Femtosecond laser flash photolysis of 3-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-ylidene)tetrahydrofuran produces singlet 3-oxacyclopentylidenecarbene which reacts with acetonitrile solvent to form an ylide. This is the first direct detection of ylide formation by an alkylidenecarbene. This new type of ylide was observed to have a broad absorption band in the visible region with λmax â¼450 nm and a lifetime of â¼13.5 ps. As with other "conventional" carbenes (the divalent carbon atom is separately bound to two substituents), this ylide formation method could be also useful for detecting alkylidenecarbenes, especially those that do not absorb at wavelengths suitable for direct observation. Furthermore, the mechanisms by which 3-oxacyclopentylidenecarbene forms the ylide and the overall favorability of ylide formation, vis-à-vis ring expansion of the carbene to strained 3-oxacyclohexyne, were supported by results from density functional theory calculations.
Photolysis of a 7,7-difluoro-1,4-diphenyl-2,3-diazabicyclo[2.2.1]hept-2-ene derivative (AZ1) using a 365 nm light-emitting diode in an Ar matrix at 4 K resulted in the formation of a planar singlet 2,2-difluoro-1,3-diphenylcyclopentane-1,3-diyl diradical derivative, S-DR1-pl (λmax = 520 nm). A singlet cyclopentane-1,3-diyl diradical system (S-DR1-pl) was directly detected by steady-state infrared (IR) spectroscopy. Due to the photolability of S-DR1-pl, initial photolysis of AZ1 also yielded the ring-closed product ret-CP1 and migration products trans-MG1 and/or cis-MG1, which were observed using IR spectra. Monitoring of prolonged photolysis using IR and ultraviolet-visible (UV-vis) spectra demonstrated the formation of the allylic cation CT1 (λmax = 470 nm). On the other hand, photolysis of a 7,7-dimethoxy-1,4-diphenyl-2,3-diazabicyclo[2.2.1]hept-2-ene derivative (AZ2) yielded a puckered conformer (instead of planar) of the corresponding diradical S-DR2-puc, which was detected by IR and UV-vis spectroscopy in an Ar matrix at 4 K. This spectroscopic characterization opens a new strategy to obtain more detailed information about the structure and reactivity of singlet cyclopentane-1,3-diyl diradicals.
o-Tolylmethylene 1 is a metastable triplet carbene that rearranges to o-xylylene 2 even at temperatures as low as 2.7â K via [1,4] H atom tunneling. Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopical techniques were used to identify two conformers of 1 (anti and syn) in noble gas matrices and in frozen organic solutions. Conformer-specific kinetic measurements revealed that the rate constants for the rearrangements of the anti and syn conformers of 1 are very similar. However, the orbital alignment in the syn conformer is less favorable for the hydrogen transfer reaction than the orbital configuration in the anti conformer. Our spectroscopic and quantum chemical investigations indicate that anti 1 and syn 1 rapidly interconvert via efficient quantum tunneling forming a rotational pre-equilibrium. The subsequent second tunneling reaction, the [1,4] H migration from anti 1 to 2, is rate-limiting for the formation of 2. We here present an efficient strategy for the study of such tunneling equilibria.
Hydrogen , Electron Spin Resonance Spectroscopy , Kinetics , Temperature
A metal-free and operationally simple strategy for the generation of various α-monofluoroalkyl radicals has been developed. A combination of 1,4-bis(diarylamino)naphthalene photocatalyst and sulfoximine-based fluoroalkylating reagents is the key to success. The protocol can be applied to modular synthesis of ß-monofluoroketones through radical monofluoroalkylation of alkenyl acetates.
To expand the utility of α-cleavage at cryogenic temperatures, we investigated the photoreactivity of 2-azido-2-phenyl-1,3-indandione (1). EPR spectroscopy revealed that irradiating 1 in 2-methyltetrahydrofuran (mTHF) matrices forms alkylnitrene 32, which has zero-field splitting parameters (D/hc = 1.5837 cm-1; E/hc = 0.0039 cm-1) typical of an alkylnitrene. IR spectroscopy demonstrated that irradiating 1 in argon matrices results in the concurrent formation of 32, imine 3, benzocyclobutenedione 4, and benzonitrile 5.
To clarify the cis-trans isomerization mechanism of simple alkenes on the triplet excited state surface, the photochemistry of acyclic and cyclic vinyl ketones with a p-methoxyacetophenone moiety as a built-in triplet sensitizer (1 and 2, respectively) was compared. When irradiated, ketone 1 produces its cis-isomer, whereas ketone 2 does not yield any photoproducts. Laser flash photolysis of ketone 1 yields a transient spectrum with λmax â¼ 400 nm (τ â¼ 125 ns). This transient is assigned to the first triplet excited state (T1) of 1, which presumably decays to form a triplet biradical (1BR) that is shorter lived than the triplet ketone. In comparison, laser flash photolysis of 2 reveals two transients (τ â¼ 20 and 440 ns), which are assigned to T1 of 2 and triplet biradical 2BR, respectively. Density functional theory calculations support the characterization of the triplet excited states and the biradical intermediates formed upon irradiation of ketones 1 and 2 and allow a comparison of the physical properties of the biradical intermediates. As the biradical centers in 2BR are stabilized by conjugation, 2BR is more rigid than 1BR. Therefore, the longer lifetime of 2BR can be attributed to less-efficient intersystem crossing to the ground state.
