The reaction of allyl iodide with ketones in the presence of cerium amalgam. An efficient method for the preparation of homoallylic alcohols

Regio- and Stereochemistry on the Electrophilic Trapping of Allylic Samariums Generated by Reductive Cleavage of Allylic Ethers with (C5Me5)2Sm(thf)n

“…Interestingly, the present regioselectivity is opposite to that observed for allylic lanthanides generated by transmetalation 5,6b and halogen−metal exchange, 2a,3a wherein ratios of the least to the most substituted homoallylic alcohols were about 100:0 and 60:40, respectively . In addition, a 1,3-disubstituted allylic species corresponding to 4h was reported to yield the other regioisomer 10h exclusively 6b.…”

“…Although there is no mechanistic proposal for the predominant formation of linear homoallylic alcohols from allylsamariums prepared by other methods, ,,, these results could be rationalized by the four-membered transition state. In fact, recent ab initio molecular orbital study on the reaction of Cp 2 Sm(η 3 -C 3 H 5 ) with formaldehyde demonstrates that the four-membered transition state is more favorable than the six-membered one…”

“…Allylic organometallics are useful reagents in organic synthesis, and thus, extensive studies on their preparation, structure, and reactivity have been reported . Of the allylic metals, allyllanthanides used for synthetic reactions can be generated in situ by various methods, which include halogen−metal exchange of allylic halides using samarium diiodide or cerium amalgam, reduction of allylic phosphates with samarium diiodide, and transmetalation such as allylic lithium with cerium trichloride and allylic palladium with samarium diiodide . However, the structure of these reagents has not been confirmed unambiguously.…”

“…In regard to regioselectivity on allylation of carbonyl compounds with allylic lanthanides, it has been concluded that the reaction takes place from the least substituted terminus to yield linear products. − Only one exception to this category is observed for the reaction with the Cp complexes in which blanched products are formed selectively . Research addressing the stereoselectivity on their addition to a prochiral center has been limited …”

Allyl Iodide

“…8 In contrast, allyl iodide with Al and Ce/Hg in THF at 0 • C under N 2 for 5 h gives excellent yields of the tertiary alcohol with a range of ketones. 9 Other such reductive allylation methods use CrCl 3 + 0.5 mol LiAlH 4 , which seems to provide Cr II which with RI or RBr in THF gives allyl Cr II species. These may lead to biallyl formation or to the Barbier-Grignard reaction with ketones and aldehydes, but not with nitriles or carboxylates (eq 2).…”