- Homolytic base-promoted aromatic alkylations by alkylmercury halides
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Electron transfer chain reactions leading to substitution in electronegatively substituted benzene derivatives can be observed with alkylmercury halides in the presence of proton accepters such as DABCO. Promotion by base involves the abstraction of a proton from the substituted cyclohexadienyl adduct radical to form a radical anion which readily transfers an electron to RHgX with the regeneration of R.. Aromatic substitutions involving t-Bu. are highly regioselective and yield products of only para attack for PhCHO, PhCOCH3, PhCOCMe3, PhCOPh, PhCN, phthalimides, or 1,2-dicyanobenzene. The ortho/para substitution products are observed for isophthaldehyde or 1,3-dicyanobenzene, while 1,4-dicyanobenzene yields the ortho substitution product. At 25-35°C substitution by t-Bu. ortho to an ester group is not observed and m- or p-cyanobenzoate esters yield only products of substitution ortho to the cyano group. With the isopropyl radical substitution ortho to the ester function is observed with diethyl isophthalate. Intramolecular radical cyclizations of the radical adducts of 1-aryl-4-penten-1-ones leading to α-tetralones is also promoted by the presence of DABCO. When the aryl group contains a para ester function, spirocyclizatien occurs leading to a rearrangement acyl radical which can be oxidized by t-BuHgCl to the acyl cation and the carboxylic acid.
- Russell, Glen A.,Chen, Ping,Kim, Byeong Hyo,Rajaratnam, Ragine
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- Homolytic base-promoted aromatic alkylations by alkyl halides
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Electron-transfer chain reactions leading to regioselective alkylations of benzenes bearing electron-withdrawing substituents can be observed with alkyl halides in the presence of the radical initiator (Bu3Sn)2 and the proton acceptor 1,4-diazabicyclo[2.2.2]octane (DABCO). Yields vary from low to high depending on the benzene derivatives. The role of DABCO is to abstract a proton from the substituted cyclohexadienyl adduct radical to form a radical anion which then transfers an electron to RX and is converted to the alkylated product itself. The rate of this electron-transfer step is probably not fast enough to sustain a good radical chain reaction so that further generation of R· from excess (Bu3Sn)2 and RX is necessary.
- Wang, Chen,Russell, Glen A.,Trahanovsky, Walter S.
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- Electron Transfer Photoinduced Cleavage of Acetals. A Mild Preparation of Alkyl Radicals
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Electron transfer from 2-alkyl- and 2,2-dialkyldioxolanes as well as from open-chain ketals to singlet excited benzene-1,2,4,5-tetracarbonitrile (TCNB) is followed by fragmentation of the donors radical cation to yield alkyl radicals and dialkoxy carbocations.The first species are trapped by TCNB to yield alkylbenzenetricarbonitriles (substitution of a second cyano group can be obtained sequentially) and in a minor path are reduced to alkanes, while the latter ones react with nucleophiles to give ortho acid derivatives.In view of the results of radical clock experiments, it is assumed that part of the process is a concerted (radical cation cleavage-addition to the aromatic) reaction, while another part involves the free-radical cation.On the other hand, intersystem crossing from the singlet radical ion pair to the triplet manifold causes cleavage of the acetal to the corresponding carbonyl derivative.This reaction offers a mild method for the preparation of alkyl radicals via C-C bonds cleavage.
- Mella, M.,Fasani, E.,Albini, A.
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p. 3051 - 3057
(2007/10/02)
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