science/technology
Theory and experiment agree on stereomutation mechanism
Weston T. Borden and graduate student Kinetics study shows disrotatory Scott B. Lewis of the ring opening is favored University of Wash Conrotatory Disrotatory ington, Seattle \J. Am. Chem. Soc., 120, 6187 H3C CH2CH3 In work where the antecedents stretch (1998)]. H H FyF back 30 years, theory and experiment The experimental s-trans,s-trans Gri2Crl3 HgC^/VH H3C have been shown to agree on the mech findings verify a 1994 H CH2CH3 anism by which a substituted cyclopro theoretical prediction cis trans pane ring opens and recloses. by Borden and co In an experimental study, researchers workers that concert Conrotatory Disrotatory Fv>F show that thermal ring-opening and re- ed disrotation would closing of a 1,1-difluorocyclopropane oc be favored in the ther H3C Η curs by concerted disrotation—a mecha mal ring-opening of s-cis,s-trans nism in which the ring opens by simulta 1,1-difluorocycloproneous rotations in opposite directions of pane. They also rep the molecule's two nonfluorinated meth resent an extension ylene groups. The reaction is a thermal of a 1968 prediction by chemistry pro sor of chemistry Jerome A. Berson of stereomutation—a heat-induced reaction fessor Roald Hoffmann of Cornell Uni Yale University, whose research inter in which the cyclopropane ring opens versity that the ring of unsubstituted cy ests include reaction mechanisms and and then recyclizes to give a stereoiso clopropane would open by concerted rearrangements. "There are not many mer of the reactant. conrotation (simultaneous rotation in the cases in which nonintuitive predictions The work was carried out by chemis same direction). Hoffmann's original pre based on all-out quantum mechanical try professor William R. Dolbier Jr. and diction has proven difficult to confirm calculations have been confirmed by ex periment. Experimentalists and theoreti graduate students Feng Tian and Michael experimentally. The experiment is "interesting and cians should be happy with this result D. Bartberger of the University of Florida, Gainesville, and chemistry professor very significant," says emeritus profes- because it tells us that we're on the same page." Thirty years ago, Hoffmann first pre dicted that ring opening and closing of cyclopropane would occur by coupled conrotation of two of the compound's A LEADER IN THE P R O D U C T I O N methylene groups, yielding a diradical triO F ALIPHATIC A M I N E S A N D methylene intermediate. He calculated DERIVATIVES INCLUDING: that the energetics of interaction be tween orbitals of the intermediate's two • Primary Ether Amines & Diamines terminal CH2 groups with orbitals of the • Ethoxylated Ether Amines central CH2 group would favor that • Quaternaries · Amine Oxides mechanism. • Amphoterics · Specialty Surfactants "Hoffmann was the first to point out
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34 JUNE 29, 1998 C&EN
Borden: theoretical prediction verified
opening of the difluorocyclopropanes, and now [Dolbier, Borden, and coworkers] are the first to demonstrate the point experimentally. It's a case where agreement between theory and experiment has been achieved—and one really likes to see that. I welcome these results." Baldwin also stresses the importance of Hoffmann's original contribution. "He was one of the first to have the imagination—and courage—to propose that theory might have something to say about alternative, closely competitive reaction
Dolbier: experimental corroboration
that kind of interaction and how important it could be for this kind of thermal reaction," says Berson. However, experiments on dideuterocyclopropane by Berson's group and others in subsequent years to test Hoffmann's prediction led to contradictory results, and the validity of Hoffmann's proposal has remained unclear. The new findings have not resolved that standoff. But in 1994, Borden and coworkers extended Hoffmann's idea that orbital interactions can determine the course of a reaction to a different class of molecules— 1,1-difluorocyclopropanes. They calculated that the preference for concerted disrotation would be much enhanced in 1,1-difluorocyclopropanes, compared with the conrotation predicted for unsubstituted cyclopropanes. This prediction on the reaction mechanism of 1,1-difluorocyclopropanes is what has now been confirmed experimentally. Dolbier, Borden, and their colleagues studied the kinetics of the thermal stereomutation of two alkylated derivatives. They found that a chiral ds-l,l-difluoro-2-ethyl-3methylcyclopropane reacts faster to give its enantiomer than does the corresponding trans compound. The results indicate that the cis compound opens to form the more stable s-trans,s-trans diradical, that the trans compound forms the less stable frcis,s-trans diradical, and that both compounds therefore open by a disrotatory mechanism. "It's a very nice demonstration of the effect picked up in the earlier theoretical study," comments chemistry professor John E. Baldwin of Syracuse University, who also has worked on the cyclopropane stereomutation problem. Borden and coworkers "were the first to predict this disrotatory preference for the
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paths," he says. "We take that for granted today, but at the time that was just way out in front of everyone." Baldwin adds, "Hoffmann's work led to an outpouring of very good theory, and it really set a new standard for what we mean by understanding a reaction. One wants discriminating experimental and theoretical evidence, and when the two disagree, one works hard tofindout where the problem is. Today, that's part of our culture, but in 1968 it was a big step forward." Stu Borman
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