Guilford Techno Consultants, Inc
Horner-Wadsworth-Emmons Reaction
Thursday, November 16, 2023 by Guilford Techno Consultants, Inc. | Name Reactions
A few of my students are learning about aldehydes and ketones in Organic Chemistry II. Carbonyl chemistry makes up a large portion of the second semester and can often be overwhelming because of the many reactions and mechanisms that must be learned. One of the reactions that I have always found interesting is the Wittig reaction, discovered by Georg Wittig, which involves the conversion of an aldehyde or ketone into an alkene through reaction of the carbonyl group with a resonance stabilized carbanion known as an ylide (Wittig reagent):
When the ylide contains a simple alkyl group, the reaction produces predominately the Z isomer of the alkene:
The ylide may, however, contain an electron withdrawing group, which stabilizes the reagent through an additional resonance structure:
Ylides containing electron withdrawing groups are referred to as stabilized Wittig reagents. These stabilized Wittig reagents afford mainly the E isomer of the alkene:
A variation of the Wittig reaction which is touched upon in the second semester is the Horner-Wadsworth-Emmons reaction in which a phosphonate ester carbanion (HWE reagent) is used in place of a stabilized Wittig reagent. Like the stabilized Wittig reagent, the HWE reagent produces the E isomer of the alkene as the major product:
Both the Wittig and HWE reactions proceed through the fragmentation of an oxaphosphetane. I have included a mechanism for the HWE reaction in the following presentation, which also includes practice problems.
Click Here for the Horner-Wadsworth-Emmons Reaction
Staudinger Reaction
Sunday, October 15, 2023 by Guilford Techno Consultants, Inc. | Name Reactions
The Staudinger reaction, discovered by Hermann Staudinger involves the reduction of organic azides by a phosphine to yield an amine. If triphenylphosphine is used, nitrogen gas and triphenylphosphine oxide are produced as by-products:
The reaction occurs in two steps. In the first step, the azide is reacted with triphenylphosphine to produce an iminophosphorane intermediate and nitrogen gas. Subsequent reaction of the iminophosphorane with water yields the desired amine product and triphenylphosphine oxide.
The Staudinger reaction is similar to the Wittig reaction in which an aldehyde or ketone is converted to an alkene through reaction with a phosphorous ylide:
Phosphorous ylides, also known as Wittig reagents, are resonance stabilized carbanions:
The mechanism for the Staudinger reaction, along with practice problems, can be found in the following presentation.
Baeyer-Villiger Oxidation
Sunday, August 20, 2023 by Guilford Techno Consultants, Inc. | Name Reactions
Several mechanisms of the second semester of Organic Chemistry involve the migration (shift) of an alkyl group to give a rearranged product. The Hofmann rearrangement, which I wrote about in my last post, is one such mechanism. Another reaction that proceeds through an alkyl shift is the Baeyer-Villiger oxidation of a ketone by a peroxyacid to yield an ester. The reaction was discovered by Adolf von Baeyer and Victor Villiger in 1899 and involves the insertion of an oxygen atom of the peroxyacid in between the carbonyl of the ketone and one of it’s alkyl groups:
If the ketone is cyclic, a lactone (cyclic ester) is formed:
The mechanism involves the formation of an uncharged tetrahedral intermediate after attack of the protonated carbonyl by the peroxyacid. The carbonyl is reformed through migration of an alkyl group and subsequent deprotonation. The peroxyacid is converted to a carboxylic acid during the process. If the ketone is asymmetric, the reaction is regiospecific with respect to which group will migrate:
H > 3°C > 2°C, phenyl > 1°C > CH3
In other words, hydrogen will migrate faster than any other group, while methyl will migrate slower than any other group. The mechanism can be found below, along with practice problems.
