Química Orgânica 2 · Adição e condensação aldólica
3-Hydroxy-1,3-diphenylpropan-1-one by a Mukaiyama aldol
Condições: TiCl4, CH2Cl2, −78 °C; then H3O+
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TiCl4 activates benzaldehyde
The aldehyde oxygen donates a lone pair to titanium, a strong Lewis acid. The coordinated carbonyl is much more electrophilic, which a neutral silyl enol ether needs.
As setas curvas
- The carbonyl oxygen lone pair bonds to titanium.
The silyl enol ether attacks
The silyl ether oxygen lone pair pushes the C=C electrons onto the activated carbonyl carbon, forming the C–C bond. The titanium-bound oxygen becomes an alkoxide and the enol ether becomes a silyloxocarbenium ion.
As setas curvas
- The ether oxygen lone pair forms the C=O+ bond.
- The C=C π pair forms the new C–C bond.
- The C=O+ π pair moves onto oxygen.
Why the Lewis acid?
Silyl enol ethers are neutral, weak nucleophiles.
Chloride transfer to silicon
A Ti–Cl bond pair transfers chloride to silicon, cleaving the Si–O bond. This releases chlorotrimethylsilane and the ketone, and leaves a neutral titanium alkoxide.
As setas curvas
- The Ti–Cl pair forms a Cl–Si bond.
- The Si–O pair returns to oxygen, giving the ketone.
Workup: protonation of the titanium alkoxide
Aqueous acid (workup, assumed) protonates the alkoxide oxygen bound to titanium.
As setas curvas
- The alkoxide oxygen lone pair takes a proton from hydronium.
- The O–H pair stays on oxygen as water.
Titanium leaves
The O–Ti bond breaks heterolytically toward the more electronegative oxygen, releasing the free β-hydroxy ketone; titanium leaves as a chlorotitanium cation that is hydrolysed in the aqueous layer.
As setas curvas
- The O–Ti pair stays on oxygen, releasing titanium as a cation.
3-Hydroxy-1,3-diphenylpropan-1-one
3-Hydroxy-1,3-diphenylpropan-1-one forms as a racemate, with chlorotrimethylsilane and titanium salts as by-products. The low temperature and the absence of base prevent dehydration to chalcone.
O que você vai praticar
Draw the TiCl4-mediated Mukaiyama aldol of the trimethylsilyl enol ether of acetophenone with benzaldehyde: Lewis-acid activation, C–C bond formation, silyl transfer and aqueous workup to 3-hydroxy-1,3-diphenylpropan-1-one.
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