Stereoselectivity lithium carbenoids

Recently, two more stereoselective cases of bromine-lithium exchange reactions have been observed. Both the glyceraldehyde-derived bromoalkene 45 and lactaldehyde-derived 47 yield the -configurated lithium carbenoids 46 and 48, respectively, when treated with... 

The chemistry of lithium carbenoids, organohthium compounds with an electronegative heteroatom in the a-position, has been developed in two respects during the last two decades. First, the ambiphilic character of these partly short-lived and mostly thermally sensitive compounds is well understood today, due to structure determinations, spectroscopic investigations and theoretical calculations. Second, many of the lithium carbenoids, formerly considered as exotic species, have developed into useful reagents that proved to be particularly fruitful in stereoselective syntheses. 

Fig. 14.32. Aldehyde alkyne chain elongation via [l,2]-rearrangement of a vinyl carbenoid (Corey—Fuchs procedure). The aldehyde and phosphonium ylide A generated in situ undergo a Wittig olefina-tion and form the 1,1-dibro-moalkene (B). In the second stage, the dibromoalkene is reacted with two equivalents of n-BuLi and the vinyl carbenoid D is formed stereoselectively. The carbenoid undergoes H migration to form the alkyne C. The alkyne C reacts immediately with the second equivalent of n-BuLi to give the lithium acetylide and is reconstituted by reprotonation during aqueous workup.

Single mono-adducts were obtained from (66) and (70) but (68) and (69) gave all four possible mono-adducts. Butyl-lithium reacts with hexachloro-cyclobutene (71) in the presence of an excess of trans-but-2-ene and gives the Spiro [2,3] hex-4-ene (72) stereospecifically with cis-but-2-ene, however, there was no measure of stereoselectivity. The predominant carbenoid from 3/f-pentachlorocyclobutene resulted from a-dechlorination and it reacted with isobutene in a highly stereoselective manner to give (73) exclusively. 

---