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

Abstract

The kinetic hydroboration of allenylsilane 5 with (dIpc)2BH at −40 °C provides allylborane 9Z with ≥12:1 selectivity. When the hydroboration is performed at temperatures above −40 °C, 9Z isomerizes to the thermodynamically more stable allylborane 9E with >20:1 selectivity. Subsequent treatment of 9Z or 9E with aldehydes at −78 °C provides syn- or anti-β-hydroxyallylsilanes, 7 or 8, respectively.

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

“…13 However, the conformation of the aldehyde α-stereocenter in TS-4 and TS-5 is opposite to what is predicted by the Felkin-Anh model and gauche-pentane interactions 18 between the methyl or PhMe 2 Si group of the crotyl units and the R group of the aldehydes could occur in these two transition states (indicated in blue). Considering the relatively longer Si-C bond (estimated to be 1.85Å) compared to a C-C bond (1.54Å), 10 the steric repulsion in TS-5 is attenuated compared to TS-4 , which likely renders TS-5 [crotylboration of 20 with ( S )- E - 15 ] to be the most favored transition state for the mismatched double asymmetric crotylboration of aldehyde 20 . Moreover, the PhMe 2 Si group is in a more hindered position, adjacent to the bulky ( d Ipc) 2 B- group in TS-4 .…”

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

“…It is worth noting that, while the resting state of the products of hydroboration of monosubstituted allenylsilane 12 with ( d Ipc) 2 BH are the ( E )- or ( Z )-γ-silyl-allylboranes, 14 E or 14 Z (depending on the temperature of the hydroboration reaction), 10 the product resting state for the hydroboration of the 3-methyl substituted allenylsilane 4 with ( d Ipc) 2 BH is ( E )-α-silyl-crotylborane ( S )- E - 5 . Indeed, 1 H NMR studies of the hydroboration reaction of ( P )- 4 with ( d Ipc) 2 BH shows that ( S )- E - 5 is produced cleanly.…”

“…13 However, the conformation of the aldehyde α-stereocenter in TS - 4 and TS - 5 is opposite to what is predicted by the Felkin–Anh model and gauche -pentane interactions 18 between the methyl or PhMe 2 Si group of the crotyl units and the R group of the aldehydes could occur in these two transition states (indicated in blue). Considering the relatively longer Si–C bond (estimated to be 1.85 Å) compared to a C–C bond (1.54 Å), 10 the steric repulsion in TS - 5 is attenuated compared to TS - 4 , which likely renders TS - 5 [crotylboration of 20 with ( S )- E - 15 ] to be the most favored transition state for the mismatched double asymmetric crotylboration of aldehyde 20 . Moreover, the PhMe 2 Si group is in a more hindered position, adjacent to the bulky ( d Ipc) 2 B– group in TS - 4 .…”

Diastereo‐ and Enantioselective Synthesis of (E)‐δ‐Boryl‐Substituted anti‐Homoallylic Alcohols in Two Steps from Terminal Alkynes

“…[2] Recently,a ne nantioselective allylation reaction using allylic boron reagents functionalized with another metalloid element, such as tin, silicon, and boron, has attracted much attention. [3][4][5][6] Advantageously in these cases,t he metalloid elements remaining in the products lead to avariety of further synthetic transformations.Roush and co-workers have developed au nique method for generating (S)-a-stannyl allylic boron species in situ from allenylstannane and (À)-diisopinocampheylborane [( d Ipc) 2 BH] through hydroboration/1,3boratropic shift. [7] Subsequent allylation reaction of aldehydes affords (E)-d-stannyl-substituted homoallylic alcohols with high enantioselectivity [ Figure 1a].…”

“…d-Boryl-substituted homoallylic alcohol 5aa was isolated in 84 %y ield with an E/Z ratio of over 95:5 [Eq. (4)]. M oreover,ahigh level of enantioselectivity (95 % ee)w as observed with (E)-5aa.…”