Hydrosilylation of 2-methyl-1-phenyl-pentan-1-one with Et_3SiH in the presence of catalytic amounts of B(C_6F_5)_3 gave the anti-product, (1R^*,2R^*)-2-methyl-1-phenyl-1-triethylsilyloxy-pentane, with slight predominance over syn-product. In contrast, the syn-product, (4R^*,5S^*)-4-methyl-5-phenyl-5-triethylsilyloxypent-1-yne, was obtained stereoselectively in the reaction of 2-methyl-1-phenyl-pent-4-yn-1-one. The investigation of other related systems also supported the above result; the reduction of 2-methyl-3-alkynyl aryl ketones or 2-methyl-3-alkynyl alkyl ketones with R_3SiH/cat. B(C_6F_5)_3 produced the corresponding syn-alcohols either exclusively or predominantly, while the reduction of the saturated analogue, 2-methyl-1-phenyl aryl ketone, with the same reducing agent afforded the corresponding anti-alcohol. The observed anti-selectivity in the latter case can be explained by the ordinary Felkin-Anh model. On the other hand, the unusual syn-selectivities can be accounted for by the σ-π chelation by R_3Si^+, in which both the lone pair (σ) of the carbonyl group and the π-electrons of the alkyne coordinate to the silylium ion.