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

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 emax = 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).