Allylpotassium

Allylpotassium

Names
IUPAC name Potassium prop-2-en-1-ide
Other names Potassium prop-1-ene, Potassium 2-Propenyl, Allylpotassium, Potassium allyl
Identifiers
CAS Number	7329-38-6
3D model (JSmol)	Interactive image
ChemSpider	35786254
PubChem CID	12859842
CompTox Dashboard (EPA)	DTXSID50511691
InChI InChI=1S/C3H5.K/c1-3-2;/h3H,1-2H2;/q-1;+1 Key: ZPQJASJRBFXYKB-UHFFFAOYSA-N
SMILES [K+].[CH2-]C=C
Properties
Chemical formula	C3H5K
Molar mass	80.171 g/mol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Allylpotassium (US: /'ælaɪl pəˈtæsiəm/ (AL-yl puh-TASS-ee-uhm), UK: /'ælɪl pəˈtæsɪəm/ (AL-il puh-TASS-ee-uhm) is an ionic chemical compound with the molecular formula C₃H₅K, containing potassium ions K⁺ and allyl ligands C₃H−5. It is a versatile reagent in organic synthesis commonly used in carbonyl allylation, alkylation, and cyclization reactions, and can exhibit catalytic properties, especially in transition-metal catalysis, cross-coupling reactions, and olefin isomerization.

General properties

Allylpotassium (C₃H₅K) is a highly nucleophilic organopotassium compound due to its negatively charged, resonance-stabilized allyl anion, making it reactive toward electrophiles.

The large ionic radius of potassium ion (K⁺) leads to π-bonding (equivalent to η³), more specifically to a μ: η³: η³ mode with multiple allyl ligands (C₃H−5), which can create polymeric or oligomeric chains. Solvent molecules like tetrahydrofuran can also easily bind to potassium, helping to stabilize the structure of allylpotassium.

The most thermodynamically stable structure of allypotassium in the endo Z form, due to intramolecular hydrogen bonding with different solvent molecules or proton donors and the negatively charged allyl group or heteroatoms (such as oxygen in coordinated tetrahydrofuran). This effect stabilizes the molecule, lowering its energy.

Allylpotassium has a high rotation barrier around its carbon-carbon bonds measured at 16.7 ± 2.0 kcal/mol, a high value explained by the stabilizing role of potassium in the structure of the allyl ion. Due to a high sensitization of allylpotassium with oxygen, especially humidity, allylpotassium is often handled under an inert atmosphere (argon or nitrogen) and is stored in anhydrous solvents.

Reactivity

Synthesis

Figure representing one of the synthesis reaction of Allylpotassium

Allylpotassium can be synthesized in a metalation reaction. Using hexane as a solvent, at -45 °C, n-butyllithium will metalate propene with potassium tert-butoxide to yield 63% of allylpotassium as well as the by product lithium tert-butoxide.

Nucleophilic reaction

Figure representing a nucleophelic reaction with Allylpotassium

In this reaction, allylpotassium is the key reagent that inserts into pyridine at the 1,4-insertion (product 2), in tetrahydrofuran at ambient temperature, forming an N-metalated dihydropyridine. Then, an excess of'[D5]pyridine is added to quickly produce the [D5]-2 a dihydropyridine. However, the product is not stable in tetrahydrofuran or pyridine. This reaction highlights the versatility of the metalation of pyridine using alkali metals and provides insights into the reactivity of alkali metal pyridine adducts, which are important in organometallic chemistry and synthesis. Finally, it allows the synthesis of 1,4-dihydropyridines, which are widely used as selective reducing agents in organic chemistry and represent an important class of pharmacologically active compounds.

Catalytic reaction

Figure representing a catalytic reaction with Allylpotassium

Allypotassium can be used as a catalyst in the silylation reaction of allylic, more specifically in the C(sp)3-H bond. This reaction mediated by allylpotassium complexes which react with chlorosilanes, giving linear allylsilanes with a high selectivity and thermodynamic control of the stereoselectivity. The mechanism is based on the catalytic generation of nucleophilic allylpotassium species with LICKOR (nBuLi/tBuOK) from alkenes (1 or 2-alkene).

It involves the formation of a η³-allylpotassium complex, which undergoes transmetalation with silydiazenes and is followed by the decomposition to release the desired allylsilane.