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Direct studies of 1,1-diazenes. Syntheses, infrared and electronic spectra, and kinetics of the thermal decomposition of N-(2,2,6,6-tetramethylpiperidyl)nitrene and N-(2,2,5,5-tetramethylpyrrolidyl)nitrene

Hinsberg, William D., III and Schultz, Peter G. and Dervan, Peter B. (1982) Direct studies of 1,1-diazenes. Syntheses, infrared and electronic spectra, and kinetics of the thermal decomposition of N-(2,2,6,6-tetramethylpiperidyl)nitrene and N-(2,2,5,5-tetramethylpyrrolidyl)nitrene. Journal of the American Chemical Society, 104 (3). pp. 766-773. ISSN 0002-7863. http://resolver.caltech.edu/CaltechAUTHORS:20160511-154702150

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Abstract

The syntheses, direct spectroscopic observation, and kinetics of thermal decomposition of the persistent 1,1-diazenes, N-(2,2,6,6-tetramethylpiperidyl)nitrene (4) and N-(2,2,5,5-tetramethylpyrrolidyl)nitrene (5) are reported. The electronic absorption spectrum of 4 at -78 °C reveals a structured absorption for the n,π* transition: λmax= 543 nm, λ_(0,0) = 620 nm, and εmax = 18 ± 3 in Et_2O; λmax= 541 nm and λ_(0,0) = 610 nm in CH_2Cl_2; λmax= 526 nm and λ_(0,0) = 592 nm in i-PrOH. The infrared spectrum of 4 shows a strong absorption at 1595 cm^(-1)(R_2^(14) = ^(14)N stretch) and provides evidence that 1,1-diazene 4 has considerable N=N double-bond character in the ground state. The infrared spectrum of 5 shows a strong absorption at 1638 cm^(-1)(R_2^(14) = ^(14)N stretch). The unimolecular rate of thermal decomposition of 4 is sensitive to solvent, the rate increasing with decreasing solvent polarity (k_(re1) = 1.0, 1.7, 4.8 in THF, Et_2O, and hexane, respectively). The activation parameters for the unimolecular fragmentation of 1,1-diazene 4 are as follows: log A = 11.6 • 0.5 and E_a= 16.9 ± 0.7 kcal mo1^(-1) in hexane; log A = 13.7 ± 0.3 and E_a = 20.0 ± 0.4 kcal mo1^(-1) in Et_2O; log A = 13.6 ± 0.3 and E_a = 20.l ± 0.4 kcal mol^(-1) in THF. The activation parameters for the bimolecular dimerization of 4 are log A = 3.8 ± 0.7 and E_a = 6.4 ± 0.9 kcal mol^(-1) in CDC1_3. The unimolecular rate of thermal decomposition of 5 is sensitive to solvent, the rate increasing with decreasing solvent polarity, k_(rel) = 1.0, 2.4, and 5.1 for THF, Et_2O, and hexane, respectively. The activation parameters for the unimolecular fragmentation of 1,1-diazene 5 are as follows: log A = 10.9 ± 0.3 and E_a = 16.8 ± 0.5 kcal mo1^(-1) in hexane; log A = 12.4 ± 0.4 and E_a = 19.0 ± 0.6 kcal mo1^(-1) in Et_2O; log A= 12.1 ± 0.3 and E_a = 19.l ± 0.4 kcal mo1^(-1) in THF. At -41.1 °C the bimolecular rate constant for the dimerization of 5 is 8.5 X 10^(-5) L/(mol s), 90 times slower than that found for 4. The change from a six-membered to a five-membered ring 1,1-diazene causes a shift to higher energy for the n,π'* transition and a shift to increased wavenumber (cm^(-1)) for the N=N stretching frequency, not unlike that of the isoelectronic ketones, tetramethylcyclohexanone and tetramethylcyclopentanone. Similar E_a values for the unimolecular thermal fragmentation of 4 and 5 may indicate the strain energy difference between 4 and 5 is also small. An approximate value of 30.5 kcal mo1^(-1) for the heat of formation of the 1,1-diazene 5 is estimated, indicating the 1,1-diazene 5 has a higher heat of formation than the cis-1,2-diazene isomer by 20 kcal mo1^(-1).


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http://dx.doi.org/10.1021/ja00367a020DOIArticle
http://pubs.acs.org/doi/abs/10.1021/ja00367a020PublisherArticle
Additional Information:© 1982 American Chemical Society. Received July 2, 1981. We are grateful to the National Science Foundation for generous support. Contribution No. 6292 from the Crellin Laboratory of Chemistry, California Institute of Technology, Pasadena, California 91125.
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Crellin Laboratory of Chemistry6292
Record Number:CaltechAUTHORS:20160511-154702150
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20160511-154702150
Official Citation:Direct studies of 1,1-diazenes. Syntheses, infrared and electronic spectra, and kinetics of the thermal decomposition of N-(2,2,6,6-tetramethylpiperidyl)nitrene and N-(2,2,5,5-tetramethylpyrrolidyl)nitrene William D. Hinsberg III, Peter G. Schultz, and Peter B. Dervan Journal of the American Chemical Society 1982 104 (3), 766-773 DOI: 10.1021/ja00367a020
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:67033
Collection:CaltechAUTHORS
Deposited By: Victoria Brennan
Deposited On:19 May 2016 22:08
Last Modified:19 May 2016 22:08

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