Reformatsky reagents with alkenyl halides

The reaction of aryl and alkenyl halides with the Reformatsky reagent 237 in polar solvents affords a-arylacetates [105], Iodides of heterocyclic compounds such as pyridine, quinoline and pyrimidine react smoothly with Reformatsky reagents. The cross-coupling of 237 with 4-iodo-2,6-dimethyl- and 2-iodo-4,6-dimethylpyrimidine (236) occurs smoothly to form 238. But no reaction of 5-iodo-2,4-dimethylpyrimidine (239) takes place [106],... 

The Reformatsky reagents, i.e. zinc enolates of esters, undergo Ni catalysed cross-coupling with aryl halides.53 The Ni catalysed reaction of arylzincs with a-bromoacetates also permits a-arylation of esters54 (Scheme 11.13). However, a-alkenylation of enolates of ketones, aldehydes, and esters has been less satisfactory. Its further development is clearly desirable. Alternatively, a-alkenylation of a-iodoenones in conjunction with conjugate reduction discussed earlier should be considered. 

The insight that zinc ester enolates can be prepared prior to the addition of the electrophile has largely expanded the scope of the Reformatsky reaction.1-3 Substrates such as azomethines that quaternize in the presence of a-halo-esters do react without incident under these two-step conditions.23 The same holds true for acyl halides which readily decompose on exposure to zinc dust, but react properly with preformed zinc ester enolates in the presence of catalytic amounts of Pd(0) complexes.24 Alkylations of Reformatsky reagents are usually difficult to achieve and proceed only with the most reactive agents such as methyl iodide or benzyl halides.25 However, zinc ester enolates can be cross-coupled with aryl- and alkenyl halides or -triflates, respectively, in the presence of transition metal catalysts in a Negishi-type reaction.26 Table 14.2 compiles a few selected examples of Reformatsky reactions with electrophiles other than aldehydes or ketones.27... 

---