Captodative effect

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Resonance contributors of the 2-(dimethylamino)propanenitrile free radical, adapted from Anslyn Resonance contributors of 2-(dimethylamino)propanenitrile radical.jpg
Resonance contributors of the 2-(dimethylamino)propanenitrile free radical, adapted from Anslyn

The captodative effect is the stabilization of radicals by a synergistic effect of an electron-withdrawing substituent and an electron-donating substituent. [2] [3] The name originates as the electron-withdrawing group (EWG) is sometimes called the "captor" group, whilst the electron-donating group (EDG) is the "dative" substituent. [3] Olefins with this substituent pattern are sometime described as captodative. [2] Radical reactions play an integral role in several chemical reactions and are also important to the field of polymer science. [4]

Contents

When EDGs and EWGs are near the radical center, the stability of the radical center increases. [1] The substituents can kinetically stabilize radical centers by preventing molecules and other radical centers from reacting with the center. [3] The substituents thermodynamically stabilize the center by delocalizing the radical ion via resonance. [1] [3] These stabilization mechanisms lead to an enhanced rate for free-radical reactions. [5] In the figure at right, the radical is delocalized between the captor nitrile (-CN), and the dative secondary amine (-N(CH3)2), thus stabilizing the radical center. [3]

Substituent effect on reaction rates

Certain substituents are better at stabilizing radical centers than others. [6] This is influenced by the substituent's ability to delocalize the radical ion in the transition state structure. [3] Delocalizing the radical ion stabilizes the transition state structure. As a result, the energy of activation decreases, enhancing the rate of the overall reaction. According to the captodative effect, the rate of a reaction is the greatest when both the EDG and EWG are able to delocalize the radical ion in the transition state structure. [7]

Ito and co-workers observed the rate of addition reactions of arylthiyl radical to disubstituted olefins. [6] The olefins contained an EWG nitrile group and varying EDGs and the effect of varying EDGs on the rate of the addition reactions was observed. The process studied was:

Captodative 2017.svg

The rate of the addition reaction was accelerated by the following EDGs in increasing order: H < CH3 < OCH2CH3. When R = OCH2CH3, the rate of the reaction is the fastest because the reaction has the smallest energy of activation (ΔG‡). The ethoxy and cyano groups are able to delocalize the radical ion in the transition state, thus stabilizing the radical center. The rate enhancement is due to the captodative effect. When R = H, the reaction has the largest energy of activation because the radical center is not stabilized by the captodative effect. The hydrogen atom is not able to delocalize the radical ion. Thus, the reaction is slow relative to the R = OCH2CH3 case. When R = CH3, the rate of the reaction is faster relative to when R = H because methyl groups have more electron donating capability. [6] However, the reaction rate is slower relative to when R = OCH2CH3 because the radical ion is not delocalized over the methyl group . Thus, the captodative does not influence the reaction rate if the radical ion is not delocalized onto both the EWG and EDG substituents. Each of these cases is illustrated below:

Thiophenoxide acrylonitrile transition states.svg

Uses in synthesis

The term "captodative ethylenes" has been used in the context of cycloaddition reactions involving captodative radical intermediates – for example, the thermal [2+2] head-to-head dimerization of 2-methylthioacrylonitrile occurs readily at room temperature; formation of the equivalent cyclobutane derivative of acrylonitrile is "sluggish". [8] Intramolecular [2+2] cyclizations have also been reported to be enhanced by captodative effects, [8] as shown below:

2plus2 cyclodimerisation and intramolecular cyclisation of captodative olefins.svg

Similar effects have been discussed for other cycloadditions such as [3+2], [4+2], and [3+4] for captodative ethylenes. [9] Effects have also been reported in cases like Diels-Alder and Friedel-Crafts reactions in cases where nucleophilic olefins react inefficiently, attributed to the transition state being close to a biradical and thus stabilized. [8] [10] These studies have revealed a direct dependence on Δω, difference in electrophilicity, and the polar nature of the reaction. They have been used because of their highly reactive, stereoselective, regioselective nature within these reactions. [9] [11]

Comparison between Diels-Alder and captodative-enhanced Friedel-Crafts reaction.svg

Captodative olefins in reactions also show interfering effects with the typical kinetic isotope effect, allowing atypical reactions to occur with isotope-labeled molecules [12] and demonstrating that the mechanisms and transition states of these reactions have been influenced.

