Trialkyl stannanes

Lewis acids, particularly the boron trifluroride diethyl ether complex, are used to promote the reaction between allyl(trialkyl)- and allyl(triaryl)stannanes and aldehydes and ketones52-54. The mechanism of these Lewis acid promoted reactions may involve coordination of the Lewis acid to the carbonyl compound so increasing its reactivity towards nucleophilic attack, or in situ transmetalation of the allyl(trialkyl)stannane by the Lewis acid to generate a more reactive allylmetal reagent. Which pathway operates in any particular case depends on the order of mixing of the reagents, the Lewis acid, temperature, solvent etc.55- 58. 

Propynyl bromides behave similarly63. Reactions between allyl(trialkyl)stannanes and aldehydes are facilitated by high pressure64. 

Another sequence involves a dibromomethyl(trialkyl)stannane as the starting material. On reaction with CrCl2, addition to the aldehyde is followed by reductive elimination.151... 

Allylic Sn(II) species are believed to be involved in reactions of allylic trialkyl stannanes in the presence of SnCl2. These reactions are particularly effective in acetonitrile, which appears to promote the exchange reaction. Ketones as well as aldehydes are reactive under these conditions.171... 

FIGURE 72. Calculated transition states at MP2 of the hydrogen abstraction reactions of trialkyl-stannane with hydrogen and alkyl radicals which are shown in Figure 71. Bond distances are in A, angles in deg. The calculated activation energies (kcal mol 1) at QCISD//MP2 (MP2 values in parentheses) refer to the forward reaction (A /i 1 i ) and the reverse reaction (A /i2 ), respectively. Reproduced by permission of The Royal Society of Chemistry from Reference 191... 

The stereochemical course of the thermally promoted addition of allylic trialkyl-stannanes to aldehydes is dependent upon the geometry of the 2-butenyl unit [67, 69], The reaction is believed to proceed via a cyclic, six-membered, chair-like transition structure. Reaction of an -2-butenylstannane provides the anti homoallylic alcohol, while an Z-2-butenylstannane affords the corresponding syn homoallylic alcohol (Scheme 10-37). The allylation of aldehydes with allylic stannanes has also been performed under high pressure and neutral conditions [70]. The stereochemical outcome of the reaction of E- and Z-2-butenylstannanes with aldehydes under high pressure was almost identical to the results obtained thermally. 

Trialkyl stannanes and trialkyl alanes react according to the equation ... 

AIkoxy(trialkyl)stannanes, which also add by various polar mechanisms,453 do not react with styrene or with methacrylic or maleic esters, but with ketene undergo smooth addition to give stannylacetic esters 454... 

Dialkoxyphosphines can be prepared in high yields by rednction of chlorophosphites with sodium borohydride or trialkyl stannanes (6.222). 

A variety of azides have been used for this chemistry. The substituent on the azide could be aryl or alkyl. Several different salts were described in Finnegan s initial report. In addition, trialkyl stannanes and silanes have been employed. Due to sterics, the 2,5-disubstituted isomer was reported to be isolated when trialkyl stannanes were employed. This chemistry was... 

As one descends Group IVB of the Periodic Table from Si to Ge to Sn to Pb, the ease of addition of the hydrides (e.g. R3MH) to olefins increases markedly with the decrease in strength of the M—H bonds. Thus trialkyl-silanes require either heating to about 300X under olefin pressures of at least 300 atm, or UV irradiation, or catalysis by, for example, peroxides, certain metal salts or tertiary amines. Trialkylgermanes and trialkyl-stannanes add to activated terminal carbon-carbon double bonds at 120°C and 90 C respectively, and a catalyst is not necessary, e.g. 

In parallel with the developments involving chiral allylation reagents, there have been a number of exciting reports of catalytic enantioselective processes [24, 30, 32, 119]. To date, these have most efficiently been achieved for chiral Lewis acid catalysts combined with type 2 allyl trialkylsilanes and trialkyl-stannanes. In addition, there have also been recent impressive reports of the use of Lewis basic catalysts with aUyl trihalosilanes. 

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