Aryl-tin bond

The mechanism of the reaction was postulated as being either synchronous attack of the RO" ion (R = Me or H) at tin and breaking of the aryl-tin bond, or rapid formation of a pentacoordinate tin intermediate [ROMe3SnPh-] followed by rate-determining breaking of the aryl-tin bond. In either case the phenyl-carbanion could separate as such and then react rapidly with solvent, or it could react with solvent as it separated. 

This prediction has now been confirmed by the observation (R. Alexander, C. Eaborn and T. G. Traylor, /. Organometal. Chem., 21 (1970) P65) that a solvent-isotope effect is obtained in base cleavage of benzyl- and aryl-tin bonds by NaOH in MeOH-MeOD. 

The earlier work on acidolysis of the aryl-tin bond is reviewed in reference (97). Attachment of the proton to the aryl ring is rate-determining, and the Hammett p-factor for the reaction has been shown to... 

These reactions to form aryl tin bonds could occur by initial oxidative addition of the aryl halide or the distannane. The stoichiometric reaction between [(PPh3)2Pd(Ph)(I)] and Me3SnSnMe3 in the presence of chloride generated good yields of the aryltin product. This result suggests that the reactions occur by initial oxidative addition of aryl halide. 

Eaborn and co-workers28 have reported product isotope ratios for the cleavage of benzyl-silicon, benzyl-tin, and aryl-tin bonds using an equimolar mixture of MeOH and MeOD as the solvent. A free carbanion would not be expected to discriminate between MeOH and MeOD in the fast step (24), and hence28 the product isotope ratio kH/kD should be unity. The values of kJkD in Table 11 indicate that the carbanions are not entirely free, but that some degree of electrophilic attack by the solvent at the benzyl carbon atom takes place, as in (V). 

The relative reactivities of aryl-tin bonds in (< -An)2SnPh2 (290) and (o-Tol)2SnPh2 toward iodine were determined686. The lower [o-AnI]/PhI value (5.2 1) compared to the [o-TolI]/PhI ratio (68 5.1) is considered to indicate an enhanced reactivity of the Ph—Sn bond in 290, arising from the nucleophilic assistance by an o-anisyl group during the Ph—Sn bond cleavage. 

Radiobromination of aromatic compounds by cleavage of aryl-tin bonds... 

The electronic effect of the butadiynyl group in electrophilic aromatic substitutions has been determined by measuring the rates of acid-catalyscd cleavage of the aryl-tin bonds in the aryltrimethylstannancs 4 The reaction (equation 2) proceeds by way of the benzenonium intermediate 5, and the relative rates (Table 2) give an indication... 

The aryl-tin bond, like the alkyl-tin bond, is stable to air and moisture, but it is more susceptible to cleavage by electrophiles. Melting points of some aryltin compounds are given in Table 7-1. 

The first group of reactions (13-1-13-3) can be carried out by mixing aliquots of the tin oxide or hydroxide and acid in a solvent such as methanol (e.g. equation 13-7).3 The water can often be conveniently removed by azeotropic dehydration in benzene or toluene, but with aryltin compounds it may be better to use molecular sieves, to avoid the risk of cleaving the aryl-tin bonds.4... 

As trialkyl(aryl)stannanes have never been reported to undergo oxidative addition with a palladium(O) or nickel(O) complex, the corresponding arylstannylation is not known. As shown in Scheme 5.7.24, Kosugi and coworkers found that the use of aryl(trichloro)stannanes instead of the trialkyl derivatives enables the palladium-catalyzed arylstannylation of norbomene. The reaction using other unsaturated hydrocarbons including ordinary alkenes has not been reported, but the observation that aryl-tin bonds can be activated by palladium(O) complexes has certain significance, and will lead to future development of other arylstannylations. 

The method works equally well for the meta-series [2]. The isolation of the tin derivatives (m- and p-Mea SnCg H4 C CH) is noteworthy since it is doubtful whether the aryl-tin bonds would survive the reactions usually employed for synthesizing arylacetylenes. Arylbutadiynes may be similarly protected [4]. 

