Biomedical subjects
Publications and source records attributed to Robert S Sheridan.
[reaction: see text] Different conformers of two fused-ring cyclopropylchlorocarbenes are shown to undergo distinct, wavelength-selectable, photochemistry in low-temperature matrixes. Conformers with H and Cl syn give ring expansion predominantly, whereas those with H and Cl anti mainly fragment.
[reaction: see text] Vinylchlorocarbenes (R = Me or Cl) cyclize to cyclopropenes with k = (2.4-5.1) x 10(7) s(-1) or (3.1-4.5) x 10(6) s(-1), respectively, in solution at room temperature. The corresponding E(a) values are 6.6 and 5.7 kcal/mol. Excited diazirines are also likely precursors of the cyclopropenes.
[reaction: see text] The highly strained cyclic allene 2,3-didehydro-2H-thiopyran was generated by irradiation of matrix-isolated 2-benzothienylchlorocarbene and characterized by IR and UV/vis spectroscopy, in situ trapping, and DFT modeling. Calculations indicate that the allenic moiety in this system, although less aromatic in character, is relatively more stable than in the corresponding oxa system. It is suggested that the thio ring system can more readily accommodate the allenic geometry.
The first direct spectroscopic characterization of singlet vinylcarbenes is reported. Several vinyl amidines were converted to the corresponding vinylchlorodiazirines through straightforward Graham oxidation. Irradiation of the diazirines at 8 K in N2 matrices gave the corresponding singlet vinylchlorocarbenes, which could be characterized by IR, UV/vis, and calculational modeling. Hence, vinylchlorocarbene, 1-methylvinylchlorocarbene, and 1-cyclopentenylchlorocarbene were all generated and investigated. The spectra are consistent with localized carbene structures compared to the very delocalized triplet vinylcarbenes. In all cases, the carbenes readily cyclize to the corresponding cyclopropenes on visible irradiation, together with H-shift in the parent vinylchlorocarbene. In the cyclopentenyl system, cyclization leads to a highly strained bicyclo[3.1.0]hexene.
Irradiation of 3-(3-noradamantyl)-3-chlorodiazirine produced the corresponding noradamantylchlorocarbene, which could be detected in solution with laser flash photolysis via its pyridinium ylide, and in N2 or Ar matrixes at 9 K with IR and UV/vis spectroscopy. At ambient temperatures, or on irradiation in the cryogenic matrixes, the carbene rearranged to 2-chloro-1-adamantene, which could be trapped in solution and which was characterized by IR and UV/vis at low temperatures. In the dark at 9 K, the carbene also slowly ring-expanded to the chloroadamantene, at a rate ca. 10111 times faster than predicted by its B3LYP-calculated activation barrier. It is proposed that the low-temperature rearrangement occurs through carbon quantum mechanical tunneling.
We observed ring expansion of 1-methylcyclobutylfluorocarbene at 8 kelvin, a reaction that involves carbon tunneling. The measured rate constants were 4.0 x 10(-6) per second in nitrogen and 4 x 10(-5) per second in argon. Calculations indicated that at this temperature the reaction proceeds from a single quantum state of the reactant so that the computed rate constant has achieved a temperature-independent limit. According to calculations, the tunneling contribution to the rate is 152 orders of magnitude greater than the contribution from passage over the barrier. We discuss environmental effects of the solid-state inert-gas matrix on the reaction rate.
Irradiation of N(2) matrix-isolated 3-chloro-3-(2-benzoxazolyl)diazirine gives a mixture of syn- and anti-benzoxazolylchlorocarbenes which could be characterized by IR, UV/vis, and B3LYP modeling. Subsequent irradiation of the carbene caused ring opening to the corresponding quinoimine, which was similarly characterized. In turn, the quinoimine photochemically underwent ring-closure to a novel, highly strained cyclic ketenimine. Unrestricted singlet DFT calculations were required to fit the IR spectrum of the ketenimine, suggesting significant diradical character. Photolysis of the ketenimine led to ring cleavage in yet another fashion, to give an isonitrile-phenoxychlorocarbene, whose spectra were similar to those of the previously characterized phenoxychlorocarbene. Finally, at shorter wavelengths, the phenoxycarbene underwent the expected rearrangement to the corresponding benzoyl chloride.