Koelsch radical explained

The Koelsch radical (also known as Koelsch's radical and 1,3-bisdiphenylene-2-phenylallyl or α,γ-bisdiphenylene-β-phenylallyl, abbreviated BDPA)[1] [2] is a chemical compound that is an unusually stable carbon-centered radical, due to its resonance structures.

Properties

BDPA is an unusually stable radical compound due to the extent to which its electrons are delocalized through resonance structures. The unpaired electron is located predominantly at the 1 and 3 positions.[3] Steric effects from the biphenyl substituents also contribute to the compound's stability.[4]

Uses

BDPA and closely related compounds are used as molecular standards in electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) experiments,[5] [6] and as a polarizing agent in dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) experiments.[7] [8] Because BDPA itself is hydrophobic, derivatives have been developed that are more soluble in aqueous solution.[9]

History

The compound was first synthesized by C. Frederick Koelsch while he was a postdoctoral fellow at Harvard University in the 1930s. He attempted to publish a paper describing the compound, but the paper was rejected on the grounds that the described properties, particularly stability, were unlikely to be those of a radical. Subsequent experimental evidence and quantum mechanics calculations suggested his interpretation of the original experiment was correct, resulting in the publication of the paper in 1957, nearly 25 years after the original experiments.[10] [11] Although the original report described stability on the order of years, modern experiments suggest that this family of compounds, while unusually stable for radicals, shows measurable degradation in months after preparation.

Notes and References

  1. Book: Zard . Samir Z. . Radical reactions in organic synthesis . 2003 . Oxford University Press . Oxford . 9780198502401 . 3.
  2. Matsui . Yasunori . Shigemori . Minoru . Endo . Toshiyuki . Ogaki . Takuya . Ohta . Eisuke . Mizuno . Kazuhiko . Naito . Hiroyoshi . Ikeda . Hiroshi . Spectroscopic and electrical characterization of α,γ-bisdiphenylene-β-phenylallyl radical as an organic semiconductor . Research on Chemical Intermediates . August 2018 . 44 . 8 . 4765–4774 . 10.1007/s11164-018-3282-7.
  3. Azuma . Nagao . Ozawa . Takehiro . Yamauchi . Jun . Molecular and Crystal Structures of Complexes of Stable Free Radical BDPA with Benzene and Acetone . Bulletin of the Chemical Society of Japan . January 1994 . 67 . 1 . 31–38 . 10.1246/bcsj.67.31.
  4. Breslin . David T. . Fox . Marye Anne . Characterization of the excited-state reactivity of a persistent aryl-substituted allyl free radical . The Journal of Physical Chemistry . December 1993 . 97 . 50 . 13341–13347 . 10.1021/j100152a045.
  5. Bennati . M. . Farrar . C.T. . Bryant . J.A. . Inati . S.J. . Weis . V. . Gerfen . G.J. . Riggs-Gelasco . P. . Stubbe . J. . Griffin . R.G. . Pulsed Electron-Nuclear Double Resonance (ENDOR) at 140 GHz . Journal of Magnetic Resonance . June 1999 . 138 . 2 . 232–243 . 10.1006/jmre.1999.1727.
  6. Goldfarb . Daniella . Lipkin . Yaakov . Potapov . Alexey . Gorodetsky . Yehoshua . Epel . Boris . Raitsimring . Arnold M. . Radoul . Marina . Kaminker . Ilia . HYSCORE and DEER with an upgraded 95GHz pulse EPR spectrometer . Journal of Magnetic Resonance . September 2008 . 194 . 1 . 8–15 . 10.1016/j.jmr.2008.05.019.
  7. Mandal . Sucharita . Sigurdsson . Snorri Th. . On the Limited Stability of BDPA Radicals . Chemistry – A European Journal . 10 June 2020 . 26 . 33 . 7486–7491 . 10.1002/chem.202001084.
  8. Lumata . Lloyd . Ratnakar . S. James . Jindal . Ashish . Merritt . Matthew . Comment . Arnaud . Malloy . Craig . Sherry . A. Dean . Kovacs . Zoltan . BDPA: An Efficient Polarizing Agent for Fast Dissolution Dynamic Nuclear Polarization NMR Spectroscopy . Chemistry - A European Journal . 19 September 2011 . 17 . 39 . 10825–10827 . 10.1002/chem.201102037. 3366484 .
  9. Dane . Eric L. . Swager . Timothy M. . Synthesis of a Water-Soluble 1,3-Bis(diphenylene)-2-phenylallyl Radical . The Journal of Organic Chemistry . 21 May 2010 . 75 . 10 . 3533–3536 . 10.1021/jo100577g. 2882179 .
  10. Koelsch . C. F. . Syntheses with Triarylvinylmagnesium Bromides. α,γ-Bisdiphenylene-β-phenylallyl, a Stable Free Radical . Journal of the American Chemical Society . August 1957 . 79 . 16 . 4439–4441 . 10.1021/ja01573a053.
  11. Noland . Wayland E. . The Life and Times of Professor Emeritus C. Frederick Koelsch . The Minnesota Chemist . November 1975 . 27 . 7 . 7–17 . 29 December 2020.

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