What is #"umpolung"#?

1 Answer
anor277
Jan 8, 2018

An old German word....#"umpolung"-="reversal"#

Explanation:

In the context of organic synthesis, #"umpolung"# means #"reversal of polarity"#. I think the term was introduced by E. J. Corey.

A classic example of umpolung is given by dithio esters of aldehydes. Now we know that aldehydes react with strong nucleophiles such as Grignards or lithium reagent...:

#RC(=O)H+R'-Li rarr RC(-O^(-)Li^+)HLi#

Now we can make the cyclic dithio ester of an aldehyde by treating this with #"1,3-dithiopropane"#....(and this stuff truly stinks!)...

#RC(=O)H+HSCH_2CH_2CH_2SH stackrel(H^+)rarr RC(S_2(CH_2)_2CH_2)stackrel"*"H +H_2O#

Now the starred carbon was originally the #alpha-"hydrogen"# of an aldehyde. And now, as the dithio ester substituent it can be abstracted by strong base....

#RC(S_2(CH_2)_2CH_2)stackrel"*"H +Bu^(n)Li rarr Rstackrel(-)C(S_2(CH_2)_2CH_2)Li^+ +Bu^(n)stackrel"*"H#

The lithiated dithiane is now a potent nucleophile (and note that this carbon centre was electrophilic in the parent aldehyde), and may be reacted with aldehydes, or benzyl halides, or ketones, to give carbon-carbon bond formation....

If we reacted it with say cyclohexanone....#C_6H_10O#...we would get #C_6H_10(OH)CH_2R# after workup, where the added #CH_2R# was originally electrophilic aldehyde...i.e. the normal polarity has been reversed.

Anyway, please consult your organic text for more details... Note also that Grignard reagents also provide an example of #"umpolung"#....how and why?

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