Bidentate chelates with larger spacers: Chelating Lewis acids prepared by double hydroalumination of dialkynylsilanes

Chelating Lewis acids with a geminal arrangement of two acceptor functions have been shown to coordinate halide, thiolate, or benzoate anions. The remarkable efficacy of the chelating coordination of hydride ions by two aluminum atoms by the formation of persistent carbocations through C-H bond activation is also known. However, often the acceptor atoms occupy geminal positions at a bridging carbon atom, which results in relatively strained four-membered heterocycles upon coordination of single-atom donors.

Therefore, a team of scientists led by Werner Uhl of the University of Münster (Germany) were very much interested in synthesizing compounds that have larger spacers between the acceptor functions in order to obtain more flexible backbones and hence better coordinating properties. The twofold hydroalumination of silicon-centered dialkynes was employed as a facile route for the preparation of such compounds, as reported in the European Journal of Inorganic Chemistry.

During the double hydroalumination of dialkynylsilanes, mixed alkenyl–alkynyl compounds resulting from the reduction of only one C?C triple bond were obtained as intermediates, two of which were isolated and characterized. Hydroalumination of the remaining C?C triple bond yielded dialkenyl species that were ideally preorganized to be applied as chelating Lewis acids, which was demonstrated by the chelation of chloride ions. In addition, an alkenyl–alkynylsilane intermediate gave a silacyclobutene derivative by 1,1-carbalumination; this is the second time such a reaction has been observed. The mechanism of this reaction was investigated by quantum chemical calculations.

This study reports an easy way to synthesize chelating Lewis acids with two geminal acceptor aluminum atoms. The chelating coordination of chloride ions by both aluminum atoms to give a six-membered ClAl2C2Si heterocycle was demonstrated.

More information: Werner Uhl, Hydroalumination of Bis(alkynyl)silanes: Generation of Chelating Lewis Acids, Their Application in the Coordination of Chloride Ions and a 1,1-Carbalumination Reaction, European Journal of Inorganic Chemistry, http://dx.doi.org/ … ic.201100890

Provided by Wiley (news : web)

Chemists transform acids into bases

Chemists at the University of California, Riverside have accomplished in the lab what until now was considered impossible: transform a family of compounds which are acids into bases.

As our chemistry lab sessions have taught us, acids are substances that taste sour and react with metals and bases (bases are the chemical opposite of acids). For example, compounds of the element boron are acidic while nitrogen and phosphorus compounds are basic.

The research, reported in the July 29 issue of Science, makes possible a vast array of chemical reactions – such as those used in the pharmaceutical and biotechnology industries, manufacturing new materials, and research academic institutions.

"The result is totally counterintuitive," said Guy Bertrand, a distinguished professor of chemistry, who led the research. "When I presented preliminary results from this research at a conference recently, the audience was incredulous, saying this was simply unachievable. But we have achieved it. We have transformed boron compounds into nitrogen-like compounds. In other words, we have made acids behave like bases."

Bertrand's lab at UC Riverside specializes on catalysts. A catalyst is a substance – usually a metal to which ions or compounds are bound – that facilitates or allows a chemical reaction, but is neither consumed nor altered by the reaction itself. Crucial to the reaction's success, a catalyst is like the car engine enabling an uphill drive. While only about 30 metals are used to form catalysts, the binding ions or molecules, called ligands, can number in the millions, allowing for numerous catalysts. Currently, the majority of these ligands are nitrogen- or phosphorus-based.

"The trouble with using phosphorus-based catalysts is that phosphorus is toxic and it can contaminate the end products," Bertrand said. "Our work shows that it is now possible to replace phosphorus ligands in catalysts with boron ligands. And boron is not toxic. Catalysis research has advanced in small, incremental steps since the first catalytic reaction took place in 1902 in France. Our work is a quantum leap in catalysis research because a vast family of new catalysts can now be added to the mix. What kind of reactions these new boron-based catalysts are capable of facilitating is as yet unknown. What is known, though, is that they are potentially numerous."

Bertrand explained that acids cannot be used as ligands to form a catalyst. Instead, bases must be used. While all boron compounds are acids, his lab has succeeded in making these compounds behave like bases. His lab achieved the result by modifying the number of electrons in boron, with no change to the atom's nucleus.

