Crossed beams study of the reaction 1CH2+C2H2-->C3H3+H

H Floyd Davis1, Jinian Shu, Darcy S Peterka

  • 1Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853-1301, USA.

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Crossed Aldol Reactions: Overview01:04

Crossed Aldol Reactions: Overview

Crossed aldol addition is the reaction between two different carbonyl compounds under acidic or basic conditions. Here, both the carbonyl compounds function as nucleophiles and electrophiles. As shown in Figure 1, such a reaction yields a mixture of products, two of which are formed via self-condensation, while the remaining two are formed via crossed-condensation. Without adjustment, the reaction's usefulness in organic chemistry is decreased.
Crossed Aldol Reaction Using Weak Bases01:14

Crossed Aldol Reaction Using Weak Bases

This lesson deals with the crossed aldol reaction using weak bases. The self-condensation of an aldehyde having α hydrogen is prevented by adding it slowly to a mixture of formaldehyde and weak bases like hydroxide and alkoxide. Upon slow addition of the aldehyde, the base deprotonates the α carbon of the aldehyde to form the corresponding enolate. The enolate subsequently attacks the formaldehyde to form a single crossed product. Figure 1 depicts the aforementioned reaction.
Crossed Aldol Reaction Using Strong Bases: Directed Aldol Reaction00:56

Crossed Aldol Reaction Using Strong Bases: Directed Aldol Reaction

The reaction between two different carbonyl compounds comprising α hydrogen in the presence of a strong base like lithium diisopropylamide (LDA) to form a crossed aldol product is known as a directed aldol reaction. The directed aldol reaction is depicted in Figure 1.
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β-Dicarbonyl Compounds via Crossed Claisen Condensations

Crossed Claisen condensations are base-promoted reactions between two different ester molecules producing β-dicarbonyl compounds. The reaction involving esters, with both containing α hydrogen, results in a mixture of four different products that are difficult to isolate. This reduces the synthetic utility of the reaction.
Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene01:17

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Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene01:14

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