Couplings of Aryltins

The aryltins discussed in this section are all derivatives of ferrocene, which have been the object of much study by Ma et ol. [242]. They have carried out reactions of tributylstannylferrocene with bromopyridines, bromothiophenes [243], and other haloheterocycles [244], as well as of a diaminomethyl-substituted staimylferrocene [245]. Similar reactions of bis(tributylstannyl)ferrocene with heterocyclic bromides have also been reported [246-248]. Bis(trimethylstannyl)ferrocene has been allowed to undergo coupling with enantiopure 2,2 -diiodo-l,l -binaphthyl [249]. The yield, however, was poor and the enantiomeric excess was zero the main product (46%, 0% ee) was that of a methyl transfer to the binaphthyl moiety. 

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The two most important reactions of aryltin compounds are first, their reaction with electrophiles, when either the tin or the non-tin product may be the aim, or, second, the coupling reactions that they show in the presence of certain transition metals (Equation (51)). 

The Sonogashira coupling of halopyridines was also achieved by other means. Resin bound bromopyridine derivatives, for example, underwent smooth coupling with acetylenes as well as with arylboronic acids and aryltin reagents.44 The advantageous effect of microwave irradiation on the coupling of halopyridines and trimethylsilylacetylene was also reported.45... 

Vinyllithiums cf. 13, 142).10 An improved route to vinyllithiums involves coupling of vinyl and aryl triflates with tributyl- or trimethylstannylcuprates of the formula (R3Sn)2Cu(CN)Li2, prepared from LDA, R,SnH, and CuCN in THF at -50° to -60°. The higher-order cuprates couple with vinyl and aryl triflates at -20° to provide a vinyltin intermediate. Work-up of the intermediate with AgOAc provides a vinyltin, with bromine a vinyl bromide, or with CF,COOH an aryltin. 

Cross-coupling of allyl halides with vinyltin reagents. Allyl halides couple with vinyl- or aryltin reagents in the presence of this catalyst in uniformly high yields (>80%). A wide variety of functional groups is tolerated. The geometry of the double bond in both partners is retained, but the carbon-to-carbon bond is formed with inversion at the allylic position. 

A significant decrease of the efficiency was observed when the resin was recycled, together with a coloration of the resin, possibly due to contamination with the palladium catalyst (Pd(PPhj)4). This observation was subsequently verified in the cross-coupling of supported aryltins with 2-halogenopyridines by washing the resin with trithiocyanuric acid, which led to a recovery of the cross-coupling efficiency at least on the first reuse. ... 

The Stille coupling involves heating a halide, a stannane and a catalyst in a suitable solvent, such as toluene or DMF. No base is required. The conditions are relatively straightforward, with little overall variation, apart from the catalyst. However, if a triflate is used instead of a halide, the reaction may not succeed, as transmetallation of aryltin compounds with arylpalladium triflates is often difficult. Addition of a halide source, such as lithium chloride, usually solves this problem as it allows the formation of the arylpalladium chloride, which can undergo transmetallation. 

Scheme 34. Stille coupling of (hetero)aryl halides with (hetero)aryltin reagents Kelly s molecular brake (68, [22]) and nicotelline (69, [124a]) a) Pd(PPh3)4,b) xylene, A...

In 1998, Oi and Inoue [161] reported early examples of a C-H/C-M coupling to make biaryl compounds. They conducted the coupling of 2-phenylpyridine with an aryltin compound (tetraphenylstaimane) in the presence of a rhodium complex in tetrachloroethane at 120 °C to afford monoarylated compound 120 and diarylated compound 121 in 56% and 20% yield, respectively (Scheme 17.34). 

The ortho-arylation of 2-arylpyridines with aryltin reagents is mediated by Wilkinson s catalyst. This cross-coupling procedure occurs at high temperatures and consequently, the prevention of double phenylation represents a major hurdle, which is often achieved by adding a methyl group to either the pyridine or aryl group (Equations (125) and (126)).1... 

Unactivated aryl iodides undergo the conversion Arl — ArCHj when treated with tris(diethylamino)sulfonium difluorotrimethylsilicate and a palladium catalyst.131 A number of methods, all catalyzed by palladium complexes, have been used to prepare unsymmetrical biaryls (see also 3-16). In these methods, aryl bromides or iodides are coupled with aryl Grignard reagents,152 with arylboronic acids ArB(OH)2,153 with aryltin compounds Ar-SnR3,154 and with arylmercury compounds.155 Unsymmetrical binaphthyls were synthesized by photochemically stimulated reaction of naphthyl iodides with naphthoxide ions in an SrnI reaction.156 Grignard reagents also couple with aryl halides without a palladium catalyst, by the benzyne mechanism.157 OS VI, 916 65, 108 66, 67. 

Hexamethylditin is a suitable reagent for Pd-catalyzed metallation of aryl-halides to furnish aryltin compounds that in turn can react in Stille couplings with aryl halides to form biaryl derivatives. Hitchcock and coworkers [ 126] have shown that 2-pyridyl triflate and (hetero)aryl bromides can be coupled to 2-(hetero)aryl pyridines 174 in moderate to good yields (Scheme 66). The underlying principle of this heterocoupling of two aryl (pseudo)hahdes is the selective stannylation of the triflate that is undergoing a faster oxidative addition to the Pd(0) complex than the bromide. 

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