Ultrafast Time Resolved and Computational Studies of Diazo and Diazirine Excited States, and of Carbenes

Ab initio quantum calculations and ultrafast time-resolved laser flash photolysis techniques have been used to study singlet carbenes and the photochemistry of diazirines and diazo compounds. After a brief introduction of carbene chemistry in Chapter 1, the photophysics and photochemistry of aryldiazirines are investigated in Chapters 2 through 6. Detailed theoretical calculations begin with parent phenyldiazirine and its isomer phenyldiazomethane. The structures of the ground and electronic excited states (S₁, S₂, and S₃) of both compounds are optimized with RI-CC2 and DFT methods. The denitrogenation of both phenyldiazirine and phenyldiazomethane to produce singlet phenylcarbene, and the isomerization between both compounds, are investigated mechanistically on their potential energy surfaces. These predictions support the spectroscopic assignment in ultrafast studies of arylalkyldiazirines in chapters 3 through 6 and the accuracy of these theoretical methods are calibrated by the excellent agreement with experimental data. In Chapter 3 we present the first direct observation of singlet phenylcarbene and measurement of its lifetime in solution using ultrafast time-resolved infrared spectroscopy. In Chapter 4 we provide the first direct observation of the S₁ excited state of para-methoxy-3-phenyl-3-methyl diazirine (p-CH₃OC₆H₄CN₂CH₃) with both IR and UV–vis detection techniques. The S₁ state of the diazirine decays into the diazo compound directly. The S₂ excited state is populated with 270 nm light and decays directly into singlet arylcarbene and diazo compound, as well as the S₁ state, via internal conversion. A Hammett study of the S₁ excited states is discussed in Chapter 5. An excellent linear correlation is obtained between the S₁ lifetimes of arylchlorodiazirines and their para- substituent σ_p⁺ parameters. The effect of substitution of b-hydrogens on the S₁ state lifetimes is examined in Chapter 6 and is consistent with the RIES mechanism. The wavelength dependence effect on the photochemistry of aryldiazirines was discussed.

In Chapter 7 we present the first direct observation of a singlet vinylcarbene and study its cyclization to a cyclopropene product in solution. Calculations predict that singlet vinylcarbene is highly delocalized over the CC double bond.

The photochemistry of N,N-diethyldiazoacetamide is detailed in Chapter 8. We concluded that the excited state of the diazoamide precursor undergoes direct intramolecular CH insertions in forming both β- and γ-lactams, as well as denitrogenation to produce singlet carbene. The relaxed singlet carbene decays by isomerizing into γ-lactam in chloroform, and in methanol, this path is suppressed by intermolecular OH insertion reactions.