Moieties containing this heteroatom “direct” the regioselectivity of elimination in a variant of the Nazarov cyclization. The anti-Markovnikov addition of this heteroatom across a double bond is described by the Chalk-Harrod mechanism and became more common with the invention of the platinum-containing Speier’s catalyst. Enol ethers containing this heteroatom (15[1])are added to Lewis acid-activated (15[1])carbonyls in the (*) Mukaiyama aldol reaction. (10[1])A pentavalent intermediate with this element at its center is formed just before the transmetalation step of Hiyama coupling. Increased reactivity at carbons two atoms away from this heteroatom is observed in the beta-[this element] effect. (10[1])A molecule containing this element bound to four methyl groups is assigned a chemical shift (10[1])of 0 ppm (10[1])(“parts per million”) in NMR. (10[1])For 10 points, name this element located below carbon on the periodic table. ■END■