Mechanism of reaction between Grignard reagents and nitroarenes. Product distribution and relative reactivities of Grignard reagents with nitronaphthalene system

Abstract

The reaction of 2-methoxy-1 nitronaphthalene with various Grignard reagents has been examined. Alkylmagnesium halides such as CH₃MgBr, PhCH₂MgBr, PhCH₂CH₂MgBr, C₂H₅MgBr, and i-C₃H₇MgBr give 1,6-addition products almost exclusively, while PhMgBr gives instead comparable amounts of 1,4- addition and reductive 1,2-addition products. Hex-5-enylmagnesium bromide reacts giving two 1,6- addition products, one containing a straight chain and the other one a cyclized alkyl fragment, where the ratio of the two decreases with decreasing temperature. The reactivity order (i-C₃H₇ > PhCH₂≃ C₂H₅ > PhCH₂CH₂ > CH₃) established by competitive reactions along with the reactivrty pattern shown by hex-5-enylmagnesium bromide was taken as clear evidence for a single-electron transfer (s.e.t.) process. A mechanism involving s.e.t. from Grignard reagent to nitroarene followed by collapse within the solvent cage of the two radicals thus formed (geminate combination) or, to a lesser extent, out of the cage (non-geminate combination), is suggested. The reaction of 1-nitronaphthalene with methyl-, isopropyl- and hex-5-enyl-magnesium bromides indicates that the distribution of isomeric 2- and 4- alkylated products is determined by the reactivity of the ring positions for both geminate and non-geminate combination. No firm mechanistic conclusions were reached regarding the reaction of PhMgBr.