Torquoselectivity
Encyclopedia
Torquoselectivity is a special kind of stereoselectivity
observed in electrocyclic reaction
s in organic chemistry
, defined as "the preference for inward or outward rotation of substituents in conrotatory
or disrotatory
electrocyclic reaction
s." Torquoselectivity is not to be confused with the normal diastereoselectivity seen in pericyclic reaction
s, as it represents a further level of selectivity beyond the Woodward-Hoffman rules. The name derives from the idea that the substituent
s in an electrocyclization appear to rotate over the course of the reaction, and thus selection of a single product is equivalent to selection of one direction of rotation (i.e. the direction of torque on the substituents). The concept was originally developed by Kendall N. Houk.
For ring closing reactions, it is an example of enantioselectivity, wherein a single enantiomer
of a cyclization product is formed from the selective ring closure of the starting material. In a typical electrocyclic ring closing, selection for either conrotatory or disrotatory
reactions modes still produces two enantiomers. Torquoselectivity is a discrimination between these possible enantiomers that requires asymmetric induction
.
Torquoselectivity is also used to describe selective electrocyclic ring openings, in which different directions of rotation produce distinct structural isomers. In these cases, steric strain is often the driving force for the selectivity. Studies have shown that the selectivity can also be changed by the presence of electron donating and electron withdrawing groups.
Other mechanisms by which torquoselectivity can operate include chiral
Lewis acid
catalysts, induction via neighboring stereocenter
s (in which case the torquoselectivity is a case of diastereoselectivity), and axial-to-tetrahedral chirality transfer. An example of the latter case is shown below for the torquoselective Nazarov cyclization reaction
of a chiral allenyl vinyl ketone.
Stereoselectivity
In chemistry, stereoselectivity is the property of a chemical reaction in which a single reactant forms an unequal mixture of stereoisomers during the non-stereospecific creation of a new stereocenter or during the non-stereospecific transformation of a pre-existing one...
observed in electrocyclic reaction
Electrocyclic reaction
In organic chemistry, an electrocyclic reaction is a type of pericyclic rearrangement reaction where the net result is one pi bond being converted into one sigma bond or vice-versa...
s in organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
, defined as "the preference for inward or outward rotation of substituents in conrotatory
or disrotatory
Disrotatory
In a conrotatory mode of an electrocyclic reaction the substituents located at the termini of a conjugated double bond system move in the same direction during ring opening or ring closure...
electrocyclic reaction
Electrocyclic reaction
In organic chemistry, an electrocyclic reaction is a type of pericyclic rearrangement reaction where the net result is one pi bond being converted into one sigma bond or vice-versa...
s." Torquoselectivity is not to be confused with the normal diastereoselectivity seen in pericyclic reaction
Pericyclic reaction
In organic chemistry, a pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted fashion. Pericyclic reactions are usually rearrangement reactions...
s, as it represents a further level of selectivity beyond the Woodward-Hoffman rules. The name derives from the idea that the substituent
Substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...
s in an electrocyclization appear to rotate over the course of the reaction, and thus selection of a single product is equivalent to selection of one direction of rotation (i.e. the direction of torque on the substituents). The concept was originally developed by Kendall N. Houk.
For ring closing reactions, it is an example of enantioselectivity, wherein a single enantiomer
Enantiomer
In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are non-superposable , much as one's left and right hands are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without...
of a cyclization product is formed from the selective ring closure of the starting material. In a typical electrocyclic ring closing, selection for either conrotatory or disrotatory
Disrotatory
In a conrotatory mode of an electrocyclic reaction the substituents located at the termini of a conjugated double bond system move in the same direction during ring opening or ring closure...
reactions modes still produces two enantiomers. Torquoselectivity is a discrimination between these possible enantiomers that requires asymmetric induction
Asymmetric induction
Asymmetric induction in stereochemistry describes the preferential formation in a chemical reaction of one enantiomer or diastereoisomer over the other as a result of the influence of a chiral feature present in the substrate, reagent, catalyst or environment...
.
Torquoselectivity is also used to describe selective electrocyclic ring openings, in which different directions of rotation produce distinct structural isomers. In these cases, steric strain is often the driving force for the selectivity. Studies have shown that the selectivity can also be changed by the presence of electron donating and electron withdrawing groups.
Other mechanisms by which torquoselectivity can operate include chiral
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
catalysts, induction via neighboring stereocenter
Stereocenter
A stereocenter or stereogenic center is an atom, bearing groups such that an interchanging of any two groups leads to a stereoisomer.A chirality center is a stereocenter consisting of an atom holding a set of ligands in a spatial arrangement which is not superposable on its mirror image...
s (in which case the torquoselectivity is a case of diastereoselectivity), and axial-to-tetrahedral chirality transfer. An example of the latter case is shown below for the torquoselective Nazarov cyclization reaction
Nazarov cyclization reaction
The Nazarov cyclization reaction is a chemical reaction used in organic chemistry for the synthesis of cyclopentenones. The reaction is typically divided into classical and modern variants, depending on the reagents and substrates employed...
of a chiral allenyl vinyl ketone.