All posts by David Palomas

I’m David, I’m from Spain and like many other chemists I’ve been wandering for a while. I’ve been a NO-MAD for the last 12 years since I left my home town in Spain , I have lived in 7 different cities from 3 different countries and for the moment I’m settled in London.

2019 Celebrates Chemistry during the International Year of the Periodic Table

Since the first of January 2019 we are officially in the International Year of the Periodic Table, another year for the celebration of science and especially a year for the celebration of chemistry. The proposal initially sponsored by UNESCO and the Russian Federation was finally approved on 20 December 2017 when the UN General Assembly proclaimed 2019 the International Year of the Periodic Table of Chemical Elements.

Why 2019? What is special about 2019?

imageIn 1869, Dmitri Mendeleev published his book Principles of Chemistry where he describes his theory of the periodic table. Mendeleev is considered one of the parents of the modern chemistry, 2019 is the 150 anniversary of the publication of his seminal work and is also the centenary of the International Union of Pure and Applied Chemistry (IUPAC), the recognised world authority in developing standards for the naming of the chemical elements and compounds.

Why do we celebrate the International Year of the Periodic Table?

First to recognise Dmitri Mendeleev and his work. In the very own words of UNESCO, The periodic table of chemical elements:

• is one of the most significant achievements in science,
• captures the essence of not only of chemistry but also of physics and biology among other sciences
• is a uniting scientific concept, which promotes international cooperation in the basic sciences and catalyses scientific breakthrough and excellence.

Mendeleev should have won the Nobel. He was nominated in 1905 and 1906 but didn’t win. Mendeleev died in 1907 and the Nobel Prize can only be awarded to alive scientists.

The second main reason for celebrating the International Year of the Periodic Table is promoting the role and benefits of science in our society. Despite everything we owe to chemistry, this science has always had a bad press and part of the problem is a lack of information. Among other objectives, UNESCO’s proposal aims at:

• enhancing the understanding and appreciation of periodic law and chemistry in general among the public;
• promoting the role of chemistry in contributing to solutions to many global problems, such as climate change and the preservation of natural resources;
• promoting awareness of the interdisciplinary nature of twenty-first century science, and emphasize how interactions between different thematic areas of the basic sciences will be increasingly needed in future research and education, and in the achievement of the 2030 Agenda for Sustainable Development;
• enhancing international cooperation by coordinating activities between learned societies, educational establishments and industry, focusing specifically on new partnerships and initiatives in the developing world;
• establishing durable partnerships to ensure that these activities, goals and achievements continue in the future beyond the International Year of Periodic Table of Chemical Elements.

International Year of Periodic Table’s Agenda

There is an official website of the International Year of Periodic Table (www.iypt2019.org) where you can find all the events happening around the world. Just to name one ongoing activity, IUPAC is hosting an online quiz about the Periodic Table of the Elements. The online challenge is aimed at a global audience of young students. Questions about the elements quiz their knowledge and inform them of the relevant work of IUPAC. The global challenge was launched in January 2019 and is available all year until the end of 2019.

And to finish, a trivia fact. The world’s biggest periodic table is on the side of the chemistry faculty building at the University of Murcia (Spain). A beautiful piece of approximately 150 m2.

Murcia

World’s biggest Periodic Table at the chemistry faculty of the University of Murcia (Spain)

NOTES 

With the hope of reaching a wider readership, this post is a translation from the original one that I wrote for the Spanish blog Dciencia: “2019, celebra la química con el Año Internacional de la Tabla Periódica“,

REFERENCES

Proposal for the proclamation by the United Nations of 2019 as an International Year of the Periodic Table of Chemical Elements. Conference:UNESCO. Executive Board, 2017, 202nd [29716].

Website of The International Year of the Periodic Table.

 

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Official Opening of Our New Teaching Labs

David Palomas, PhD

On the 9th of November of 2018, the newly refurbished Chemistry Teaching Lab was officially opened by the Principal Colin Bailey. Prof Marina Resmini, Head of the Chemistry Department had the honour of cutting the ribbon.

