Samarium reaction with oxygen

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. 

A reaction of dibromoacetic acid with different aldehydes promoted by Sml2, followed by an elimination reaction also promoted by samarium diiodide, affords ( > ,/l-unsaturalcd carboxylic acids with total stereoselectivity (Scheme 8).42 A mecha- (g) nism that involves chelation of the Sm(III) centre with the oxygen atom of the alcohol group through a six-membered chair-like transition state has been described. 

The lanthanides react slowly with cold water (more rapidly with hot water) to form hydrogen gas, and readily bum in air to form oxides. Oxides are substances in which a metal and oxygen have chemically combined to form a compound. For example, samarium and oxygen combine to form the compound samarium oxide. Yttrium has a natural protective oxide coating, making it much more resistant. The lanthanides form compounds with many nonmetals, such as hydrogen, fluorine, phosphorous, sulfur, and chlorine, and heating may be required to induce these reactions. 

Scheme 21 shows the reaction of the 2-deoxy-2-amino sugar 100 reacting with samarium iodide and cyclohexanone to give the desired C-glycoside 101 as the major compound [29]. When the azide was used in lieu of the protected amine fimction, only the product of elimination was formed. This indicates a possible complexation between the acetimido oxygen atom (Scheme 21, second equation) and the samarium atom, resulting in stabilization of the a-organosamarium species, since normally. 

The overall reaction gives the ester 165 and not, as we might expect, a diol or an acetal. The alcohol 164 reacts with acetaldehyde to form a hemiacetal 166. It is the hydrogen on this hemiacetal that is delivered as a hydride to reduce the neighbouring carbonyl but only after a Sm chelate 167 is formed. Compare this transition state 167 with 162 where trisacetoxyborohydride donated the hydride ion. They are very similar both transfer a hydride via a six-membered cyclic transition state. In 162 the boron holds the two oxygens in a ring while in 167 it is samarium.41... 

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