Efforts toward Synthetic Photosynthesis: Visible Light-Driven CO₂ Valorization

Abstract

Current methods of urethane preparation from amines invariably involve high-energy and often toxic or cumbersome molecules to make the process exergonic. CO 2 aminoalkylation using olefins and amines represents an attractive albeit endergonic alternative. We report a moisture-tolerant method that uses visible light energy to drive this endergonic process (+25 kcal/mol at STP) using sensitized arylcyclohexenes. They convert much of the photon's energy to strain upon olefin isomerization. This strain energy greatly enhances alkene basicity, allowing for sequential protonation by and interception of ammonium carbamates. Following optimization steps and amine scope evaluation, an example product arylcyclohexyl urethane underwent transcarbamoylation with some demonstrative alcohols to form more general urethanes with concomitant regeneration of the arylcyclohexene. This represents a closure of the energetic cycle, producing H 2 O as the stoichiometric byproduct.

Probing the Visible Light‐Driven Geometrical Isomerization of 4‐Arylbut‐3‐ene‐2‐amines

“…These rates were then converted to ζ p with the help of the Ru(bpy) 3 Cl 2 -9,10-diphenylanthracene actinometer [18] which determined I 0 to be 30 nmol hν/s in a prior investigation in our lab. [19] Proximity and relative orientation conducive to orbital overlap are prerequisites for efficient electron exchange energy transfer between an excited donor and acceptor. [13] 2 and 4 had lower ζ p than 1, which may be indicative of a reduced accessibility of their frontier orbitals to successful collision with excited sensitizer.…”

“…I 0 (the photon flux) was found to be 30 nmol/s from a previous study. [19] ζ p experiments were all carried out with 68,000 nmol of amine. The moles of Z formed are estimated as the product of the mole fraction of Z-isomer (determined by 1 H NMR) and the total moles of amine present (68,000 nmol).…”

“…The authors have cited additional references, elaborated on experimental details, and provided NMR spectra supporting tabulated PS ratios and purified E-compounds within the Supporting Information. [19,21] Raw data files and full annotated spectra are available from the corresponding author upon reasonable request.…”

Exploiting Visible Light Triggered Formation of trans-Cyclohexene for the Contra-thermodynamic Protection of Alcohols

“…An important distinguishing feature of photochemistry (or photocatalysis) is the ability to pump energy into the system and molecular motion that is generally inaccessible on the ground state surface, recognized in the early days of the 20th century. − Chemists have shown that visible light can drive alkene classes away from their thermodynamic equilibrium. − During the 1960s and 1970s Kropp, Bonneau, and Dauben studied the geometrical isomerization of cyclic alkenes which enabled a number of unique transformations that were due to captured strain. − Our recent efforts have focused on exploiting visible light to generate such strain and using it to accomplish synthetic objectives. − …”

“…We are particularly interested in the development of molecules that can effectively, and repeatedly, convert photochemical energy to useable ground state potentialan energy “currency” . To this end, we are drawn toward cyclohexene, which is predicted to harvest a large portion of the available photochemical energy, approximately 52 kcal/mol. − Indeed, while transient, its existence has been established .…”