Organotin halides synthesis

The synthesis of organotin halides is of paramount importance for organotin chemistry, because these compounds are considered as the starting materials for the preparation of a great number of organotin substances. 

The oldest method for the synthesis of the organotin halides is the reaction of alkyl or aryl halides with elemental tin. This reaction exhibits numerous variants and uses a wide variety of catalysts however, yields are usually unsatisfactory and the method is no longer common nowadays. 

The Kocheshkov redistribution reaction between tetra-organotins and tin(rV) halides, in appropriate mole ratios, is a general method of synthesis of organotin halides (equations 59 and 60). It is a particularly useful reaction, since it is difficult to control the alkylation of SnX4 by organolithium or Grignard reagents to the desired extent. 

Recently we and others have shown that (tBu2SnO)3 [1] acts as a convenient anhydrous source of 0 in reactions with organotin halides allowing the controlled synthesis of new organotin clusters, e. g.,. [RClSn(CH2)3SnClR]0 4 [2] and [R(0)Sn]2CMe2 2 [3],... 

Almost every useful method for the synthesis of organopolytin compounds has involved organotin halides or hydrides as starting materials. The classic Wurtz synthesis employing a triorganotin halide and sodium in benzene, toluene, or xylene has been used frequently for the preparation of ditin compounds 195-200). 

Many methods of synthesis are available, and all find their use for special casesis. The halogenation of tetra-organotins may be carried out by a number of reagents as outlined below, and possibly represents the most extensively used synthesis for the organotin halides. 

Another very widely used synthesis of the organotin halides involves the proportionation reaction between tetra-organotins and tin(IV) halidesi . In these reactions, illustrated below, it is to be noted that there is conservation ofthe whole organic content of the reaction as the required organotin halide, in contrast to the losses of organic groups in the above equations. 

The ionization of these compounds in solution may be formulated as M+SnRs, with the R3Sn ion exactly analogous to the organic carbonion. Thus these powerfully nucleophilic ions complement the RsSn cations present in solutions of the organotin halides, and where a synthesis is not possible with one, it can normally be accomplished by the other. 

Various organotin reagents react with acyl and aroyl halides under mild conditions without decarbonylation to give carbonyl compounds[390,39l]. Alkyl- or alkenyltin reagents react with acyl and aroyl chlorides to give ketones[548.733,734]. One example is the preparation of the a,/3-dnsaturated 7-keto esters 860 and 861, carried out under a CO atmosphere[735]. The reaction has been applied intramolecularly to the synthesis of the macrocyclic keto... 

For reviews of organotin hydrides, see Neumann, W.R Synthesis, 1987,665 Kuivila, H.G. Synthesis, 1970,499, Acc. Chem. Res., 1968,1,299. Tributyltin hydride also reduces vinyl halides in the prescence of a palladium catalyst. See Uenishi, J. Kawahama, R. Shiga, Y Yonemitsu, O. Tsuji, J. Tetrahedron Lett., 1996, 37, 6759. 

In basic aqueous media, a kinetic study of the reaction between stannate(II) ions and alkyl halide shows that mono- and disubstituted organotin compounds are formed (Eq. 6.12a).27 The monosubstituted organotin compound is obtained after a nucleophilic substitution catalyzed by a complexation between the tin(II) and the halide atom. The disubstituted compound results from an electrophilic substitution coupled with a redox reaction on a complex between the monosubstituted organotin compound and the stannate(II) ion. Stannate(IV) ions prevent the synthesis of the disubstituted compound by complexation. Similarly, when allyl bromide and tin were stirred in D2O at 60° C, allyltin(II) bromide was formed first. This was followed by further reaction with another molecule of allyl bromide to give diallyltin(IV) dibromide (Eq. 6.12b).28... 

The dehalogenation of organic halides by organotin hydrides takes place in most cases with a free-radical mechanism [1, 84, 85], The stereospecific reduction of 1,1-dibromo-l-alkenes with Bu3SnH discovered by Uenishi and coworkers [86-89], however, did not occur in the absence of palladium complexes and did not involve radicals. For the synthesis of (Z)-l-bromo-l-alkenes, [(PPh3)4Pd] proved to be the most effective catalyst which could also be generated in situ. The reaction in Eq. (7) proceeded at room temperature and a wide range of solvents could be used. 

Halogenated pyrroles can serve as the aryl halide in Stille couplings with organotin reagents. Scott has used this idea to prepare a series of 3-vinylpyrroles, which are important building blocks for the synthesis of vinyl-porphyrins, bile pigments, and indoles [77]. Although 3-chloro-and 3-bromopyrroles fail completely or fared poorly in this chemistry, 3-iodopyrroles 101 work extremely well to yield 3-vinylpyrroles 102. 

One of the first cross-coupling reactions performed on solid supports was the Stille reaction [250] which is a paUadium-catalyzed reaction of a trialkylaryl or trialkylalkenyl stannane with an aromatic iodide, bromide or triflate. In contrast to the process in liquid-phase, the organotin reagent is easily removed from the solid-phase because of the subsequent washing processes. Immobilized aryl halides have been frequently coupled with aryl and alkenylstannanes, whereas stan-nanes attached to the solid support have been used less frequently for the StiUe reaction. An example is the synthesis of a benzodiazepine library by EUman et al. Recently, a Stille cross-couphng reaction has been employed in the synthesis of al-kenyldiarylmethanes (ADAM) series of non-nucleoside HlV-1 Reverse Transcriptase Inhibitors (Scheme 3.14) [251]. 

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