Chemical Science Blog

Microwaves have been used to promote organic reactions since the 1980s and they can lead to higher yields and shorter reaction times than conventional heating, but why? Conventional wisdom says that it’s the heat that promotes the reactions, but new research shows that the waves could be interacting with the molecules directly.

Gregory Dudley and colleagues from Florida State University, Tallahassee, in the US, discovered a microwave effect occurring during a Friedel-Crafts benzylation reaction in a microwave. The microwave-absorbing polar reactant – a pyridinium – reacted in a non-polar and largely non-microwave-absorbing solvent – toluene. Previously, a non-polar reaction system had to be doped with ionic liquids, which converted the microwaves to heat. ‘Polar molecules interact more strongly with microwaves than non-polar molecules,’ explains Dudley. ‘So we tried to design a system in which only the reaction substrate would interact with the incident radiation energy.’

The team found that the microwave irradiation induces reactivity levels that can’t be duplicated by conventional heating, without reaching the temperature required in conventional heating. ‘It’s hard to speculate what exactly is going on,’ says Dudley. ‘Our focus was to identify an effect.’ Dudley adds that there is still much to learn about the differences between activating reactions with microwaves and conventional heating.

Defying conventional wisdom, new research shows it could be microwaves that promote microwave-assisted reactions, not heat

Read the full story in Chemistry World

Link to journal article
On the rational design of microwave-actuated organic reactions
Michael R. Rosana ,  Yuchuan Tao ,  Albert E. Stiegman and Gregory B. Dudley
Chem. Sci., 2012, Advance Article
DOI: 10.1039/C2SC01003H

There are very few examples of non-macromolecular gene carriers to take DNA through lipid membranes. Now, a remarkably small, trivalent, dipeptide-based gene carrier has been developed by scientists in Germany to bind to DNA to transport DNA into human cells.

The carrier consists of 6 amino acids and 3 guanidinocarbonyl pyrrole moieties; it carries 3–4 positive charges at physiological pH, which is unprecedented for non-viral gene carriers. Also, the carrier doesn’t require additional helper molecules, for example, phospholipids.

Reference:
Efficient Gene Delivery into Cells by a Surprisingly Small Three-Armed Peptide Ligand
H Y Kuchelmeister et al, Chem. Sci., 2012
DOI: 10.1039/c2sc01002j

US scientists have made multifunctional lactones – normally a challenging process – in an efficient way, under mild conditions and without protecting groups or by-products.

Multifunctional molecules are attractive for cellular targeting, imaging and drug delivery or as synthetic mimics of biological structure and function.

The team began with a symmetrical trifunctional molecule – benzotrifuranone – and sequentially functionalised its three reactive sites by aminolysis. The functional groups were designed in such a way that each functionalisation diminishes the reactivity of the remaining sites so that the groups only functionalise one reactive site each time.

Reference:
Molecular multifunctionalization via electronically coupled lactones
M B Baker, I Ghiviriga and R K Castellano, Chem. Sci., 2012
DOI: 10.1039/c2sc00943a

A new class of tunable molecular wires has been made by researchers in the US. The team has made new cyano-functionalised oligoenes and they report the longest oligoene crystal ever made.

Cyano groups decrease oligoenes’ band gaps and stabilise them for study under normal laboratory conditions while preserving the conjugation along the poly-olefin backbone. This particular class of oligoenes has not been studied much.

Functionalised oligoenes are ideal structures for nanoscale electronic and structural components and they could be used in photovoltaics because their band gaps are easy to tune and their absorptions cover the visible spectrum.

Reference:
Functionalizing Molecular Wires: A Tunable Class of α,ω-diphenyl- μ,ν-dicyano-oligoenes
J S Meisner et al, Chem. Sci., 2012
DOI: 10.1039/c2sc00770c

Researchers from Wuhan University and the Lanzhou Institute of Chemical Physics have described a sulfur-promoted cleavage of aryl–iodo bonds by a Pd(ll) species.

Aiwen Lei and co-workers showed that treatment of thioimido substrates (1) with a catalytic amount of a “pincer” Pd(ll) complex (2) at 80 ˚C could form thiazoles (3) in excellent yield.

In Nature, sulfur plays an important role in tuning the electronic properties of metals found in proteins. In analogy to the role of cysteine (a sulfur-containing amino acid), the incorporation of a donating sulfur ligand in the starting material (1) is thought to promote reaction of the Pd(ll) centre with the aryl-iodo bond. The reaction is thought to proceed via formation of complex A, whose structure has been confirmed by single crystal analysis.

Further investigations into the mechanism of this reaction are ongoing, but the researchers are hopeful that this work will enable new opportunities for the use of Pd(ll)–Pd(lV) chemistry, a powerful yet under-investigated catalysis mode.

Find out more by downloading Lei’s Edge article today.

Computation experiments by researchers from the University of Geneva have illuminated their understanding of chemistry they previously reported and, importantly, have led to an improvement of the original methodology.

Alexandre Alexakis’  group developed a copper-catalysed asymmetric allylic alkylation, which transforms a racemic starting material into an enantioenriched product.

Computational modelling led to a revision of the original mechanistic explanation for the reaction outcome. The researchers propose that each enantiomer of the starting material undergoes divergent reactivity, where (R)-1 reacts through anti-S_N2’ oxidative addition whilst its antipode (S)-1 reacts through anti-S_N2. The regiodivergent oxidative addition leads to the formation of a common Cu(III) intermediate 2, which undergoes rapid reductive elimination to give the product (R)-3.

This work clearly demonstrates the importance of acertaining mechanistic insight in order to improve the practical application of organic chemistry.

Download Alexakis’ Edge article to find out more.