Thermolysis of exo- and endo-5-methoxy-2,3-diazabicyclo[2.2.1]-2-heptene

Allred, Evan L.; Smith, Richard L.

doi:10.1021/ja01002a063

Cited by 37 publications

(30 citation statements)

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“…First, experiments showed that N 2 loss results in bicyclo products 3 and 4 with a nonstatistical preference for the exo product in a 4.7:1 ± 0.9 (exo:endo) ratio. , The statistical ratio would be 1:1 if the intervening 1,3-singlet diradical intermediate 2 were to undergo complete IVR prior to C–C bond formation. Second, several qualitative theories for the origin of selectivity have been proposed, beginning with Roth and Martin, , Allred and Smith, and culminating with Carpenter’s in-depth description that the nonstatistical exo:endo ratio results from a relatively low barrier (∼2 kcal/mol) for transition-state TS2 ( endo-TS2 and exo-TS2 , see Scheme ), which results in C–C bond formation without IVR. , More specifically, Carpenter’s description focused on the out-of-plane bending motion of the methylene (CH 2 ) bridge group that leads to the unexpected excess of exo product 3 , and this rationale was qualitatively supported by classical trajectory analyses and experiments in supercritical propane . Third, density functional theory (DFT) quasiclassical trajectories have not been reported.…”

Section: Introductionmentioning

confidence: 99%

Machine Learning Analysis of Direct Dynamics Trajectory Outcomes for Thermal Deazetization of 2,3-Diazabicyclo[2.2.1]hept-2-ene

et al. 2020

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Experimentally, the thermal gas-phase deazetization of 2,3-diazabicyclo[2.2.1]hept-2-ene (1) results in the loss of N 2 and the formation of bicyclo products 3 (exo) and 4 (endo) in a nonstatistical ratio, with preference for the exo product. Here, we report unrestricted M06-2X quasiclassical trajectories initialized from the concerted N 2 ejection transition state that were able to replicate the experimental preference to form 3. We found that the 3:4 ratio results from the relative amounts of very fast (ballistic) exotype trajectories versus trajectories that lead to the 1,3-diradical intermediate 2. These quasiclassical trajectories provided a set of transition-state vibrational, velocity, momenta, and geometric features for the machine learning analysis. A selection of popular supervised classification algorithms (e.g., random forest) provided poor prediction of trajectory outcomes based on only transitionstate vibrational quanta and energy features. However, these machine learning models provided more accurate predictions using atomic velocities and atomic positions, attaining ∼70% accuracy using initial conditions and between 85 and 95% accuracy at later reaction time steps. This increased accuracy allowed the feature importance analysis to reveal that, at the later-time analysis, the methylene bridge out-of-plane bending is correlated with trajectory outcomes for the formation of either the exo product or toward the diradical intermediate. Possible reasons for the struggle of machine learning algorithms to classify trajectories based on transitionstate features is the heavily overlapping feature values, the finite but very large possible vibrational mode combinations, and the possibility of chaos as trajectories propagate. We examined this chaos by comparing a set of nearly identical trajectories that differed by only a very small scaling of the kinetic energies resulting from the transition-state reaction coordinate.

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Section: Introductionmentioning

confidence: 99%

Machine Learning Analysis of Direct Dynamics Trajectory Outcomes for Thermal Deazetization of 2,3-Diazabicyclo[2.2.1]hept-2-ene

et al. 2020

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“…Since 1965, mechanistic studies have revealed that the structures of short-lived and elusive diradicals were essential to understand the selective formation of inversion product inv- CP . − Computational studies at the CASPT2//CASSCF level of theory predict that 1,3-diradicals DR and 1,5-diazenyl diradicals DZ are responsible for the selectivity between the product with retained stereochemistry, ret- CP , and its inverted counterpart, inv- CP (Scheme ). − Homolytic substitution (S H 2) involving the equatorial diazenyl diradical, eq- DZ , offers a plausible pathway for the formation of inv- CP . , The Closs-type planar singlet diradical intermediate, pl- DR , , is common to both ret- CP and inv- CP .…”