Polymer science application

Free-radical polymerization, where radicals are the chain carriers in the propagation of the process, accounted for 40 billion of the 110 billion pounds of polymers produced in the United States in 2001. [13] Captodative olefins have a specific advantage of being responsive to solvent effects without the effect of destabilizing the radical. [4] They have also shown to undergo their radical transformation spontaneously which allows them to be useful in polymerization mechanism elucidation and better understood through NMR Studies. Furthermore, captodative ethanes are initiators with unique properties giving higher molecular weight distribution and forming block copolymers through the known radical mechanisms. The polymers obtained from captodatively substituted starting materials exhibit "desirable" properties such as optical activity, differences in polarity, solvent affinity, thermal and mechanical stabilities.

Substituents on the monomer can affect solvent affinities Solvent Affinity effects of substituents.jpg
Substituents on the monomer can affect solvent affinities
How a captodative monomer can form a polar polymer Polar effect.jpg
How a captodative monomer can form a polar polymer
  1. Polymers with polar substituents are known to have interesting applications including within electrical and optical materials.
  2. These polymers are typically transparent.
  3. The Tdi (initial decomposition) of these polymers are relatively low compared to their analogues, but have relatively higher Tdm (maximum rate of weight change temperatures). Meaning although they will start to melt quicker, they will take longer to fully change phases.
  4. Polymers with large captodative stabilizations starting materials can quickly “unzip” to their starting monomer upon heating.
  5. Bifunctional polymers, with two different functional groups at every monomer unit, are commonly formed from the captodative monomers.
    1. Dative groups substantially alter the solubility through Hydrogen bonding in specific bifunctional polymers( see figure above). However no clear correlation has been developed at this time, since not all combinations of substituents and solubilities have been investigated.
  6. Captodative polymer is highly functional in chelates with certain metals. [4]

Related Research Articles

Aromatic compounds are those chemical compounds that contain one or more rings with pi electrons delocalized all the way around them. In contrast to compounds that exhibit aromaticity, aliphatic compounds lack this delocalization. The term "aromatic" was assigned before the physical mechanism determining aromaticity was discovered, and referred simply to the fact that many such compounds have a sweet or pleasant odour; however, not all aromatic compounds have a sweet odour, and not all compounds with a sweet odour are aromatic. Aromatic hydrocarbons, or arenes, are aromatic organic compounds containing solely carbon and hydrogen atoms. The configuration of six carbon atoms in aromatic compounds is called a "benzene ring", after the simple aromatic compound benzene, or a phenyl group when part of a larger compound.

In organic chemistry, the Diels–Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene derivative. It is the prototypical example of a pericyclic reaction with a concerted mechanism. More specifically, it is classified as a thermally-allowed [4+2] cycloaddition with Woodward–Hoffmann symbol [π4s + π2s]. It was first described by Otto Diels and Kurt Alder in 1928. For the discovery of this reaction, they were awarded the Nobel Prize in Chemistry in 1950. Through the simultaneous construction of two new carbon–carbon bonds, the Diels–Alder reaction provides a reliable way to form six-membered rings with good control over the regio- and stereochemical outcomes. Consequently, it has served as a powerful and widely applied tool for the introduction of chemical complexity in the synthesis of natural products and new materials. The underlying concept has also been applied to π-systems involving heteroatoms, such as carbonyls and imines, which furnish the corresponding heterocycles; this variant is known as the hetero-Diels–Alder reaction. The reaction has also been generalized to other ring sizes, although none of these generalizations have matched the formation of six-membered rings in terms of scope or versatility. Because of the negative values of ΔH° and ΔS° for a typical Diels–Alder reaction, the microscopic reverse of a Diels–Alder reaction becomes favorable at high temperatures, although this is of synthetic importance for only a limited range of Diels-Alder adducts, generally with some special structural features; this reverse reaction is known as the retro-Diels–Alder reaction.

