立体効果

WordNet

act so as to bring into existence; "effect a change"
a symptom caused by an illness or a drug; "the effects of sleep loss"; "the effect of the anesthetic"
(of a law) having legal validity; "the law is still in effect" (同)force
the central meaning or theme of a speech or literary work (同)essence, burden, core, gist
an impression (especially one that is artificial or contrived); "he just did it for effect"
produce; "The scientists set up a shock wave" (同)effectuate, set_up
property of a personal character that is portable but not used in business; "she left some of her personal effects in the house"; "I watched over their effects until they returned" (同)personal_effects

PrepTutorEJDIC

〈C〉〈U〉『結果』 / 〈C〉〈U〉(…への)『効果』,効きめ,影響《+『on』(『upon』)+『名』》 / 〈C〉(色・音・形などの)印象,感銘 / 《複数形で》品物;身の回り品;動産,財産 / (結果として)…‘を'『もたらす』;〈目的など〉‘を'果たす,遂行する

Wikipedia preview

出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2016/02/28 21:26:10」(JST)

wiki en

[Wiki en表示]

See also: intramolecular forces

The steric effect of tri-(tert-butyl)amine makes electrophilic reactions, like forming the tetraalkylammonium cation, difficult. It is difficult for electrophiles to get close enough to allow attack by the lone pair of the nitrogen (nitrogen is shown in blue)

Steric effects arise from a fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds (Pauli or Born repulsion), and this may affect the molecule's preferred shape (conformation) and reactivity.

Types

Steric hindrance

Steric hindrance occurs when the large size of groups within a molecule prevents chemical reactions that are observed in related molecules with smaller groups. Although steric hindrance is sometimes a problem (it prevents S_N2 reactions with tertiary substrates from taking place), it can also be a very useful tool, and is often exploited by chemists to change the reactivity pattern of a molecule by stopping unwanted side-reactions (steric protection) or by leading to a preference for one stereochemical reaction course as in diastereoselectivity. Steric hindrance between adjacent groups can also restrict torsional bond angles. However, hyperconjugation has been suggested as an explanation for the preference of the staggered conformation of ethane because the steric hindrance of the small hydrogen atom is far too small.^[1]^[2] This is the effect responsible for the observed shape of rotaxanes.

Regioselective dimethoxytritylation of the primary 5'-hydroxyl group of thymidine in the presence of a free secondary 3'-hydroxy group as a result of steric hindrance due to the dimethoxytrityl group and the ribose ring (Py = pyridine).^[3]

When a Lewis acid and Lewis base cannot combine due to steric hindrance, they are said to form a frustrated Lewis pair.^[4]

Other steric effects

Steric shielding occurs when a charged group on a molecule is seemingly weakened or spatially shielded by less charged (or oppositely charged) atoms, including counterions in solution (Debye shielding). In some cases, for an atom to interact with sterically shielded atoms, it would have to approach from a vicinity where there is less shielding, thus controlling where and from what direction a molecular interaction can take place.

Steric attraction occurs when molecules have shapes or geometries that are optimized for interaction with one another. In these cases molecules will react with each other most often in specific arrangements.

Chain crossing: A chain, ring, or a set of rings cannot change from one conformation to another if it would require a chain (or ring - a ring is a cyclic chain) to pass through itself or another chain. This effect, or lack of, is responsible for the shape of catenanes and molecular knots.

Steric repulsions between different parts of a molecular system were found to be of key importance in governing the direction of transition-metal-mediated transformations and catalysis. Steric effects can even induce a mechanism switch in the catalytic reaction.^[5] Steric repulsion is also largely responsible for the stabilizing of colloids by coating the surface with a polymer, and can also cause bond length shortening, and compressional frequency enhancement in the IR spectrum.^[6]

Steric inhibition of resonance is present only in benzene rings. The presence of any group at the ortho position in benzoic acid will throw the carboxylic acid group out of the plane, and thus its mesomeric connection with the benzene ring vanishes. This means that ortho-substituted benzoic acids are stronger than meta- and para-substituted benzoic acids.

Steric inhibition of protonation

Consider 2,2,6,6-Tetramethylpiperidine and in,in-diphosphine.^[7]

Distinction from electronic effects

The structure, properties, and reactivity of a molecule is dependent on straightforward bonding interactions including covalent bonds, ionic bonds, hydrogen bonds and lesser forms of bonding. This bonding supplies a basic molecular skeleton that is modified by repulsive forces. These repulsive forces include the steric interactions described above. Basic bonding and steric are at times insufficient to explain many structures, properties, and reactivity. Thus steric effects are often contrasted and complemented by electronic effects implying the influence of effects such as induction, conjunction, orbital symmetry, electrostatic interactions, and spin state. There are more esoteric electronic effects but these are among the most important when considering structure and chemical reactivity.

A special computational procedure was developed to separate electronic and steric effects of an arbitrary group in the molecule and to reveal their influence on structure and reactivity.^[8]

Significance

Understanding steric effects is critical to chemistry, biochemistry and pharmacology. In organic chemistry, steric effects are nearly universal and affect the rates and activation energies of most chemical reactions to varying degrees. Steric effects often dictate reaction pathways in organic synthesis because there are fewer configurations in which molecules can collide and successfully react. In surface chemistry, steric effects manifest in an entropic contribution to the overall Gibbs energy. Hence, colloidal stability can be obtained by steric shielding from polymers, often made possible via block co-polymer adsorption or grafting polymer onto charged nanoparticles. This spatial shielding effect is large-scale in comparison to Debye shielding due to the size of the attached polymer strands. In biochemistry, steric effects are often exploited in naturally occurring molecules such as enzymes, where the catalytic site may be buried within a large protein structure. In pharmacology, steric effects determine how and at what rate a drug will interact with its target bio-molecules.

