Interaction of the carbon-germanium or carbon-tin bond with positive charge on a .beta. carbon

The acid-catalyzed demetalation of 1-(tri-n-butylstannyl)-2-phenylethyne. A surprisingly small β-stannyl effect

“…Comparison with the corresponding trimethylsilyl analog, l-(trimethylsilylj-2-phenylethyne (7), shows the stannyl substituent to be better than the silyl group by a factor of 210 and gives a differential P-stannyVP-silyl effect of 6AG' = 3.2 kcal mol-l. These are substantial effects, but they pale in comparison to the much larger P-stannyl stablizations observed in other systems (2,3). The difference between the present result and the previously reported stannyl/silyl rate ratio of 5.1 x lo7, corresponding to ~AG'= 10.5 kcal mol-', is especially striking, for in both cases the reaction examined involved proton addition to a carbon-carbon triple bond.…”

“…l), and other Group IV A metals lying below silicon in the periodic table have even more powerful effects. For example, a stannyl substituent in the P-position was found to accelerate the rate of solvolysis of a cyclohexanol ester over that of the unsubstituted substrate by a factor much in excess of 1 0 '~ (2), and the remarkably large stannyllsilyl rate ratio of lo8 was observed in the protonation of metalated acetylenes (3). We were surprised to discover, therefore, that the stannyl group in 1-(tri-rz-butylstanny1)-2-phenylethyne raises the rate of vinyl cation formation by protonation of this substance, eq.…”

Enantioselective Syntheses of syn- and anti-β-Hydroxyallylsilanes Via Allene Hydroboration-Aldehyde Allylboration Reactions

“…Although it is well known that both R 3 Si– and R 3 Sn– can stabilize a β —carbocation, 16 the capacity of these two groups to stabilize the β -carbocation is clearly different. For the intermediate α -stannylallylborane 3 , the hyperconjugative interaction between the Bu 3 Sn– group and the boron atom (isoelectronic to a carbocation) must be strong enough to override the steric repulsion between the Bu 3 Sn– and the ( d Ipc) 2 B– units.…”

Reprint of: Enantiodivergent hydroboration reactions of a racemic allenylsilane with diisopinocampheylborane and Curtin–Hammett controlled double asymmetric crotylboration reactions of (S)-E-α-phenyldimethylsilyl(ddiisopinocampheyl)-crotylborane

“…Other crotylborane species, such as ( R )- Z - 9 or ( S )- E - 15 are not observed (Scheme 4). Presumably a combination of the hyperconjugative effect between the PhMe 2 Si– group and the adjacent boron atom (analogous to a silyl-stabilized β-carbocation) 11 and steric effects account for ( S )- E - 5 being the only observed (and most stable) crotylborane product of the hydroboration of ( P )- 4 . Steric repulsion between the methyl group and the ( d Ipc) 2 B– unit in either ( R )- Z - 9 or ( S )- E - 15 destabilizes these two species at equilibrium.…”