Carbocation

A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium CH+
3, methanium CH+
5 and vinyl C
2H+
3 cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountered.

In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carries a partial positive charge, or have an atom that does not have an octet of electrons.

A carbanion is an anion in which carbon is trivalent and bears a formal negative charge.

In an electrophilic aromatic substitution reaction, existing substituent groups on the aromatic ring influence the overall reaction rate or have a directing effect on positional isomer of the products that are formed. An electron donating group (EDG) or electron releasing group is an atom or functional group that donates some of its electron density into a conjugated π system via resonance (mesomerism) or inductive effects —called +M or +I effects, respectively—thus making the π system more nucleophilic. As a result of these electronic effects, an aromatic ring to which such a group is attached is more likely to participate in electrophilic substitution reaction. EDGs are therefore often known as activating groups, though steric effects can interfere with the reaction.

In chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is R-(C:)-R' or R=C: where the R represent substituents or hydrogen atoms.

The 1,3-dipolar cycloaddition is a chemical reaction between a 1,3-dipole and a dipolarophile to form a five-membered ring. The earliest 1,3-dipolar cycloadditions were described in the late 19th century to the early 20th century, following the discovery of 1,3-dipoles. Mechanistic investigation and synthetic application were established in the 1960s, primarily through the work of Rolf Huisgen. Hence, the reaction is sometimes referred to as the Huisgen cycloaddition. 1,3-dipolar cycloaddition is an important route to the regio- and stereoselective synthesis of five-membered heterocycles and their ring-opened acyclic derivatives. The dipolarophile is typically an alkene or alkyne, but can be other pi systems. When the dipolarophile is an alkyne, aromatic rings are generally produced.

Arynes or benzynes are highly reactive species derived from an aromatic ring by removal of two substituents. The most common arynes are ortho but meta- and para-arynes are also known. o-Arynes are examples of strained alkynes.

A transition metal carbene complex is an organometallic compound featuring a divalent organic ligand. The divalent organic ligand coordinated to the metal center is called a carbene. Carbene complexes for almost all transition metals have been reported. Many methods for synthesizing them and reactions utilizing them have been reported. The term carbene ligand is a formalism since many are not derived from carbenes and almost none exhibit the reactivity characteristic of carbenes. Described often as M=CR2, they represent a class of organic ligands intermediate between alkyls (−CR3) and carbynes (≡CR). They feature in some catalytic reactions, especially alkene metathesis, and are of value in the preparation of some fine chemicals.

Azomethine ylide

Azomethine ylides are nitrogen-based 1,3-dipoles, consisting of an iminium ion next to a carbanion. They are used in 1,3-dipolar cycloaddition reactions to form five-membered heterocycles, including pyrrolidines and pyrrolines. These reactions are highly stereo- and regioselective, and have the potential to form four new contiguous stereocenters. Azomethine ylides thus have high utility in total synthesis, and formation of chiral ligands and pharmaceuticals. Azomethine ylides can be generated from many sources, including aziridines, imines, and iminiums. They are often generated in situ, and immediately reacted with dipolarophiles.

Neighbouring group participation (NGP) in organic chemistry has been defined by IUPAC as the interaction of a reaction centre with a lone pair of electrons in an atom or the electrons present in a sigma bond or pi bond contained within the parent molecule but not conjugated with the reaction centre. When NGP is in operation it is normal for the reaction rate to be increased. It is also possible for the stereochemistry of the reaction to be abnormal when compared with a normal reaction. While it is possible for neighbouring groups to influence many reactions in organic chemistry this page is limited to neighbouring group effects seen with carbocations and SN2 reactions.

