Archive for the ‘Organometallics’ Category

An Organometallic Toolbox for the Study and Synthesis of Unique N-Heterocyclic Carbenes

N-heterocyclic carbenes (NHCs) are an interesting example of chemical curiosity turned commonplace. NHCs are stable singlet carbenes located within an N-heterocycle, in which the carbon centre bears an sp2 hybridised pair of electrons. As early as 1835 chemists were thinking about carbenes, with Dumas’ optimistic (if unsuccessful) attempt to synthesise the methylene carbene by dehydrating methanol. For many years the intentional study of carbenes was considered too demanding because of their instability, and so they remained in relative obscurity. A number of seminal papers changed this preconception; in particular, a report by Wanzlick in 1968 reporting the synthesis of the first NHC-metal complex using mercury and the first synthesis of a stable and isolable NHC by Arduengo in 1991.

Intensification in research and interest in NHCs over the past thirty years may have originated with these seminal reports, but it continues because of the success of NHCs in catalysis: both as strongly σ-donating metal ligands and nucleophilic organocatalysts. One of the most valuable features of NHCs is the ability to tailor their steric and electronic properties by altering the heterocyclic ring and N-bound substituents. Accordingly, the study of NHC reactivity and the development of methods to functionalise NHCs are essential for continued innovation in this field.

Drs Marina Uzelac and Eva Hevia at the University of Strathclyde, Scotland, have written a review article summarising organometallic methods to metallate N-heterocyclic carbenes. The work summarises metallation of all three components of the NHC: the carbenic carbon, the heterocyclic backbone and the N-bound substituents.

The lithiated complex (1), synthesised by treatment of the N-heterocyclic carbene NHC with nBuLi, can be transmetallated at the C4 position by a number of main group elements to give a variety of bimetallic complexes (2). These complexes can be selectively quenched to generate NHC complexes with unconventional regiochemistry (3).

The lithiated complex (1) can be transmetallated at the C4 position by a number of main group elements to give a variety of bimetallic complexes (2). These complexes can be selectively quenched to generate NHC complexes with unconventional regiochemistry (3).

To exemplify the breadth of research discussed; beginning with 2,6-diisopropylphenyl (dipp) substituted imidazole-2-ylidenes, the reactivity of the NHC can be unlocked by initial addition of an alkali metal such as lithium, sodium or potassium (see figure). Metallation at the C4 position occurs by deprotonation of the vinyl protons in the NHC backbone, while a second metal coordinates to the carbene electron pair at the C2 position. From this species (1) it is possible to transmetallate the C4 position with a less-polar metal such as zinc, aluminium, gallium, boron or iron to furnish a bi-metallic NHC (2). Interestingly, addition of an electrophilic methyl or proton source to this species exclusively quenches the C2 position, generating a suite of unconventional complexes (3) with the carbene electron pair positioned on the C4 carbon.

Lithiation of NHC complexes: a) deprotonation of the backbone of NHC-borane complex; b) co-complexation of NHC-zinc complex with alkyllithium affording lithium zincate; c) deprotonation of the abnormal carbene complex.

Reactivity of main-group NHC complexes towards lithiation.

Further studies investigate how different reagents influence the regioselectively and extent of metallation, how metallated NHCs can activate small-molecules such as carbon dioxide, conditions which can lead to the metallation of N-dipp substitutents, as well as products and speciation following treatment of NHCs with a variety of bimetallic reagents.

In addition to expanding the knowledge of NHC reactivity, the work summarised in this review provides a reference and inspiration to researchers seeking to tailor NHCs for unique applications in synthesis and catalysis.

To find out more please read:

Polar organometallic strategies for regioselective C-H metallation of N-heterocyclic carbenes

Marina Uzelac and Eva Hevia.
Chem. Commun., 2018, Advance Article
DOI: 10.1039/c8cc00049b

About the author:

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

 

 

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Capturing C60 in a Crystalline Copolymer Chain

Since its structural realisation in 1985, C60 has garnered much attention in the chemical world for not only its spherical shape, but also its stability, electronic properties and the ability to do chemistry on its surface.

