Reactions of tin hydrides

These normally occur in a regio- and stereoselective manner and provide a useful alternative to the well-known AlBN-catalyzed radical addition. Cochran et al. [194] demonstrated this clearly in the case of the addition of Me3SnH to diphenylacetylene, the radical addition proceeding in a tram manner and the Pd-catalyzed reaction cis. Rossi et al. carried out additions of Bu3SnH to 1,3-diynes to give 2-stannyl-l-en-3-ynes [195] and to esters of substituted propynoic acids [196]. A basically similar reaction was reported by Sai et al. [197]. Paley et al. [198] obtained enantiomerically pure dienyl sulfoxides by first 

Xiang el al. added Bu3SnH to an alkynyl triflone [176] in the presence of Pd(PPh3)4. Casson and Kocienski obtained a-alkoxyalkenyltins by addition of Bu3SnH to the corresponding alkynyl ethers [199]. 

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Stannyl radicals are usually generated by homolytic substitution at hydrogen in a tin hydride, or at tin in a distannane, or, conjugatively, at the y-carbon atom in an allylstannane.453 The initiator is commonly AIBN at ca. 80 °C. In the presence of a trace of air, organoboranes are oxidized by a radical chain mechanism, and triethylborane is now commonly used as an initiator at temperatures down to —78°C,519 and it can be used in aqueous solution.520 9-Borabicyclo[3.3.1]nonane (9-BBN) has similarly been used to initiate the reaction of tin hydrides at 0 and —78°C,521 and diethylzinc works in the same way.522... 

Table VIII contains rate constants for reactions of tin hydrides with carbon-centered radicals. A striking feature of Table VIII in comparison to other tables in this work is the high percentage of reactions for which Arrhenius parameters were determined by direct LFP or the LFP-clock method. These results are expected to be among the most accurate listed in this work. Scores of radical clocks have been studied with Bu3SnH, but the objectives of those studies were to determine rate constants for the clocks using tin hydride trapping as the calibrated basis reaction.

The limited kinetic data for reactions of tin hydride with nitrogen-centered radicals apparently demonstrates the combined effects of the enthalpies of the reactions and polarization in the transition states for H-atom transfer. The aminyl and iminyl radicals are electron-rich, and the N-H bonds formed are relatively weak these radicals react relatively slowly with tin hydride. On the other hand, the electrophilic amidyl and aminium cation radicals form strong N-H bonds and react rapidly with the tin hydride reagents. 

It is well known that kH is similar for all alkyl-substituted radicals but rate constants for reaction of tin hydride with carbonyl-substituted radicals are not known. Substituents can effect the rate constant for hydrogen transfer. For example, the benzyl radical is about 50 times less reactive than a primary alkyl radical. 

Of particular interest with respect to addition of the Sn—H bond are the reactions of tin hydrides with areneM(CO)3 (M = Cr, Mo, W) complexes246. In these reactions the Sn—H bond coordinates to the metal in the 2-fashion as illustrated for Cr in equation 101. 

There are many individual reactions of tin hydrides with metal complexes that are not readily categorized. For example, the salt [ R u 3 (/r -NO) (CO) i o 1 [ PPN]+ reacts with one or two equivalents of HSnR3 to form the oxidative addition products 83 and 84 (R = Bu, Ph) (equation 106)262. Similarly, the reaction between RuH(772-H2BH2)(CO)(P(Pr-/)3)2 and HSnPh3, in a 1 3 molar ratio under a hydrogen atmosphere, results in the formation of 85 in which the H2 ligand can be easily displaced by CO or P(OMe)3263 (equation 107). 

The reactions of tin hydrides with metal-metal bonded clusters has proven to be a successful route to a range of metal-tin bonded complexes. Products usually result in the cleavage of the M—M bond however, under special conditions, for example with bridging ligands, the bond can remain intact and result in either bridging or terminal tin groups, as shown for some ruthenium and osmium clusters (equations 102 and 103). 

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... 

The principal addition reactions of tin hydrides to unsaturated systems are listed in Table 15-3. These reactions are discussed in the sections dealing with the products, as noted in Column 3. 

Scheme 1.1.2 Synthesis of organotin compounds based on the reactions of tin hydrides or stannylmetallic compounds...

The principal alternative route into the organotin(IV) manifold is based on the reaction of tin hydrides or stannylmetallic compounds, as shown in Scheme 1.1.2. The most common (radical) mechanism of hydrostannation of alkenes and alkynes with organotin hydrides is outlined, for alkenes, in Equations (1.1.5) and (1.1.6). 

Nakamura reported that the ultrasound-promoted reaction of tin hydride with alkenes in the presence of air results in the addition of stannyl and hydroxyl groups across the double bond (hydroxystannylation) (Scheme 16). This procedure is the first example of the conversion of alkenes to hydroxylated organotin compounds... 

Tin hydrides containing an electronegative group attached to the tin atom are relatively unstable, and only three appear to be known. Amberger (25) prepared chlorotin trihydride by the reaction of tin hydride with hydrogen chloride at — 70° C, Eq. (3) it is unstable even at this temperature and decomposes to metallic tin, stannous chloride, and hydrogen, Eqs. (4-5). Alternative mechanisms for the decomposition can be visualized. 

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