Homoaromaticity Special case of aromaticity in which conjugation is interrupted by a single sp³ hybridized carbon atom

Homoaromaticity, in organic chemistry, refers to a special case of aromaticity in which conjugation is interrupted by a single sp3 hybridized carbon atom. Although this sp3 center disrupts the continuous overlap of p-orbitals, traditionally thought to be a requirement for aromaticity, considerable thermodynamic stability and many of the spectroscopic, magnetic, and chemical properties associated with aromatic compounds are still observed for such compounds. This formal discontinuity is apparently bridged by p-orbital overlap, maintaining a contiguous cycle of π electrons that is responsible for this preserved chemical stability.

The polar effect or electronic effect in chemistry is the effect exerted by a substituent on modifying electrostatic forces operating on a nearby reaction center. The main contributors to the polar effect are the inductive effect, mesomeric effect and the through-space electronic field effect.

The Hammett equation in organic chemistry describes a linear free-energy relationship relating reaction rates and equilibrium constants for many reactions involving benzoic acid derivatives with meta- and para-substituents to each other with just two parameters: a substituent constant and a reaction constant. This equation was developed and published by Louis Plack Hammett in 1937 as a follow-up to qualitative observations in a 1935 publication.

Physical organic chemistry, a term coined by Louis Hammett in 1940, refers to a discipline of organic chemistry that focuses on the relationship between chemical structures and reactivity, in particular, applying experimental tools of physical chemistry to the study of organic molecules. Specific focal points of study include the rates of organic reactions, the relative chemical stabilities of the starting materials, reactive intermediates, transition states, and products of chemical reactions, and non-covalent aspects of solvation and molecular interactions that influence chemical reactivity. Such studies provide theoretical and practical frameworks to understand how changes in structure in solution or solid-state contexts impact reaction mechanism and rate for each organic reaction of interest.

Oxocarbenium

An oxocarbeniumion is a chemical species characterized by a central sp2-hybridized carbon, an oxygen substituent, and an overall positive charge that is delocalized between the central carbon and oxygen atoms. A oxocarbenium ion is represented by two limiting resonance structures, one in the form of a carbenium ion with the positive charge on carbon and the other in the form of an oxonium species with the formal charge on oxygen. As a resonance hybrid, the true structure falls between the two. Compared to neutral carbonyl compounds like ketones or esters, the carbenium ion form is a larger contributor to the structure. They are common reactive intermediates in the hydrolysis of glycosidic bonds, and are a commonly used strategy for chemical glycosylation. These ions have since been proposed as reactive intermediates in a wide range of chemical transformations, and have been utilized in the total synthesis of several natural products. In addition, they commonly appear in mechanisms of enzyme-catalyzed biosynthesis and hydrolysis of carbohydrates in nature. Anthocyanins are natural flavylium dyes, which are stabilized oxocarbenium compounds. Anthocyanins are responsible for the colors of a wide variety of common flowers such as pansies and edible plants such as eggplant and blueberry.

Vinyl cation

The vinyl cation is a carbocation with the positive charge on an alkene carbon. Its empirical formula is C
2H+
3. More generally, a vinylic cation is any disubstituted, trivalent carbon, where the carbon bearing the positive charge is part of a double bond and is sp hybridized. In the chemical literature, substituted vinylic cations are often referred to as vinyl cations, and understood to refer to the broad class rather than the C
2H+
3 variant alone. The vinyl cation is one of the main types of reactive intermediates involving a non-tetrahedrally coordinated carbon atom, and is necessary to explain a wide variety of observed reactivity trends. Vinyl cations are observed as reactive intermediates in solvolysis reactions, as well during electrophilic addition to alkynes, for example, through protonation of an alkyne by a strong acid. As expected from its sp hybridization, the vinyl cation prefers a linear geometry. Compounds related to the vinyl cation include allylic carbocations and benzylic carbocations, as well as aryl carbocations.

Boranylium ions

In chemistry, a boranylium ion is an inorganic cation with the chemical formula BR+
2, where R represents a non-specific substituent. Being electron-deficient, boranylium ions form adducts with Lewis bases. Boranylium ions have historical names that depend on the number of coordinated ligands:

Stereoelectronic effect

In chemistry, primarily organic and computational chemistry, a stereoelectronic effect is an effect on molecular geometry, reactivity, or physical properties due to spatial relationships in the molecules' electronic structure, in particular the interaction between atomic and/or molecular orbitals. Phrased differently, stereoelectronic effects can also be defined as the geometric constraints placed on the ground and/or transition states of molecules that arise from considerations of orbital overlap. Thus, a stereoelectronic effect explains a particular molecular property or reactivity by invoking stabilizing or destabilizing interactions that depend on the relative orientations of electrons in space.