Triplet cyclopentane-1,3-diyl diradical (T-DR) was generated via photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene (AZ) under low-temperature matrix conditions. Temperature independency of T-DR decay and the kinetic isotope effect of T-DR-d6 provided experimental evidence in favor of heavy-atom (carbon) tunneling process during the decay of T-DR to bicyclo[2.1.0]pentane (CP) via singlet S-DR. For the first time, the formation of CP was confirmed using low-temperature infrared spectra. Computations of the heavy-atom tunneling process using the small-curvature tunneling method demonstrated a fast reaction from S-DR to CP. Moreover, we observed heavy-atom tunneling during denitrogenation of AZ. Stereoselectivity in the tunneling process of T-DR-d6 was observed at 7 K to form retention-CP-d6 in higher amounts compared to inversion-CP-d6. Photolysis of AZ-d6 yielded inv-CP-d6 and ret-CP-d6 in environment- and temperature-dependent ratios. Moreover, because of the prominent matrix effect, T-DR decayed more rapidly in Ar than in glassy organic matrices.
Photolysis of 2,3-diazidonaphthalene-1,4-dione (1) in methyltetrahydrofuran matrices forms 2-(λ1-azaneyl)-3-azidonaphthalene-1,4-dione (vinylnitrene 32), as confirmed by electron paramagnetic resonance spectroscopy. The zero-field splitting (zfs) parameters for 32 (D/hc = 0.5338 cm-1, and E/hc = 0.0038 cm-1) reveal significant 1,3-biradical character. Irradiating 32 yields 2-(λ1-azaneyl)-1,3-dioxo-2,3-dihydro-1H-indene-2-carbonitrile (alkylnitrene 33), which has zfs parameters typical of a cycloalkylnitrene (D/hc = 1.57 cm-1, and E/hc = 0.00071 cm-1). Photolysis of 1 in argon matrices verifies that 32 forms 33.
Irradiation of nanocrystals of azide 1 results in a solid-to-solid reaction that forms imine 2 in high chemical yield. In contrast, solution photolysis of azide 1 yields a mixture of products, with 7 as the major one. Laser flash photolysis (LFP) of a nanocrystalline suspension of azide 1 in water shows selective formation of benzoyl radical 4 (λmax â¼ 400 nm), which is short-lived (τ = 833 ns) as it intersystem crosses to form imine 2. In comparison, LFP of azide 1 in methanol reveals the formation of triplet alkylnitrene 10 (λmax â¼ 340 nm). The selectivity observed in the solid-state is related to stabilization of the triplet ketone with (n,π*) configuration by the crystal lattice, which results in α-cleavage being favored over triplet energy transfer to the azido chromophore. Both the solid-state and solution reaction mechanisms are further supported by density functional theory calculations. Thus, laser flash photolysis has been used to effectively elucidate the medium dependent reaction mechanisms of azide 1.
Photolysis of 3-azido-1-indenone (1) with a light-emitting diode (LED, λ = 405 nm) or mercury arc lamp (Pyrex) resulted in the formation of heterodimer 3 in excellent yield, through dimerization of triplet vinylnitrene 32. At ambient temperature, vinylnitrene 32 (λmax at 340 and 480 nm) was detected directly with laser flash photolysis of vinyl azide 1. The vinylnitrene intermediate was also characterized directly with IR and ESR spectroscopy in cryogenic matrices. The ESR spectrum of vinylnitrene 32 yielded a zero-field splitting parameter |D/hc| of 0.460 cm-1 and |E/hc| of 0.015 cm-1, which reveals that vinylnitrene 32 has significant 1,3-biradical character. The proposed mechanism for the formation and reactivity of triplet vinylnitrene 32 was supported with density functional theory (DFT) calculations, which highlight that the steric demand of the five-membered ring in vinylnitrene 32 prevents intersystem crossing to the corresponding azirine (10). CASSCF and CASPT2 calculations showed that the energy gap between the singlet and triplet configurations of vinylnitrene 2 is only 10 kcal/mol. In spite of this small energy gap, vinylnitrene 32 does not decay by intersystem crossing, but rather by dimerization. Thus, triplet vinylnitrenes can be selectively formed with visible light and used to form new C-N bonds in synthetic applications.
