RESEARCH
NMR reveals cycloheptatriene features CWRU chemists observe three distinct methyl resonances for the tert-butyl group in an organic molecule Room temperature l-methyl-7-tert-butyl-cycloheptatriene
7-tert-butyl 1-methyl
KK********"
- 2 4 ° C. 1-methyl (axial conformer)
7-tert-butyl (equatorial conformer) 7-tert-butyl (axial conformer)
1-methyl (equatorial conformer) 7-methine (axial conformer)
7-methine (equatorial conformer)
-104° C. 3 methyl
groups of 7-tert-butyl y (equatorial \ conformer) \
ing magnetic field
NMR spectrum for cycloheptatriene derivative changes as temperature drops
Two chemists at Case Western Reserve University have shown by nuclear magnetic resonance spectroscopy that a simple cycloheptatriene derivative undergoes slow ring inversion at room temperature. The result is surprising considering that cycloheptatriene itself must be cooled to an extremely low temperature (—170° C.) to measure its rate of interconversion. Low-temperature NMR studies of the derivative ( l-methyl-7-tert-butyl-cycloheptatriene) also show unique conformational features. Moreover, the studies provide the first direct NMR observation of three distinct methyl resonances for the tert-buty\ group in an organic molecule, according to Dr. Chris A. Cupas and William E. Heyd [/. Am. Chem. Soc, 91, 1559 (1969)]. The NMR spectrum of l-methyl-7tert-butyl-cycloheptatriene at room temperature shows, in addition to complex resonances of the olefinic protons, a broad absorption band from the 7-tert-butyl group and a singlet for the 1-methyl group, Dr. Cupas says. However, as the temperature of the sample is progressively lowered, some very unusual changes occur in the spectrum, he notes. For example, at —24° C , the tert-butyl resonance sharpens and gives rise to two singlets of unequal intensity in the spectrum. At the same time, the
1-methyl proton band undergoes a similar change and exhibits two signals of unequal intensity. Furthermore, two new signals appear in the spectrum—a doublet downfield from the other bands and a doublet appearing upfield between the two singlets of the tert-butyl resonances—attributable to 7-methine protons, Dr. Cupas explains. The most reasonable interpretation of these results, he feels, is that 1methyl-7-tert-butyl-cycloheptatriene exists as an equilibrating mixture of nonplanar conformers, with one of these conformers present in greater concentration. Two possible conformers of the compound exist—one with a syn or "axial" tert-butyl group, and one with an anti or "equatorial" tertbutyl group, Dr. Cupas says. Analogous conformers exist for any 7-substituted cycloheptatriene. Although the groups are not strictly axial or equatorial, it simplifies their identification to designate them so. Previous studies of cycloheptatriene systems by other chemists have shown that the axial-7-proton is shielded and therefore occurs upfield in the spectrum in comparison to the equatorial7-proton. By considering the areas of the bands in the —24° C. spectrum and the relative positions of the 7methine signals, Dr. Cupas and Mr. Heyd assign peaks in the spectrum
Cycloheptatriene derivative exists as equilibrating mixture of nonplanar conformers
"Axial" conformer
'Equatorial" conformer
l-methyl-7-tert-butyl-cycloheptatriene
42 C&EN APRIL 7, 1969
One of these
Experimental Molybdenum
Chemicals NONBONDED INTERACTION. Ν MR data of Dr. C. Cupas illustrate the im portance of nonbonded interactions in determining conformational preferences
C&EN—Ernie Carpenter
for 7-methine, 1-methyl, and 7-tertbutyl protons for both the axial and equatorial conformers of l-methyl-7terf-butyl-cycloheptatriene. The striking conclusion from exam ining the spectrum, Dr. Cupas says, is that the conformer with the axial7-ferf-butyl group is more stable. This is in contrast to the fact that in 7-terf-butyl-cycloheptatriene, the equatorial conformer is more stable. Holding his flexible model of 1-methyl7-ter£-butyl-cycloheptatriene and twisting it back and forth between the axial and equatorial forms, Dr. Cupas tells C&EN that these results indicate that the 1-methyl group's interaction with the tert-butyl group hinders ring inversion from the axial form to the equatorial form and thus illustrates the importance of nonbonded inter actions (allylic strain) in determining conformational preferences. The CWRU chemists determine the
CWRU group finds three magnetically nonequivalent methyl groups in 7-tert-butyl group of equatorial conformer
