Effect of HMPA on the Composition of Organocyanocuprates. First Observation of a 2 J 13 C - 13 CN Coupling Constant in a Vinylcyanocuprate

Variable temperature 1H- and 13C-NMR spectroscopic analysis of THF and THF−HMPA solutions of stannylcyanocuprates, as well as their addition products to 1-alkynyl ethers have been conducted. When prepared in the presence of HMPA, the reagents and addition products formed exist as mixtures in which some free lithium cyanide is present and the remaining cyanide is otherwise complexed. When stannylcyanocuprates were prepared in THF solutions no LiCN was detected. Heteronuclear NMR of the intermediates formed in the low-temperature reaction of a 13C-labeled acetylenic ether with a THF−HMPA solution of stannylcyanocuprate (Bu3Sn)2Cu(13CN)Li2 reveals a 13CN−13C-vinyl carbon coupling consistent with a vinylcuprate intermediate containing cyanide and a vinyl group bound to a common center. Coupling between a metal bound 119Sn and 13C-vinyl carbon was observed in the intermediate suggesting these two atoms are bound to a common center. The lack of 119Sn−13CN coupling in the intermediate suggested 119Sn and 13CN were bound to different atoms. Requirements are met in both higher order and lower order cuprate structures. When “Me2Cu(13CN)Li2” is prepared in THF, no free LiCN is observed and neither is it generated on addition of HMPA. When Me2Cu(13CN)Li2 is prepared in HMPA-containing solutions it exists with the corresponding cyanide-free species and LiCN.