Tin-hydrogen bonds

Organotin hydrides are advantageous as reductants in terms of their facile availability, stability, and reactivity. - Almost all tin hydrides are liquids, and stoichiometric tin-hydrogen bonds can be used. In general, the tin hydride reductions have been performed under radical conditions using initiators such as azobisisobutyroni-trile (AIBN), triethylborane, and UV irradiation. The reduction of organic halides and pseudohalides by tri- -... 

The tin-hydrogen bond of tributyltin hydride (2.43) is relatively weak and undergoes fission in the presence of AIBN (2.37). Since tin reagents are highly toxic, other methods ... 

The special characteristics of organotin hydrides as reducing agents are rationalized by the fact that the tin-hydrogen bond is both weaker and less polar than the B—H or A1—H bonds. These characteristics are manifested in reactions that proceed by either a free radical chain or polar mechanism, depending on the substrate, catalyst and reaction conditions. 

Trimethylsilyl groups at triple bonds are easily removed by mild bases such as alkali metal carbonates to yield 35 with a terminal triple bond. The following reaction is a Pd-catalyzed reductive addition of tributyl stannane, HSnBu3, to the triple bond, which forms the -configured vinyl stannane 19. One possible rationalization of the outcome is the mechanism shown below First, the catalytically active species 36 inserts into the tin-hydrogen bond to form 37. Then, cis addition takes place after coordination to the alkyne triple bond, generating species 39. Reductive elimination affords the vinyl stannane 19 and regenerates the catalyst 36. ... 

The mechanism of this process is not clear, but it is presumed that the di-n-butylstannylene, BuaSn generated from the decomposition of (3-ethoxyethyl(di-n-butyl)stannane with lithium diisopropylamide (LDA), inserts into the tin-hydrogen bond in the chain growth step. The air-sensitive colorless oligomers are separated by reverse-phase HPLC using dichloromethane/acetonitrile solvent mixture. Short chain oligomers, n = 3,4, and 5, dominate in the mixture of oligomers, n = 1-15. 

Owing to the low configurational stability of tin compounds, optically active tin radicals have not been observed. The racemization of optically active hydrostannanes at room temperature or in the presence of AIBN is due to a slow inversion of the trivalent tin radical produced by the homolytic scission of the tin-hydrogen bond. 

The cleavage of the phosphorus-hydrogen and tin-hydrogen bonds was reported with interesting synthetic applications. For the sake of comparison with the cases discussed above, the energies of the P-H and Sn-H bonds are in the range of 310-320 kJmoH. 

Any extensive study of the chemistry of the tin-hydrogen bond is extremely difficult in SnH4 due to its inherent instability. In the case of the organotin hydrides, this difficulty is considerably allayed. The stability of the organotin hydrides increases with decreasing number of tin-hydrogen linkages in the moleculeis. 

The increase in activity in the field of organotin hydrides is mostly due to the facility of addition of the tin-hydrogen bond to unsaturated systems and to the multiple mechanistic investigations of the hydrostannation reaction. Hev is a number of organometallic reactions, i.e., addition to transition metal complexes7 condensation with metal alkyls yielding tin-metal derivatives and with metal amides for synthesis of compounds with longer tin chains. 

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