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

Chen, Ping
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[alpha],[beta]-Unsaturated nitriles undergo radical chain addition reactions with tert-butylmercury 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 NH4+ or by NaBH4. The addition reaction involves the attack of the adduct radical upon the ate complex, t-BuHgI2-. Proton donors also promote the reactions by protonating the adduct radicals to form keteniminyl radicals cations which accept an electron from t-BuHgI2- 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/t-BuHgI 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 t-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 cis-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 t-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 (1,4-diazabicyclo (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 -CO2Et, 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 t-BuHgX to afford the product and regenerate t-Bu·.

Chemistry, Chemistry, Organic, Organic chemistry