If you are interested in working with Prof Johnson, please email him at Adam_Johnson@hmc.edu to set up an appointment to discuss the research opoportunities in more detail.

Organometallic chemistry and asymmetric catalysis. Organometallic chemistry has it all: synthesis, characterization, physical methods, catalysis, mechanistic studies, plus, you get to study molecules containing carbon and metals. My research involves the design and synthesis of amino alcohol ligands with tunable steric and electronic properties in order to develop better organometallic catalysts for interesting organic transformations. We use the standard techniques of organic synthesis as well a glove box or Schlenk line for working with air sensitive transition metal complexes. We use our complexes for a variety of catalytic asymmetric reactions such as intramolecular hydroamination and aldehyde alkylation. We study these reactions by a variety of methods, including variable temperature one- and two-dimensional NMR spectroscopy, kinetics experiments and theoretical modeling.

Synthesis of chiral ligands from amino acids: Chiral bi- and tridentate chelators are synthesized starting from commercially available amino acids. The syntheses are relatively short (3-5 steps, typically) and high-yielding, and use a variety of important organic trans¬for¬mations such as reductive amination, Grignard alkylation, and amide bond formation. Some of ligands my group is currently targeting are shown here.  Our “first-generation” ligands have small substituents at the alkohol carbon (R = H), but later work has increased the steric bulk at that position in our “second-generation” ligands.

Structural characterization. A major part of our research program is the determination of molecular structure through singe crysal X-ray diffraction. We grow the crystals at –35°C in our glove box, and then send them to one of my collaborators for characterization. A sample crystal structure is shown below.

R = iPr or c-C6H11;  R* = Ph or CH2Ph.

Catalytic alkylation of benzaldehyde. A simple reaction is used to rapidly test our ligands and complexes for catalytic activity. Benzaldehyde reacts with dialkyl zinc reagents (in the presence of a Lewis Acid catalyst such as Ti(OiPr)4 to form racemic alkylation products (diethylzinc gives 1-phenyl-1-propanol, shown below). Our titanium complexes are chiral, and we therefore observe an excess of one enantiomer over the other, this is known as enantiomeric excess: (%major enantiomer - %minor enantiomer). Our catalysts, derived from L-amino acids, give the R product with ee%’s ranging from the low teens for our “first-generation” catalysts, and up into the 70s for our “second-generation” catalysts.

Catalytic hydroamination chemistry of chiral titanium complexes. Titanium complexes catalyze the intramolecular hydroamination of 1,3-aminoallenes as shown here:

Our complexes with “first-generation” ligands form product 1 in high yield (100% when R’ = Me), although the enantiomeric excess is low. We have not yet tested the “second-generation ligands” in the hydroamination reaction. In order to help verify that the above mechanism is operating with our catalysts, we need to carry out kinetic experiments. The synthesis and isolation or trapping of a titanium imido complex (Ti=NR bond) would show that our ligand system can support imido formation. A possible synthesis of an imido is shown below.

New substrates for catalytic hydroamination. All of our hydroamination work has used methyl substituted aminoallene substrates, and we hope to expand the substrate scope to include bulkier substituents, longer chains, and aminoalkenes. Some substrate targets are shown here. Substrates can be synthesized in a straightforward multi-step organic synthesis.