Tag Archives: Paul M

Organotextile catalysis

Ji-Woong Lee, Thomas Mayer-Gall, Klaus Opwis, Choong Eui Song, Jochen Stefan Gutmann, and Benjamin List
Science
DOI: 10.1126/science.1242196

list1

I’ve written a research highlight for Nature Chemistry talking about Opwis’ and List’s recent paper on organotextile catalysis in Science. Take a look.

Catalytic cloth

Image credit: © 2013 American Association for the Advancement of Science

Stereoinversion of tertiary alcohols to tertiary-alkyl isonitriles and amines

Sergey Pronin, Christopher Reiher, and Ryan Shenvi
Nature
DOI: 10.1038/nature12472

shenvi1

The bimolecular nucleophilic substitution (SN2) reaction is a well understood and widely used chemical transformation that allows a chemist to affect useful functional group interconversions or combine two molecules. A significant advantage of the SN2 reaction over the related (and often competitive) unimolecular nucleophilic substitution (SN1) reaction is that it provides predictable stereoinversion at the electrophilic carbon centre. SN2 processes do, however, suffer from a significant limitation: intolerance of tertiary electrophilic carbon atoms, where steric crowding inhibits the approach of the nucleophilic reacting partner. This drawback limits the stereochemical complexity of possible reaction substrates and the utility of nucleophilic substitution in the synthesis of challenging chiral molecules.

Now, a team of researchers from California led by Ryan Shenvi have developed a process that allows the stereochemical inversion of tertiary alcohols with nitrogen-based nucleophiles. The key reaction in their discover y involves Lewis-acid-catalyzed solvolysisof a tertiary alcohol derivative – trifluoroacetate or perfluoroalkanoate esters — in the presence of excess trimethylsilyl cyanide. Addition of the cyanide nucleophile to the carbocation of a postulated contact ion pair, generated by solvolysis, occurs with high enantioselectivity, giving tertiary isonitrile products in a remarkably simple transformation. The researchers compare this process to proposed biosynthetic pathways of nitrogen-based marine terpenoids, which are known to derive from the addition of inorganic cyanide to highly substituted carbon centres.

shenvi2

As well as extending the scope of traditional SN2 processes, Lewis-acid-catalyzed solvolysis also provides complementary reactivity. Because the reaction is conceptually related to the SN1 reaction in the generation of a reactive carbocation in the contact ion pair, activated primary and secondary alcohols do not undergo solvolysis even over extended reaction times. The utility of the process was further demonstrated by converting the isonitrile products into various useful nitrogen-based compounds, such as amines, formamides and isothiocyanates.

The influence of steric crowding on this SN2-like reaction has not, however, been eliminated; branched tertiary alcohols react with lower stereoselectivity than minimally substituted analogues. Despite this, the reported transformation fills a gap in modern synthetic methodology and may lead to further developments in the synthesis of other stereo-defined tertiary-substituted compounds.

Uncovering alkenes: complex products from all-carbon substrates

alkenerev1

Molecules containing simple alkene functionality are attractive substrates in synthesis. Carbon-carbon double bonds are relatively stable groups, lacking polarisation, they can be carried through synthetic processes involving acid, base, mildly oxidative and reductive conditions. But, the availability of the π-electrons for reaction embodies remarkable potential for constructing substituted contiguous stereocentres and the possibility of introducing complex functionality to an all-carbon system.

In recent years, functionalisation of unactivated alkenes has blossomed with developments in robust metal-catalysed processes and new oxidating agents. In the last few weeks several reports of new alkene functionalisations have been published that cover many aspects of this broad area, and show the diverse utility of alkenes in synthesis.

