Tin-halogen bonds

Mechanism a) operates if the tin-halogen bond of this trigonal bipyramidal complex is broken. 

Ionization of tin-halogen bonds can occur in triorganotin halides containing two intramolecular donor groups, e.g. (18). Diorganotin dUialides and bis-pseudohalides also exhibit strong tendencies to form aggregated species with... 

Table 4. " Sn NMR parameters of compounds with tin-halogen bonds. ... 

One short (primary, covalent) and one long (secondary) tin-halogen bond are observed in each trigonal bipyramidal unit. The short bonds are within the normal limits expected from the sum of the covalent radii and are only slightly longer than the standard values measured in tetrahedral monomeric compounds. The interatomic distances for the secondary bonds vary within broader limits, and sometimes are close to the sum of the van der Waals radii. 

The fact that unsaturated halides have undergone reduction rather than hydrostannation could mean that reaction (30) is faster than reaction (27). Since carbon-halogen and tin-halogen bond energies can vary greatly, it would be expected that the rate of Eq. (30) can vary over a large range with the result that the addition reaction could be made to compete under appropriate circumstances (see Section D). 

Chemical Properties. The most impoitant reactions which tetraorganotins undergo are heterolytic, ie, electrophilic and nucleophilic, cleavage and Kocheshkov redistribution (81—84). The tin—carbon bond in tetraorganotins is easily cleaved by halogens, hydrogen hahdes, and mineral acids ... 

Acid, hydrogen haUde, or halogen cleavage of tetraorganotins is not used except on a laboratory scale because they are wasteful of tin—carbon bonds and uneconomical on a commercial scale. 

Heterolytic cleavage of the tin-carbon bond is reviewed in references (94-96). Cleavage by electrophiles (e.g, HgXj or halogen) is dominated by electrophilic attack at carbon, and cleavage by nucleophiles principally involves nucleophilic attack at tin. Much of the interest in these processes centers on the intermediate mechanisms that may exist between these extremes, in which electrophilic attack is accompanied by some nucleophilic assistance, and vice versa. Allylic, al-lenic, and propargylic compoimds show a special reactivity by a special (Se2 or SE2y) mechanism. 

Besides carbon-halogen bonds, Bu3SnH is used routinely for hydrogenolysis of various carbon-heteroatom bonds. The deoxygenation from alcohols has been performed by a Barton-McCombie reaction where the addition of tin hydride to the triple bond is suppressed (Equation (2)).29-31... 

Another important homolytic reaction of tin hydrides is the reduction of carbon-halogen bonds the reaction is promoted by initiators and retarded by radical traps (Scheme 13). Reactivity decreases in the sequence X = I > Br > Cl and BusSnH > Bu2SnH2 Ph3SiiH > BuSnH3. Other groups X in RX that can be reduced by tin hydrides include -OC(S)R, SR, SePh, TePh, NC, and NO2. The intermediate radical, R in Scheme 13, can be trapped by additional substances, e.g. alkenes, or may undergo... 

---