NMR Spectroscopic Investigation of the Adducts Formed by Addition of Cuprates to Ynoates and Ynones: Alkenylcuprates or Allenolates?
✍ Scribed by Karolina Nilsson; Thomas Andersson; Christina Ullenius; Andreas Gerold; Norbert Krause
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 165 KB
- Volume
- 4
- Category
- Article
- ISSN
- 0947-6539
No coin nor oath required. For personal study only.
✦ Synopsis
The adducts obtained by reaction of Me 2 CuLi ´LiI, tBuCu(CN)Li, and tBu 2 CuLi ´LiCN with ynoates and ynones were characterized by determination of their 13 C NMR chemical shifts and 13 C, 13 C coupling constants. Alkenylcopper adducts were formed from ynoates and Me 2 CuLi ´LiI or tBu-Cu(CN)Li, whereas allenolates are obtained from ynoates and tBu 2 CuLi ĹiCN, as well as from ynones and Me 2 CuLi ´LiI. The transformations can therefore be regarded as carbocupration of an alkyne rather than a Michael addition. The equilibrium between alkenylcuprates and allenolates can be shifted towards lithium allenolates by addition of organolithium compounds. In the case of adducts formed from Me 2 CuLi ´LiI, isomerization of cis-to trans-alkenylcuprates via the corresponding allenolate can be prevented by removal of LiI or by the use of THF as solvent.
Whereas the protonation of alkenylcuprates to the corresponding alkenes proceeds stereospecifically, it is difficult to control the steric course of the protonation of allenolates. Addition of chlorotrimethylsilane to the adducts gives silyl ketene acetals or enol ethers in all cases.