Reactivity of mixed organozinc and mixed organocopper reagents: 5-A kinetic insight to compare the transfer ability of the same group in copper catalyzed alkylation of mixed and homozincates

A detailed kinetic investigation and activation parameters are reported for copper catalyzed coupling reaction of mixed zincate, n-BuPh2ZnMgBr and homozincate, Ph3ZnMgBr with n-pentyl bromide in THF at 25–65 °C. An empirical rate law can be expressed as rate = k[zincate]0 [alkyl bromide]1 [CuI]1. The reaction rate of transferable phenyl group in mixed catalytic cuprate, n-BuPhCuMgBr derived from mixed zincate is higher than the rate of catalytic homocuprate Ph2CuMgBr derived from homozincate. A catalytic cycle and the mechanism which accommodates the kinetic data and activation parameters is given. These results show that the reaction rate of transferable group changes depending on the residual group in the reactions of mixed diorganocuprates and also provide a kinetic support for the commonly accepted hypothesis regarding the dependence of the R1 group transfer ability on the strength of R2–Cu bond in reactions of R1R2CuMgBr reagents.

Graphical abstractRate = k[zincate]0 [alkyl bromide]1 [CuI]1, k for mixed zincate, n-BuPh2ZnMgBr > k for homozincate, Ph3ZnMgBr. Active organocopper intermediates: n-BuPhCuMgBr and Ph2CuMgBr. The kinetic results provide another support for the commonly accepted hypothesis for mixed diorganocuprates.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Cu catalyzed coupling of n-BuPh2ZnMgBr with alkyl bromide takes place with Ph transfer. ► An empirical rate law can be expressed as rate = k[zincate]0 [alkyl bromide]1 [CuI]1. ► The coupling rate of CuI catalyzed n-BuPh2ZnMgBr is higher that the rate of Ph3ZnMgBr. ► A catalytic cycle based on the reaction of n-BuPhCuMgBr and Ph2CuMgBr is proposed. ► The kinetic results are in accordance with the reactivity of mixed diorganocuprates.