3- Thienylzinc

Many aryl- and heteroarylzinc reagents have been coupled with halopyridines. Examples include arylzincate 20 derived from ethyl benzoate [3], thienylzinc reagent 21 [16], 2-imidazolylzinc chloride 22 [17], pyrazolylzinc reagent 23 [18], and 2-pyridylzinc reagent 24 [19]. 

The 2-thienylzinc bromide compound was prepared according to the same process described above in a 45% yield (equation 32)39. However, a different procedure has been used for the preparation of the 3-thienylzinc bromide compound41. 

This last electrochemical process is carried out in an undivided electrolysis cell fitted with a sacrificial magnesium anode and a nickel foam as cathode. The reaction is conducted in dimethylformamide in the presence of both NiBr2(bpy) as the catalyst and dried ZnBr2 (1.1 molar equivalents with respect to bromothiophene), which is used both as supporting electrolyte and as a zinc(II) ion source. The other conditions are the same as those described in the section concerning the aromatic halides. The yield of 3-thienylzinc bromide was roughly 80%, as determined by GC analysis after treatment with iodide (equation 34). 

Thienylzinc bromide prepared under mild conditions by electroreduction of a DMF solution containing 3-bromothiophene, zinc bromide and a catalytic amount of nickel-bipyridine complex, using a magnesium rod as the sacrificial anode as described in... 

Electrochemical synthesis of 3-thienylzinc bromide and its coupling with iodobenzene synthesis of 3-phenylthiophene... 

Miyasaka and Rajca used PdfPiz-BuJjJj to couple 3-bromothiophene to 3-thienylzinc chloride and 3-benzo[b]thienyl chloride to yield thiophene 69 and benzothiophene 70 in 86 and 60% yield, respectively [61]. Unlike the Fu example, the reaction was run at 40°C owing to the greater reactivity of 3-bromothiophene over 3-chlorothiophene. 

A useful technique for the preparation of n and Cu powders) is by electrolysis with puls< efficiency in allylation achieved by using s involves reduction of ZnCl with Na in liquid 1 Ketones. Rieke zinc produced by the r reacts with alkyl halides, and the organozin treatment with CuCN and acid chlorides. S readily prepared in this direct manner. a-Chkj method using chloroacetyl chloride in the couf 3-Thienylzinc halides. These reagenii Rieke zinc. They undergo Ni-catalyzed coupln... 

Thienylzinc haUdes. These reagents are obtained from 3-thienyl halides and Rieke zinc. They undergo Ni-catalyzed coupling with aryl iodides. 

The palladium(0)-catalysed coupling of 2-bromocyclopentene-l-carboxaldehyde NJf-dimethylhydrazone with vinylzinc-, 2-furyl-, or 2-thienylzinc halides provides a route to the corresponding dimethylhydrazones, which cyclize thermally to pyridines, e.g. 9 <95T(51)9119>. 

The electrochemical conversion of 2,5-dibromo-3-substituted thiophenes to the corresponding thienylzinc species has been achieved by a similar method in an undivided cell fitted with a zinc sacrificial anode using catalytic amounts of NiBr2(bpy) as the catalyst43. The overall reaction giving (11) and (12) is presented in equation 35. The results are reported in Table 3. 

TABLE 3. Yields and regioselectivity for the conversion of 3-sub-stituted 2-dihalothiophenes into thienylzinc species... 

The palladium-catalyzed reaction of 2-bromocyclopentene-l-carbaldehyde derivatives with vinylzinc, 2-furyl, or 2-thienylzinc halides leads to the dimethylhydrazones which cyclize thermally to pyridines (Scheme 97) <1995T9119>. Palladium-catalyzed hetero-annelation reactions of internal alkynes to pyridines were reported... 

Thienylzincates are available using lithium di-zW-butyltetramethylpiperidinozincate (TMP-zincate) <1999JA3539> thus, ethyl thiophene-3-carboxylate is easily metallated at C(2) at room temperature, subsequent reaction with iodine giving ethyl 2-iodothiophene-3-carboxylate in 89% yield. Similarly ethyl thiophene-2-carboxylate gives the 5-iodo derivative in 62% yield. 

The ligands 6,6 -bis(2-hydroxyphenyl)-bpy, (44), and 6,6 -bis(2"-thienyl)-bpy, (45), each have two additional groups that may chelate. Ligand (44) has been shown to be tetradentate in some copper complexes, and was prepared from 6,6 -diacetyl bipyridine in 90% yield.237 The mono-substituted phenolic bipyridine is also known.238 The dithienyl substituted bipyridine was synthesized via Negishi coupling of 6,6 -dibromo bipyridine and 2-thienylzinc chloride.235... 

The example below illustrates that the Negishi reaction for thiophenes can also be chemoselective. In the case of 5-bromo-5 -iodobithiophene (71), thienylzinc reacted at the iodo position in the presence of Pd(dppf)Cl2 to deliver terthiophene 72 in 77% yield [62],... 

Propylzinc bromide Butylzinc bromide Cyclohexylzinc bromide 3-Ethoxy-3-oxopropylzinc bromide Phenylzinc bromide 2-Pyridylzinc bromide 2-Thienylzinc bromide... 

Mercury, Zinc, and Copper. The thermal decomposition of 2-thienylmercury thiocyanate, azide, acetate, and trifluoromethylsulphonate has been investigated. Thienylmercury derivatives have been cross-coupled with primary and secondary alkyl- and alkenyl-cuprate reagents. 2-Thienylzinc chloride has been coupled with iodobenzene and vinyl bromide, using Pd catalysis. ... 

There are by now many examples involving palladium(0)-catalysed couplings using halothiophenes, thiophene boronic acids, thienylstannanes, and thienylzinc... 

Thienylzinc iodides can be prepared from iodothiophenes by treatment with activated zinc on graphite. These can be transmetalated with CuCN 2LiCl and reacted subsequently with an allylic halide, or acylated with acid chlorides <94TL1047>. 

Novel quinones with interesting amphoteric redox properties have been synthesized by making use of the cross-coupling reaction between thienylzinc chlorides and iodobenzenes <91JA4576>. 

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