L_3_09 - primary alcohol CH 3 MgBr adds to acetaldehyde to...

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318N – Krische, Lecture 3: Tu - 01/27/09, Aldehydes and Ketones We begin by reviewing acetal/ketal formation and hydrolysis and the formation of thio-acetals (i.e. 1,3-dithianes) The mechanism for acetal/ketal hydrolysis is simply the reverse of acetal/ketal formation – KNOW THE MECHANISM. Thus far, we’ve examined the addition of Oxygen- and Sulfur-nucleophiles to aldehydes and ketones. Next, we discuss additions involving Carbon-nucleophiles. Some carbon-centered nucleophiles: organolithiums, Grignard reagents, acetylides, cyanide and dithianes. The preparation of organometallic reagents was discussed in class, along with several problem in synthesis that exploit such reagents. Carbanions will add to the positive end of the carbonyl dipole, i.e. the carbonyl carbon. The immediate product of addition is an alkoxide. By quenching the reaction with a large excess of water, one obtains the corresponding alcohol. EXAMPLE : CH 3 MgBr adds to formaldehyde to give ethanol (a
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Unformatted text preview: primary alcohol). CH 3 MgBr adds to acetaldehyde to give iso-propanol (a secondary alcohol), and CH 3 MgBr adds to acetone to give tert-butanol (a tertiary alcohol). CH 3 MgBr adds to carbon dioxide to give acetic acid. With the exception of phosphorus ylides, all carbon-nucleophiles we discuss will behave in an analogous fashion. • The Wittig Reaction : Phosphorus ylides (e.g. Ph 3 P=CH 2 ) can be represented as a zwitterionic species in which the carbon bears a negative charge and the phosphorus bears a positive charge. Ylides will add to the carbonyl carbon of an aldehyde or ketone. The resulting alkoxide will “bite back” onto phosphorus to generate the famous “oxaphosphetane” intermediate: a 4-membered ring containing oxygen and phosphorus. This strained ring will collapse to give an alkene and triphenylphosphine oxide. The Wittig reaction can be thought of as the reverse of an ozonolysis....
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