Section: Introductionmentioning

confidence: 99%

Impact of Diradical Spin State (Singlet vs Triplet) and Structure (Puckered vs Planar) on the Photodenitrogenation Stereoselectivity of 2,3-Diazabicyclo[2.2.1]heptanes

et al. 2016

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Versatile transformations of azo compounds are utilized not only in synthetic organic chemistry but also in materials science. In this study, a hitherto unknown stereoselectivity was observed by low-temperature in situ NMR spectroscopy for the photochemical denitrogenation of a cyclic azoalkane (2,3-diazabicyclo[2.2.1]heptane) derivative. Direct (singlet) photodenitrogenation at 188 K afforded two products, the configurationally retained ring-closed compound (ret-CP) and the inverted compound (inv-CP), in a ratio of 82/18 (±3) (ret-CP/inv-CP), with an overall yield of >95%. Triplet-sensitized denitrogenation at 199 K using benzophenone ((3)BP*) or xanthone ((3)Xan*) selectively produced inv-CP, with a ret-CP/inv-CP ratio of 7/93 (±3). Thermal isomerization of inv-CP into ret-CP was observed by low-temperature NMR spectroscopy. Transient absorption spectroscopy revealed that two distinct singlet diradicals are involved in the formation of CP during direct photodenitrogenation, that is, puckered puc-(1)DR and planar pl-(1)DR diradicals. The former produces ret-CP, whereas the latter affords inv-CP. Kinetic analysis using the integrated profiles method was used to determine the molecular absorption coefficient of pl-(1)DR (ε560 = 4900 ± 250 M(-1) cm(-1)) for the first time. The involvement of the puckered singlet diradical resolves the mechanistic puzzle of stereoselective denitrogenation of diazabicycloheptane-type azoalkanes.

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“…6 Rate constant for decay of sensitizer triplet (see Herkstroeter and Hammond in footnote a). 6 Intersystem crossing efficiency. d Uncorrected for direct photolysis.…”

Section: Resultsmentioning

confidence: 99%

“…d Uncorrected for direct photolysis. 6 Value in ethanol: C. A. Parker and T. A. Joyce, Trans. Faraday Soc., 62, 2785 (1966).…”

Section: Resultsmentioning

confidence: 99%

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Sensitized photolysis of acyclic azo compounds. Singlet energy transfer

Engel¹,

Bartlett²

1970

J. Am. Chem. Soc.

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Azomethane is decomposed by direct photolysis in hexane and in toluene with quantum yields of 0.1 5 and 0.088, and with cage effects (ethane-nitrogen product ratio) of 68 and 76%, respectively. Sensitized photolysis is observed with phenanthrene, triphenylene, pyrene, anthracene, and acetone, with cage effects indistinguishable from those in direct photolysis. Thioxanthone gave a 9% lower cage effect than direct photolysis. With benzophenone, benzanthrone, and acridine the quantum yields were only of the order of magnitude attributable to some direct light absorption by the azomethane. In the case of azo-2-methyl-2-propane, sensitized photolysis is observed with six sensitizers, listed in Table 111, with four aromatic ketones failing to show appreciable sensitized decomposition of the azo compound. Azo-2-methyl-2-propane quenches the fluorescence of five aromatic hydrocarbons with Stern-Volmer slopes from 6.6 for perylene to 435 for phenanthrene. The highest quantum yield for sensitized decomposition is equal to that for direct photolysis at the same substrate composition, 0.02 M . Efficient triplet quenchers do not affect the sensitized photolysis. Selective irradiation of benzophenone (triplet energy (ET) = 68.5 kcal) in the presence of azo-2-methyl-2-propane and excess triphenylene (ET = 66.6 kcal) results in no nitrogen formation. The quantum yield of decomposition of azo-2-methyl-2-propane sensitized by triphenylene increases with increasing substrate concentration to a limit of 0.40, equal to that of direct photolysis under the same conditions. These results are interpreted as meaning that the azo compounds efficiently accept energy from T,T* excited singlets of sensitizers. Possible sequences of states leading to the cis,trans isomerization and decomposition of azo compounds are discussed. The exceptionally easy transfer of singlet energy to azo compounds accounts for previous failures to observe spin correlation in radical pairs from azo sources. ~~~~~ ~~ a Sensitizer triplet energy: W. Herkstroeter and G. S. Hammond, J . Amer. Chem. SOC., 88, 4769 (1966); J. Calvert and J. Pitts, "Photo-Rate constant for decay of sensitizer triplet (see Herkstroeter and Hammond in footnote e Value in ethanol:

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Thermolysis of exo- and endo-5-methoxy-2,3-diazabicyclo[2.2.1]-2-heptene

Cited by 37 publications

References 0 publications

Machine Learning Analysis of Direct Dynamics Trajectory Outcomes for Thermal Deazetization of 2,3-Diazabicyclo[2.2.1]hept-2-ene

Machine Learning Analysis of Direct Dynamics Trajectory Outcomes for Thermal Deazetization of 2,3-Diazabicyclo[2.2.1]hept-2-ene

Impact of Diradical Spin State (Singlet vs Triplet) and Structure (Puckered vs Planar) on the Photodenitrogenation Stereoselectivity of 2,3-Diazabicyclo[2.2.1]heptanes

Sensitized photolysis of acyclic azo compounds. Singlet energy transfer

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