One such avenue that has proven popular in recent times is the incorporation of C60 into one-, two- and three-dimensional arrays, either covalently or non-covalently, in attempts to control the distribution of the molecules in the solid- or solution-phase.  One problem that arises in the synthesis of these extended frameworks, however, is that there often a large amount of disorder and void space in the structure, so it can be difficult to ascertain with much degree of certainty how these C60 molecules are oriented. This uncertainty can consequentially result in the properties and behaviours of the new materials remaining unidentified.

Now, researchers from the University of California, DavisMarilyn Olmstead and Alan Balch – have shown that coordination chemistry can be used to not only generate polymers that covalently link molecules of functionalised C60 in such a manner that can they can be studied crystallographically, but also that these polymers can be used to capture free C60 and C70.

Initially, polymers of C60 were synthesised through the mono-functionalisation of C60 with a piperazyl group, which, on account of its two tertiary amines, can coordinate in a linear fashion with transition metal ions, in this case rhodium(II) acetate. Upon the combination of these two components, a linear one-dimensional polymer was formed, in which it could be seen crystallographically that the C60 moieties were positioned on alternating sides of the polymer chain. These polymer chains were further found to extend into two dimensions through the interdigitation of neighbouring chains in a zipper-like fashion. C60-Rh(II) polymers can capture free C60

Perhaps more interestingly is that when these polymer chains were synthesised in the presence of either C60 or C70, free molecules of C60 or C70 were seen to occupy the void spaces between the C60 molecules of the polymer. Additionally, if a mixture of C60 and C70 was present in the polymer synthesis, it was observed that only C60 was captured by the polymer, most likely as a result of a better geometric match between the polymer and the spherical C60 in preference to the more elongated shape of C70.

This work elegantly demonstrates the generation of not only a self-assembling C60-containing polymer that can be characterised structurally in the solid state, but of one  that can entrap free molecules of C60 selectively over molecules of C70. Based on the properties of free C60 and transition metal complexes, the electronic and chromophoric properties of such a crystalline system could also be expected to offer some noteworthy results.

Read this HOT ChemComm article in full!

Zipping up fullerenes into polymers using rhodium(II) acetate dimer and N(CH2CH2)2NC60 as building blocks
Amineh Aghabali, Marilyn M. Olmstead and Alan L. Balch
Chem. Commun., 2014, Advance Article.
DOI: 10.1039/C4CC06995A

Biography

Anthea Blackburn is a guest web writer for Chemical Communications. Anthea is a graduate student hailing from New Zealand, studying at Northwestern University in the US under the tutelage of Prof. Fraser Stoddart (a Scot), where she is exploiting supramolecular chemistry to develop multidimensional systems and study the emergent properties that arise in these superstructures. When time and money allow, she is ambitiously attempting to visit all 50 US states before graduation.

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A new, functionally tolerant route to organo-aluminium reagents

Paul Knochel and colleagues at the Ludwig Maximilians University in Munich have reported a new general synthesis of aryl and heteroaryl aluminium reagents.  The route described allows a larger range of functional groups to be incorporated, compared with the more usual approach of inserting Al into aryl halide bonds directly.  The synthetic methodology uses di-isobutyl aluminium chloride and n-BuLi at -78C in an exchange reaction with a functionalised aryl or heteroaryl halide.

General scheme for preparation and derivitisation of aryl aluminium reagents

The synthesis of a group of derivatives is described, via the reaction of the aluminium reagents with a variety of electrophiles.  Typical cross coupling reactions using palladium catalysis, as well as copper-catalysed Michael additions, allylation and acylations are reported, involving a rich variety of incorporated functional groups. Importantly, further derivitisation of the organo-aluminium reagents includes no further transmetalation steps.

