Photostimulated tert-butylations of a,b-unsaturated nitriles, cyclizations of 1,6-dienes and olefinic nitriles, and homolytic aromatic alkylations

Abstract

a,P-Unsaturated nitriles undergo radical chain addition reactions with tertbutylmercury halides in the presence of iodide ions. The mercury adduct products are formed in high yields and can be converted to the reductive alkylation products with NPU' or by NaBfit. The addition reaction involves the attack of the adduct radical upon the ate complex, r-BuHgl2'. Proton donors also promote the reactions by protonating the adduct radicals to form keteniminyl radicals cations which accept an electron from f-BuHgl2 to produce the alkylation products. Acrylonitrile, crotononitrile and fumaronitrile give nearly quantitative yields of the products in DMSO via photolysis or even in the dark at room temperature. Three-component condensation reactions occur for mixtures of acrylonitrile/fBuHgl and reagents such as CH2=CHCH2Br or CH2=CHCH2SPh or upon further reaction of the mercury adducts with I2, NBS, PhCH=CHI, PhSeSePh, or iminium ions. Cyclization reactions of 1,6-dienes with f-BuHgX have been investigated. A radical chain process provides the cyclized organomercurials via 5-exo mode cyclization. Cis and trans isomeric cyclized mercury compounds are obtained with the cw-isomers preferred. The measured initial kinetic chain length for the substrates are 3-10. Photolysis of the mercury compounds with PhSSPh leads to the corresponding sulfides. Allyl vinylphosphonates react with r-BuHgX very readily. However, the cis/trans product ratio is 1:1. Alkylmercury halides are very suitable reagents for alkylation of electron-deficient aromatics. In the presence of Dabco (l,4-diazabicycIo[2.2.2]octane), the disubstituted benzenes undergo a regioselective radical alkylation reaction. For 1,3-dicyanobenzene, the initial kinetic chain length is 22 and not only the mono-alkylated but also the dialkylated product is produced in high yield. With larger groups meta or para to a cyano group such as -CHO and -C02Et, the second alkylation is inhibited. Dabco promotes the chain reaction by removing a proton from adduct cyclohexadienyl radical. The radical anion formed is oxidized by r-BuHgX to afford the product and regenerate f-Bu*.