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Instant insight: Cosmic dust as chemical factories
01 June 2010
Daren Caruana and Katherine Holt discuss how electrochemistry could be the missing link to understanding chemistry in space
The search of the origin of life's molecular building blocks has extended into interstellar space to find evidence of life forming chemicals. As yet no positive identification of molecules that can be considered as precursors to living systems on earth has been made. But, a rich mixture of complex chemicals has been discovered in the spaces between stars in our galaxy, which brings the question of how are these chemicals synthesised?
Mostly, the space between the stars is a vacuum with a light peppering of atoms and molecules. However, high concentrations of chemical species exist in some areas forming dense clouds also containing dust. These dusty clouds appear to be an incubation centre for the synthesis of quite a large variety of molecular species. More than 140 molecular species have been identified in dust clouds, using a variety of spectroscopic techniques. Sophisticated computer models that describe the abundance of chemical species present in clouds have been developed to investigate the formation of these species. Yet, these models fail to describe the formation of many polyatomic molecules, and their origin remains one of the longstanding enigmas of interstellar chemistry.
The involvement of dust grains in the synthesis of complex polyatomic molecules is generally accepted to be important in astrochemistry. For example, molecular hydrogen - the most abundant molecule in the interstellar medium (ISM) - is formed on the surface of grains. So far computer models assume dust grains act only as benign surfaces, where reactants are adsorbed on the surface before reacting. But it is likely that dust grains could play a more active role in chemical synthesis.
Interstellar space
Recently, a new hypothesis has been proposed that connects chemical synthesis on the surface of dust grains with electrical charging of the grains by known physical phenomena in the ISM. Individual grains can be negatively charged by attaching to electrons in areas of the gas that is ionised, or positively charged in the periphery of dust clouds where electromagnetic radiation is powerful enough to eject electrons from grains.
The heart of this hypothesis hinges on the fact that electrically charged surfaces can be reducing when negative and oxidising when positive. So depending on the charging processes the balance of electrons in grains can drive surface reduction or oxidation reactions, which may otherwise be thermodynamically unfavourable.
At the moment there is little understanding of redox reactions at the solid-gas interface, but by combining knowledge of redox process at the solid-liquid interface, this hypothesis can be developed into a possible processing mechanism involving solid dust grains found in the ISM. Here the individual grains act as 'single electrode' electrochemical reactors in the gas phase. The number of electrons in the grain changes the overall potential (or Fermi energy) - for a 10 nm diameter dust grain attachment of 32 electrons changes the potential difference of the particle by 1 eV. A voltage of this magnitude applied to a solid conducting surface is known to be able to drive many chemical reactions in liquid phase electrochemistry. Therefore, it is likely that a polarised interface in the gas phase can drive similar chemical reactions.
There is speculation that the formation of some larger molecules (> 6 atoms) may be synthesised through mechanisms that involve coupled surface electrochemical reduction or oxidation reactions, which could occur at spatially distinct areas of a dust cloud. Once developed this surface electrochemical mechanism in conjunction with other gas phase based chemical models may account for the unexpected abundances of certain polyatomic chemical species. First, the understanding of redox reactions at the solid/gas interface needs to be furthered. But in the future, this electrochemical mechanism may also be applied to other environments such as chemistry in the upper atmosphere.
Structural order gained over conducting polymer
01 June 2010
Scientists in Canada and the US have shown how it is possible to assemble ordered arrays of short chains of a commercially important conducting polymer on a metal surface. The finding is significant because the molecular order of conducting polymers can have a profound effect on their electronic properties. The researchers believe that their work is an important step towards the creation of flat sheets of such molecules, a single molecule deep. This would be analogous to graphene - a two-dimensional wafer of carbon one atom thick which has a range of intriguing physical, chemical and electronic properties.
The researchers used crystalline copper as the surface for growing the polymer poly(3,4-ethylenedioxythiophene) (PEDOT), which is the most widely used conducting polymer. A precursor molecule is deposited on the copper surface which acts as both a template that dictates the direction of the growth of the polymer, and as a catalyst for promoting the polymerisation.
Using scanning tunnelling microscopy, the team showed that the monomer precursor molecules align themselves along a crystal plane of the copper in a long chain. However, initially they remain unbonded. Several chains align themselves in parallel, where adjacent precursors then link covalently. The effect of this is to form multiple arrays of oligomers, ranging from two to 14 molecules long, lying adjacent to each other.