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Aryne-Mediated Arylation of Hantzsch Esters: Access to Highly Substituted Aryl-hydropyridines

David Palomas, PhD

https://www.thieme-connect.de/media/synthesis/EFirst/i_z0578_ga_10-1055_s-0037-1611065.gif

Synthesis, 2018, Accepted DOI: 10.1055/s-0037-1611065

This is a full account of our studies into the generation of highly functionalised 2-aryl-1,2-dihydropyridines and 2-methylene-3-aryl-1,2,3,4-tetrahydropyridines via intermolecular aryne ene reactions of Hantzsch esters. Furthermore, exposure to excess aryne revealed unusual 3′-aryl-spiro[benzocyclobutene-1,1′-(3′,4′-dihydropyridines)]. Mechanistic insights are provided by deuterium-labelling studies and DFT calculations, whilst preliminary cytotoxicity investigations reveal that the spirocycles are selective against colon carcinomas over ovarian cancer cell lines and that all the compounds have high selectivity indices with regards to non-cancer cells.

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#TitrationDay

David Palomas, PhD

First day in the labs with our 1st year students. Today we will be doing acid-base titrations to determine the amount of citric acid in a sample of a powdered fruit drink. Students will use a standard solution of NaOH as a titrating agent and Phenolphthalein as indicator.

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Organic Synthesis Under Inert Atmosphere

David Palomas, PhD

It is September again and as usual, we start the semester with the advanced practicals in chemistry with our Master students. It seems that after the summer break our students are in a very good mood and organic synthesis under nitrogen atmosphere becomes a balloon party!!!

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Intermolecular Aryne Ene Reaction of Hantzsch Esters: Stable Covalent Ene Adducts from a 1,4-Dihydropyridine Reaction

David Palomas, PhD

Abstract Image

Org. Lett. 2017, 19174644-4647

The reaction of arynes with 1,4-dihydropyridines affords 2-aryl-1,2-dihydropyridines or 2-methylene-3-aryl-1,2,3,4-tetrahydropyridines via a regioselective C-2 or C-3 arylation. These compounds are the first series of isolable and bench-stable covalent ene adducts formed between dihydropyridines and unsaturated substrates. Experimental studies and DFT calculations provide mechanistic support for a concerted intermolecular aryne ene process, which may have implications for NAD(P)H model reactions.

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On the road to the full control of the stereochemistry of dihalogenation reactions

The dihalogenation reactions of alkenes proceed with almost exclusive anti-diastereoselectivity and opposed to other well-known text book reactions, an enantioselective version seems quite elusive. In the last few years, I have been fascinated by two great advances towards the full control of the stereochemistry in reactions of dihalogenation of alkenes. First the discovery of enantioselective dihalogenation reactions and second the discovery of a catalytic syn-dichlorination of alkenes.

anti mechanism
The development of enantioselective versions is very elusive because racemization can easily occur via olefin-to-olefin halenium transfer and non-regioselective nucleophilic opening of the halonium intermediates. The first successful approach to this problem was done by the group of Scott Snyder who developed a stoichiometric dichlorination reaction using a chiral borane auxiliary that forms an in-situ complex with the substrate[1]. The chlorinated olefin was obtained with 87% e.e. and that was a tremendous success due to the difficulty of the reaction. A catalytic version adds the nonenantioselective background reaction to the existing problems. Later discoveries of catalytic enantioselective dichorinaton, dibromination and heterodihalogenation reactions of olefins by the groups of Nicolau, Burns and more recently Borhan are simply spectacular [2–5]. Nicolau and Borhan approach the problem using Sharpless dihydroxylation catalyst (DHQD)2PHAL while Burns uses a combination of a Titanium salt with a chiral promoter. But all these approaches have in common a wise election of substrates (allyl alcohols and amides) that can serve as an anchor to the catalysts to overcome the regioselectivity problem.

dihalogenation of olefins
The almost set-in-stone mechanism of the dihalogenation of alkenes involves the formation of a cyclic halonium ion followed by a backside nucleophilic opening that leads to the formation of the well stablished anti-product. Overturning the anti-diastereospecificity of this process is a major challenge. The group of Scott Denmark was able to develop the first syn-dichlorination of alkenes with a strategy based on a first step of anti-addition of an in-situ generated PhSeCl3 to an alkene followed by a nucleophilic displacement with a chloride ion source leading to a syn-dichlorination product overall.6
The process is made catalytic by adding an oxidant to the Ph-Se-Se-Ph precatalyst.
All these discoveries are impressive and I cannot help finding certain parallelism with the already Nobel Prize awarded hydrogenation and dihydroxylation reactions by Noyori, Knowles and Sharpless. I am very much looking forward to seeing further advances.

