Alkenyl-benzyl cross-coupling, benzylation reactions

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

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... 

Oxidative addition [1, 38] of 1-alkenyl, i-alkynyl, allyl, benzyl, and aiyl halides to a palladium(O) complex affords a stable rra .s-<7-palladium(II) complex (11). The reaction proceeds with complete retention of configuration for alkenyl halides and with inversion for allylic and benzylic halides. Alkyl halides having /3-hydrogens are rarely useful because the oxidative addition step is very slow and may compete with /3-hydride elimination from the a-organopalladium(II) species. However, it has been recently shown that iodoalkanes undergo the cross-coupling reaction with organoboron compounds (Section 2.4.5). 

In view of some highly satisfactory Pd-catalyzed cross-coupling reactions between alkenyl-, aryl-, or alkynyhnetals and allyl, benzyl, or propargyl electrophiles discussed in Sects. III.2.8.2 and III.2.9, it is not unreasonable to explore related Pd-catalyzed allyl-allyl, benzyl-allyl, propargyl (or allenyl)-allyl coupling and related reactions. Most of the efforts have been focused on the Pd-catalyzed allyl-allyl coupling involving Sn. 

There are six discrete combinations for Pd-catalyzed cross-coupling between homoallyl-, homopropargyl-, or homobenzyhnetals and alkenyl or aryl electrophiles (Scheme 4). In addition to these processes, those involving alkyl, allyl, benzyl, propargyl, alkynyl, and acyl electrophiles are conceivable, and some have indeed been observed. However, these reactions are discussed in other pertinent sections. 

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