Photochemical and thermal reactions of intermediates in the phenylnitrene rearrangement inside a hemicarcerand.
Broadband irradiation (lambda > 320 nm) of hemicarceplex H(.)1 between -74 degrees C and -84 degrees C, produces encapsulated didehydroazepine (2), triplet phenylnitrene ((3)PN), 2-azabicyclo[3.2.0]hepta-1,3,6-triene (6), and 4-azaspiro[2.4]hepta-1,4,6-triene (7). The highly strained anti-Bredt imine 6 is formed from 2 via a photochemical four-electron electrocyclization. Under the irradiation conditions, 6 rearranges further to azaspirene 7. In addition, 6 thermally rearranges to 7 via a 1,5-sigmatropic shift (DeltaG(267K) = 20.0 +/- 0.5 kcal/mol), yielding a final equilibrium composed of [7]/[6] = 5 at room temperature. The observation of a photochemical rearrangement of 2 to 6 contrasts earlier results of narrow band irradiations (lambda = 334 nm) of matrix-isolated 2, which gave (3)PN (Hayes, J. C.; Sheridan, R. S. J. Am. Chem. Soc. 1990, 112, 5879-5881). Encapsulated (3)PN is remarkably stable due to the prevention of its dimerization by the surrounding hemicarcerand. Above 255 K, it slowly decays with a rate constant k = 10(7.7+/-0.4) s(-1) x exp {(13300 +/- 500 cal/mol)/RT}. The isolation of substantial amounts of a hemicarcerand lacking one acetal spanner suggests that (3)PN decays preferentially by inserting into an inward-pointing acetal C-H bond of H.
Warmuth
Ralf
R
Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA. warmuth@rutgers.edu
Makowiec
Slawomir
S
D000465:Algorithms; D003500:Cyclization; D019281:Dimerization; D004583:Electrons; D007097:Imines / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D009584:Nitrogen / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry*; D010777:Photochemistry; D011760:Pyrrolidinones / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D013141:Spiro Compounds / Q000737:chemistry; D013696:Temperature; D013816:Thermodynamics
D007097:Imines; D011760:Pyrrolidinones; D013141:Spiro Compounds; C469812:azaspirene; C017621:phenylnitrene; D009584:Nitrogen; D010100:Oxygen