Controlling Reaction Selectivity Via Molecular Dynamics | Chemical & Engineering News
Volume 93 Issue 46 | p. 27 | Concentrates
Issue Date: November 23, 2015

Controlling Reaction Selectivity Via Molecular Dynamics

Reaction Mechanisms: When one transition state leads to two products, controlling timing and molecular motion allows for product selection
Department: Science & Technology
News Channels: JACS In C&EN, Organic SCENE
Keywords: selectivity, rearrangement, dynamics, trajectory

The traditional way to produce only one out of several possible products of a reaction is to stabilize the transition state that leads to that desired compound. But in some reactions a single transition state leads to multiple products, requiring a different approach to selectivity. Such is the case with [1,2]- versus [2,3]-sigmatropic rearrangements. Working with an ammonium ylide containing an enolate group (Y, shown), Bibaswan Biswas and Daniel A. Singleton of Texas A&M University have demonstrated that they can select for either the [1,2] or [2,3] rearrangement product by choosing reaction conditions geared toward promoting particular molecular dynamics paths (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b08635). Alkali-metal bases that do not hydrogen bond to the ylide allow for early formation of the transition state with dynamic motion that favors fragmentation and [1,2] rearrangement. In contrast, methanol or protonated 1,8-diazabicycloundec-7-ene does form a hydrogen bond to the enolate oxygen, increasing the reaction barrier and promoting later formation of the transition state with dynamic motion that favors [2,3] rearrangement.

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ONE WAY OR ANOTHER
Early motion of an ammonium ylide transition state favors [1,2]-sigmatropic rearrangement, whereas later motion favors a [2,3] rearrangement.
Reaction scheme showing dynamic motion in an ammonium ylide.
 
ONE WAY OR ANOTHER
Early motion of an ammonium ylide transition state favors [1,2]-sigmatropic rearrangement, whereas later motion favors a [2,3] rearrangement.
 
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