We have shown that we can get structural order on an important conjugated polymer,' says team member Federico Rosei of the University of Quebec. The team is now investigating ways of lengthening the covalently bonded chains and modifying the precursor so that it can form bonds in two directions, with the aim of linking the adjacent chains to form a two-dimensional sheet, which could then be etched off the copper surface. 'Ultimately we would like to create analogues of graphene made with more sophisticated building blocks,' Rosei says.
Colin Raston, director of the Centre for Strategic Nano-Fabrication at the University of Western Australia, says that the new work 'has implications in forming a diverse array of different aligned conjugated polymers, with high stereochemical control, as well as control over the chain length and overall 2D structures, which is not possible in conventional solution processing.'
Raston adds, 'the results have exciting possibilities in studying selectivity of cross coupling reactions, with the "laboratory" combinatorial library down to nanometre dimensions, using copper and other metals, and indeed metal alloys.'
Column: Totally Synthetic
Decarestrictine D
Totally Synthetic
Although its chemistry is mature and varied, my use of silicon reagents in my synthetic forays has been limited to a somewhat clumsy use of hydroxyl protecting groups. Worse still, I have neglected its group 14 colleague, tin, even further - only using stannanes in the ubiquitous Stille coupling. This now feels like an opportunity lost, as a team lead by Varinder Aggarwal of the University of Bristol, UK, demonstrates their synthetic utility beautifully in a short formal synthesis of the cholesterol biosynthesis inhibitor, decarestrictine D.1 With four stereocentres clustered around an unsaturated bond, this small but complex lactone is the perfect challenge to showcase a powerful method.
beta-hydroxy allylsilanes have long been used as powerful reagents in organic chemistry, but their synthesis can be quite awkward. This problem is only enhanced when the desired allylsilane is chiral - a problem the group set out to solve. Their plan was to combine allylsilane synthesis with a boron-mediated aldehyde addition to produce a novel three-component coupling - an ambitious goal, but very appealing.
Their synthesis started with construction of a chiral building block (figure 1), containing one silyl-protected secondary alcohol and a carbamate-protected primary alcohol (denoted OCb). Treating this with a strong base - in the presence of a chiral diamine to direct the deprotonation - allowed the team to form a chiral carbon-tin bond in high diastereomeric excess. In this case, the source of chirality was a '(+)-sparteine surrogate'. (-)-Sparteine is an alkaloid found in plants of the lupin family and a very useful chiral reagent, but the (+)-enantiomer is not easily available. Thanks to this recently developed substitute,2 both options are now open to chemists.
With their chiral stannane fragment complete, they moved on to a pair of coupling reactions to build up much of the target's complexity. Exchanging the tin for lithium gave an asymmetric carbanion, which attacked the vinyl borane and rearranged to substitute the carbamate for the vinyl silane group. Addition of a small aldehyde fragment allowed formation of a cyclic chair-type transition state (figure 2). As many readers will remember from undergraduate organic chemistry, this type of transition state often gives excellent stereoselectivity, and after a little tinkering this proved to be no exception. The result of their efforts was the fluid installation of a cis double bond, along with the creation of a new stereocentre. The group performed this chemistry on a wide variety of substrates outside the scope of the total synthesis - indeed this is fundamentally a methodology paper - but I was quite taken with the rest of their synthesis.
An elegant ending
The team weren't quite done with their manipulation of group 14 elements. Using some silicon chemistry developed decades earlier, they were rather cunningly able to use an acidic Peterson rearrangement to install both a hydroxyl stereocentre and create the desired trans double bond.3 This worked by first oxidising the cis olefin with meta-chloroperbenzoic acid (mCPBA), which was highly stereoselective - directed by the bulky silicon group. Treatment of the now fragile epoxide intermediate with a little acid resulted in a stereospecific elimination of the silicon group, and creation of the required trans alkene (figure 3).
From this point, the group could complete the target in five steps; but three steps was enough to consititute a formal synthesis (in which their route converged with a previous total synthesis). Importantly, this route is not only shorter than previous efforts, but also acts as the perfect test of a new and useful method. Perhaps I should brave the smell of rotting leaves (that's what silicon reagents remind me of) more of
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