Ultrafast Dynamics of Photochemical Nitrile Imine Formation

The chemical reactivity of nitrile imines is of great utility in organic synthesis with applications rapidly expanding into the materials and life sciences. Yet, our understanding of the electronic and molecular structures of nitrile imines remains incomplete and the elementary mechanism of their photoinduced generation is entirely unknown. Here, femtosecond infrared spectroscopy after 266 nm‐excitation of 2,5‐diphenyltetrazole has been carried out to temporally resolve the formation and structural relaxation dynamics of the nascent diphenylnitrile imine in liquid solution under ambient conditions. The infrared‐spectroscopic evolution is interpreted by an initial sequence of intersystem crossings within 250 fs followed by the cleavage of N₂ with formation of a structurally relaxed nitrile imine on the adiabatic ground‐state singlet surface within a few tens of picoseconds. The infrared spectrum supports the notion of a “floppy” nitrile imine molecule whose equilibrium character ranges from fully propargylic to fully allenic in the room temperature liquid solution.

Water-Involved Ring-Opening of 4-Phenyl-1,2,4-triazoline-3,5-dione for "Photo-Clicked" Access to Carbamoyl Formazan Photoswitches In Situ

Cyclic azodicarbonyl derivatives, particularly 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD), commonly serve as arenophile, dienophile, enophile and electrophile. Perplexed by its instability in aqueous environment, there are few studies focused on the transient intermediate produced by hydrolysis of PTAD to achieve synthetic significance. Herein, we describe a "photo-click" method that involves nitrile imine (NI) from diarylsydnone to capture the diazenecarbonyl-phenyl-carbamic acid (DACPA) generated by water-promoted ring-opening of PTAD. DFT calculation reveal that H-bonding interactions between PTAD and water are vital to form DACPA which exhibited an umpolung effect during ligation by nature bond orbit (NBO) analysis. The ultra-fast ligation resulted in carbamoyl formazans, as a unique Z↔E photo-switchable linker on target molecules, including peptide and drugs, with excellent anti-fatigue performance. This strategy is showcased to construct highly functionalized carbamoyl formazans in situ for photo-pharmacology and material studies, which also expands the chemistry of PTAD in aqueous media.

Transition-Metal-Free Coupling of 1,3-Dipoles and Boronic Acids as a Sustainable Approach to C-C Bond Formation

The need for alternative, complementary approaches to enable C-C bond formation within organic chemistry is an on-going challenge in the area. Of particular relevance are transformations that proceed in the absence of transition-metal reagents. In the current study, we report a comprehensive investigation of the coupling of nitrile imines and aryl boronic acids as an approach towards sustainable C-C bond formation. In situ generation of the highly reactive 1,3-dipole facilitates a Petasis-Mannich-type coupling via a nucleophilic boronate complex. The introduction of hydrazonyl chlorides as a complementary nitrile imine source to the 2,5-tetrazoles previously reported by our laboratory further broadens the scope of the approach. Additionally, we exemplify for the first time the extension of this protocol into another 1,3-dipole, through the synthesis of aryl ketone oximes from aryl boronic acids and nitrile N-oxides.

A Terminal Iron Nitrilimine Complex: Accessing the Terminal Nitride through Diazo N-N Bond Cleavage

A novel method for the N-N bond cleavage of trimethylsilyl diazomethane is reported for the synthesis of terminal nitride complexes. The lithium salt of trimethylsilyl diazomethane was used to generate a rare terminal nitrilimine transition metal complex with partially occupied d-orbitals. This iron complex 2 was characterized by CHN combustion analysis, 1 H and 13 C NMR spectroscopic analysis, single-crystal X-ray crystallography, SQUID magnetometry, 57 Fe Mössbauer spectroscopy, and computational analysis. The combined results suggest a high-spin d 6 (S=2) electronic configuration and an allenic structure of the nitrilimine ligand. Reduction of 2 results in release of the nitrilimine ligand and formation of the iron(I) complex 3, which was characterized by CHN combustion analysis, 1 H NMR spectroscopic analysis, and single-crystal X-ray crystallography. Treatment of 2 with fluoride salts quantitatively yields the diamagnetic FeIV nitride complex 4, with concomitant formation of cyanide and trimethylsilyl fluoride through N-N bond cleavage.

Singlet Photoreactivity of 3-Methyl-2-phenyl-2H-azirine

Irradiation of 3-methyl-2-phenyl-2H-azirine (1) at 254 nm in argon matrices results in ylide 6. Similarly, laser flash photolysis (λ = 266 nm) of azirine 1 in acetonitrile yields ylide 6, which has a transient absorption with λmax at ~340 nm and a lifetime of 14 μs. Density functional theory calculations were preformed to support the characterisation of ylide 6 in solution and argon matrices. Irradiation of azirine 1 above 300 nm has previously been reported (J. Org. Chem. 2014, 79, 653) to yield triplet vinylnitrene in solution and ketenimine in cryogenic argon matrices. Thus, the photochemistry of azirine 1 is dependent on the irradiation wavelength.

Sydnone Photochemistry: Direct Observation of Earl's Bicyclic Lactone Valence Isomers (Oxadiazabicyclo[2.1.0]pentanones), Formation of Carbodiimides, Reaction Mechanism, and Photochromism

The matrix photolyses of 3-phenyl-, 3-pyridyl, and 3,4-diphenylsydnones 16, 19, and 22 were investigated by matrix-isolation infrared spectroscopy. The formation of the neutral, bicyclic lactone valence isomers postulated by Earl – the oxadiazabicyclo[2.1.0]pentanones exo-17 and exo-20 – was clearly observed in the first two cases and is also likely in the case of exo-23 (C=O absorptions in the IR at 1881, 1886, and 1874 cm–1, respectively). The efficient photodecomposition of sydnones to carbodiimides RN=C=NR′ (18, 21, and 24) and CO2 was established in all three cases. The formation of benzonitrile 27 and azacycloheptatetraene 29 in the matrix photolysis of diphenylsydnone 22 is indicative of diphenylnitrile imine PhCNNPh 26 as an intermediate (2340 cm–1). Neither bicyclic lactones nor carbodiimides have been observed previously in sydnone photochemistry. A general reaction mechanism for the formation of carbodiimides, nitrile imines, and photochromism is put forward.