A di-pinene borane complex of this functional group is commonly used to enantio·selectively add it to aldehydes. Phenyl ethers of this functional group can undergo two consecutive Claisen rearrangements to give a para-substituted phenolic product. Nucleophilic addition to this functional group can proceed with inversion or retention of configuration based on the hardness of the nucleophile in one reaction, which proceeds through formation of a “pi”-[this functional group] complex with a (*) palladium(0) (10[1])catalyst. The beta-silicon effect stabilizes a carbo·cation intermediate in the addition of this functional group to a carbonyl in the Hosomi–Sakurai reaction. The presence of an alcohol [emphasize] geminal to one of these groups is essential for the Sharpless asymmetric epoxidation. For 10 points, name this functional group that consists of a methylene group adjacent to an alkene. ■END■ (10[2]0[7])