"It's almost like changing one atom into another atom," Bertrand said.

His research group stumbled upon the idea during one of its regular brainstorming meetings.

"I encourage my students and postdoctoral researchers to think outside the box and not be inhibited or intimidated about sharing ideas with the group," he said. "The smaller these brainstorming groups are, the freer the participants feel about bringing new and unconventional ideas to the table, I have found. About 90 percent of the time, the ideas are ultimately not useful. But then, about 10 percent of the time we have something to work with."

The research was supported by grants to Bertrand from the National Science Foundation and the U.S. Department of Energy.

An internationally renowned scientist, Bertrand came to UCR in 2001 from France's national research agency, the Centre National de la Recherche Scientifique (CNRS). He is the director of the UCR-CNRS Joint Research Chemistry Laboratory.

A recipient of numerous awards and honors, most recently he won the 2009-2010 Sir Ronald Nyholm Prize for his seminal research on the chemistry of phosphorus-phosphorus bonds and the chemistry of stable carbenes and their complexes.

He is a recipient of the Japanese Society for Promotion of Science Award, the Humboldt Award, the International Council on Main Group Chemistry Award, and the Grand Prix Le Bel of the French Chemical Society. He is a fellow of the American Association for the Advancement of Sciences, and a member of the French Academy of Sciences, the European Academy of Sciences, Academia Europea, and Academies des Technologies.

He has authored more than 300 scholarly papers and holds 35 patents.

Provided by University of California - Riverside (news : web)

Chemists transform acids into bases: Research offers vast family of new catalysts for use in drug discovery, biotechnology

Chemists at the University of California, Riverside have accomplished in the lab what until now was considered impossible: transform a family of compounds which are acids into bases.

As our chemistry lab sessions have taught us, acids are substances that taste sour and react with metals and bases (bases are the chemical opposite of acids). For example, compounds of the element boron are acidic while nitrogen and phosphorus compounds are basic.

The research, reported in the July 29 issue of Science, makes possible a vast array of chemical reactions -- such as those used in the pharmaceutical and biotechnology industries, manufacturing new materials, and research academic institutions.

"The result is totally counterintuitive," said Guy Bertrand, a distinguished professor of chemistry, who led the research. "When I presented preliminary results from this research at a conference recently, the audience was incredulous, saying this was simply unachievable. But we have achieved it. We have transformed boron compounds into nitrogen-like compounds. In other words, we have made acids behave like bases."

Bertrand's lab at UC Riverside specializes on catalysts. A catalyst is a substance -- usually a metal to which ions or compounds are bound -- that facilitates or allows a chemical reaction, but is neither consumed nor altered by the reaction itself. Crucial to the reaction's success, a catalyst is like the car engine enabling an uphill drive. While only about 30 metals are used to form catalysts, the binding ions or molecules, called ligands, can number in the millions, allowing for numerous catalysts. Currently, the majority of these ligands are nitrogen- or phosphorus-based.

"The trouble with using phosphorus-based catalysts is that phosphorus is toxic and it can contaminate the end products," Bertrand said. "Our work shows that it is now possible to replace phosphorus ligands in catalysts with boron ligands. And boron is not toxic. Catalysis research has advanced in small, incremental steps since the first catalytic reaction took place in 1902 in France. Our work is a quantum leap in catalysis research because a vast family of new catalysts can now be added to the mix. What kind of reactions these new boron-based catalysts are capable of facilitating is as yet unknown. What is known, though, is that they are potentially numerous."

Bertrand explained that acids cannot be used as ligands to form a catalyst. Instead, bases must be used. While all boron compounds are acids, his lab has succeeded in making these compounds behave like bases. His lab achieved the result by modifying the number of electrons in boron, with no change to the atom's nucleus.

"It's almost like changing one atom into another atom," Bertrand said.

His research group stumbled upon the idea during one of its regular brainstorming meetings.

"I encourage my students and postdoctoral researchers to think outside the box and not be inhibited or intimidated about sharing ideas with the group," he said. "The smaller these brainstorming groups are, the freer the participants feel about bringing new and unconventional ideas to the table, I have found. About 90 percent of the time, the ideas are ultimately not useful. But then, about 10 percent of the time we have something to work with."

