File Name: list of electron withdrawing groups and donating groups .zip
Electrophilic Aromatic Substitution: Introduction.
- Activating and Deactivating Groups In Electrophilic Aromatic Substitution
- 2011, Vol.84, No.1
- Electrophilic aromatic directing groups
Activating and Deactivating Groups In Electrophilic Aromatic Substitution
E-mail: michael. The present review is devoted to summarizing the recent advances — in the field of metal-catalysed group-directed C—H functionalisation.
In order to clearly showcase the molecular diversity that can now be accessed by means of directed C—H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure.
The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C—H functionalisation chemistry to provide a comprehensive overview of the progress made in the last years and, even more so, all organic chemists who want to introduce the C—H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them.
Carlo Sambiagio. Patrick McGowan. After that he spent two year as a postdoc in the Maes group in Antwerp BE , mostly exploring aerobic oxidation chemistry. His interests generally include metal-catalysed transformations and the development of new methodologies for organic synthesis.
Joanna Wencel-Delord. After postdoctoral studies with Prof. Colobert University of Strasbourg, France. Her research focuses on the transition metal-catalysed asymmetric C—H activation.
Tatiana Besset. In , she joined the group of Prof. Reek Amsterdam University, Netherlands , as a postdoctoral fellow in collaboration with Eastman company. Her research program is focused on transition metal-catalysed C—H activation and the development of new strategies to access fluorinated molecules. Bert U. Anny Jutand, studying fundamental mechanisms in catalysis.
In he was appointed assistant professor docent in the Department of Chemistry at UAntwerpen. His research interests include heterocyclic chemistry, organometallic chemistry, homogeneous catalysis and sustainable chemistry. Peter Stanetty. During his PhD-studies, he was on a 4 month sabbatical in the group of Prof. He was then post doc with Prof.
Subsequently, he was promoted to Associate Professor for Organometallic Chemistry in , a position he still holds. DG-assisted C—H functionalisation started to gain momentum in the mids after a landmark contribution by the group of Murai. Since then, a huge number of papers on the topic have been published, expanding the scope of potential DGs and of chemical transformations which can be carried out.
In case of C—H functionalisation, the situation is much more complex due to the higher diversity in coupling partners which often react via distinctively different reaction mechanisms, and a large variety of specific additives which these reactions require often complex mixtures of reagents and additives are required for each transformation. To facilitate the understanding and the choice of a specific procedure, below are listed the different types of additives and their roles in C—H functionalisation reactions.
It is important to stress that, although the main classes of additives and roles are relatively clear, many compounds can have more than one distinct role in the reaction, therefore their choice and combinations is not straightforward. In these cases the Ag acts as a halide scavenger, and the counteranion usually OTf, NTf 2 , or SbF 6 promotes the in situ formation of cationic metal catalysts in solution.
When a suitable preformed cationic metal catalyst is used, the addition of such additives is generally not required. Their role is mostly related to their ability to deprotonate the desired C—H bond, often via a concerted metallation—deprotonation CMD pathway also termed ambiphilic metal ligand activation AMLA. These can be phosphines, carbenes, mono-protected aminoacids MPAA or other bidentate ligands. These can display different roles, including the activation of coupling partners e. It is worth noting that common bases used for cross coupling reactions, such as Cs 2 CO 3 , are instead not often encountered in C—H functionalisation, while Na, K, or Li counterparts are generally more effective Ag 2 CO 3 is often used as both oxidant and base.
The variety of additives and reaction conditions encountered in this field is also reflected by the range of metals which are applied as catalysts. There is no such thing as a single dominant metal species, but Pd, 48,49 Rh, 50,51 and Ru 52—54 share together the top spot.
Moreover, increasing examples using Ni, 55—58 Co, 55,59—64 Ir, 64—66 Cu, 67—69 Fe, 3,55,63,70,71 and Mn 72—74 are appearing in the literature. Naturally, such a manifold of metals with often distinctively different properties adds a lot of complexity to the field. This is of course an advantage for method development, since it is only logical that a larger pool of catalyst candidates allows also for a larger variety of possible transformations.
On the other hand, the complexity has reached a level where it is very difficult to keep track of the developments and to get an overview of what is actually possible in C—H functionalisations. What does this mean for a synthetic chemist planning a multistep synthesis of a molecule? The retrosynthetic disconnections might frequently lead to situations in which C—H functionalisation chemistry could be applied.
However, because of the diversity of the field and the lack of a general and easy to consult overview, its application is unfortunately not even considered in many cases. This is the shortcoming which the authors attempted to address by this review.
This should be very useful for taking this research field to the next step, the more frequent application in syntheses, also at late stages of prolonged synthetic sequences. Another important aspect of the review is the stress that has been placed on the DG removal. The removal of DGs is an important drawback in C—H functionalisation chemistry, as the DG might not be required after the directed step, and its removal is therefore necessary which is an extra synthetic step. Several DGs are actually not removable depending on the substrate as well which is no problem when they anyway constitute a precursor part of the target molecule, while others are easily removed or modified.
We tried to incorporate all the possible protocols for the removal or modification of the DGs in each section, so that the reader will be immediately aware of what can currently be done with the DG in use.
This information is often not reported in reviews dealing with C—H functionalisation. The removal of the DG has also been listed as a specific advantage in each procedure, in case this is specifically investigated by the authors.
So not being mentioned does not mean it could not be done, it is just not reported in the original publication. It has to be mentioned that sometimes different publications use different terms for the very same transformation.
