| Abstract
| - C,C-Dicyanoketenimines 10a−c were generated by flash vacuum thermolysis of ketene N,S-acetals9a−c or by thermal or photochemical decomposition of α-azido-β-cyanocinnamonitrile 11. In thelatter reaction, 3,3-dicyano-2-phenyl-1-azirine 12 is also formed. IR spectroscopy of the ketenimininesisolated in Ar matrixes or as neat films, NMR spectroscopy of 10c, and theoretical calculations(B3LYP/6-31G*) demonstrate that these ketenimines have variable geometry, being essentiallylinear along the CCN−R framework in polar media (neat films and solution), but in the gas phaseor Ar matrix they are bent, as is usual for ketenimines. Experiments and calculations agree thata single CN substituent as in 13 is not enough to enforce linearity, and sulfonyl groups are lesseffective that cyano groups in causing linearity. C,C-Bis(methylsulfonyl)ketenimines 4−5 and aC-cyano-C-(methylsulfonyl)ketenimine 15 are not linear. The compound p-O2NC6H4NCC(COOMe)2 previously reported in the literature is probably somewhat linearized along the CCNRmoiety. A computational survey (B3LYP/6-31G*) of the inversion barrier at nitrogen indicates thatelectronegative C-substituents dramatically lower the barrier; this is also true of N-acyl substituents.Increasing polarity causes lower barriers. Although N-alkylbis(methylsulfonyl)ketenimines are notcalculated to be linear, the barriers are so low that crystal lattice forces can induce planarity inN-methylbis(methylsulfonyl)ketenimine 3.
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