Radical and Nitrenoid Reactivity of 3‑Halo-3-phenyldiazirines

3-Halo-3-phenyl-3<i>H</i>-diazirines (halogen = Br or Cl) undergo a dissociative single-electron transfer from alkyllithiums (RLi) in THF-based solvent mixtures. The resulting 3-phenyldiazirinyl radical, observed by EPR spectroscopy, is eventually transformed to benzonitrile. In Et₂O, 2 equiv of RLi add to both nitrogens of halodiazirine NN bond, affording <i>N,N</i>′-dialkylbenzamidines. The nitrenoid reactivity of some <i>N</i>-alkyl-1<i>H</i>-diazirine intermediates is manifested by their insertion into the α-C–H bond of THF or Et₂O

Evidence for the Cyclic CN₂ Carbene in Solution

Diazirinylidene (<i>c</i>-CN₂) is formally the simplest of the N-heterocyclic carbenes. The intermediacy of this elusive species in the fragmentation of butyl 3-bromodiazirine-3-car­box­ylate (<b>1a</b>) with pent-4-en-1-ols and their sodium alkoxides in DMF is supported by the formation of 2-oxa­bicyclo­[4.1.0]­heptanes and di­pen­tenoxy­methanes. These products result from an intramolecular [2 + 1] cycloaddition and O–H insertion, respectively, of penten­oxy­methyl­enes suggested to originate from the reaction of the electrophilic <i>c</i>-CN₂ with an alkoxide ion. The reaction of <b>1a</b> with primary or secondary amines in methanol affords the corresponding 3-bromo­diazirine-3-carbox­amides

Magnetic Circular Dichroism Evidence for an Unusual Electronic Structure of a Tetracarbene–Oxoiron(IV) Complex

In biology, high valent oxo–iron­(IV) species have been shown to be pivotal intermediates for functionalization of C–H bonds in the catalytic cycles of a range of O₂-activating iron enzymes. This work details an electronic-structure investigation of [Fe^IV(O)­(L^NHC)­(NCMe)]²⁺ (L^NHC = 3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohexacosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene, complex <b>1</b>) using helium tagging infrared photodissociation (IRPD), absorption, and magnetic circular dichroism (MCD) spectroscopy, coupled with DFT and highly correlated wave function based multireference calculations. The IRPD spectrum of complex <b>1</b> reveals the Fe–O stretching vibration at 832 ± 3 cm^–1. By analyzing the Franck–Condon progression, we can determine the same vibration occurring at 616 ± 10 cm^–1 in the E­(d_<i>xy</i> → d_<i>xz</i>,yz) excited state. Both values are similar to those measured for [Fe^IV(O)­(TMC)­(NCMe)]²⁺ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclo­tetradecane). The low-temperature MCD spectra of complex <b>1</b> exhibit three pseudo <i>A</i>-term signals around 12 500, 17 000, and 24 300 cm^–1. We can unequivocally assign them to the ligand field transitions of d_<i>xy</i> → d_<i>xz</i>,yz, d_<i>xz</i>,yz → d_z2, and d_<i>xz</i>,yz → d_{<i>x</i>2‑<i>y</i>2}, respectively, through direct calculations of MCD spectra and independent determination of the MCD <i>C</i>-term signs from the corresponding electron donating and accepting orbitals. In comparison with the corresponding transitions observed for [Fe^IV(O) (SR-TPA)­(NCMe)]²⁺ (SR-TPA = tris­(3,5-dimethyl-4-methoxypyridyl-2-methy)­amine), the excitations within the (FeO)²⁺ core of complex <b>1</b> have similar transition energies, whereas the excitation energy for d_<i>xz</i>,yz → d_{<i>x</i>2‑<i>y</i>2} is significantly higher (∼12 000 cm^–1 for [Fe^IV(O)­(SR-TPA)­(NCMe)]²⁺). Our results thus substantiate that the tetracarbene ligand (L^NHC) of complex <b>1</b> does not significantly affect the bonding in the (FeO)²⁺ unit but strongly destabilizes the d_{<i>x</i>2‑<i>y</i>2} orbital to eventually lift it above d_z2. As a consequence, this unusual electron configuration leads to an unprecedentedly larger quintet–triplet energy separation for complex <b>1</b>, which largely rules out the possibility that the H atom transfer reaction may take place on the quintet surface and hence quenches two-state reactivity. The resulting mechanistic implications are discussed