For the sake of simplicity, in this review each transformation has the same name in all entries. For example, the coupling of olefins with arenes to give alkylated arenes is either classified as alkylation or hydroarylation in literature.
It was decided for this review to look at the reaction product and see what happened to the part of the molecule which carried the DG. Hence, in case the arene carried the DG, the reaction is always classified as an alkylation reaction.
This review aims at being comprehensive in the regard that all DGs and potential transformations with the DG are listed, however, reporting every single variant of a given transformation would just be beyond the scope of this review.
In such cases a selection had to be made by critically looking at the reported protocols and choosing representative examples to give a clear flavour on chemical diversity searched for by the synthetic chemist.
It is clear that such a selection will always be biased and readers might find that one or the other example should have been selected differently. The speed in which new contributions are brought forward in the field is amazing. This shows the high relevance of this area of research and the potential of the field.
For writers of a review it brings certain problems as well, most importantly which time frame to cover and which examples to include? As a starting point, we used the very comprehensive review by Zhanxiang Liu and Yuhong Zhang published in early , which covers literature until the end of This is certainly due to the manifold of opportunities of further manipulation a ketone allows, and also the ease of introduction via various methods, e.
Friedel—Crafts acylation. From the very beginning, Ru proved to be especially well suited for ketone-directed C—H activation reactions followed by Rh, with other metals playing only a minor role. Again using boronic esters as aryl sources, Lu and Sun developed a Rh catalysed protocol for arylating alkyl—aryl ketones exclusively at the C sp 2 —H bond. In these cases, the more electron-rich ketone had to be used in excess.
This is a significant improvement to traditional routes towards the same type of products. An example which can be counted to arylation reactions as well is the synthesis of fullerene-fused tetralones via a Pd catalysed protocol.
Ramana investigated the C3-arylation of 2-arylbenzofurans using either arylboronic acids or the corresponding potassium trifluoroborates as the aryl source. Recently, an Fe catalysed alkylation protocol was published by Kakiuchi and co-workers Scheme 2D. Also cyclic alkyl—aryl ketones e. Diaryl ketones did not react as efficiently and required higher catalyst loading and trialkylation was observed as the major product.
The proposed mechanism followed the initial suggestion by Murai. One rare example of ortho methylation was recently disclosed by Ilies and Nakamura Scheme 2E. As usual in most of such alkylation reactions, mixtures of mono- and di-alkylated products were obtained if permitted by the substrate structure. In the same paper also carboxylic acids were used as DGs see Section 4. All examples so far gave exclusively the synthesis of linear products.
However, Ramana and co-workers showed that tuning the catalytic system allowed to switch between linear and branched products in the C3-alkylation of 2-aroylbenzofurans with acrylates Scheme 3A. The transition state towards the branched product turned out to be significantly lower in energy, which goes in line with the experimental observations.
Furthermore, the method was extended to the C2-alkylation of 3-aryloxybenzofurans. An interesting example has been reported by Wan and Li.
Key was the unprecedented presence of an oxidizing C—N bond in the substrates which allowed this overall redox-neutral process. Detailed mechanistic studies were carried out, including DFT calculations. Bakthadoss et al. Again on tetraphenylenes, Zhang also reported alkenylation reactions Scheme 4C.
Besides acrylates, also alkenyl sulfoxides and -phosphates as well as styrene derivatives could be used. Szostak's group showed the ketone-directed hydroarylation of internal alkynes leading to alkenylated arenes Scheme 4D.
In this contribution, the DG showed significant variation as opposed to many contributions which relied primarily on the acetyl group. Further elaboration of the obtained alkenylated products to a series of useful compounds was also demonstrated. Prabhu and co-workers showed that a ketone DG in position 3 of indoles leads to alkenylated products either at the C2- or C4-position. An ester or an aldehyde as DG also gave C4 alkenylation.
2011, Vol.84, No.1
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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. An electron withdrawing group EWG draws electrons away from a reaction center. When this center is an electron rich carbanion or an alkoxide anion, the presence of the electron-withdrawing substituent has a stabilizing effect. An electron releasing group ERG or electron donating groups EDGs releases electrons into a reaction center and as such stabilizes electron deficient carbocations.
Indeed, it depends greatly on the conditions of the reaction. For example, the chlorination of File Size: KB. Eastman, Victoria H. Description: Poly arylene-ethynylene -alt-poly arylene-vinylene PAE-PAV oligomers were synthesized with electron withdrawing end groups in order to determine the effects on their relative fluorescence quantum yields and absorbance. The effect of electron donating and withdrawing groups on the morphology and optical properties of Alq3. The arenium ion arising from meta attack has no such highly unstable resonance stucture. By the usual reasoning we would also expect the transition state leading to the meta substituted arenium ion to be the least unstable and, therefore, that meta attack would be favored.
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Electrophilic aromatic directing groups
A substituent on a benzene ring can effect the placement of additional substituents on that ring during Electrophilic Aromatic Substitution. How do we know where an additonal substituent will most likely be placed? The answer to this is through inductive and resonance effects. Inductive effects are directly correlated with electronegativity. Substituents can either be meta directing or ortho-para directing.
E-mail: michael. The present review is devoted to summarizing the recent advances — in the field of metal-catalysed group-directed C—H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C—H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure.
China E-mail: cechwei scut. Regulating the synthesis of photocatalytic materials at the molecular level could affect the absorption of light and guide the synthesis of highly efficient photocatalysts for the photocatalytic degradation organic pollutants.
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