Vinylcuprate

The simplicity of operation and flawless rendering of (E)-geometry make this an attractive alternative to vinylcuprate additions. 

Another recent disclosure examined silicon-to-copper transmetallation as a mild means of synthesizing alkenyl-copper reagents from stable precursors. The method requires activation of the silyl group by an allylic alcohol. Again, the silanes in this work are produced by circuitous means but should be accessible by ruthenium-catalyzed hydrosilylation. Treatment of the silyl alcohol with a stoichiometric amount of copper(l) /rz -butoxide results in the C-to-O migration of the silyl group to produce a vinylcuprate shown to be competent for subsequent allylation to produce 1,4-diene products (Scheme 17). 

This reaction can be used to effect a [3 + 2] or a [3 + 2 + 2] annelation to a cyclopentenone ketal or a cycloheptadienone ketal, respectively. Thus the adduct of a vinylcuprate with 1 rearranges thermally (240-290°) to 3-cyclopentenone ketals (3) in 50-80% yield (equation I). [3 + 2 + 2]Annelation involves preparation of... 

Scheme 3.16. Silylcupration of alkynes with (t-BuPh2Si)2CuLi-LiCN and electrophilic trapping of the vinylcuprate reagent [69].

Scheme 3.18. Ring-formation by intramolecular trapping of the vinylcuprates resulting from silylcupration of alkynes with magnesium silylcuprates [75a] or lithium silylcuprates (mCPBA = m-chloroperbenzoic acid) [75bj.

Scheme 3.19. Stannylcupration of acetylene and trapping of the vinylcuprate with electrophiles [79c]...

Scheme 3.23. Cyclobutene and alkylidine cyclobutane synthesis by stannylcupration of alkynes and trapping of the resultant vinylcuprate with epoxides [15c],...

Another example is found in the total synthesis of iso[7]-levuglandin D2 (30) by Salomon et al. [24]. The cyanocuprate 27 was prepared by transmetalation of multifunctional vinylstannane 26 with Me2CuLiLiCN (Scheme 9.6) [25]. Addition of the enone 28 to the multifunctional vinylcuprate 27 provided the conjugate addition product 29 in 65% yield (based on the enone consumed). 

ESR and CIDNP studies intended to detect the radical intermediates failed [63], Conjugate addition of a vinylcuprate reagent to an enone takes place with retention of the vinyl geometry indicating that no vinyl radical intermediate is involved [64, 65], Kinetic isotope effects and substituent effects in cuprate addition to benzophenone indicate that C-C bond formation is rate-determining, which is not consistent with the involvement of a radical ion pair intermediate [66]. 

This mixed cuprate is useful for preparation of 2-methyl-l-alkenes from 1-alkynes (equation II). In the absence of LiBr the intermediate vinylcuprate a decomposes to... 

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