Photochemical Generation of Iminoquinone Methides by 1,4-Hydrogen Migration in Derivatives of o-Tolylnitrene

Abstract

The photochemistry of a series of derivatives of o‐tolyl azide, bearing a variety of substituents in the benzylic positions, has been investigated using matrix isolation spectroscopy and density functional calculations. It has been found that introduction of any substituent possessing a lone pair (i.e., R = Br, Cl, MeO, Me2N) allows a 1,4‐hydrogen shift to take place, yielding iminoquinone methides. Additional methyl groups in the benzylic position, however, do not promote a photochemical conversion into iminoquinone methides. If the benzylic substituent itself is part of a ring system, the size of this ring plays an important role. Thus, 2‐methyl‐8‐nitrenotetrahydroisoquinoline rearranges very easily, whereas 4‐nitrenophthalan does not give the reaction. Density functional calculations [B3LYP/6‐31G(d)] have been used to gain an understanding of the reaction. It has been found that the activation energies depend strongly on the nature of the substituent, being lowest if R = NMe2. Incorporation of the benzylic substituent into a ring reduces the flexibility of the system and results in significantly raised barriers.

A Laser Flash Photolysis Study on 2-Azido-N,N-diethylbenzylamine − The Reactivity of Iminoquinone Methides in Solution

“…Ϫ Azide 1 was obtained as described in the previous contribution. [1] Ϫ Calculations were performed using Gaussian 98 [56] or Titan [57] software, using the hybrid B3LYP method. [58] All optimized structures were characterized as minima or transition states by calculating vibrational spectra.…”

“…[1] Didehydroazepine 5, on the other hand, was only formed in minute amounts. Low-temperature photochemistry of 1 in argon thus represents a very clean reaction, yielding a welldefined set of products.…”

“…Iminoquinone methides sa-3 and aa-3 are formed photochemically (λ exc ϭ 320 nm) from azide 1, matrix-isolated in Ar at T ϭ 10 K. [1] While the data obtained with matrix isolation spectroscopy thus draw a clear picture, the situation is definitely less obvious when looking at the product studies, LFP data, and calculations presented in this contribution. Certainly, a significant part of the photochemistry of 1 runs via the well-established pathways of conventional singlet nitrene chemistry, eventually yielding methyleneazepine 9.…”

“…If cyclohexane or n-hexane were used as solvent, the transient behavior detected upon LFP (λ exc ϭ 355 nm) of 1 was again very sim- The transient spectrum of transient X obtained by LFP of 1 is very similar to the UV/Vis spectrum assigned to iminoquinone methide sa-3, matrix-isolated in Ar. [1] For sa-3, the reactivity of a reactive diene, undergoing facile DielsϪAlder cycloadditions and addition of nucleophiles, was anticipated. Quenching experiments were therefore performed, using dienophiles such as 2-chloroacrylonitrile (CAN) and nucleophiles such as methanol.…”

A Laser Flash Photolysis Study on 2-Azido-N,N-diethylbenzylamine − The Reactivity of Iminoquinone Methides in Solution

“…Ϫ Azide 1 was obtained as described in the previous contribution. [1] Ϫ Calculations were performed using Gaussian 98 [56] or Titan [57] software, using the hybrid B3LYP method. [58] All optimized structures were characterized as minima or transition states by calculating vibrational spectra.…”

“…[1] Didehydroazepine 5, on the other hand, was only formed in minute amounts. Low-temperature photochemistry of 1 in argon thus represents a very clean reaction, yielding a welldefined set of products.…”

“…Iminoquinone methides sa-3 and aa-3 are formed photochemically (λ exc ϭ 320 nm) from azide 1, matrix-isolated in Ar at T ϭ 10 K. [1] While the data obtained with matrix isolation spectroscopy thus draw a clear picture, the situation is definitely less obvious when looking at the product studies, LFP data, and calculations presented in this contribution. Certainly, a significant part of the photochemistry of 1 runs via the well-established pathways of conventional singlet nitrene chemistry, eventually yielding methyleneazepine 9.…”

“…If cyclohexane or n-hexane were used as solvent, the transient behavior detected upon LFP (λ exc ϭ 355 nm) of 1 was again very sim- The transient spectrum of transient X obtained by LFP of 1 is very similar to the UV/Vis spectrum assigned to iminoquinone methide sa-3, matrix-isolated in Ar. [1] For sa-3, the reactivity of a reactive diene, undergoing facile DielsϪAlder cycloadditions and addition of nucleophiles, was anticipated. Quenching experiments were therefore performed, using dienophiles such as 2-chloroacrylonitrile (CAN) and nucleophiles such as methanol.…”

Photochemistry of an Azido-Functionalized Cryptand: Controlling the Reactivity of an Extremely Long-Lived Singlet Aryl Nitrene by Complexation to Alkali Cations