Note the organic chemical example of Stereochemistry of ketonization of enols and enolates where diastereoselectivity results from steric hindrance.

References

^ Pophristic, Vojislava; Goodman, Lionel (2001). "Hyperconjugation not steric repulsion leads to the staggered structure of ethane". Nature 411 (6837): 565–8. doi:10.1038/35079036. PMID 11385566.
^ Weinhold, Frank (2001). "Chemistry. A new twist on molecular shape". Nature 411 (6837): 539–41. doi:10.1038/35079225. PMID 11385553.
^ Gait, Michael (1984). Oligonucleotide synthesis: a practical approach. Oxford: IRL Press. ISBN 0-904147-74-6.
^ Stephan, Douglas W. "Frustrated Lewis pairs": a concept for new reactivity and catalysis. Org. Biomol. Chem. 2008, 6, 1535-1539. doi:10.1039/b802575b
^ Ananikov, Valentin P.; Szilagyi, Robert; Morokuma, Keiji; Musaev, Djamaladdin G. (2005). "Can Steric Effects Induce the Mechanism Switch in the Rhodium-Catalyzed Imine Boration Reaction? A Density Functional and ONIOM Study". Organometallics 24 (8): 1938. doi:10.1021/om049156o.
^ Zong, J., J. T. Mague, and R. A. Pascal, Jr., Exceptional Steric Congestion in an in,in-Bis(hydrosilane), J. Am. Chem. Soc. 2013, 135, 13235-13237.http://DOI: 10.1021/ja407398w
^ Zong, J., Mague, J. T., Kraml, C. M., & Pascal Jr, R. A. (2013). "A Congested in, in-Diphosphine." Organic letters, 15(9), 2179-2181. DOI: 10.1021/ol400728m
^ Ananikov, Valentine P.; Musaev, Djamaladdin G.; Morokuma, Keiji (2007). "Critical Effect of Phosphane Ligands on the Mechanism of Carbon–Carbon Bond Formation Involving Palladium(II) Complexes: A Theoretical Investigation of Reductive Elimination from Square-Planar and T-Shaped Species". European Journal of Inorganic Chemistry 2007 (34): 5390. doi:10.1002/ejic.200700850.

External links

www.chem.swin.edu.au/modules/mod4/qsarwebp4.html
Steric A Program to Calculate the Steric Size of Molecules

UpToDate Contents

全文を閲覧するには購読必要です。 To read the full text you will need to subscribe.

1. 不安定ヘモグロビン変異体 unstable hemoglobin variants
2. 高プロラクチン血症の臨床症状および評価 clinical manifestations and evaluation of hyperprolactinemia
3. 早期興奮症候群における副伝導路の解剖学、病態生理、および局在 anatomy pathophysiology and localization of accessory pathways in the preexcitation syndrome
4. 心不全患者における利尿剤の使用 use of diuretics in patients with heart failure
5. 心不全の予後 prognosis of heart failure

English Journal

Porphyrinic metal-organic framework as electrochemical probe for DNA sensing via triple-helix molecular switch.

Ling P1, Lei J2, Ju H1.
Biosensors & bioelectronics.Biosens Bioelectron.2015 Sep 15;71:373-9. doi: 10.1016/j.bios.2015.04.046. Epub 2015 Apr 20.
An electrochemical DNA sensor was developed based on the electrocatalysis of porphyrinic metal-organic framework (MOF) and triple-helix molecular switch for signal transduction. The streptavidin functionalized zirconium-porphyrin MOF (PCN-222@SA) was prepared as signal nanoprobe via covalent method
PMID 25950931

Investigation of the pellets produced from sugarcane bagasse during liquid hot water pretreatment and their impact on the enzymatic hydrolysis.

Wang W1, Zhuang X1, Yuan Z2, Yu Q1, Qi W1.
Bioresource technology.Bioresour Technol.2015 Aug;190:7-12. doi: 10.1016/j.biortech.2015.04.059. Epub 2015 Apr 21.
In the process of liquid hot water (LHW) pretreatment, there are numbers of pellets formed on the lignocellulosic surface. The characteristics and effect of pellets on the enzymatic hydrolysis of LHW-treated sugarcane bagasse (SCB) were investigated. After SCB was treated with LHW at 180°C, the pel
PMID 25916262

Rheological behavior of cellulose nanowhisker suspension under magnetic field.

Kim DH1, Song YS2.
Carbohydrate polymers.Carbohydr Polym.2015 Aug 1;126:240-7. doi: 10.1016/j.carbpol.2015.03.026. Epub 2015 Mar 20.
We investigated the influence of a magnetic field on the rheology of cellulose nanowhisker (CNW) suspension. The morphology of CNWs was analyzed by using polarized optical microscopy (POM) and transmission electron microscopy (TEM). The findings show that the application of the magnetic field leads
PMID 25933545