Thorpe-Ingold effect

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The Thorpe-Ingold effect or gem-dimethyl effect, or angle compression is an effect observed in organic chemistry where increasing the size of two substituents on a tetrahedral center leads to enhanced reactions between parts of the other two substituents. The effect was first reported by Beesley, Thorpe and Ingold in 1916 as part of a study of cyclization reactions [1].

A common application of this effect is addition of a quaternary carbon (e.g., a gem-dimethyl group) in an alkyl chain to increase the reaction rate and/or equilibrium constant of cyclization reactions. An example is this is an olefin metathesis reaction:[2]

image:tiemethathesis.PNG

One proposed explanation for this effect is that the increased size of the substituents increases the angle between them. As a result, the angle between the other two substituents decreases. By moving them closer together, reactions between them are accelerated. It is thus a kinetic effect.

image:Thorpeingoldeffect.PNG


The effect also has some thermodynamic contribution, the in silico strain energy decreases on going from cyclobutane to 1-methylcyclobutane and 1,1-dimethylcyclobutane by a value between 8 kcal/mole [3] and 1.5 kcal/mole [4].


[edit] References

  1. ^ CXIX.—The formation and stability of spiro-compounds. Part I. spiro-Compounds from cyclohexane Richard Moore Beesley, Christopher Kelk Ingold and Jocelyn Field Thorpe, J. Chem. Soc., Trans., 1915, 107, 1080 doi:10.1039/CT9150701080
  2. ^ Fürstner, A; Langemann, K. (1996). "A Concise Total Synthesis of Dactylol via Ring Closing Metathesis". J. Org. Chem. 61 (25): 8746–8749. doi:10.1021/jo961600c. 
  3. ^ Conventional Strain Energy in Dimethyl-Substituted Cyclobutane and the gem-Dimethyl Effect Ashley L. Ringer† and David H. Magers J. Org. Chem. 2007, 72, 2533-2537 doi:10.1021/jo0624647
  4. ^ The gem-Dimethyl Effect Revisited Steven M. Bachrach J. Org. Chem. 2008, 73, 2466-2468 doi:10.1021/jo702665r
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