Pentadienyl

In organic chemistry, pentadienyl refers to the unsaturated five-carbon chain with the formula C5H7. Three versions are discussed, all formally derived from 1,4-or 1,3-pentadiene C5H8:

  • pentadienyl radical with the formula H2CCHCHCHCH2
  • pentadienyl anion with the formula H2CCHCHCHCH−2, an approximate description of pentadienyl lithium and related species.
  • pentadienyl cation with the formula H2CCHCHCHCH+2, which is rarely invoked carbocation.

The situation is closely related to the allyl (C3H5) derivatives, which also are discussed in three states, being derived from propylene.

Production and structure

Pentadienyl "anion"

1,4-or 1,3-Pentadiene can be deprotonated with butyl lithium, providing lithium pentadienyl.[1]

Pentadienyl radical

Pentadienyl radicals are intermediates in the degradation of 1,4-dienes, as found in some fats. Like the pentadienyl anions, the radicals are planar.[2]

Organometallic chemistry

In organometallic chemistry, the pentadienyl anion is a ligand, the acyclic analogue of the more-common cyclopentadienyl anion. The pentadienyl anion is generated by deprotonation of pentadiene. A number of complexes are known, including bis(pentadienyl) iron, Fe(C5H7)2, the "open" analog of ferrocene. Only few pentadienyl complexes feature simple C5H7 ligands. More common is the dimethyl derivative 2,4-Me2C5H5. Additionally, many pentadienyl ligands are cyclic, being derived from the addition of hydride to η6-arene complexes or hydride abstraction from cyclohexadiene complexes.[3][4]

The first pentadienyl complex to be reported was derived from protonolysis of a complex of pentadienol:[5]

Fe(C5H7OH)(CO)3 + H+ → [Fe(C5H7)(CO)3]+ + H2O

Treatment of this cation with sodium borohydride gives the pentadiene complex:

[Fe(C5H7)(CO)3]+ + H− → Fe(C5H8)(CO)3

References

  1. ^ Seyferth, Dietmar; Pornet, Jacques (1980). "(2,4-Pentadienyl)trimethylsilane: A useful pentadienylation reagent". The Journal of Organic Chemistry. 45 (9): 1721–1722. doi:10.1021/jo01297a053.
  2. ^ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 978-0-471-72091-1
  3. ^ Lothar Stahl; Richard D. Ernst (2007). "Pentadienyl Complexes of the Group 4 Transition Metals". Advances in Organometallic Chemistry. 55: 137–199. doi:10.1016/S0065-3055(07)55003-3. ISBN 978-0-12-373978-0.
  4. ^ Richard D. Ernst (1988). "Structural and reactivity patterns in transition-metal-pentadienyl chemistry". Chem. Rev. 88 (7): 1255–1291. doi:10.1021/cr00089a013.
  5. ^ Mahler, J. E.; Pettit, R. (1962). "Pentadienyl and Hexadienyl Carbonium Ions as Ligands in Stable Complex Cations". J. Am. Chem. Soc. 84 (8): 1511–1512. doi:10.1021/ja00867a051.


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