Organocopper, reagents preparation

The organocopper reagents prepared in this way cleave epoxides in high yield with substitution of the alkyl group at the less hindered site. This cleavage has been used to effect intramolecular cyclization (second example). 

Reaction of Organocopper Reagents Prepared from Organozinc Reagents with Externai Chirai Ligands... 

Organocopper reagents prepared from the more readily available organozincs have found much use in the synthesis of highly functionalized molecules. For example, Ni-catalyzed hydrozincation of allylic alcohols initiates the preparation of a-(4-hydroxyalkyl)acrylates when the cuprate intermediates are used in the coupling with the a-bromomethylacrylic esters." ... 

Direct transmetalation of organoboranes to organocopper reagents is not a general reaction. Because of dieir similar bond energies and electronegativities, diis trans-nietalation is linided to die preparation of alkenylcopper and unfiinctionalized... 

For use of other organocopper reagents in converting carboxylic acid chlorides to ketones, see G. H. Posner and C. E. Whitten, Tetrahedron Lett., 1815 (1973) G. H. Posner, C. E. Whitten, and P. E. McFarland, J. Amer. Chem. Soc., 94, 5106 (1972). For a recent report on direct and convenient preparation of lithium phenylthio (alkyl)-cuprate reagents, see G H Posner, D J Brunelle, and L. Sinoway, Synthesis, 662 (1974). 

Organocopper reagents can also be prepared from Grignard reagents, which are generated and used in situ by adding a Cu(I) salt, typically the bromide, iodide, or cyanide. 

Buynak et al. reported the synthesis of representative 7-vinylidenecephalosporine derivatives bearing an axial allene chirality (Scheme 4.5) [9]. A chiral allene 24 was prepared stereoselectively utilizing the reaction of an organocopper reagent with propargyl triflate 23, obtained by a diastereoselective ethynylation of the ketone 22 with ethynylmagnesium bromide. Terminally unsubstituted allene 26 was synthesized via bromination of the triflate 23 followed by reduction of the bromide 25 with a zinc-copper couple. 

Here an alkynyl sulfoxide 55 is first carbocuprated with an organocopper reagent 56 to provide a vinylcopper intermediate 57, which is then zinc homologated with the primary zinc sp3-carbenoid 58 to yield the allylzinc intermediate 59. This, in a spontaneous syw-/)-climination, gives the corresponding allene 60. This protocol could also be adopted to the preparation of chiral allenes. 

If the reaction was carried out in THF with R2CuMgX or [RCuBr]MgX as the or-ganometallic reagent, a vinylic compound was prepared upon protonation. The E Z ratio of the product depends strongly on the type of organocopper reagent used. The reaction may proceed via the attack of Cu on the central carbon atom of the allene moiety, followed by transfer of the R group from Cu to the terminal carbon atom [54]. 

---