Organosodium chemistry

Organosodium chemistry is the chemistry of organometallic compounds containing a carbon to sodium chemical bond.^[1][2] The application of organosodium compounds in chemistry is limited in part due to competition from organolithium compounds, which are commercially available and exhibit more convenient reactivity.

The principal organosodium compound of commercial importance is sodium cyclopentadienide. Sodium tetraphenylborate can also be classified as an organosodium compound since in the solid state sodium is bound to the aryl groups.

Organometal bonds in group 1 are characterised by high polarity with corresponding high nucleophilicity on carbon. This polarity results from the disparate electronegativity of carbon (2.55) and that of lithium 0.98, sodium 0.93 potassium 0.82 rubidium 0.82). The carbanionic nature of organosodium compounds can be minimized by resonance stabilization, for example, Ph₃CNa. One consequence of the highly polarized Na-C bond is that simple organosodium compounds tend to aggregate and are often poorly soluble.

Synthesis and structure

The usual method for preparing organosodium compounds involves deprotonation. Sodium cyclopentadienide is thus prepared by treating sodium metal and cyclopentadiene:^[3]

2 Na + 2 C₅H₆ → 2 NaC₅H₅ + H₂

Trityl sodium can be prepared by metal-halogen exchange. This method is used in preparing trityl sodium:^[4]

Ph₃CCl + 2 Na → Ph₃CNa + NaCl

Sodium also reacts with polycyclic aromatic hydrocarbons via one-electron reduction. With solutions of naphthalene, it forms the deeply coloured radical sodium naphthalenide, which is used as a soluble reducing agent:

C₁₀H₈ + Na → Na⁺[C₁₀H₈]^-•

In the original work the alkylsodium compound was accessed from the dialkylmercury compound by transmetallation. For example diethylmercury in the Schorigin reaction or Shorygin reaction:^[5][6][7]

(C₂H₅)₂Hg + 2 Na → 2 C₂H₅Na + Hg

The low solubility of lithium alkoxides in hexane defines another useful synthetic route:^[8]

LiCH₂SiMe₃ + NaO–t–Bu → LiO–t–Bu + NaCH₂SiMe₃

Simple organosodium compounds such as the alkyl and aryl derivatives are generally insoluble polymers. X-ray quality single crystals have been obtained when the substituents become bulky and especially in the presence of chelating ligands like TMEDA. For example [NaCH₂SiMe₃]TMEDA is soluble in hexane. Crystals have been shown to consist of chains of alternating Na(TMEDA)⁺ and CH₂SiMe₃⁻ groups with Na–C distances ranging from 2.523(9) to 2.643(9) Å.^[8]

Reactions

Organosodium compounds are traditionally used as strong bases,^[4] although this application has been supplanted by other reagents such as sodium bis(trimethylsilyl)amide.

The higher alkali metals are known to metalate even some unactivated hydrocarbons and are known to self-metalate:

2 NaC₂H₅ → C₂H₄Na₂ + C₂H₆

In the Wanklyn reaction (1858) ^[9][10] organosodium compounds react with carbon dioxide to give carboxylates:

C₂H₅Na + CO₂ → C₂H₅CO₂Na

Grignard reagents undergo a similar reaction.

Some organosodium compounds degrade by beta-elimination:

NaC₂H₅ → NaH + C₂H₄

Industrial applications

Although organosodium chemistry has been described to be of "little industrial importance", it once was central to the production of tetraethyllead.^[11] A similar Wurtz coupling-like reaction is the basis of the industrial route to triphenylphosphine:

3 PhCl + PCl₃ + 6 Na → PPh₃ + 6 NaCl

The polymerization of butadiene and styrene is catalyzed by sodium metal.^[5]

Organic derivatives of the heavier alkali metals

The higher alkali metals, organopotassium, organorubidium, and organocaesium, are even more reactive than organosodium compounds and of limited utility. A notable reagent is Schlosser's base, a mixture of n-butyllithium and potassium tert-butoxide. This reagent reacts with propene to form the compound allylpotassium (KCH₂CHCH₂). cis-2-Butene and trans-2-butene equilibrate when in contact with alkali metals. Whereas isomerization is fast with lithium and sodium, it is slow with the higher alkali metals. The higher alkali metals also favor the sterically congested conformation.^[12] Several crystal structures of organopotassium compounds have been reported, establishing that they, like the sodium compounds, are polymeric.^[8]

References