Química Orgânica 1 · Adição a alcenos e regioquímica
Make an ether without a methyl shift
Condições: 3,3-Dimethylbut-1-ene with Hg(OAc)₂ in methanol, followed by NaBH₄. Draw the alkoxymercuration stage only: ionization, mercurinium formation, methanol attack, and deprotonation by a second methanol. The NaBH₄ demercuration is summarized, not arrow-pushed. OAc = acetate; Ac is drawn as one group.
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Ionize mercuric acetate
Mercury(II) acetate partly dissociates in the aqueous or alcoholic solvent. The resulting ⁺HgOAc is the electrophile that the alkene attacks.
As setas curvas
- The Hg–O pair leaves with the acetate oxygen, giving acetate and the ⁺HgOAc cation.
Form the mercurinium ion
The alkene attacks mercury to give a three-membered mercurinium ion instead of an open carbocation, much as Br₂ gives a bromonium ion. Without a free carbocation there is no carbocation rearrangement. The bond between mercury and the more substituted carbon is longer and weaker because that carbon can better carry positive charge.
As setas curvas
- The alkene π pair supplies one of the two carbon–mercury bonds.
- Mercury’s drawn electron pair supplies the other bond; exchanging the two carbon assignments gives the same bridge.
Methanol opens the bridge
The solvent oxygen bonds to the more substituted carbon, which carries more of the positive charge in the bridged ion. Attack from the side opposite mercury makes the addition anti; that relationship is lost later in demercuration.
Nota: Use a solvent oxygen as the nucleophile.
As setas curvas
- A methanol oxygen lone pair bonds to the more substituted carbon from the face opposite mercury.
- The breaking C–Hg bond pair returns to mercury, which stays bonded to the terminal carbon.
Deprotonate the oxonium ion
The solvent removes the proton, leaving a neutral organomercury ether.
As setas curvas
- A lone pair on a second methanol molecule accepts a proton from the positive oxygen.
- The O–H pair stays on the positive oxygen, which becomes neutral.
Methoxy organomercurial, then 3-methoxy-2,2-dimethylbutane
The methoxy group bonds to the more substituted alkene carbon and HgOAc to the terminal carbon; the neighboring quaternary carbon still carries all three methyl groups. With H⁺/CH₃OH, a secondary cation next to that quaternary carbon would undergo a methyl shift to a tertiary cation and give a rearranged ether. The mercurinium ion is never a free carbocation, so it does not rearrange. NaBH₄ (not drawn) then replaces HgOAc with H, giving 3-methoxy-2,2-dimethylbutane (racemic at its methoxy-bearing carbon).
O que você vai praticar
Use alkoxymercuration to add methanol across an alkene whose carbocation would rearrange, keeping the original carbon skeleton.
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