A Little Hard Time Does Photochemistry Good

Daniel S. Burgess

A technique under development at Tulane University and the University of British Columbia in Vancouver, Canada, promises to advance asymmetric photochemistry. Although current approaches allow chemists to produce unique variants of asymmetric molecules, they are insufficiently selective. The new method, which cages the reactants as they are irradiated, enables the chemists to better control the results so that up to 90 percent of the products are of the desired type.

Vaidyanathan Ramamurthy, a professor of organic chemistry at Tulane, and his research partners, including John R. Scheffer at the University of British Columbia, reported in the Journal of the American Chemical Society and Organic Letters that a commercially available zeolite can be used to select the results of photochemical reactions.

Zeolite prison blues

Organic molecules can be enantiomeric (mirror images of each other), or they can be diastereoisomeric and display asymmetry but not chirality. The asymmetric structures determine the differing properties of these chemically identical molecules, such as optical rotation, and photochemical reactions create them in specific proportions.

Typically, Ramamurthy said, only one form is suitable for an application, and the challenge is to maximize the excess of the favorable type. Performing a reaction in the confined space of a zeolite, they discovered, favors the formation of one variant of its products.

For example, when a 450-W mercury lamp irradiates tropolone ether and ephedrine in solution or on a silica surface, they produce two variants of diastereoisomeric molecules in a ratio of 1:1. When the same photochemical reaction occurs inside the NaY zeolite, however, the researchers found that it favors one over the other by 9:1. "It's like a prisoner in a jail cell," Ramamurthy said. "Maybe inside he is a nice guy. His behavior is different than what it would be outside."

The researchers are working to better understand the processes involved. In principle, the right combinations of zeolite and chiral inductor can boost excesses and enable chemists to select other products of photoreactions. Potential applications of the technique include the synthesis and separation of chemicals for photography.