Andy S. Tsai

Rhodium catalyzed chelation-assisted C-H bond functionalization reactions

Over the last several decades, researchers have achieved remarkable progress in the field of organometallic chemistry. The development of metal-catalyzed cross-coupling reactions represents a paradigm shift in chemical synthesis, and today synthetic chemists can readily access carbon-carbon and carbon-heteroatom bonds from a vast array of starting compounds. Although we cannot understate the importance of these methods, the required prefunctionalization to carry out these reactions adds cost and reduces the availability of the starting reagents. The use of C-H bond activation in lieu of prefunctionalization has presented a tantalizing alternative to classical cross-coupling reactions. Researchers have met the challenges of selectivity and reactivity associated with the development of C-H bond functionalization reactions with an explosion of creative advances in substrate and catalyst design. Literature reports on selectivity based on steric effects, acidity, and electronic and directing group effects are now numerous. Our group has developed an array of C-H bond functionalization reactions that take advantage of a chelating directing group, and this Account surveys our progress in this area. The use of chelation control in C-H bond functionalization offers several advantages with respect to substrate scope and application to total synthesis. The predictability and decreased dependence on the inherent stereoelectronics of the substrate generally result in selective and high yielding transformations with broad applicability. The nature of the chelating moiety can be chosen to serve as a functional handle in subsequent elaborations. Our work began with the use of Rh(I) catalysts in intramolecular aromatic C-H annulations, which we further developed to include enantioselective transformations. The application of this chemistry to the simple olefinic C-H bonds found in α,β-unsaturated imines allowed access to highly substituted olefins, pyridines, and piperidines. We observed complementary reactivity with Rh(III) catalysts and developed an oxidative coupling with unactivated alkenes. Further studies on the Rh(III) catalysts led us to develop methods for the coupling of C-H bonds to polarized π bonds such as those in imines and isocyanates. In several cases the methods that we have developed for chelation-controlled C-H bond functionalization have been applied to the total synthesis of complex molecules such as natural products, highlighting the utility of these methods in organic synthesis.

Rhodium(III)-Catalyzed Arylation of Boc-Imines via C−H Bond Functionalization

The first rhodium-catalyzed arylation of imines proceeding via C-H bond functionalization is reported. Use of a non-coordinating halide abstractor is important to obtain reactivity. Aryl-branched N-Boc-amines are formed, and a wide range of functionality is compatible with the reaction.

Rh(III)-catalyzed oxidative coupling of unactivated alkenes via C-H activation.

Oxime directed aromatic C-H bond activation and oxidative coupling to alkenes is reported using a cationic Rh(III) catalyst. Significantly, the method can be used to oxidatively couple unactivated, aliphatic alkenes.

Asymmetric synthesis of (-)-incarvillateine employing an intramolecular alkylation via Rh-catalyzed olefinic C-H bond activation.

An asymmetric total synthesis of (-)-incarvillateine, a natural product having potent analgesic properties, has been achieved in 11 steps and 15.4% overall yield. The key step is a rhodium-catalyzed intramolecular alkylation of an olefinic C-H bond to set two stereocenters. Additionally, this transformation produces an exocyclic, tetrasubstituted alkene through which the bicyclic piperidine moiety can readily be accessed.

Atomic Layer Deposition of L-Alanine Polypeptide

L-Alanine polypeptide thin films were synthesized via atomic layer deposition (ALD). Instead of using an amino acid monomer as the precursor, an L-alanine amino acid derivatized with a protecting group was used to prevent self-polymerization, increase the vapor pressure, and allow linear cycle-by-cycle growth emblematic of ALD. The successful deposition of a conformal polypeptide film has been confirmed by FTIR, TEM, and Mass Spectrometry, and the ALD process has been extended to polyvaline.

Correction to Chiral Brønsted Acid Catalyzed Enantioselective Synthesis ofanti-Homopropargyl Alcohols via Kinetic Resolution-Aldehyde Allenylboration Using RacemicAllenylboronates

The structures for 12 and 13 in the published Scheme 4 were inadvertently drawn incorrectly, with the methyl and cyclohexyl groups transposed in the original structures. The corrected Scheme 4 appears below.