Gauche effect

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The term "gauche" refers to conformational isomers (conformers) where two vicinal groups are separated by a 60° torsion angle. IUPAC defines groups as gauche if they have a "synclinal alignment of groups attached to adjacent atoms".[1]

In stereochemistry, gauche interactions hinder bond rotation. For example, sighting along the C2-C3 bond in staggered butane, there are two possible relative potential energies. The two methyl groups can be in an anti-bonding relationship, or offset at sixty degree dihedral angles. In the latter configuration, the two methyls are said to be in a gauche relationship, and the relative potential energy of each methyl-methyl gauche interaction is 0.9 kilocalories per mole (4 kJ/mol). In general a gauche rotamer is less stable than an anti-rotamer.

Gauche effect[edit]

The Gauche effect characterizes any gauche rotamer which is actually more stable than the anti rotamer.[2] This effect is present in 1,2-difluoroethane (H2FCCFH2) for which the gauche conformation is more stable by 2.4 to 3.4 kJ/mole in the gas phase. Another example is 1,2-dimethoxyethane.

Gauche effect in 1,2-difluoroethane, left: anti and right: gauche
Hyperconjugation model for explaining the gauche effect in 1,2-difluoroethane

There are two main explanations for the gauche effect: hyperconjugation and bent bonds. In the hyperconjugation model, the donation of electron density from the C–H σ bonding orbital to the C–F σ* antibonding orbital is considered the source of stabilization in the gauche isomer. Due to the greater electronegativity of fluorine, the C–H σ orbital is a better electron donor than the C–F σ orbital, while the C–F σ* orbital is a better electron acceptor than the C–H σ* orbital. Only the gauche conformation allows good overlap between the better donor and the better acceptor.

Key in the bent bond explanation of the gauche effect in difluoroethane is the increased p orbital character of both C-F bonds due to the large electronegativity of fluorine. As a result electron density builds up above and below to the left and right of the central C-C bond. The resulting reduced orbital overlap can be partially compensated when a gauche conformation is assumed, forming a bent bond. Of these two models, hyperconjugation is generally considered the principal cause behind the gauche effect in difluoroethane.[3][4]

The molecular geometry of both rotamers can be obtained experimentally by high resolution infrared spectroscopy augmented with in silico work.[2] In accordance with the model described above, the carbon - carbon bond length is higher for the anti-rotamer (151.4 pm vs. 150). The steric repulsion between the fluorine atoms in the gauche rotamer causes increased CCF bond angles (by 3.2°) and increased FCCF dihedral angles (from the default 60° to 71°).

In the related 1,2-difluorodiphenylethane (two protons replaced by phenyl) the threo isomer is found (by X-ray diffraction and from NMR coupling constants) to have an anti conformation between the two phenyl groups and the two fluorine groups and a gauche conformation is found for both groups for the erythro isomer.[5] According to in silico results this conformation is more stable by 0.21 kcal/mol (880 J/mol).

A gauche effect has also been reported for a molecule featuring 4 successive fluor syn substituents, the last one introduced by deoxofluorinating agent bis(2-methoxyethyl)aminosulfur trifluoride:[6]

Synthesis of an All-syn Four Vicinal Fluorine Motif

The gauche effect is very sensitive to solvent effects, due to the large difference in polarity of the two conformers. For example, 2,3-dinitro-2,3-dimetylbutane, which in the solid state exists only in the gauche conformation, prefers the gauche conformer in benzene solution by a ratio of 79:21, but in carbon tetrachloride it prefers the anti conformer by a ratio of 58:42.[7] Another case is trans-1,2 difluorocyclohexane, which has a larger preference for the eq/eq conformer in more polar solvents.[4]

A related effect is the alkene cis effect.

See also[edit]

References[edit]

  1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "gauche".
  2. ^ a b Contribution to the Study of the Gauche Effect. The Complete Structure of the Anti Rotamer of 1,2-Difluoroethane Norman C. Craig, Anthony Chen, Ki Hwan Suh, Stefan Klee, Georg C. Mellau, Brenda P. Winnewisser, and Manfred Winnewisser J. Am. Chem. Soc.; 1997; 119(20) pp 4789 - 4790; (Communication) doi:10.1021/ja963819e
  3. ^ Goodman, L.; Gu, H.; Pophristic, V.. Gauche Effect in 1,2-Difluoroethane. Hyperconjugation, Bent Bonds, Steric Repulsion. J. Phys. Chem. A. 2005, 109, 1223-1229. doi:10.1021/jp046290d
  4. ^ a b David O'Hagan. Understanding organofluorine chemistry. An introduction to the C–F bond. Chem. Soc. Rev. 2008 doi:10.1039/b711844a
  5. ^ The vicinal difluoro motif: The synthesis and conformation of erythro- and threo- diastereoisomers of 1,2-difluorodiphenylethanes, 2,3-difluorosuccinic acids and their derivatives O'Hagan D, Rzepa H, Schuler M, Slawin A Beilstein Journal of Organic Chemistry, 2006 2:19 ( 2 October 2006 ) doi:10.1186/1860-5397-2-19
  6. ^ Enantioselective Synthesis of an All-syn Four Vicinal Fluorine Motif Luke Hunter, David O'Hagan, and Alexandra M. Z. Slawin J. Am. Chem. Soc.; 2006; 128(51) pp 16422 - 16423; (Communication) doi:10.1021/ja066188p
  7. ^ Smith, Michael. B.; March, J. March's Advanced Organic Chemistry, 5th edition. Wiley, 2001. ISBN 0-471-58589-0