Ch_322b_19.07

Ch_322b_19.07 - C R H S S + CH 3 CH 2 CH 2 CH 2 Li...

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Alkylation of 1,3-Dithianes: The Corey-Seebach Method A useful method for converting aldehydes to ketones was developed around 1970 by E.J. Corey and D. Seebach. Several steps are involved in this transformation which begins with reacting the aldehyde with 1,3-propanedithiol to produce the 1,3-dithiane. The mechanism of the acid-catalyzed formation of the 1,3-dithiane is similar to the mechanism of ordinary acetal/ketal formation. RCH O = + HSCH 2 CH 2 CH 2 SH 1,3-propanedithiol H + C R H S S 1,3-dithiane (a thioacetal) RCH O aldehyde RCR' O ketone
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Alkylation of the 1,3-Dithiane The electron-withdrawing sulfur atoms enhance the acidity of the ! -H, which allows deprotonation with bases such as organolithium reagents in ether solvents. Alkylation of the dithiane anion with a primary alkyl halide, followed by hydrolysis of the dithiane (thioketal) yields a ketone. HgCl 2 CH 3 OH, H 2 O hydrolysis In the hydrolysis reaction, the mercury (II) chloride acts as a Lewis acid catalyst while the methanol helps solubilize the dithiane.
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Unformatted text preview: C R H S S + CH 3 CH 2 CH 2 CH 2 Li butyllithium THF C R S S-Li + + CH 3 CH 2 CH 2 CH 3 C R S S-Li + + R'-X (-LiX) C R R' S S alkylation C R R' O = + HSCH 2 CH 2 CH 2 SH ketone Examples of Synthetic Applications of Dithiane Anions C 4 H 9 Li ether C S S-Li + + C 4 H 10 C S S-Li + alkylation CH 3 I (-LiI) C CH 3 S S HgCl 2 CH 3 OH, H 2 O hydrolysis addition to acyl carbon (i) CH 3 CH O = (ii) H 3 O + C CHOH S S CH 3 HgCl 2 CH 3 OH, H 2 O hydrolysis CH O benzaldehyde H + HSCH 2 CH 2 CH 2 SH C H S S dithiane CCH 3 O acetophenone CCHCH 3 O OH 1-phenyl-1-oxo-2-propanol (CH 3 ) 2 CHC-H S S (CH 3 ) 2 CHC: Li S S-+ (CH 3 ) 2 CHCCH 2 C 6 H 5 S S (CH 3 ) 2 CHCCH 2 C 6 H 5 O = 1-phenyl-3-methyl-2-butanone Quiz 19.07 Provide the missing structures in the following Corey-Seebach synthesis. (CH 3 ) 2 CHCHO 2-methylpropanal + HSCH 2 CH 2 CH 2 SH H + n-BuLi/ether C 6 H 5 CH 2 Cl HgCl 2 CH 3 OH/H 2 O...
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Ch_322b_19.07 - C R H S S + CH 3 CH 2 CH 2 CH 2 Li...

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