Sml2-mediated Barbier and Grignard Reactions

In Chapter 4, we saw that Sml2 mediates the dehalogenation of a range of alkyl halides. These reactions proceed by reduction to alkyl radicals that are then 

Carbon-Carbon Bond-forming Reactions Using Sml2 

As organosamariums are formed from radical intermediates, it is important to note that competing radical processes may be problematic if they proceed at a rate that is greater than the rate of reduction of the intermediate radical to the organosamarium (see Chapter 3, Section 3.2). In general, Barbier conditions should be used when the organosamarium intermediate is prone to dimerisation (e.g. allylic and benzylic organosamariums), decomposition (e.g. 

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The Mechanism of Sml2-mediated Barbier and Grignard Reactions... 

A variety of functionalised organic halides have been found to be compatible with the Sml2-mediated Barbier and Grignard reactions. For example, ben-zyloxymethyl chloride can be used in conjunction with Sml2 for the alkoxy-methylation of ketones.120 This addition has been found to work well even for hindered ketones such as 101, an intermediate in White s 1987 approach to 2-desoxystemodinone (Scheme 5.70).121 It is important to note that, in contrast to analogous organometallics, decomposition via a-elimination is not observed. 

The Barbier-type reaction of aldehydes and ketones with allyl halides (485) in the presence of Smh, leading to homoallyl alcohols (486), has received recent interest as a one-step alternative to the Grignard reaction. However, the reactions require the use of stoichiometric amounts of the reducing Sm(III) species. Recently, the electroreductive Barbier-type allylation of carbonyl compounds in an Sml2-mediated reaction has been developed [569]. The electrolysis of (485) is carried out in a DMF-SmCl3-(Mg/Ni) system in an undivided cell to give the adduct (486) in 50 85% yields (Scheme 168) [569]. Electrosynthesis of y-butyrolactones has been achieved by the reductive coupling of ethyl 3-chloropropionate with carbonyl compounds in the presence of a catalytic amoxmt of SmCfi [570]. 

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