Reactions with Aryl Zinc Compounds

PhMgBr (1.5 equiv.) ZnBra (0.75 equiv.) LiBr (1.5 equiv.) 

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Biaryls have also been prepared by coupling support-bound aryl halides with aryl-zinc compounds (Figure 5.20) or with aryl(fluoro)silanes [203]. As with Suzuki or Stille couplings, these reactions also require transition metal catalysis. An additional strategy for coupling arenes on solid phase is the oxidative dimerization of phenols (Figure 5.20). 

Allylic substitutions with nonstabilized C-nucleophiles are an important domain of organocopper chemistry [51]. However, on close inspection of the literature, it becomes apparent that regioselectivity in favor of the branched allylic alkylation products is only obtained with alkyl copper compounds, while aryl copper compounds mainly give the linear alkylation products. This observation was an incentive for Alexakis et al. [52] to probe the reactions of aryl zinc hahdes in the Ir-catalyzed allylic substitution (Scheme 9.18). 

When the metallic additive to the intermediate 374 was zinc dihalide (or another Lewis acid, such as aluminum trichloride, iron trichloride or boron trifluoride), a conjugate addition to electrophilic olefins affords 381 . In the case of the lithium-zinc transmetallation, a palladium-catalyzed Negishi cross-coupling reaction with aryl bromides or iodides allowed the preparation of arylated componnds 384 ° in 26-77% yield. In addition, a Sn2 allylation of the mentioned zinc intermediates with reagents of type R CH=CHCH(R )X (X = chlorine, bromine) gave the corresponding compounds 385 in 52-68% yield. ... 

The first examples of microwave-assisted cross-couplings with organo-zinc compounds were reported in 2001, as shown in Scheme 70. Aryl- as well as alkylzinc bromides were effectively coupled with short reaction times [155]. 

A -Arylation of pyrroles can be achieved by conversion of 1-lithiopyrroles into the corresponding zinc compounds and then reaction with aryl bromides using palladium(O) catalysis or by direct reaction of the pyrrole with an aryl halide in the presence of base and the palladium catalyst. ... 

In many instances, the scope of the reactions listed above is limited as to the functional groups that are unaffected. Except for aryl-stannanes and arylfluorosilanes, carbonyl functionalities are often not compatible with the reactions. A number of cross-coupling reactions are known of aryl triflates [64 a] and arylfluorosulfonates [64b] with aryl stan-nanes catalysed by Pd(0)-complexes (Stille reaction) [65], aryl zinc chlorides [64b], and aryl boron compounds (boronic acids and esters, Suzuki reaction [66]). Aryl stannates are relatively stable to air and moisture and many funetional groups are tolerated. The coupling reaction using Ar-X/Ar-SnR3 can be accelerated by Cu(I) [67] or Ag(I) [68] catalysis. 

Two examples for nickel-catalyzed cross-coupling reactions of triflales and mesylates with the requisite zinc and boron species are outlined in Scheme 12.26. In 1994, Snieckus reported the Ni(0)-catalyzed reaction of aryl zinc reagents with aryl triflates and obtained the desired biaryl compounds in... 

The direct reaction of zinc metal with organic iodides dates back to the work of Frankland(67). Several modifications have been suggested since that time to increase the reactivity of the metal. The majority of these modifications have employed zinc-copper couples(68-72), sodium-zinc alloys(73), or zinc-silver couples(77). Some recent work has indicated that certain zinc-copper couples will react with alkyl bromides to give modest yields of dialkylzinc compounds(74,73). However, all attempts to react zinc with aryl iodides or bromides have met with failure. The primary use of zinc couples has been in the Simmons-Smith reaction. This reaction has been primarily used with diiodomethane as 1,1-dibromides or longer chain diiodides have proven to be too unneactive even with the most reactive zinc couples. 

The coupling of aryl- and alkenylzinc reagents with various halides has widespread use in the cross-coupling of aromatic rings [101]. The reactions of zinc derivatives of aromatic and heteroaromatic compounds with aryl and heterocyclic halides have wide synthetic applications [103,104],... 

With the development of the cross coupling methodology, many 6-C-substituted purines have been prepared in the past decade. Thus, 6 halopurine derivatives react with arylmagnesium halides,25 alkyl(aryl)zinc or tin reagents,26 trialkylaluminum,27 or alkylcuprates28 to give the 6-alkylpurine derivatives. Also a reverse approach based on the reaction of purine-6-zinc iodide with aryl or vinyl halides has recently been described.29 For the synthesis of 6-arylpurines, an alternative approach makes use of radical photochemical reactions of adenine derivatives with aromatic compounds,30 but this method is very unselective and for substituted benzenes, mixtures of ortho-, meta-, and para substituted derivatives were obtained. 

Palladium catalysed coupling reaction of a heteroaromatic zinc compound with a functionalized aryl iodide preparation of N-benzyl-6-(4-ethylcarboxyphenyi)-purine33... 

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