The photolysis of 2-azido-1,4-naphthoquinone (1) in argon matrices at 8 K results in the corresponding triplet vinylnitrene (3)2, which was detected directly by IR spectroscopy. Vinylnitrene (3)2 is stable in argon matrices but forms 2-cyanoindane-1,3-dione (3) upon further irradiation. Similarly, the irradiation of azide 1 in 2-methyltetrahydrofuran (MTHF) matrices at 5 K resulted in the ESR spectrum of vinylnitrene (3)2, which is stable up to at least 100 K. The zero-field splitting parameters for nitrene (3)2, D/hc = 0.7292 cm(-1) and E/hc = 0.0048 cm(-1), verify that it has significant 1,3-biradical character. Vinylnitrene (3)2 (λmax â¼ 460 nm, τ = 22 µs) is also observed directly in solution at ambient temperature with laser flash photolysis of 1. Density functional theory (DFT) calculations support the characterization of vinylnitrene (3)2 and the proposed mechanism for its formation. Because vinylnitrene (3)2 is relatively stable, it has potential use as a building-block for high-spin assemblies.
Nanosecond laser flash photolysis of o-hydroxyacetophenone (1a) and 2,4-dihydroxyacetophenone (1b) in ethanol and acetonitrile results in absorption due to triplet biradicals 2a (λmax 430 nm, τ ≈ 3 µs) and 2b (λmax 400 nm, τ ≈ 1 µs), respectively. Triplet biradical 2a intersystem crosses to form Z-3a (λmax 400 nm, τ ≈ 10 µs), whereas 2b forms both Z-3b and E-3b (λmax 350 nm, τ ≈ 5 and 72 µs). Quenching studies demonstrate that 3a,b are formed on both the singlet and triplet excited surface of 1a and 1b. In ethanol at 77 K, o-hydroxyacetophenone derivatives 1a and 1b show phosphorescence, as is typical for triplet ketones with (n,π*) configuration. The mechanism for the photoreactivity of 1a,b is supported by density functional calculations.
Photolysis of vinylazide 1, which has a built-in acetophenone triplet sensitizer, in argon-saturated toluene results in azirine 2, whereas irradiation in oxygen-saturated toluene yields cyanide derivatives 3 and 4. Laser flash photolysis of azide 1 in argon-saturated acetonitrile shows formation of vinylnitrene 1c, which has a λmax at â¼300 nm and a lifetime of â¼1 ms. Vinylnitrene 1c is formed with a rate constant of 4.25 × 10(5) s(-1) from triplet 1,2-biradical 1b. Laser flash photolysis of 1 in oxygen-saturated acetonitrile results in 1c-O (λmax = 430 nm, τ ≈ 420 µs acetonitrile). Density functional theory (DFT) calculations were used to aid in the characterization of the intermediates formed upon irradiation of azide 1 and to validate the proposed mechanism for its photoreactivity.
Photolysis of 3-methyl-2-phenyl-2H-azirine (1a) in argon-saturated acetonitrile does not yield any new products, whereas photolysis in oxygen-saturated acetonitrile yields benzaldehyde (2) by interception of vinylnitrene 5 with oxygen. Similarly, photolysis of 1a in the presence of bromoform allows the trapping of vinylnitrene 5, leading to the formation of 1-bromo-1-phenylpropan-2-one (4). Laser flash photolysis of 1a in argon-saturated acetonitrile (λ = 308 nm) results in a transient absorption with λ(max) at ~440 nm due to the formation of triplet vinylnitrene 5. Likewise, irradiation of 1a in cryogenic argon matrixes through a Pyrex filter results in the formation of ketene imine 11, presumably through vinylnitrene 5. In contrast, photolysis of 2-methyl-3-phenyl-2H-azirine (1b) in acetonitrile yields heterocycles 6 and 7. Laser flash photolysis of 1b in acetonitrile shows a transient absorption with a maximum at 320 nm due to the formation of ylide 8, which has a lifetime on the order of several milliseconds. Similarly, photolysis of 1b in cryogenic argon matrixes results in ylide 8. Density functional theory calculations were performed to support the proposed mechanism for the photoreactivity of 1a and 1b and to aid in the characterization of the intermediates formed upon irradiation.
Photolysis of 1 in argon-saturated acetonitrile yields 2, whereas in oxygen-saturated acetonitrile small amounts of benzoic acid and benzamide are formed in addition to 2. Similarly, photolysis of 2 in argon-saturated acetonitrile results in 1 and a trace amount of 3, whereas in oxygen-saturated acetonitrile the major product is 1 in addition to the formation of small amounts of benzoic acid and benzamide. Laser flash photolysis of 1 results in an absorption due to triplet vinylnitrene 4 (broad absorption with λ(max) at 360 nm, τ = 1.8 µs, acetonitrile) that is formed with a rate constant of 1.2 × 10(7) s(-1) and decays with a rate constant of 5.6 × 10(5) s(-1). Laser flash photolysis of 2 in argon-saturated acetonitrile likewise results in the formation of triplet vinylnitrene 4 but also ylide 5 (λ(max) at 440 nm, τ = 13 µs). The rate constant for forming 4 in argon-saturated acetonitrile is 1.6 × 10(7) s(-1). In oxygen-saturated acetonitrile, vinylnitrene 4 reacts to form the peroxide radical 6 (λ(max) 360 nm, ~0.7 µs, acetonitrile) at a rate of 2 × 10(9) M(-1) s(-1). Density functional theory calculations were performed to aid in the characterization of vinylnitrene 4 and peroxide 6 and to support the proposed mechanism for the formation of these intermediates.