nobel prize2

References:
1. Snyder, S. A., Tang, Z.-Y. & Gupta, R. Enantioselective Total Synthesis of (-)-Napyradiomycin A1 via Asymmetric Chlorination of an Isolated Olefin. J. Am. Chem. Soc. 131, 5744–5745 (2009).
2. Nicolaou, K. C., Simmons, N. L., Ying, Y., Heretsch, P. M. & Chen, J. S. Enantioselective Dichlorination of Allylic Alcohols. J. Am. Chem. Soc. 133, 8134–8137 (2011).
3. Hu, D. X., Shibuya, G. M. & Burns, N. Z. Catalytic Enantioselective Dibromination of Allylic Alcohols. J. Am. Chem. Soc. 135, 12960–12963 (2013).
4. Hu, D. X., Seidl, F. J., Bucher, C. & Burns, N. Z. Catalytic Chemo-, Regio-, and Enantioselective Bromochlorination of Allylic Alcohols. J. Am. Chem. Soc. 137, 3795–3798 (2015).
5. Soltanzadeh, B. et al. Highly Regio- and Enantioselective Vicinal Dihalogenation of Allyl Amides. J. Am. Chem. Soc. 139, 2132–2135 (2017).
6. Cresswell, A. J., T.-C., E. & Denmark, S. E. Catalytic, stereospecific syn-dichlorination of alkenes. Nat Chem 7, 146–152 (2015).

The Making of #ChemisTree 2016

It is again this time of the year, the semester has been very long. One more week of practicals and we are all ready for the Christmas break!!!

This year to honour the tradition we have our ChemisTree in the teaching labs. In this post I’m unveiling the secrets behind the colours of our 2016 ChemisTree.

Blue: A pinch of Copper (II) sulfate pentahydrate in water

Red: a few drops of Ferroin indicator in water

Green:  A pinch of Nickel(II) nitrate hexahydrate in water

Deep purple: A pinch of potassium Permanganate in water

Violet: Literally 1 mg of Crystal Violet per Liter of water. Crystal Violet is a very strong dye.

Pink: A few drops of Phenolphthalein in a slightly basidified solution of NaOH in water. The colour fades away after a few minutes, long enough for a nice picture.

Many thanks to Indigo and Fosca for their help and creativity.

Superbases in FLP chemistry

A new post for the series I am crazy about Frustrated Lewis Pairs (FLPs). If you are new to this exciting field I recommend you to read first one of my previous posts: “Frustration to a Good End”.superbases2The extraordinary reactivity of FLPs allows the stoichiometric and catalytic activation of small molecules being the most important application the heterolytic cleavage of hydrogen. The spectrum of Lewis bases used in FLP chemistry is wider compared to the Lewis acid partners. Phosphines, amines, N-heterocyclic carbenes or carbodiphosphoranes have been used, with a few exceptions, combined with boranes such as B(C6F5)3 and derivatives.

strategy

The activation of the strong H-H bond has been commonly achieved using a type of the Lewis bases mentioned above in the presence of strong Lewis acidic boranes. Krempner and co-workers have recently showed that the use of strong Lewis acids is not strictly necessary for the activation of hydrogen. In this new approach denominated as “Inverse” Frustrated Lewis Pairs, organic superbases combined with rather moderate and weak Lewis acids are capable to reversibly cleavage hydrogen. Organic superbases have an enormous proton affinity and are very strong due to the great stability of their conjugated acids once they are protonated. In their recent work Krempner and co-workers activate hydrogen using phosphazene based superbases with BPh3, HBMes2 or 9-BBN and extend the concept of Inverse Frustrated Lewis Pairs to the catalytic hydrogenation of organic molecules using N-Benzylidenaniline as model substrate.catalysisDo you want to know more? Read the original paper

Suresh Mummadi, Daniel K. Unruh Jiyang Zhao, Shuhua Li and Clemens Krempner. “Inverse” Frustrated Lewis Pairs – Activation of Dihydrogen with Organosuperbases and Moderate to Weak Lewis Acids. J. Am. Chem. Soc., 2016, 138 (10), pp 3286–3289.

Are you a newcomer to the topic? Take a look to this papers

Seminal work by Douglas Stephan’s group

Gregory C. Welch, Ronan R. San Juan, Jason D. Masuda, Douglas W. Stephan. Reversible, Metal-Free Hydrogen Activation.Science, 2006, 314, 1124-1126.

A couple of reviews for newcomers to the topic

Stephan, Douglas W. “Frustrated Lewis pairs”: a concept for new reactivity and catalysis Organic & Biomolecular Chemistry (2008), 6(9), 1535-1539.

Stephan, Douglas W.; Erker, Gerhard. Frustrated Lewis Pairs: Metal- free Hydrogen Activation and More. Angewandte Chemie, International Edition (2010), 49(1), 46-76.