Radical barbier-type reaction

These examples again have some mechanistic implications in that they appear to rule out cyclization via 5n2 displacement of the halide by a samarium ketyl. However, one cannot distinguish between a mechanism based on allylsamarium addition to the carbonyl versus an electron transfer mechanism as outlined for the alkyl hdide substrates above. Both mechanisms allow for isomerization of the double bond (via 1,3-allylic transposition in the case of an allylmetallic, or configurational instability in an allylic radical in a diradical coupling mechanism) and also provide reasonable routes for generation of butadiene. Further mechanistic work is clearly required in order to provide a more detailed understanding of all of these intramolecular Barbier-type reactions. 

A cydization/Barbier-type reaction was reported by Curran and coworkers [21] in 2004 in the total synthesis of penitrem D 46. In this work, the aryl radical generated from the iodoarene 41 and Smij proceeded to attack the tethered cyclobutene to from a cyclobutyl radical 42. Subsequently, reaction with Smij led the organo-samarium species 43, which underwent a Barbier-typie reaction with acetone to give the tertiary alcohol 44 in 40% yield. The product contains the BCD ring system of penitrem D 46 (Scheme 5.12). 

Since the observation that allylation of carbonyl compounds could be mediated by tin in aqueous medium [77], there has been an intensive development of the Barbier-type allylation reaction in water. Three metals were particularly investigated zinc, tin, and indium. In the aqueous zinc-promoted allylation, allylzinc species are considered unlikely. The initiation of the reaction could be attributed to the formation of an allylic radical anion on the metal surface this radical surface could then react with the carbonyl compound to give an alkoxide radical, which could add an electron and form the alcohols [82]. Allyl bromide or even chloride reacts with aldehydes and ketones in the presence of commercial zinc powder in a mixture of tetrahydrofuran and saturated ammonium chloride aqueous solution (Eq. 7) [83]. 

Although the details of the mechanism are still under debate, the Luche reaction probably proceeds via radical mechanism, similar to that of Barbier-Grignard-type reaction occurring in aqueous solution, as outlined here. 

Comparing the yields of the different products in these reactions with the stability of the cage radicals, the conclusion was drawn that the Barbier-type... 

As C—M bonds of most post-transition metals have a strong covalent character and because many reactions can occur also via radical and radical anion processes on metal surfaces, it is not surprising that many other metals have been foimd to mediate the Barbier-Grignard-type reactions in water. 

One of the benefits of the Barbier reaction over the Grignard reaction is generally the production of products with little or no R-R dimer formation. However, because of the radical nature of the reaction Wurtz-type dimer formation can be the primary reaction in the absence of an electrophilic coreagent. Some Russian chemists have reported the dimerization of halogenated alkenes and alkanes using iodine-activated magnesium with silver bromide [Eqs. (27) and (28) 64]. 

Initial formation of a radical anion of the type [RX] seems highly probable as in the Barbier reaction. The radical ion absorbed on the metal surface, more or less readily cleaved in a radical, is trapped by the a,P-unsaturated carbonyl compound, affording an a-keto radical, which is further rapidly... 

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