References

  1. 1 2 3 Anslyn, E. V.; Dougherty, D. A. (2006). Modern Physical Organic Chemistry (Dodr. ed.). Sausalito, CA: University Science Books. ISBN   9781891389313.
  2. 1 2 Concise Dictionary of Chemistry. V&S Publishers. 2012. p. 51. ISBN   9789381588628.
  3. 1 2 3 4 5 6 Viehe, H. G.; Janousek, Z.; Merényi, R.; Stella, L. (1985). "The Captodative Effect". Accounts of Chemical Research . 18 (5): 148–154. doi:10.1021/ar00113a004.
  4. 1 2 3 Tanaka, H. (2003). "Captodative Modification in Polymer Science". Progress in Polymer Science . 28 (7): 1171–1203. doi:10.1016/S0079-6700(03)00013-3.
  5. ↑ Sustmann, R.; Korth, H.-G. (1990). Advances in Physical Organic Chemistry. San Diego, CA: Academic Press. pp. 131–172. ISBN   0120335263.
  6. 1 2 3 Ito, Osamu; Arito, Y.; Matsuda, M. (1988). "Captodative Effects on Rate of Addition Reactions of Arylthiyl Radical to Disubstituted Olefins". Journal of the Chemical Society, Perkin Transactions 2 (6): 869–873. doi:10.1039/P29880000869.
  7. ↑ Creary, X.; Mehrisheikh-Mohammadi, M. E. (1985). "Captodative Rate Enhancement in the Methylenecyclopropane Rearrangement". Journal of Organic Chemistry . 51 (14): 2664–2668. doi:10.1021/jo00364a009.
  8. 1 2 3 Stella, L. (1986). "Captodative Substituent Effects in Cycloaddition Reactions". In Viehe, H. G.; Janousek, Z.; Merényi, R. (eds.). Substituent Effects in Radical Chemistry. Springer. pp. 361–370. ISBN   9789027723406.
  9. 1 2 Herrera, R.; Jimenez-Vazquez, H. A.; Delgado, F.; Soderberg, B. C. G.; Tamariz, J. (2005). "1-Acetyvinyl Acrylates: New Captodative Olefins Bearing and Internal Probe for the Evaluation of the Relative Reactivity of Captodative against Electron-Deficient Double Bond in Diels-Alders and Friedel-Crafts Reaction". Journal of the Brazilian Chemical Society . 16 (3A): 456–466. doi: 10.1590/S0103-50532005000300021 .
  10. ↑ Stella, L.; Boucher, J.-L. (1982). "Capto-dative Substituent Effects. 121 - New Ketene Equivalents for Diels-Alder Cycloadditions". Tetrahedron Letters . 22 (9): 953–956. doi:10.1016/S0040-4039(00)86992-0.
  11. ↑ Domingo, L.; Chamorro, E.; Pérez, P. (2008). "Understanding the Reactivity of Captodative Ethylenes in Polar Cycloaddition Reactions. A Theoretical Study". Journal of Organic Chemistry . 73 (12): 4615–4624. doi:10.1021/jo800572a. hdl: 10533/139635 . PMID   18484771.
  12. ↑ Wood, M.; Bissiriou, S.; Lowe, C.; Windeatt, K. M. (2013). "Synthetic Use of the Primary Kinetic Isotope Effect in Hydrogen Atom Transfer 2: Generation of Captodatively Stabilised Radicals". Organic and Biomolecular Chemistry . 11 (16): 2712–23. doi:10.1039/C3OB40275D. PMID   23479029.
  13. ↑ Odian, G. (2004). Principles of Polymerization (4th ed.). New York: Wiley-Interscience. ISBN   9780471274001.
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