Palladium-catalyzed asymmetric diene cyclization/hydrosilylation employing functionalized silanes and disiloxanes.
Pentasubstituted disiloxanes and silanes of the form HSiMe(2)CH(x)Ph(3-x)(x = 1 or 2) reacted with dimethyl diallylmalonate (1) and other functionalized 1,6-dienes in the presence of a catalytic 1:1 mixture of (N-N)Pd(Me)Cl [N-N = (R)-(+)-4-isopropyl-2-(2-pyridinyl)-2-oxazoline] [(R)-2] and NaBAr(4) [Ar = 3,5-C(6)H(3)(CF(3))(2)] to form the corresponding silylated cyclopentanes in good yield with high diastereoselectivity. The enantioselectivity of cyclization/hydrosilylation of 1 with disiloxanes and functionalized silanes at -20 degrees C increased in the following order: HSiMe(2)OSiMe(3) (75% ee) < HSiMe(2)OSiMe(2)-t-Bu (80% ee) < HSi(i-Pr)(2)OSiMe(3) (86% ee) = HSiMe(2)Bn (86% ee) < HSiMe(2)OSi(i-Pr)(3) (89% ee) < HSiMe(2)OSiPh(2)-t-Bu (91% ee) < HSiMe(2)CHPh(2) (93% ee). Silylated cyclopentanes derived from HSiMe(2)OSiMe(3) were oxidized with excess KF and peracetic acid at room temperature for 48 h to form the corresponding hydroxymethylcyclopentanes in good yield (82-95%). Silylated cyclopentanes derived from HSiMe(2)OSiPh(2)t-Bu were oxidized with a mixture of tetrabutylammonium fluoride and either H(2)O(2) or peracetic acid to form the corresponding alcohols in 48-76% yield. Silylated carbocycles generated from benzhydryldimethylsilane were oxidized with a mixture of TBAF/KHCO(3)/H(2)O(2) in 71-98% yield. Asymmetric cyclization/hydrosilylation/oxidation employing benzhydryldimethylsilane tolerated allylic and terminal olefinic substitution and a range of functional groups.
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