User:Bmalbrecht/sandbox

EK

In organic chemistry, asymmetric hydrogenation is a chemical process that adds a molecule of hydrogen to one face of an unsaturated bond. A more specific technical definition could term it as the hydrogenation of prochiral substrates to preferentially produce one of two chiral products. First achieved with a heterogeneous palladium catalyst deposited on silk,[1] this field has since flourished predominantly in the realm of homogeneous catalysis. It was in this field that William Knowles and Ryōji Noyori were recognized with one half of the 2001 Nobel Prize in Chemistry for their pioneering contributions. Since that time, the field has continued to expand and a wide range of unsaturated starting materials and catalytic systems have been investigated and are now available for use. At its best, the technology is a remarkable example of a case where small, designed organometallic systems can mimic both the selectivity and productivity of enzymes.[2] It generates little waste and can be a cheap, effective and clean source of enantiomerically-pure materials that are otherwise difficult or impossible to derive from nature: characteristics that have led to its use as a technique in many commercial processes.[3][4]

LEG

Asymmetric hydrogenation is a chemical reaction that adds two atoms of hydrogen to a target (substrate) molecule with spatial selectivity. Critically, this selectivity does not come from the target molecule itself but from other reagents or catalysts present in the reaction, allowing this spatial information (what chemists refer to as chirality or stereoisomerism) to transfer from one molecule to the target. Most commonly, the chiral information is contained in a catalyst and in this case, the information in a single molecule of catalyst may be transferred to many substrate molecules, effectively amplifying the amount of chiral information present. This is akin to processes that occur in nature, where a single chiral molecule like an enzyme can produce many other chiral molecules, like glucose or amino acids, that a cell needs to function. By imitating this process, chemists can generate many novel synthetic molecules that interact with biological systems in specific ways, leading to new pharmaceutical agents and agrochemicals. The success of this reaction both in academia and industry is such that two of its pioneers, William Standish Knowles and Ryōji Noyori were awarded the 2001 Nobel Prize in Chemistry for their contributions to the field.

STored text

A remarkable feature of living organisms is that they generally synthesize and use only one enantiomer of a chiral molecule. Even very simple organisms like bacteria produce and consume solely the "L" enantiomer of amino acids and the "D" enantiomer of glucose. Pharmaceuticals, agrochemicals, and other molecules that scientists use to influence biological systems must often also be single enantiomers to ensure that they produce as specific an effect as possible. To synthesize these molecules, chemists often turn to asymmetric reactions that selectively produce one enantiomer of a target molecule and of these, one of the oldest and most widely used is asymmetric hydrogenation.

Asymmetric hydrogenation allows chemists to generate chiral molecules with high enantiomeric purity and overall yield.

Stored text

Research into asymmetric hydrogenation with heterogeneous catalysts has generally focused on three areas. The oldest, dating back to the first asymmetric hydrogenation with palladium deposited on a silk support, involves modifying a metal surface with a chiral molecule, usually one that can be harvested from nature. Alternatively, researchers have used various techniques to attempt to immobilize what would otherwise be homogeneous catalysts on heterogeneous supports or have used synthetic organic ligands and metal sources to build chiral metal-organic frameworks (MOFs).

Cinchonidine, one of the cinchona alkaloids

The greatest successes in chiral modification of metal surfaces have come from the use of cinchona alkaloids, though numerous other classes of natural products have been evaluated. These alkaloids have been shown to enhance the rate of substrate hydrogenation by 10–100 times, such that less than one molecule of cinchona alkaloid is needed for every reactive site on the metal and, in fact, the presence of too much of the chiral modifier can cause a decrease in the enantioselectivity of the reaction.

An alternative technique and one that allows more control over the structural and electronic properties of active catalytic sites is the immobilization of catalysts that have been developed for homogeneous catalyis on a heterogeneous support. Covalent bonding of the catalyst to a polymer or other solid support is perhaps most common, though immobilization of the catalyst may also be achieved by adsorption onto a surface, ion exchange, or even physical encapsulation. One drawback of this approach is the potential for the proximity of the support to change the behaviour of the catalyst, lowering the enantioselectivity of the reaction. To avoid this, the catalyst is often bound to the support by a long linker though cases are known where the proximity of the support can actually enhance the performance of the catalyst.

The final approach involves the construction of MOFs that incorporate chiral reaction sites from a number of different components, potentially including chiral and achiral organic ligands, structural metal ions, catalytically active metal ions, and/or preassembled catalytically active organometallic cores. This field is relatively new, and few examples exist of chiral asymmetric hydrogenation using these frameworks. One of these was reported in 2003, when a heterogeneous catalyst was reported that included structural zirconium, catalytically active ruthenium, and a BINAP-derived phosphonate as both chiral ligand and structural linker. As little as 0.005 mol% of this catalyst proved sufficient to achieve the asymmetric hydrogenation of aryl ketones, though the usual conditions featured 0.1 mol % of catalyst and resulted in an enantiomeric excess of 90.6–99.2%.

The active site of a heterogeneous zirconium phosphonate catalyst for asymmetric hydrogenation

Comments

Inevitably there is bound to be a lot of technical talk in an article like this,but I think you could help readers by trying some of the following.

• Give a bit of background before plunging into the research. Something like Unlike biological processes which often result in molecules consisting of one stereoisomer, synthetic reactions tend to produce both forms. Asymmetric hydrogenation is a process designed to produce single stereoisomers by selective reactions.
• I'd add a sentence explaining why this matters to the chemical industry. The current lead suggests it's just a few scientists messing around in labs. I'd also put back the Nobel prize bit, that's something easy for people to follow (realistically, few non-specialists are going to read past the lead, so engage them here).
• You need the lead to summarise the article, but see if any of the hard stuff can be moved or simplified in the lead.
• You need to help your readers by either linking or explaining technical terms at their first occurrence. I'd have the current lead paragraph (which I'd put after an introductory para) like this. Research into asymmetric hydrogenation with heterogeneous catalysts has generally focused on three areas. The oldest technique, dating back to an early success using the rare metal palladium deposited on a silk support, involves modifying a metal surface with a chiral molecule, usually one that can be harvested from nature. Alternatively, researchers have used various techniques to attempt to immobilize what would otherwise be homogeneous catalysts on heterogeneous supports, or used synthetic organic ligands and metal sources to build chiral metal-organic frameworks (MOFs). You can't have too many links in an article like this, we can always run a script to check for overlinking later.
• A couple of technical things, numbers should be separated with an endash (& followed immediately by ndash;) not hyphens, and numbers separated from their units by a non-breaking space(& followed immediately by  ).
• I'd omit scary reaction schemes from the lead, and add an undemanding image (one of the Nobel prize winners? A medicine made using this procedure?)

Text storage

A remarkable feature of living organsims is that they generally synthesize and use only one enantiomer of a chiral molecule. Even very simple organisms like bacteria produce and consume solely the "L" enantiomer of amino acids and the "D" enantiomer of glucose. To mimic this selectivity, chemists often turn to asymmetric reactions to produce one enantiomer of a target molecule and of these, one of the oldest and most widely used is asymmetric hydrogenation.

Hydrogenation allows chemists to form two new bonds between two atoms of hydrogen and elements like carbon, nitrogen, and oxygen. When these bonds are preferentially formed in a specific orientation relative to the substrate molecule (for example, from the top face of a flat substrate) the reaction becomes enantioselective and may be called asymmetric hydrogenation.