“…This points to a long lifetime of 9 . In the presence of 8% diethylamine, the formation of methyleneazepine 5 − is observed in addition to the formation of 4 . Product 5 is probably formed by addition of diethylamine to didehydroazepine 7 , followed by elimination of the diaza-18-crown-6 ring (Scheme ).…”

A Laser Flash Photolysis Study on 2-Azido-N,N-diethylbenzylamine − The Reactivity of Iminoquinone Methides in Solution

“…Ϫ Azide 1 was obtained as described in the previous contribution. [1] Ϫ Calculations were performed using Gaussian 98 [56] or Titan [57] software, using the hybrid B3LYP method. [58] All optimized structures were characterized as minima or transition states by calculating vibrational spectra.…”

“…[1] Didehydroazepine 5, on the other hand, was only formed in minute amounts. Low-temperature photochemistry of 1 in argon thus represents a very clean reaction, yielding a welldefined set of products.…”

“…Iminoquinone methides sa-3 and aa-3 are formed photochemically (λ exc ϭ 320 nm) from azide 1, matrix-isolated in Ar at T ϭ 10 K. [1] While the data obtained with matrix isolation spectroscopy thus draw a clear picture, the situation is definitely less obvious when looking at the product studies, LFP data, and calculations presented in this contribution. Certainly, a significant part of the photochemistry of 1 runs via the well-established pathways of conventional singlet nitrene chemistry, eventually yielding methyleneazepine 9.…”

“…If cyclohexane or n-hexane were used as solvent, the transient behavior detected upon LFP (λ exc ϭ 355 nm) of 1 was again very sim- The transient spectrum of transient X obtained by LFP of 1 is very similar to the UV/Vis spectrum assigned to iminoquinone methide sa-3, matrix-isolated in Ar. [1] For sa-3, the reactivity of a reactive diene, undergoing facile DielsϪAlder cycloadditions and addition of nucleophiles, was anticipated. Quenching experiments were therefore performed, using dienophiles such as 2-chloroacrylonitrile (CAN) and nucleophiles such as methanol.…”

Photochemistry of an Azido-Functionalized Cryptand: Controlling the Reactivity of an Extremely Long-Lived Singlet Aryl Nitrene by Complexation to Alkali Cations

“…This points to a long lifetime of 9 . In the presence of 8% diethylamine, the formation of methyleneazepine 5 − is observed in addition to the formation of 4 . Product 5 is probably formed by addition of diethylamine to didehydroazepine 7 , followed by elimination of the diaza-18-crown-6 ring (Scheme ).…”

A Laser Flash Photolysis Study on 2-Azido-N,N-diethylbenzylamine − The Reactivity of Iminoquinone Methides in Solution

“…Ϫ Azide 1 was obtained as described in the previous contribution. [1] Ϫ Calculations were performed using Gaussian 98 [56] or Titan [57] software, using the hybrid B3LYP method. [58] All optimized structures were characterized as minima or transition states by calculating vibrational spectra.…”

“…[1] Didehydroazepine 5, on the other hand, was only formed in minute amounts. Low-temperature photochemistry of 1 in argon thus represents a very clean reaction, yielding a welldefined set of products.…”

“…Iminoquinone methides sa-3 and aa-3 are formed photochemically (λ exc ϭ 320 nm) from azide 1, matrix-isolated in Ar at T ϭ 10 K. [1] While the data obtained with matrix isolation spectroscopy thus draw a clear picture, the situation is definitely less obvious when looking at the product studies, LFP data, and calculations presented in this contribution. Certainly, a significant part of the photochemistry of 1 runs via the well-established pathways of conventional singlet nitrene chemistry, eventually yielding methyleneazepine 9.…”

“…If cyclohexane or n-hexane were used as solvent, the transient behavior detected upon LFP (λ exc ϭ 355 nm) of 1 was again very sim- The transient spectrum of transient X obtained by LFP of 1 is very similar to the UV/Vis spectrum assigned to iminoquinone methide sa-3, matrix-isolated in Ar. [1] For sa-3, the reactivity of a reactive diene, undergoing facile DielsϪAlder cycloadditions and addition of nucleophiles, was anticipated. Quenching experiments were therefore performed, using dienophiles such as 2-chloroacrylonitrile (CAN) and nucleophiles such as methanol.…”

Photochemistry of an Azido-Functionalized Cryptand: Controlling the Reactivity of an Extremely Long-Lived Singlet Aryl Nitrene by Complexation to Alkali Cations

“…This points to a long lifetime of 9 . In the presence of 8% diethylamine, the formation of methyleneazepine 5 − is observed in addition to the formation of 4 . Product 5 is probably formed by addition of diethylamine to didehydroazepine 7 , followed by elimination of the diaza-18-crown-6 ring (Scheme ).…”