SYNTHESIS arylzinc reagents

The use of expensive and unstable ZnPli2 in the preparation of chiral di-arylmethanol derivatives, with electronically and sterically similar aryl rings, made this approach less attractive for the enantioselective synthesis. In order to avoid this inconvenience, other alternative preparations of arylzinc reagents were evaluated.As a first choice, Yus et al. proposed the use of arylboronic adds as a viable source of phenyl (Scheme 4.19). Thus, the reaction of various boronic acids with an excess of ZnEt2 at 70 °C gave the corresponding arylzinc intermediates (probably aryl(ethyl)zincs), which were trapped by reaction with dif-... 

The reaction was exploited very recently in a solid phase synthesis of biaryls. Aryl zinc bromides undergo palladium catalyzed coupling reactions with aryl bromides bound to a polystyrene resin. The product can be released from the resin by transesterification [44]. Ni(0) catalysed homocoupling of arylzinc reagents could also be realised using aryl triflates [45], as well as aryl tosylates and mesylates [46]. 

Alkylzinc reagents undergo transmetallation in the presence of Co(II) salts to provide the corresponding organocobalt derivatives. These intermediates are stable below —10 °C in mixtures ofTHF and NMP and have proved useful for the synthesis of symmetrical ketones [310, 311]. Cross-coupling reactions with alkylzincs can also be performed using catalytic amounts of Co(II) salts. In contrast to arylzinc reagents, however, disclosed examples [312, 313] are still rather scarce. 

Scheme 2-110. Synthesis via a B/Zn-exchange and reactions of the arylzinc reagent...

Kneisel FF, Leuser H, Knochel P (2005) Preparation and reactions of highly functionalized bis-arylzinc reagents using a Li(acac)-catalyzed iodine-zinc exchange. Synthesis 2625-2629. doi 10.1055/s-2005-872097... 

The orf/zo-alkenylarylzinc reagent obtained by the migratory arylzincation serves as a precursor to benzo[ijchalcogenophenes such as benzo[fe]thiophenes and benzo[h]selenophenes (Scheme 10.9) [28]. Treatment of the zinc intermediate with sulfur powder in the presence of Cul allows one-pot synthesis of a benzo[h]thiophene derivative (route A). Alternatively, the zinc intermediate is quenched with iodine to give ortho-alkenylaryl iodide, which is then subjected to copper-catalyzed reaction with sulfur or selenium powder to furnish the corresponding benzo[i]thiophene or benzo[(i]selenophene derivative (route B). Collectively, these approaches allow for expedient synthesis of functionalized benzo[h]thiophenes and benzo[()]selenophenes, many of which are difficult to... 

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