Aryltrialkylstannanes

Kinetic studies of the acid-catalysed cleavage of aryltrialkylstannanes, reaction (269)... 

Dichloropyridine reacts in the dark (88% of disubstitution), whereas 2,6- and 3,5-dichloropyridines require photostimulation to afford ca. 80% yield of disubstitution products [26]. Aryltrialkylstannanes are valuable intermediates in organic synthesis, and the fact that they can be easily synthesized through the SRN1 mechanism, opens up important synthetic routes to different reaction schemes. For over a decade, the Pd(0)-catalyzed cross-coupling of organotin compounds with electrophiles, known as the Stille reaction, has been a very important tool in product design [104]. 

Other organometallic compounds, including aUylic stannanes, allylic samarium, allylic germanium, and allylic indium compounds add to aldi-mines in the same manner. Aryltrialkylstannanes also add the aryl group to Al-tosyl imines using a rhodium catalyst and sonication. Catalytic enantioselective addition reactions are well known, including reactions in an ionic liquid.Allylic... 

Lam and coworkers357,358 explored the reactions of aryltrialkylstannanes as coupling partners in copper-mediated processes because of their air and moisture stability and... 

Co-condensation of aryltrialkylstannanes with metal vapour gives rise to the r 6-arene-metal complexes 7-3.66... 

Much of the recent interest in the arylstannanes, particularly the aryltrialkylstannanes, however, derives from their use in the Stille reaction in which an aryl-C bond is generated by reaction with a halide or sulfonate RX, where R = vinyl, allyl, benzyl, or aryl, to give Ar-R under catalysis by palladium68 or another transition metal.69-72 The reactions are very tolerant of various functional groups, and are finding wide application in organic synthesis, and are considered in Section 22.2. 

Because a wide range of aryltrialkylstannanes are available from the corresponding aryl bromides this reaction can be used to introduce one or more dialkylaminomethyl groups into a variety of aromatic substrates. The reaction of A(,N-dimethylmethyleneiminium chloride with l,4-bis(trimethylstannyl)benzene gives IVjVA A -tetramethyl-l, 4-diaminobenzene in 58% yield. ... 

Coupling reactions of organomanganese reagents with 2- and 3- furoyl chloride in the presence of copper or iron catalysts have been used to prepare a number of naturally occurring ketones in high yields <92TL5245> and aryltrialkylstannanes have been shown to arylate 2-furoyl chloride in the presence of a palladium catalyst <92CC1440>. 

The aryltrialkylstannanes as the key-Stille intermediates can be obtained by the following alternative reactions ... 

In similar manner, hexaalkyldistannanes allow the homo-coupling reactions of aryl iodides, bromides [104] and triflates [105] to the symmetrical biaryls. First, the aryltrialkylstannane is generated by reaction of the aryl halides with Me6Sn2 or -BugSn2, followed by the in situ Stille reaction with an excess of electrophilic reactant, since both reactions are catalysted by the same complexes and proceed at comparable reaction rates. For instance, 2-naphthyl triflate (165) was coupled to 2,2 -binaphthalene (166) in 69% yield [105], respectively, Scheme 22. 

The aryltrialkylstannanes obtained by above described methods affected the Stille cross-coupling reactions whose efficacy is presented through the following examples where arylstannanes 172-176 were reacted with aryl halides 25, 177, 178 and iodobenzene giving biaryls 179-183 in respective yields [77], Scheme 25. 

Scheme 27. Addition of cesium fluoride, as the most suitable fluoride source, was necessary to rich the high yields. The fluoride ion reacts with aryltrialkylstannane to form a hypervalent, pentacoordinative, organotin species typically more nucleophilic than its tetravalent precursor. In this manner, the fluoride ion does co-eatalyse the Stille reaction.

Synthesis of aryltrialkylstannanes by reaction of aryl bromides with hexaalkyldistannanes Preparation of 2-nitrophenyltrimethylstannane (228) 199]... 

Note Copper(lI) nitrate is also efficient reagent for the homo-coupling reaction of aryltrialkylstannanes at molar ratio ArSnR3 Cu(N03)2 = 1 5 to give high yields of symmetrical biaryls in dry DMF or THF [27,28]. 

The second method for the formation of aryllead triacetates is via transmetallation either from the corresponding aryltrialkylstannane [80] or arylboronoic acid [81] catalysed by Hg(OAc)2. This approach is the more general approach and greatly extends the scope of the aryl group to include different substitution patterns which... 

Pinhey also pioneered the use of arylboronoic acids in a similar transformation which is also catalysed by Hg(OAc)2 [81]. Although the isolated yields for the aryllead triacetates obtained via this method are generally lower that with aryltrialkylstannanes the commercial availability of a large variety of arylboronoic acids make this a very desirable approach. The lower yields can be circumvented somewhat by generation of the aryllead triacetate in situ which has proved to be a successful approach. For example, the in situ generation of a range of aryllead triacetates such a 4-trifluoromethylphenyl, 2-methoxyphenyl and 3-nitrophenyl derivatives were used in the arylation of a P-keto ester (Scheme 4.38) [81]. 

Aryltrialkylstannanes selectively transfer the aryl group rather than the alkyl group. The use of a mixed CH3CN-Et20 solvent increases the selectivity and the reaction rate ... 

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