activation energy for the inversion process existing between the axial and equatorial conformers of the cy cloheptatriene derivative by compar ing the line shapes of the tert-butyl absorption in the experimental spectra with those of theoretical spectra hav ing various values of the mean life time. From this treatment and an Arrhenius plot of the rate data, the CWRU group finds the activation energy for the ring inversion at room temperature to be 18.9 kcal./mole. Cycloheptatriene, in comparison, has an activation energy for ring inver sion of 6.4 kcal./mole, and requires a temperature of —170° C. to slow the ring inversion process enough to measure it. By studying the change in equilib rium constant as a function of tem perature, Dr. Cupas and Mr. Heyd find that despite the fact that the axial conformer is favored by a sub stantial amount in terms of groundstate enthalpy, it is disfavored in terms of entropy. The free energy difference is thus small, so that both conformers can be observed in lowtemperature spectra, Dr* Cupas says. Pursuing the NMR studies at even lower temperatures than —24° C. re sults in striking changes in the spec trum of the equatorial conformer, says Dr. Cupas. As the temperature is lowered, the terf-butyl resonance for the less-stable equatorial conformer gradually broadens and then finally sharpens (at —104° C.) into three distinct singlets of equal intensity· Dr. Cupas ascribes these resonances to the three magnetically nonequivalent methyl groups in the ierf-butyl group of the equatorial conformer. This is the first example, he says, of direct NMR observation of three distinct methyl resonances for the tert-butyl group in an organic molecule. At such a low temperature, he explains, the rotation of the terf-butyl group in the conformer slows to the point where the NMR spectrometer can detect the three methyl groups.
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RESEARCH IN BRIEF
A multimillion-volt neutron generator for cancer research has been donated by Firestone to Cleveland Clinic, Cleveland, Ohio. The generator, called Dynagen (made by Radiation Dynamics, Inc.), will be the first of its kind in the U.S. and about 40% more powerful than the only other existing one, which was installed in Germany last December. Besides be ing used for experimental cancer ther apy, the equipment may be used to produce radioactive materials for tumor diagnosis, to provide neutron activation analysis for identifying sub stances in body tissue and fluids, and to take high-contrast photographs of internal organs. The generator is ex pected to be operating later this year. Firestone, active in commercial radiation research in recent years, says that it decided to make the contribu tion because of the great promise Dynagen holds for eventual control of cancer.
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44 C&EN APRIL 7, 1969
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A nematic liquid crystal has been de veloped by Liquid Crystal Industries. The Turtle Creek, Pa., company claims that the development opens up the area of electro-optical devices to the application of liquid crystals. The crystals can now be used in devices such as optical shutters, flat screen dis plays, alpha numeric displays, and window and mirrors that automatically dim or brighten in response to outside light, according to LCI vice president Edward N. Sharpless.
The first peptide synthesizing ma chine is on the market just three months after the announcement from Rockefeller University and Merck Sharp & Dohme research laboratories of the first synthesis of an enzyme, ribonuclease (C&EN, Feb. 10, page 6 2 ) . The synthesizer is coming out this month from Schwarz BioResearch, a division of Becton, Dickinson and Co., in Orangeburg, N.Y. Under license from the Danish Institute of Protein Chemistry, the unit applies solid-phase methods—largely from Dr. R. B. Merrifield's work at Rockefel ler—to peptide synthesis. So far, Schwarz BioResearch states, several biologically interesting chemicals, in cluding hormones, have been pro duced in the synthesizer. Larger peptides, including enzymes, should be possible. The heart of the ma chine, its maker states, is an auto mated sequential control unit pro gramed to direct events in peptide synthesis. Cost of the machine, in cluding a year's service and training, is $18,500.
