Carbenic Nitrile Imines: Properties and Reactivity

Abstract

Structures and properties of nitrile imines were investigated computationally at B3LYP and CCSD­(T) levels. Whereas NBO analysis at the B3LYP DFT level invariably predicts a propargylic electronic structure, CCSD­(T) calculations permit a clear distinction between propargylic, allenic, and carbenic structures. Nitrile imines with strong IR absorptions above ca. 2150 cm^–1 have propargylic structures with a CN triple bond (RCNNSiMe₃ and R₂BCNNBR₂), and those with IR absorptions below ca. 2150 cm^–1 are allenic (HCNNH, PhCNNH, and HCNNPh). Nitrile imines lacking significant cumulenic IR absorptions at 1900–2200 cm^–1 are carbenic (R–(<i>C</i>:)–NN–R′). Electronegative but lone pair-donating groups NR₂, OR, and F stabilize the carbenic form of nitrile imines in the same way they stabilize “normal” singlet carbenes, including <i>N</i>-heterocyclic carbenes. NBO analyses at the CCSD­(T) level confirm the classification into propargylic, allenic, and carbenic reactivity types. Carbenic nitrile imines are predicted to form azoketenes <b>21</b> with CO, to form [2+2] and [2+4] cycloadducts and borane adducts, and to cyclize to 1<i>H-</i>diazirenes of the type <b>24</b> in mildly exothermic reactions with activation energies in the range 29–38 kcal/mol. Such reactions will be readily accessible photochemically and thermally, e.g., under the conditions of matrix photolysis and flash vacuum thermolysis