Torquoselectivity is a special kind of

stereoselectivity In chemistry, stereoselectivity is the property of a chemical reaction in which a single reactant forms an unequal mixture of stereoisomers during a non-stereospecific creation of a new stereocenter or during a non-stereospecific transformation ...

observed in

electrocyclic reaction In organic chemistry, an electrocyclic reaction is a type of pericyclic rearrangement where the net result is one pi bond being converted into one sigma bond or vice versa. These reactions are usually categorized by the following criteria: * Reac ...

s in

organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clay ...

, defined as "the preference for inward or outward rotation of

substituents A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as ''side ...

conrotatory An electrocyclic reaction can either be classified as conrotatory or disrotatory based on the rotation at each end of the molecule. In conrotatory mode, both atomic orbitals of the end groups turn in the same direction (such as both atomic orbital ...

disrotatory An electrocyclic reaction can either be classified as conrotatory or disrotatory based on the rotation at each end of the molecule. In conrotatory mode, both atomic orbitals of the end groups turn in the same direction (such as both atomic orbital ...

s." Torquoselectivity is not to be confused with the normal

diastereoselectivity In stereochemistry, diastereomers (sometimes called diastereoisomers) are a type of stereoisomer. Diastereomers are defined as non-mirror image, non-identical stereoisomers. Hence, they occur when two or more stereoisomers of a compound have dif ...

seen in

pericyclic reaction In organic chemistry, a pericyclic reaction is the type of organic reaction wherein the transition state of the molecule has a cyclic geometry, the reaction progresses in a concerted fashion, and the bond orbitals involved in the reaction overlap ...

s, as it represents a further level of selectivity beyond the Woodward-Hoffman rules. The name derives from the idea that the

substituent A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and '' functional group'', as well as '' ...

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 Kendall Newcomb Houk is a Distinguished Research Professor in Organic Chemistry at the University of California, Los Angeles. His research group studies organic, organometallic, and biological reactions using the tools of computational chemistry. ...

. For

ring closing reaction A cyclic compound (or ring compound) is a term for a compound in the field of chemistry in which one or more series of atoms in the compound is connected to form a ring. Rings may vary in size from three to many atoms, and include examples where ...

s, it is an example of enantioselectivity, wherein a single

enantiomer In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical ant ...

of a cyclization product is formed from the selective ring closure of the starting material. In a typical electrocyclic ring closing, selection for either

reactions modes still produces two enantiomers. Torquoselectivity is a discrimination ''between'' these possible enantiomers that requires

asymmetric induction In stereochemistry, asymmetric induction (also enantioinduction) 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 sub ...

Torquoselectivity is also used to describe selective electrocyclic ring openings, in which different directions of rotation produce distinct

structural isomers In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct bonds between them. The term met ...

. In these cases,

steric strain Van der Waals strain is strain resulting from Van der Waals repulsion when two substituents in a molecule approach each other with a distance less than the sum of their Van der Waals radii. Van der Waals strain is also called Van der Waals repul ...

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 group In chemistry, an electron-withdrawing group (EWG) is a substituent that has some of the following kinetic and thermodynamic implications: *with regards to electron transfer, electron-withdrawing groups enhance the oxidizing power tendency of th ...

Other mechanisms by which torquoselectivity can operate include

chiral Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from i ...

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

s, induction via neighboring

stereocenter In stereochemistry, a stereocenter of a molecule is an atom (center), axis or plane that is the focus of stereoisomerism; that is, when having at least three different groups bound to the stereocenter, interchanging any two different groups cr ...

s (in which case the torquoselectivity is a case of

), 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. Axialtotetrahedral transfer

References

{{Reflist Stereochemistry