Sml2 also accelerates the pinacol coupling reactions of aliphatic ketones and aldehydes. Pinacol coupling reactions are also promoted with samarium metal and a Lewis acid such as Et2AlCl or MesSiCl. Coordination of such a Lewis acid to a carbonyl oxygen facilitates the one-electron reduction by samarium. 

Diastereomeric ratios of pinacol dimers obtained by coupling of benzaldehyde and acetophenone with samarium reagents are summarized in Tables 3 and 4, respectively. In general, the stereochemical control of pinacol coupling reactions with a stoichiometric or catalytic samarium(ll) reagent is poor, giving nearly a 1 1 dljmeso mixture. 

There are several important stereochemical features in the coupling reaction of benzaldehyde with samarium(ll). 

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)). ... 

A. Chenede, E. Perrin, E. D. Rekai, and P. Sinay, Synthesis of C-disaccharides by a samarium diiodide induced intramolecular coupling reaction of tethered sugars, Synlett p. 420 (1994). 

Shiraishi, H., Nishitani, T., Sakaguchi, S. and Ishii, Y., Preparation of substituted alkylpyrroles via samarium-catalyzed three-component coupling reaction of aldehydes, amines, and nitroalkanes, /. Org. Chem., 1998, 63, 6234-6238. 

Intermolecular coupling of ketones and alkenes, promoted by SmH, occurs with excellent stereochemical control. In one such reaction, samarium(II) iodide has been used to prepare cyclobutanones and cyclobutanols from chiral, 6-oxohex-2-enoates (equation 137)520. The reaction is performed in THF in the presence of HMPT and occurs in good yield with excellent stereocontrol. If appropriately located carbonyl and alkene moieties are present in a molecule, then Sml2-HMPT can be used to form cyclooctanols by a radical cyclization process in some cases there is a reasonable degree of diastereoselectivity (equation 138)521,522. 

Several examples of intramolecular ketyl anion/unactivated olefin coupling reactions were reported by the Molander group, one of which is illustrated in equation 2884. An interesting facet of this reaction is that it is possible to further react the cyclized samarium(III) intermediate (47) with a variety of electrophiles, thereby enhancing the... 

A number of unsaturated substances, such as ketone, azobenzene, imine, a-diimine, carbodi-imide, nitrile, CO2, and so on can be reduced by divalent samarium complexes. For example, reaction of bi(methylcyclopentadienyl) samarium complex with carbodiimide generates, via reducing-coupling reaction, the first structurally characterized bimetallic oxalamidino complex (Figure 8.34) [114]. 

Scheme 10 is representative of the mechanism of these coupling reactions involving a captodatively stabhzed glycyl radical 15 from the initial reduction of the pyridyl sulfide group by the divalent lanthanide reagent. Further reduction of this carbon radical by a second equivalent of samarium diiodide leads to a Sm(lII) enolate intermediate 16 of unknown geometry, which ultimately reacts with the carbonyl compound to give 17. 

Samarium diiodide has also been utilized as a reductant to promote pinacolic coupling reactions mediated by low-valent titanium species (equation 71). Utilizing this protocol, high diastereoselectivity can be achieved, although the yields for Ais particular process were not reported. 

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