Spectroscopic Characterization of (Iodomethyl)zinc Reagents Involved in Stereoselective Reactions: Spectroscopic Evidence That IZnCH2I Is Not Zn(CH2I)2 + ZnI2 in the Presence of an Ether

We have shown that low-temperature 13 C NMR spectroscopy is an extremely powerful technique for characterizing the (iodomethyl)zinc-derived reagents involved in the cyclopropanation reactions. This technique has allowed us to spectroscopically characterize and unambigously differentiate the Furukawa reagent (EtZnCH 2 I), the Simmons−Smith reagent (IZnCH 2 I), and the Wittig reagent (Zn(CH 2 I) 2 ). Unique spectra are obtained for each of these reagents when they are complexed to a chiral diether. We have also demonstrated that IZnCH 2 I is not converted into Zn(CH 2 I) 2 + ZnI 2 in CD 2 Cl 2 in the presence of a chiral complexing agent. Furukawa's reagent “EtZnCH 2 I”, however, is in equilibrium with Et 2 Zn and Zn(CH 2 I) 2, and it eventually decomposes into PrZnI and EtZnI at room temperature. The decomposition of Zn(CH 2 I) 2 into IZnCH 2 I and of IZnCH 2 I into ZnI 2 was monitored by NMR. We have also demonstrated that the general trends observed for the various equilibria involving (iodomethyl)zinc-derived reagents follow those observed with ethylzinc-derived organometallic compounds.