Organozinc halides, with

SCHEME 80. Fe-catalyzed alkylation of organozincs with secondary alkyl halides... 

The electrochemical preparation of organozinc compounds obtained from the corresponding aromatic halides and with the use of a nickel complex as catalyst is only efficient in dimethylformamide as solvent. Moreover, in most cases and as described previously, the reaction requires the presence of excess 2,2 -bipyridine (five molar equivalents with respect to nickel) to achieve the transmetallation reaction leading to the organozinc compound and to avoid the formation of biaryl, Ar-Ar (equation 53). 

Fig. 1 Nickel-catalyzed Negishi coupling reactions of alkyl halides 1 with organozinc reagents 2...

The cross-coupling of organozinc compounds has been accomplished using a number of metal enolate complexes, namely Rh(acac)(H2C=CH2)2 (6) , Co(acac)3 (30) °, Ni(acac)2 (47) , Li(acac) (107) and Cr(acac)3 (108) . For instance, complex 47 proved to be an extremely effective catalyst system for the Negishi cross-coupling of arylzinc halides (109) with aryl (110), heteroaryl and alkenyl halides, triflates and nonaflates to give the corresponding biaryl compounds (111) (equation 30). The solvent played an important role in these reactions and optimal conditions were found with 8 1 mixtures of THF and A-ethylpyrrolidinone (nep). Only 0.05 mol% of the nickel complex... 

Competitive schemes similar to that shown in Scheme 1-51 must apply to other tandem processes in which reductive elimination is the Pd-releasing step. Indeed, the authors attempts to develop a carbonylative ketone synthesis similar to that shown in Scheme 1-50 using organozincs were largely unsuccessful due to competitive direct coupling, even though the Pd-catalyzed reaction of organozincs with acyl halides is very satisfactory [105]. 

Alkenylation. Grignard reagents or organozinc halides couple with alkenyl halides with Co(acac)2 as catalyst. The configuration of the double bond is retained. 

Coupling of organozincs with aryl halides. The method is very useful for the synthesis of biaryls,including heterobiaryls.Vinylic, " aliphatic zinc reagents, and even Zn(CN>2 are reactive toward ArX in the presence of (Ph3P)4Pd. Perfluoro-alkenylzinc bromides and cyclopropenylzinc chlorides also couple efficiently with iodoarenes. 

Similarly, the high reactivity of organozincs tends to preclude the CO insertion-crosscoupling tandem process, which proceeds well with slower reacting organotins. However, some chelation-stabilized alkylzinc derivatives do satisfactorily participate in this tandem process (Scheme 11). It should also be recalled that the Pd-catalyzed reaction of organozincs with acyl halides without the use of CO is one of the most general routes to ketones. ... 

Thus, most of such catalytic systems designed to improve efficiency in the case of the cross-coupling of C(sp )-organozincs with C(sp )-organic halides are similarly applicable with primary alkylzincs. These include the above-mentioned palladium complexes of PtBuj [188, 189, 233-236] biphenyl-based monophosphine ligands of the SPhos (148) family [131-133] or the use of PEPPSI precatalysts [136-138, 237, 238]. 

Substituting lithium 2-thienylcyanocuprate for the CuCN/LiBr complex and omitting boron trifluoride etherate from the reaction of the organozinc halide species with 2-cyclohexenone indeed gave a reasonable yield (76% GC) of the... 

The Buchwald lab has studied ligand effects in the coupling of simple primary and secondary allq l organozinc reagents with aryl halides. Organozincs are robust nucleophiles and readily undergo transmetallation to Pd(ii). Thus, in these cases, the key issue becomes what influences the reaction path toward reductive elimination over palladium hydride elimination. In the catalytic cycle with a secondary organometallic, if the catalyst... 

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