The research was supported by grants to Bertrand from the National Science Foundation and the U.S. Department of Energy.

An internationally renowned scientist, Bertrand came to UCR in 2001 from France's national research agency, the Centre National de la Recherche Scientifique (CNRS). He is the director of the UCR-CNRS Joint Research Chemistry Laboratory.

A recipient of numerous awards and honors, most recently he won the 2009-2010 Sir Ronald Nyholm Prize for his seminal research on the chemistry of phosphorus-phosphorus bonds and the chemistry of stable carbenes and their complexes.

He is a recipient of the Japanese Society for Promotion of Science Award, the Humboldt Award, the International Council on Main Group Chemistry Award, and the Grand Prix Le Bel of the French Chemical Society. He is a fellow of the American Association for the Advancement of Sciences, and a member of the French Academy of Sciences, the European Academy of Sciences, Academia Europea, and Academies des Technologies.

He has authored more than 300 scholarly papers and holds 35 patents.

Bertrand was joined in the research by Rei Kinjo and Bruno Donnadieu of UCR; and Mehmet Ali Celik and Gernot Frenking of Philipps-Universitat Marburg, Germany.

UCR's Office of Technology Commercialization has filed a provisional patent application on the boron-based ligands developed in Bertrand's lab.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by University of California - Riverside.

Journal Reference:

R. Kinjo, B. Donnadieu, M. A. Celik, G. Frenking, G. Bertrand. Synthesis and Characterization of a Neutral Tricoordinate Organoboron Isoelectronic with Amines. Science, 2011; 333 (6042): 610 DOI: 10.1126/science.1207573

Organic chemistry: Amino acids made easy

Amino acids are the building blocks of proteins. There are 22 different amino acids and they can combine in a myriad ways to form a vast array of proteins. All amino acids except glycine are chiral molecules, meaning they exist in two mirror-image, or enantiomeric, forms—only one of which is naturally occurring. These unnatural enantiomers of amino acids are in great demand by the pharmaceutical industry as the raw materials for the production of a variety of drugs, including the antibiotic amoxycillin and the anti-nausea drug aprepitant (see image).

One method that is widely used to produce amino acids generally is Strecker synthesis—a chemical reaction devised by the nineteenth-century German chemist Adolph Strecker that combines an aldehyde, ammonia and hydrogen cyanide to produce an aminonitrile that can be easily converted into an amino acid. Unfortunately, the original Strecker synthesis can only produce a mixture of the enantiomeric forms of an amino acid. For this reason, many chemists have taken an interest in the development of enantioselective, or asymmetric, catalytic reactions—reactions that use a catalyst to selectively increase the formation of a particular enantiomer.

Some catalytic enantioselective variations of Strecker synthesis have already been reported, but there are problems. Many require the use of expensive sources of cyanide—typically a compound called trimethylsilylcyanide—and very low temperatures, which can be difficult to achieve on an industrial scale.

Abdul Majeed Seayad at the A*STAR Institute of Chemical and Engineering Sciences and co-workers have now developed an asymmetric Strecker protocol that uses hydrogen cyanide as the cyanide source and which proceeds at room temperature. The new methodology still requires the use of trimethylsilylcyanide, which the researchers found to be essential to achieving an enantioselective reaction, but only a relatively small catalytic amount is required and it is regenerated in the reaction by the addition of cheaper hydrogen cyanide. Seayad and his co-workers showed that they can use their conditions to produce a variety of unnatural amino acids.

As with most methodology developments, there is room for improvement with further research. “So far we’ve tackled only amino acids with aromatic side-chains,” explains Seayad. “We would like to develop the process to produce amino acids with other side chains. Hydrogen cyanide is inexpensive but it is extremely toxic and special equipment and training are needed to handle it. We are exploring ways in which we might generate it in the reaction, which would be much safer.”

More information: Ramalingam, B. et al. A remarkable titanium-catalyzed asymmetric Strecker reaction using hydrogen cyanide at room temperature. Advanced Synthesis and Catalysis 352, 2153–2158 (2010). http://dx.doi.org/ … sc.201000462

Provided by Agency for Science, Technology and Research (A*STAR)