Organosodium chemistry

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Organosodium chemistry is the chemistry of organometallic compounds containing a carbon to sodium chemical bond.12 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:567

(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 [NaCH2SiMe3TMEDA 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) 910 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

  1. ^ Synthesis of Organometallic Compounds: A Practical Guide Sanshiro Komiya Ed. 1997
  2. ^ C. Elschenbroich, A. Salzer Organometallics : A Concise Introduction (2nd Ed) (1992) from Wiley-VCH: Weinheim. ISBN 3-527-28165-7
  3. ^ Tarun K. Panda, Michael T. Gamer, and Peter W. Roesky "An Improved Synthesis of Sodium and Potassium Cyclopentadienide" Organometallics 2003, 22, 877-878. doi:10.1021/om0207865
  4. ^ a b W. B. Renfrow, Jr and C. R. Hauser (1943), "Triphenylmethylsodium", Org. Synth. ; Coll. Vol. 2: 607 
  5. ^ a b Dietmar Seyferth "Alkyl and Aryl Derivatives of the Alkali Metals: Strong Bases and Reactive Nucleophiles. 2. Wilhelm Schlenk’s Organoalkali-Metal Chemistry. The Metal Displacement and the Transmetalation Reactions. Metalation of Weakly Acidic Hydrocarbons. Superbases" Organometallics 2009, volume 28, pp 2–33. doi:10.1021/om801047n
  6. ^ P. Schorigin, "Beiträge zur Kenntnis der Kondensationen, welche durch Natrium bewirkt werden" Chem. Berichte 1907, vol. 40, 3111. doi:10.1002/cber.19070400371
  7. ^ P. Schorigin, "Synthesen mittels Natrium und Halogenalkylen" Ber. 41, 2711. doi: 10.1002/cber.190804102208. P. Schorigin, "Über die Natriumalkyl-Verbindungen und über die mit ihrer Hilfe ausgeführten Synthesen" Berichte, 1908, vol. 41, 2717. doi: 10.1002/cber.190804102209. P. Schorigin, "Synthesen mittels Natrium und Halogenalkylen" Chem. Berichte, 1908, volume 41, 2711. doi: 10.1002/cber.190804102208. P. Schorigin, "Neue Synthese aromatischer Carbonsäuren aus den Kohlenwasserstoffen" Chem. Berichte 1908, vol. 41, 2723. doi:10.1002/cber.190804102210.
  8. ^ a b c William Clegg, Ben Conway, Alan R. Kennedy, Jan Klett, Robert E. Mulvey, Luca Russo "Synthesis and Structures of [(Trimethylsilyl)methyl]sodium and -potassium with Bi- and Tridentate N-Donor Ligands" Eur. J. Inorg. Chem. 2011, 721–726. doi:10.1002/ejic.201000983
  9. ^ J. A. Wanklyn, Ann. 107, 125 (1858)
  10. ^ The Merck index of chemicals and drugs: an encyclopedia for chemists, Paul G. Stecher
  11. ^ Rolf Ackermann, Ludwig Lange "Sodium Compounds, Organic" in Ullmann's Encyclopedia of Industrial Chemistry, 2005 Wiley-VCH, Weinheim. doi:10.1002/14356007.a24_341
  12. ^ Manfred Schlosser (1988). "Superbases for organic synthesis". Pure and Appl. Chem. 60 (11): 1627–1634. doi:10.1351/pac198860111627. 


CH He
CLi CBe CB CC CN CO CF Ne
CNa CMg CAl CSi CP CS CCl CAr
CK CCa CSc CTi CV CCr CMn CFe CCo CNi CCu CZn CGa CGe CAs CSe CBr CKr
CRb CSr CY CZr CNb CMo CTc CRu CRh CPd CAg CCd CIn CSn CSb CTe CI CXe
CCs CBa CHf CTa CW CRe COs CIr CPt CAu CHg CTl CPb CBi CPo CAt Rn
Fr CRa Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv Uus Uuo
CLa CCe CPr CNd CPm CSm CEu CGd CTb CDy CHo CEr CTm CYb CLu
Ac CTh CPa CU CNp CPu CAm CCm CBk CCf CEs Fm Md No Lr
Chemical bonds to carbon
Core organic chemistry Many uses in chemistry
Academic research, but no widespread use Bond unknown