Electrophilic reactions triethylborane

In the previous sections, the reactions of nucleophilic alkyl and acyl radicals with electron-deficient aromatics via SOMO-LUMO interaction have been described. At this point, we introduce the reactions of electrophilic alkyl radicals and electron-rich aromatics via SOMO-HOMO interaction, though the study is quite limited. Treatment of ethyl iodoacetate with triethylborane in the presence of electron-rich aromatics (36) such as pyrrole, thiophene, furan, etc. produces the corresponding ethyl arylacetates (37) [50-54]. 

This reaction comprises firstly of SH2 reaction on the iodine atom of ethyl iodoacetate by an ethyl radical, formed from triethylborane and molecular oxygen, to form a more stable Chester radical and ethyl iodide. Electrophilic addition of the a-ester radical to electron-rich aromatics (36) forms an adduct radical, and finally abstraction of a hydrogen atom from the adduct by the ethyl radical or oxidation by molecular oxygen generates ethyl arylacetate (37), as shown in eq. 5.20. Here, a nucleophilic ethyl radical does not react with electron-rich aromatics (36), while only an electrophilic a-ester radical reacts with electron-rich aromatics via SOMO-HOMO interaction. 

Hydrogallation of carbon-carbon multiple bonds with HGaCl2 proceeds in the presence of triethylborane as a radical initiator. Several functionalities do not interfere with this reaction (Scheme 143). Resulting alkenyl-and alkylgallium species can be trapped by several electrophiles.375 A novel N02/alkyl substitution of /5-nitrostylene with trialkylgallium has been found, which is considered to proceed via a radical mechanism (Equation (105)).434... 

Miyabe et al. developed a tandem addition/cycUzation reaction featuring an unprecedented addition of alkoxycarbonyl-stabihzed radicals on oxime ethers [117], and leading to the diastereoselective formation of /1-amino-y-lactone derivatives [118,119]. The reaction proceeds smoothly in the absence of toxic tin hydride and heavy metals via a route involving a triethylborane-mediated iodine atom-transfer process (Scheme 37). Decisive points for the success of this reaction are (1) the differentiation of the two electrophilic radical acceptors (the acrylate and the aldoxime ether moieties) towards the nucleophilic alkyl radical and (2) the high reactivity of triethylborane as a trapping reagent toward a key intermediate aminyl radical 125. The presence of the bulky substituent R proved to be important not only for the... 

Most organic free radicals are nucleophilic and will react with electrophilic centers. Lewis acids have been used to activate aj3-unsaturated carbonyl compounds towards addition of free radicals and also to stabilize a-keto radicals [67]. The first report of the use of a chiral Lewis acid to effect an asymmetric free-radical reaction was that of Urabe, Yamashita, Suzuki, Kobayashi, and Sato in 1995 [68]. They found that if the BINOL aluminum catalyst 313 is stoichiometrically complexed with lactone 323 and then treated with butyl iodide and tributylstannane in the presence of triethylborane the alkylated lactone 324 can be isolated in 47 % yield with 23 % ee (Sch. 40). 

The intermolecular addition of carbon-centered radicals to unactivated alkenes followed by azidation (a formal carboazidation of alkenes) has been reported. A one-pot procedure similar to the one used for intramolecular reactions gives good results (Scheme 8.28). Slow addition of benzenesulfonyl azide is not necessary because this electrophilic reagent does not react with the initial electrophilic or ambiphilic radicals. Excellent results are obtained with a-iodo and -xanthate esters. a-Bromoacetates give also satisfactory results. The carboazidation process allows to prepare pyrroUdinone derivatives in a straightforward manner (Scheme 8.28, bottom example). A tin-free version of this reaction using triethylborane instead of hexabutylditin has also been reported. ... 

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