Are Unsaturated Isocyanides so Different from the Corresponding Nitriles?

Abstract

Simple unsaturated and cyclopropylic isocyanides are synthesized by an efficient and simple approach. These compounds with gradually increasing distance between the unsaturated moiety and the isonitrile group are studied by UV photoelectron spectroscopy and quantum chemical calculations, and also compared to the corresponding nitriles. The first photoelectron band of the unsaturated compounds is linked to removal of an electron from the HOMO, which corresponds to CC multiple-bond ionization in antibonding interaction with the π-isocyanide bond (in the same plane) for conjugated systems, or in antibonding interaction with the pseudo-π-CH(2) group for isolated systems. For the 1-ethenyl derivatives, both cyano and isocyano groups act as a π-electron acceptor from the vinyl group, but the isocyano π system is much more strongly destabilized (ionization energies (IEs) shift to smaller values) by vinyl (3.12 eV) than the cyano π system is (2.70 eV). In comparison with the 1-ethynyl derivatives, a less pronounced destabilization (2.69 eV) of π(NC) by the ethynyl system (1.86 eV for π(CN)), and nearly the same order of magnitude of the energetic gap between the total antibonding (π(CC)-π(NC)) and the total bonding (π(CC)+π(NC)) IEs for ethenyl and ethynyl compounds are noted. The huge values of these last-named data for H(2)C=CH-NC (3.85 eV) and for HC≡C-NC (4.04 eV) reflect the strong interaction between the unsaturated carbon-carbon moiety and the isocyanide group, and thus more efficient conjugation than for the corresponding nitriles.

Relative stability and proton transfer reactions of unsaturated isocyanides and cyanides

“…We stated previously that ethynyl (―C ≡ CH) substituent increases the relative energy between the cyanide and isocyanide because of the triple bond position near the isocyanide or cyanide group. It has been reported based on vinyl and ethynyl substituted species that isocyanides with conjugated π systems and especially with triple bonds are more reactive than the corresponding cyanides (because of lower antibonding π CC –π NC ionization energies) . However this gives us only limited information about the absolute stabilities, as for example a stabilization was observed for the same bonding MO .…”

“…Some of these bases were additionally purified before use by fractional vacuum distillation or sublimation techniques by using microscale compatible in‐house made high‐vacuum manifold apparatus and liquid nitrogen for cooling, when needed. Isocyanides have been synthesized as previously reported . Prepared isocyanides were collected into high‐vacuum compatible flasks, and these were stored after preparation in freezer at −70 °C.…”

Gas‐Phase Infrared Spectra of Three Compounds of Astrochemical Interest: Vinyl, Allenyl, and Propargyl Isocyanides

“…Vinyl, allenyl, and propargyl isocyanides were easily prepared on a gram scale 19. The samples were evaporated in a 20 cm‐long gas cell at RT.…”

“…Of course, ethynyl isocyanide is also of a great interest, but there have been a lot of accurate spectroscopic studies [15–17] reported in the 20 years since its first synthesis 18. All these basic compounds show different interactions between the unsaturated substituent and the isonitrile group 19. Vinyl isocyanide was synthesized for the first time in 196720 and analyzed by microwave (MW)21, 22 and infrared (IR) spectroscopy 22.…”

“…At the end of the 1980′s, Kroto et al recorded the microwave spectrum of the propargyl derivative,24 but the spectrum of the allenyl derivative has been studied much more recently 25. For both compounds, the data on a few significant IR absorptions have been reported,26 and recently, the photoelectron spectra of all these species have been analyzed 19…”