Reagent Friday: Lithium Di-isopropyl Amide (LDA)

Video What is lda Organic Chemistry?Lithium Diisopropyl Amide (LDA), A Solid, Obstructed BaseIn an obvious addition to the Reagent Guide, every Friday I profile a different reagent commonly encountered in Organization 1 / Organization 2. Version 1.2 was released just last week, with a bunch of tweaks and a new page index. organic chemistry ldaIf NaNH2 is a piranha, then today’s reagent – lithium diisopropylamide (LDA) is like a hammerhead shark. It also has a strong bite, but that particular proboscis can get in the way. So LDA can’t approach tight spaces in the same way that NaNH2 can.

Formation of Enolates Less Obstructed (“Kinetic”) With LDA


In other words: The LDA is a sturdy, bulky stand. The most common use of LDA is in the formation of enolate. In the example below, notice how both atoms with the C=O bond have a CH bond? LDA will selectively remove the proton from the carbon being replaced with the least amount of carbon:lda-lithium-diisopropyl-amide-in-formation-of-kinetics-enolate-from-ketoneRead more: What does a dog taste like Also, note the temperature (-78°C). There’s nothing special about -78° compared to -72° or -60° for this to work – it’s just that cold temperatures improve selectivity, and -78°C happens to be the temperature of a cold bath that is very cheap preparation (dry ice and acetone). A common solvent for this is tetrahydrofuran (THF).

Alkylation, halogenation and aldolization of enolates obtained with Lithium Diisopropylamide

Why is LDA useful? Well, enolate is extremely useful ndrinker, which can participate in SN2 reactions with alkyl halides as well as aldol reactions (among many others). If we use NaNH2 to form an enolate like this, we might get mixture of two enolates, will give a mixture of products. LDA’s selectivity for the formation of the less-substituted enolate makes it extremely useful.

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Formation of less substituted Alkenes (“Non-Zaitsev” or “Hoffmann”) in elimination reactions

Although less common, LDA can also be used to form the “Hoffman” product in exclusion reactions. The usual basis for this is potassium t-butoxide, but LDA can do that too:da-as-a-base-for-shape-the-hofmann-less-alternative-alkenes-product

Formation of less substituted enolates with LDA: Mechanism

How it works: The diagram below shows the reaction between LDA and ketones. Note the forming bonds (NH, CC) and breaking bonds (CH, CO). The resulting enolate has a resonance isomer in which a negative charge is on the carbon. This is in some respects the more “important” form of resonance, since carbon tends to be a better nucleophile than oxygen in enolate reactions. Read more: What is endurance in league of legends.mechanism for lda used to form the least substituted ketone enolatePS You can read about the chemistry of LDA and more than 80 other reagents in undergraduate organic chemistry in the “Reagent Guide to Organic Chemistry”, available here as a downloadable PDF.icon2Read more: what is the square root of 69 | Top Q&A(Advanced) References and Further Reading

  • Stimulus α OF IRRITATIONS FROM LITHIUM-AMMONIA REDUCTION OF α, β-UNSATURATED KETONES Gilbert Stork, Perry Rosen and Norman L. Goldman Directors of the American Chemical Society In 1961, 83 (13), 2965-2966 DOIs: 10.1021/ja01474a051 This paper has one of the first descriptions of kinetic enolate formation in the literature – “The success of enolate IV ion trapping depends on a faster alkylation reaction than equilibration of enolate IV equilibria. is generated initially to the more stable II via proton transfer with some neutral alkylated ketones initially formed. ”
  • tetrahedral report number 25: Ketone enolate: specific preparation and synthetic use Jean d’Angelo Tetrahedron 197632 (24), 2979-2990 DOIs: 10.1016 / 0040-4020 (76) 80156-1 This review covers different methods for enolate formation and has data on the composition of different ketone-enolate mixtures formed under dynamic physics and thermodynamics. HO House (MIT, then Georgia Tech) published a series of papers on carbanion and enolate chemistry, detailing kinetic and thermodynamic enolate formation. Here are some selections of these papers:
  • Chemistry of the Carbanions. V. Enolate is derived from asymmetric ketones Herbert O. House and Vera Kramar The Journal of Organic Chemistry 1963, 28 (12), 3362-3379 DOI: 10.1021 / jo01047a022
  • Chemistry of the Carbanions. IX. Potassium and Lithium Enolat Derived from Cyclic Ketones Herbert O. House and Barry M. Trost The Journal of Organic Chemistry 1965 30 (5), 1341-1348 DOI: 10.1021 / jo01016a001
  • Chemistry of carbanions. XV. Stereochemistry of 4-tert-butylcyclohexanone . alkylation Herbert O. House, Ben A. Tefertiller, and Hugh D. Olmstead The Journal of Organic Chemistry In 1968, 33 (3), 935-942 DOI: 10.1021 / jo01267a002
  • Thermodynamically and Kinetic Controlled Enolat: Project for a Problem-Oriented Lab Course Augustine Silveira Jr., Michael A. Knopp, and Jhong Kim 1998, 75 (1), 78 DOIs: Paper 1021 / ed075p78A by J. Chem. Ed. covers how to demonstrate the concepts of enolate kinetics and thermodynamics in a university lab session.
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