CO2, as the ideal C1 synthon, is an abundant and renewable feedstock for the synthesis of fine chemicals. However, its high kinetic and thermodynamic stability often makes it challenging to incorporate into organic transformations under mild conditions. The emergence of photoredox catalysis, however, has enabled the straightforward incorporation of CO2 by harvesting the energy of visible light. Reactions of CO2 with C—C unsaturated bonds, C—X bonds and amines have been demonstrated under photoredox catalysis.
Given the continued interest in the biological applications of unnatural amino acids, we were intrigued by the prospect of using CO2 and imines as building blocks for amino acid synthesis. We previously demonstrated that ketimines participated in photoredox catalysis to generate intermediates with nucleophilic reactivity at the carbonyl carbon, which abstracted protons from water to afford reduction products (DOI: 10.1021/acs.orglett.8b00778) or added to electrophiles (eg. aldehydes) to achieve cross-electrophile couplings (DOI: 10.1021/acs.orglett.8b03394). We were, therefore, interested in determining if the nucleophilicity of this intermediate generated in our system was sufficiently strong enough to add to CO2 to yield α-amino acids.
To our delight, the desired amino acids were obtained in high yields with Cy2NMe as the sacrificial electron donor, leading to precipitation of the product from the reaction mixture (Figure 1). In addition to allowing isolation by simple filtration, precipitation prevents the product from undergoing photoredox-promoted decarboxylation.
The key benefits of this method include: 1) use of atmospheric pressure of CO2, 2) purification of the products by filtration (no chromatography!), and 3) utilization of visible light. These advantages enabled facile scaling of this α-amino acid synthesis with CO2 using sunlight energy. This was demonstrated with a 10-gram reaction using outdoor sunlight (87% yield).
For more details, please see: Nat Comm (2018), DOI: 10.1038/s41467-018-07351-2 by: Xinyuan Fan, Xu Gong, Mengyue Ma, Rui Wang and Patrick J. Walsh.