Substituent Effects on the Reaction Rates of Copper-Catalyzed Cyclopropanation and Aziridination of <i>para</i>-Substituted Styrenes

Abstract

Relative rates of cyclopropanation and aziridination of a series of para-substituted styrenes have been determined using Tp‘Cu(C2H4) (Tp‘ = hydridotris(3,5-dimethyl-1-pyrazolyl)borate) as the copper precatalyst in combination with N2CHCO2CH2CH3 and C6H5INSO2C6H4CH3 (PhINTs), respectively. For the cyclopropanation reaction, a linear plot of the experimental data is obtained by means of the Hammett equation, log(kX/kH) = ρσ (ρ = −0.85 ± 0.07). This correlation supports earlier mechanistic proposals of an electrophilic metal−carbene complex intermediate. In the case of the aziridination reaction, the experimental data can be fit with a two term equation of the type log(kX/kH) = ρ+σ+ + ρ•σ• (σ• are Jackson's radical substituent constants) leading to the values ρ+ = −0.28 ± 0.06 (polar contribution) and ρ• = +0.34 ± 0.13 (radical contribution). A paramagnetic copper nitrene species which behaves as an electrophilic, nitrogen-centered radical is proposed as the intermediate for the aziridination reaction.