Hayashi and co-workers have published an elegant cyclising difunctionalisation of dienes.[1] The process is an iridium-catalysed C-H activation of cyclic aryl N-sulfonyl ketimines and gives spirocyclic aminoindane scaffolds with high diastereoselectivity.

alkenerev2

We’ve already discussed an asymmetric cyclising aminofluorination reaction carried out by a chiral hypervalent iodine fluoride oxidant.[2] The reaction is endo-selective and prepares fluoropiperidine-based systems. Extension of the reaction to a racemic intermolecular process with styrene substrates was also demonstrated.

alkenerev3

In a similar example of intermolecular alkene difunctionalisation, Zhang reports aminocyanation and diamination of (predominantly) styrene substrates.[3] The principle reagent for functionalisation is NFSI (N-fluorobenzenesulfonimide), which aminates the terminal end of the double bond. Subsequent reaction with TMSCN gives the aminonitrile product. Alternatively, an alkylnitrile may be used to give the Ritter product of the second addition.

alkenerev4

The authors suggest a copper(I)-catalysed radical mechanism, generating a carbon-centered radical intermediate after amination by •N(SO2Ph)2. This reaction gives some idea of the kind of complex functionality that can be introduced to all-carbon alkene substrates by oxidative processes.

Hydrofunctionalisation of alkenes doesn’t provide the same high degree of complexity as oxidative difunctionalisation processes, but reactions in this category do offer a great deal of diversity in synthetic options while still providing regio- and stereocontrol of sp3 carbon centres. A report from Qing and co-workers describes a general and high yielding hydrotrifluoromethylation of unactivated alkenes.[4] Trifluoromethyl groups are useful moieties in pharmaceutical development of lead compounds. The group apply the method to a wide scope of unactivated alkene substrates showing tolerance to many other functionalities.

alkenerev5

Hartwig’s group in Berkeley have developed an asymmetric hydroheteroarylation of bicycloalkenes.[5] The reaction is catalysed by an iridium DTBM-Segphos complex and tolerates various heteroaromatic coupling partners.

alkenerev6

Alkenes offer chemists widely available, stable and highly tolerant substrates for synthesis. Careful application of modern reactions allows the uncovering of diverse and complex functionality from a carbon-carbon double bond, and new developments provide ever more effective methods for the preparation of desirable adducts.

References:
1. DOI: 10.1021/ja311968d
2. DOI: 10.1002/anie.201208471
3. DOI: 10.1002/anie.201209142
4. DOI: 10.1002/anie.201208971
5. DOI: 10.1021/ja312360c

A paramagnetic/diamagnetic molecular switch

Fraser Stoddart’s group have reported a catenane molecule that can be oxidised to a stable paramagnetic radical state. The oxidation states of the molecule are easily configurable, so this structure has significant potential for use in data storage on the molecular level.

From the Chemistry World article:

The key to the stability of the new radical compound is the mechanical bond that links the two macrocycles, forcing the charged species to remain close. And that proximity means that the molecule never oxidises to a fully charged species but stops at the paramagnetic species •7+. That, says Barnes, is because the molecule is trying to minimise the charge in the centre where the two catenanes link. ‘The charged units have no choice but to interact with one another,’ explains Barnes, so it holds on to that remaining electron to reduce the charge repulsion.

But while the •7+ species is paramagnetic, all the other charged species are diamagnetic, as the electrons spins pair. Using cyclic voltammetry it is possible to quickly switch between the paramagnetic and diamagnetic states by adding and removing electrons. And it is this simple switching that could be the key to a potential application of the material: memory storage.

DOI: 10.1126/science.1228429

A ring-distortion strategy to construct stereochemically complex and structurally diverse compounds from natural products

Robert W. Huigens III, Karen C. Morrison, Robert W. Hicklin, Timothy A. Flood Jr, Michelle F. Richter and Paul J. Hergenrother
Nature Chem.
DOI: 10.1038/nchem.1549

To synthetic chemists, natural products are generally viewed as end-points in synthetic projects (or occasionally as tools, such as ligands or catalysts). Rarely, are complex natural products considered as launching-off points in the synthesis of other interesting molecules. A recent report takes this rather unusual approach by applying the principles of diversity oriented synthesis (DOS) in preparing a series of molecules with drug-like molecular properties.

