Formation of an Unusual Four-Membered Nitrogen Ring (Tetrazetidine) Radical Cation

Abstract

Treatment of triphenylphosphine (Ph(3)P) with an excess of diisopropyl azodicarboxylate at 0-25 °C resulted in the formation of a symmetrical tetraalkyl tetrazetidinetetracarboxylate radical cation, containing the elusive cyclic N(4) ring system. Electron paramagnetic resonance (EPR) spectroscopy revealed a 9-line spectrum, with hyperfine coupling constants indicative of four almost magnetically equivalent nitrogen atoms. The radical species was surprisingly long-lived, and could still be observed several hours after generation and standing at 25 °C. Expansion of the central resonance revealed further splitting into a pentet (hyperfine coupling to the four methine protons). Three mechanistically plausible structures containing the tetrazetidine substructure were proposed based on the 9-line EPR spectrum. Following DFT calculations, the predicted hyperfine coupling constants were used to simulate the EPR spectra for the three candidate structures. The combined calculations and simulations were consistent with a radical cation species, but not a radical anion or radical-carbenoid structure. The lowest energy conformation of the N(4) ring was slightly puckered, with the alkyl carboxylate groups all trans and the four carbonyl groups aligned in a pinwheel arrangement around the ring. Analogous results were obtained with the original Mitsunobu reagents, Ph(3)P and diethyl azodicarboxylate, but not with Ph(3)P and di-tert-butyl azodicarboxylate. A mechanism is proposed based on a radical version of the Rauhut-Currier or Morita-Baylis-Hillman reactions.

Capturing a Tetratomic Nitrogen Ring cyclo-N4 inside an Aza[ 82 ]fullerene

“…Compared with the results of other C 3v (8)-C 82 4based metallofullerenes (Table 1), the redox property of Dy-I is distinct. The most striking differences lie in the rst oxidation ( ox E 1 ) and reduction ( red property of the N 4 ring is in accordance with the fact that the N 4 R 4 + radical cation is more stable than its neutral form in solution 50 , suggesting a unusual bonding tendency in cyclo-N 4 which is likely attributed to the strain of the four-membered nitrogen ring.…”

Thermal stability of the N10 compound with extended nitrogen chain

“…Although much effort has been devoted for developing novel poly-nitrogen compounds, only a few have been successfully obtained during the past years. [11][12][13][14] Recently, building some brand-new polynitrogen compounds, in which the azo group is directly bonded to the nitrogen atoms of the high-nitrogen heterocycles, has provided new ideas for the development of nitrogen clusters 6,15,16 (Scheme 1). Their property investigation and potential application have become a hotspot in nitrogen cluster science.…”

The mechanism of the first step of the Mitsunobu reaction

“…5,9,10 These radicals are than the other way around). The P-O betaine 6 could be formed directly by Michael-type addition to the carbonyl oxygen (Scheme 5) or by "normal" betaine formation (Scheme 1) followed by ring-closure to give the O,N-phosphorane 2 (R = Ph, R' = Et or i-Pr) and then ring-opening at the P-N bond rather than the P-O bond.…”