Phenylnitrene, phenylcarbene, and pyridylcarbenes. Rearrangements to cyanocyclopentadiene and fulvenallene.

Flash vacuum thermolysis (FVT) of phenyl azide 29 as well as precursors of 2-pyridylcarbene 34 and 4-pyridylcarbene 25 affords phenylnitrene 30 (labeled or unlabeled), as revealed by matrix isolation electron spin resonance spectroscopy. FVT of 1-C-phenyl azide 29 affords 1-cyanocyclopentadiene (cpCN) 32, which is exclusively labeled on the CN carbon, thus demonstrating direct ring contraction in phenylnitrene 30 without the intervention of cycloperambulation and 1,3-H shifts. However, the cpCN obtained by rearrangement of pyridyl-2-(C-carbene) 34 carries C label on all carbon atoms, including the CN carbon. Calculations at the B3LYP/6-31G∗ level and in part at the CASSCF/6-31G∗ and CASPT2/cc-pVDZ//CASSCF(8,8)/cc-pVDZ levels support a new mechanism whereby 2-pyridylcarbene rearranges in part via 1-azacyclohepta-1,2,4,6-tetraene 36 to phenylnitrene, which then undergoes direct ring contraction to cpCN. Another portion of 2-pyridylcarbene undergoes ring expansion to 4-azacyclohepta-1,2,4,6-tetraene 42, which then by trans-annular cyclization affords 6-azabicyclo[3.2.0]cyclohepta-1,3,5-triene 43. Further rearrangement of 43 via the spiroazirine 44 and biradical/vinylnitrene 45 affords cpCN with the label on the CN group. An analogous mechanisms accounts for the labeling pattern in fulvenallene 60 formed by ring contraction of 1-C-phenylcarbene 59 in the FVT of 1-C-phenyldiazomethane 58

Trimethylsilylnitrene and its surprising rearrangement to N-(Dimethylsilyl)methanimine via silaziridine and silaazomethine ylide

Photolysis of trimethylsilyl azide at 254 nm in Ar matrix at 15 K generates the triplet ground state trimethylsilylnitrene 2aT, 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 2aS(1A") is a discrete intermediate lying ~38 kcal/mol above the triplet. The normally expected rearrangement of the nitrene 2aS to dimethylsilanimine 3a has a high calculated barrier (33 kcal/mol), which explains why this product has never been observed. Instead, the singlet nitrene 2aS inserts into a methyl C-H bond to yield silaziridine 12 via an activation barrier of only 6 kcal/mol. 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