The authors take three widely available natural products and carry out numerous derivatisations and transformations to give 49 dissimilar molecular scaffolds, a process they dub ‘compexity-to-diversity’ (CtD). A few examples are shown below.

hergenrother1

These molecules are presented as exemplifying a new approach to preparing drug-like molecules. They are analysed in characteristics typical of drug molecules and compared with a common screening library of 150,000 molecules.

In terms of diversity, these molecules display considerable structural dissimilarity (established by calculating Tanimoto coefficients for each pair of molecules), even within derivatives of the same natural product.

The CtD molecules display superior lipophilicity (average ClogP = 2.90) compared with the screening collection (average ClogP = 3.99), with over 60% of CtD molecules in the optimal logP range of 0 to 4. Similarly, the CtD library also displays a far higher fraction of sp3 character than the screening compounds, a property indicating a high degree of 3D structure and correlated with enhanced aqueous solubility.

The authors argue that molecular complexity is advantageous in drug-like molecules as more complex molecules might bind their targets more specifically. They point to the number of stereogenic centres the molecule contains as a surrogate for molecular complexity and show that their CtD molecules contain far more stereocentres than the compounds in the screening collection.

Despite demonstrating clear drug-like properties (particularly ClogP), the molecules have not been analysed in another significant property indicative of the likelihood of observing drug-like behaviour: molecular weight. While molecular complexity offers the possibility of tighter target binding, it also decreases the chances of observing binding in any given target site. This is correlated with molecular weight, where it is estimated that each heavy atom added to the molecule increases the number of potential structures by a factor of 10.1 The chances of finding hit molecules is increased when screening molecules with lower molecular weight (due to better sampling of chemical space), albeit with potentially weaker binding (cf. fragment-based design).

The six CtD molecules displayed above lie in the molecular weight range of 353 to 505, outside of the optimal range of 200 to 350 for lead compounds, though within the boundaries of the Lipinski rule of 5 for drug-like molecules.

This approach is, of course, not limited to the derivatisation of the natural products or classes presented within this paper, but that similar structural modifications may be carried out on any readily available complex molecules to prepare diverse structures with drug-like properties.

1. A. Nadin, C. Hattotuwagama, I. Churcher Angew. Chem. Int. Ed. 2012, 51, 1114 and references 27-29 therein.

Solar-Driven Incorporation of Carbon Dioxide into α-Amino Ketones

Naoki Ishida, Yasuhiro Shimamoto, and Masahiro Murakami
Angew. Chem. Int. Ed.
DOI: 10.1002/anie.201206166

Preparation of cyclic carbonates by incorporation of CO2 into strained cyclic systems is well known and widely exploited. A standard synthesis of these compounds is through addition of epoxide into CO2 followed by ring expansion. Murakami and co-workers take a similar approach in cyclic carbonate synthesis in order to demonstrate a process involving 2 goals in green chemistry: CO2 incorporation, and sunlight activation.

The authors show that azetidine derivatives can be prepared from α-methylaminoketones through irradiation with sunlight alone. These high energy intermediates are relatively stable and their stored solar energy can be released in reaction with CO2 under mild conditions. The reaction can tolerate various aryl groups, but chain length is fixed as a hydroxypyrrolidine derivative is too stable to undergo ring opening.

Substitution of Two Fluorine Atoms in a Trifluoromethyl Group: Regioselective Synthesis of 3-Fluoropyrazoles

Kohei Fuchibe, Masaki Takahashi, and Junji Ichikawa
Angew. Chem. Int. Ed.
DOI: 10.1002/anie.201206946

We don’t often consider the trifluoromethyl group as a reactive centre. But it is, of course, a highly polarised moiety and, under the right conditions, fluoride can act as a competent leaving group. Ichikawa and coworkers employ hydrazines as nucleophiles in two successive displacements of fluoride from vinyl trifluoromethyl adducts to give 3-fluoropyrazole scaffolds.

After testing other possible reaction pathways, they postulate that the reactive intermediate in the cyclisation step, after loss of the tosyl anion, is an azomethine ylide.