Kinetic and Spectroscopic Analysis of Intermediates

Conjugate additions to a, j5-unsaturated ketones and esters are the most important cuprate reactions. Kinetic studies by Krauss and Smith on Me2CuLi and a variety of ketones revealed the following kinetic characteristics (Eq. 10.5), first order both in cuprate dimer and in the enone [60]. 

This rate expression is consistent with the reaction scheme shown in Eq. 10.6, formulated on the basis of the Krauss-Smith paper. Thus, the initially formed cuprate dimer/enone complex with lithium/carbonyl and copper/olefin coordinations [71, 72] transforms into the product via an intermediate or intermediates. A lithium/ carbonyl complex also forms, but this is a dead-end intermediate. Though detailed 

An intermediate formed on 1,6-addition of a cuprate to a dienone has recently been examined by low-temperature NMR spectroscopy. This reaction passes though a Cu/olefin re-complex intermediate A, in which cuprate binds to the a- and the j5-carbon. Further 1,3-rearrangement from another intermediate (B) to still another (C) is proposed (Eq. 10.7) [76]. 

Corey explicitly proposed a Dewar-Chatt-Duncan son (DCD) interaction for such a complex [73]. XANES investigation of a complex formed between a tr ns-cinnamate ester and Me2CuLiTiI in THE indicated elongation of the C=C double bond and an increase in the coordination number of the copper atom. NMR studies on the organic component in the complexes indicated loosening of the olefinic bond [72 74]. Very recently Krause has determined the kinetic activation energies (E = 17-18 kcal mol ) of some conjugate addition reactions for the first time [75]. 

---