Novel Green Catalysts for Organic Reactions, Organometallic Self-Assembled Systems.
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Researchers in our group consist of graduate students finishing up their degrees and postdoctorals who enjoy working on organic or inorganic projects, or on projects that span both fields. Although the research approaches they use are fundamental ones, they keep their eyes open for applications of their chemistry. This strategy has resulted in 20 patents, and industrial licenses to Aldrich and Strem to manufacture and market three of our group's "superbase"/catalysts of type 1.
It all began with the accidental discovery in our group that football-shaped molecules of type 1 are terrific non-ionic bases. Not long afterward, we accidentally discovered that these molecules are superior catalysts for a wide variety of important organic reactions. Several of these applications have produced patents bearing the names of students and post doctorals in our group as co-inventors.
One of the novel features of our catalysts is the extraordinary stability of structure 2. This stability is so great that Lewis base 1 is about 8 orders of magnitude stronger than any amine known, including commercially used DBU, DBN and Proton Sponge! We have successfully utilized molecules of type 1 in a variety of syntheses catalyzed by bases, such as alkylations, dehydrohalogenations, acylations, a variety of condensations and a series of palladium-catalyzed reactions useful for making carbon-carbon and carbon-nitrogen bonds. Our nonionic superbase catalysts 1 have the advantage that, unlike strong ionic bases, unwanted side reactions are minimized or eliminated completely.
A second novel characteristic of our bases of type 1 is that substituents Z in 3 (other than a proton) can cause only partial transannulation as shown in this structure. These substituents can be organometallic or nonmetallic groups. This flexibility in transannular P-N bonding is a crucial factor in the ability of 1 to act as a superior catalyst for a continuously widening range of important reactions such as protecting alcohol groups (e.g., of pharmaceutical intermediates) with various silyl groups during multistep syntheses, converting isocyanates to isocyanourates (which are commercially important in the stabilization of Nylon-6 during its manufacture) and the synthesis of alpha, beta-unsaturated nitriles (which are important in the synthesis of pharmaceuticals, pigments and perfumes).
Experiments are currently underway to bind our superbase/catalysts 1 to organic polymer solid supports so that product separation will be much easier. The organic linker from the polymer chain will be to one of the P-N nitrogens of 1. The phosphorus of 1 will then be free to act as a very strong base and also as an electron-rich atom for complexing metals that facilitate metal-assisted C-C and C-N couplings, for example. This achievement will lead to new applications of our super base/catalyst chemistry, because the polymer-bound catalyst system can be easily recovered for recycling via filtration. This will be particularly important in economizing certain steps in drug syntheses in the pharmaceutical industry, for example. This also allows fine tuning of the basicity of the phosphorus for wider applications of these heterogeneous catalysts, by placing groups such as =O, =S, RN= and RN=N-N= on the phosphorus atom.
We have begun a program to synthesize enantiomeric versions of 1. This has been done by making one (or more) of the R groups a chiral one, and also by having each R group be a different nonchiral substituent, thus making the phosphorus the chiral center. This opens up the possibility of carrying out asymmetric reactions (e.g., chiral deprotonation of racemic compounds to generate carbanionic nucleophiles of a specific chirality).
To further expand the scope of important transformations promoted by 1, we are interested in developing even stronger superbase/catalyst systems by placing substituents on the equatorial nitrogens of 1 that can better stabilize structures of types 2 and 3 via extensive delocalization of double bonds.
It seems counterintuitive, but a positively charged species such as 2 can behave as a Lewis acid. We have shown this to be the case when certain other substituents such as a fluorine atom or CN are placed at the apical position instead of a hydrogen. Preliminary evidence suggests that the phosphorus atom is the Lewis acidic site that becomes coordinated by a nucleophile, thus activating the electrophilic center of a reactant. We have also synthesized 4 in which we have shown that (again counter-intuitively) the Lewis acidity of the aluminum is stronger than the boron in BF3! We have shown that aldehyde and ketone oxygens can coordinate to the aluminum in 4, thus activating the electrophilic carbonyl carbon center to nucleophilic attack.
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