Trimethylsilylnitrene and Its Surprising Rearrangement to N-(Dimethylsilyl)methanimine via Silaziridine and Silaazomethine Ylide

Abstract

Photolysis of trimethylsilyl azide at 254 nm in Ar matrix at 15 K generates the triplet ground state trimethylsilylnitrene 2 aT, observed by ESR spectroscopy (|D/hc|=1.540 cm^-1 ; |E/hc|=0.0002 cm^-1 ). Calculations at the CASPT2(14,13) level reveal the open-shell singlet nitrene 2 aS(¹ A") is a discrete intermediate lying ≈38 kcal mol^-1 above the triplet. The normally expected rearrangement of the nitrene 2 aS to dimethylsilanimine 3 a has a high calculated barrier (33 kcal mol^-1 ), which explains why this product has never been observed. Instead, the singlet nitrene 2 aS inserts into a methyl C-H bond to yield silaziridine 12 via an activation barrier of only 6 kcal mol^-1 . Ring opening of 12 generates a 1-silaazomethine ylide 13, in which a facile 1,2-H shift yields N-(dimethylsilyl)methanimine 5, all with barriers well below the energy of the singlet nitrene.

Keywords: EPR spectroscopy; quantum chemistry; rearrangement; spin crossover.