The reactivity of various groups (R) follows the sequence allyl, benzyl > aryl > alkyl, and usually proceeds readily to the stage of RaSnlOSORlj (121,122), but pentafiuorophenyl- and trifluorovinyl-tin bonds are usually unreactive. The reactivity is enhanced by such ligands as bipyridyl (123). 

Electrophile-mediatedadditionofsilylatedbasestofuranoidglycalsresultedinnucleosides207,208. Thus, 13 was treated with an aryl sulfenyl chloride at —78 °C and, after the temperature had been raised to 25 °C, tin(IV) chloride and the silylated base were added. The corresponding nucleosides 14 were obtained in good yield and high d.r., independent of the size of the aryl group bonded to sulfur. The configurations of the reaction products and d.r. were determined by HNMR209. 

Deshpande [74], has carried out carbon-carbon bond formation on a solid support using a polymer-supported aryl iodide and vinyl (or aryl) tins. In the area of natural-product synthesis. Overman and co-workers have carried out total syntheses of (—) and (+)-strychnine which include an aryltin/CO/alkenyltin coupling step [75] (a technique introduced earlier by Stille [1]). Very recently, Heathcock and co-workers have reported total syntheses of (—)-papuamine and (—)-haliclonadiamine which include a key step in which a 1,3-diene unit is constructed by coupling two alkenyltin moieties thus reaction (Scheme 4-27) only proceeds in the presence of copper (I) iodide [76]. 

Metal carbon insertions of C02 occur in both main group and transition metal complexes.5,19 The reactions result in a strong M—O bond at the expense of a weak M—C bond (together with a C C a bond at the expense of a C O n bond). Insertion into t/weo-W(CO)5 CHDCHDPh gave the threo carboxylate, indicating retention of configuration at the o-carbon, as with CO insertion.29 Cis- RW(CO)4L (R = Me, Et or Ph) showed second-order kinetics toward C02 insertion, first order in anion and in C02. Replacement of CO by a phosphine or phosphite increased the insertion rate.30 Palladium catalysis of C02 insertion into unreactive Sn C bonds in allyl stannanes provided three isomeric tetra-carboxylates from tetraallyl tin. Attempted reactions of tetraalkyl, vinyl or aryl tin complexes did not proceed. Insertion into an intermediate rf -allyl palladium complex was suggested to lead to carboxylate products.31,32... 

These mechanisms are discussed, together with the reactions, below. By most of these mechanisms, the alkyl-tin bond is less reactive than the aryl-, alkenyl-, alkynyl-, allyl-, or benzyl-tin bond, and when two types of group are bonded to tin, reactions are selective for this second class of group. 

Sulfur dioxide inserts into alkyltin bonds to give stannylsulfinates, R3Sn0-S(0)R,46 but the reactivity falls off in the sequence R = allyl allenyl propargyl > phenyl > benzyl > methyl. Insertion into a vinyl-tin bond occurs with retention of configuration. The reaction with aryl and benzyl compounds is cleanly first order in each of the two reactants, and is rather insensitive to the nature of any substituents in the benzyl ring. 

The aryl-Sn bond is cleaved by electrophiles much more readily than the aryl-H bond, and aryl-tin compounds react by ipso substitution by the classic mechanism for electrophilic substitution.36 The part that symmetry-enhanced hyperconjugation plays in determining the reactivity and regioselectivity is discussed in Section 3.I.2.2.37... 

The low reactivity of the cyclohexyl-tin bond has been exploited in preparing the spacer bridged bistrichlorides, Cl3SnSpSnCl3 (Sp = alkylene, arylene, or aryl-dimethylene) by the Kocheshkov reaction of Cy3SnSpSnCy3 with S11CL4.12 From these hexachlorides, a variety of compounds R3SnSpSnR3 (e.g. R = MeOC) can be prepared. 

When conducted under mild conditions, or with only one equivalent of tin tetrachloride, the dealkylation process can be stopped after reaction of one tin center, in the case of 87. The presence of the electron-withdrawing trichlorotin group in the / ara-position of the ring deactivates the metalated aryl carbon bond to such extent that the intermediate bearing a trichlorotin and a tricyclohexyltin group can... 

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