Of note are the reactivities of electron-rich furan and thiophene bromides functionalised with ester groups, which also could remain intact during the reaction with di-isobutylaluminium chloride and butyl-lithium at -78C, yielding the desired reagents that were further derivatised, as in other examples.

N-heterocycles such as 3-bromo-quinoline also received attention, yielding the aluminium reagent in 73% yield, and smoothly converting in a palladium catalysed cross coupling reaction with 4-iodobenzonitrile.  Full NMR data for the products of the reactions described is given in the supplementary information.

In general, this Communication describes a considerable step forward in the field of organo-aluminium reagents for organic synthesis, and no doubt will be of interest to synthetic chemists in many fields.

Read this HOT ChemComm article today!

Generation of Functionalised Aryl and Heteroaryl Aluminium Reagents by Halogen/Lithium Exchange
Thomas Klatt, Klaus Groll and Paul Knochel
Chem. Commun., 2013,49, 6953-6955
DOI: 10.1039/C3CC43356K, Communication

Kevin Murnaghan is a guest web-writer for Chemical Communications. He is currently a Research Chemist in the Adhesive Technologies Business Sector of Henkel AG & Co. KGaA, based in Düsseldorf, Germany. His research interests focus primarily on enabling chemistries and technologies for next generation adhesives and surface treatments. Any views expressed here are his personal ones and not those of Henkel AG & Co. KGaA.

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Synthesising gold- and silver-NHC complexes using a weak base

Scientists from the University of Zaragoza in Spain have developed a simple and efficient method of synthesising N-heterocyclic carbene (NHC) gold and silver complexes with the use of an extremely weak base1.

Gold-NHC complexes are commercially important precursors of active, luminescent species that catalyse many useful reactions, such as cycloisomerisation, rearrangement of allylic acetates, C-H activation, carbene transfer, polymerisation, among others.  In addition, they have potentially significant applications in the synthesis of new pharmaceuticals and natural products.

Conventional methods of gold-NHC synthesis– the generation of free NHC and the Ag-carbene transfer route– present several logistic and economic limitations, such as the need for an inert atmosphere and the use of additives.  These methods are not always efficient, and typically require complicated working conditions in order to produce even moderate yields.

M. Concepción Gimeno and her team’s novel and elegant one-pot synthetic route involves isolating imidazolium salts using [AuCl(tht)] (tht = tetrahydrothiophene) in the presence of a mild base, such as K2CO3, to produce gold-NHC complexes with very high yields (91-94%) over relatively short reaction times (1.5 hours).

c3cc42919a-s2

Similarly, Gimeno et al. found that, using the same mild base protocol, silver-NHC complexes could also be efficiently synthesised using AgNO3, with vast potential significance in transmetalation.

c3cc42919a-s3

In both routes, the reactions occur under ambient conditions, eliminating the need to work in an argon atmosphere, and using readily-available technical grade solvents.

Interestingly, a mere few days later, Gimeno et al.‘s groundbreaking work was followed closely and independently by a related Communication from Steven Nolan’s group at the University of St Andrews.  In addition to testing a similar methodology, Nolan’s team compared small- and larger-scale reactions, and characterised compounds by 1H and 13C{1H} NMR spectroscopies, as well as by elemental analysis2.

To find out more about these fascinating breakthroughs in organometallics, read these HOT ChemComm articles now for free!

1.  Simple and efficient synthesis of [MCI(NHC)] (M = Au, Ag) complexes
Renso Visbal, Antonio Laguna and M. Concepción Gimeno
Chem. Commun., 2013, 49
DOI: 10.1039/C3CC42919A, Communication

2.  Straightforward synthesis of [Au(NHC)X] (NHC = N-heterocyclic carbene, X = Cl, Br, I) complexes
Alba Collado, Adrián Gómez-Suárez, Anthony R. Martin, Alexandra M. Z. Slawin and Steven P. Nolan
Chem. Commun., 2013, 49
DOI: 10.1039/C3CC43076F, Communication

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