Concurrent hydrogen migration and nitrogen extrusion in the excited states of alkylchlorodiazirines

White, W. R.; Platz, Matthew S.

doi:10.1021/jo00036a016

Cited by 70 publications

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“…There have been no previous experimental or theoretical studies pertaining to direct alkene formation in the photodissociation of alkyl-substituted ketenes. Instead, most previous work has been on the photochemistry of diazoalkanes or alkyldiazirines, − with the primary focus being the occurrence of H migration forming alkenes (RIES) or CO migration forming ketenes (Wolff rearrangement). As noted earlier, the term RIES (rearrangement in the excited state) has been applied to photoinduced reactions forming stable molecules, bypassing formation of free carbenes.…”

Section: Discussionmentioning

confidence: 99%

“…Diazoalkanes, or their more stable cyclic alkyldiazirine isomers, produce alkyl-substituted carbenes via N 2 elimination following electronic excitation. − However, because of the high enthalpies of formation and relatively short-wavelength electronic absorption features of the parent molecules, the carbene products, if produced, are very highly internally excited and rapidly isomerize to alkenes, which typically lie ∼300 kJ/mol (∼3 eV) lower in energy than ground state carbenes . The well-established, “conventional” kinetic mechanism after electronic excitation of stable precursors involves free carbene production followed by isomerization to alkenes. − However, beginning with the earliest studies in the 1960s, , “isomerization concurrent with N 2 loss” has been frequently hypothesized ,,− with the acronym “rearrangement in the excited state” (RIES), often used to denote photoinduced processes involving carbene precursors bypassing formation of free carbenes . The most extensively studied examples in which RIES is thought to occur include 1,2-hydrogen migration in alkyl-substituted carbene precursors forming alkenes ,,, and the photoinduced Wolff rearrangement in α-carbonyl-substituted carbene precursors. , A number of theoretical studies − as well as ultrafast time-resolved experiments − , have addressed the role of possible concerted and stepwise pathways for alkene production.…”

Section: Introductionmentioning

confidence: 99%

“…The well-established, “conventional” kinetic mechanism after electronic excitation of stable precursors involves free carbene production followed by isomerization to alkenes. − However, beginning with the earliest studies in the 1960s, , “isomerization concurrent with N 2 loss” has been frequently hypothesized ,,− with the acronym “rearrangement in the excited state” (RIES), often used to denote photoinduced processes involving carbene precursors bypassing formation of free carbenes . The most extensively studied examples in which RIES is thought to occur include 1,2-hydrogen migration in alkyl-substituted carbene precursors forming alkenes ,,, and the photoinduced Wolff rearrangement in α-carbonyl-substituted carbene precursors. , A number of theoretical studies − as well as ultrafast time-resolved experiments − , have addressed the role of possible concerted and stepwise pathways for alkene production. Some of this work has led to the conclusion that alkyl-substituted diazirines are inefficient photolytic sources of alkyl-substituted carbenes due to predominant direct formation of alkenes via RIES. , However, we note that several subsequent theoretical studies have not supported the RIES mechanism in these diazirine systems. ,, Owing to the potential importance of dimethylcarbene in organic synthesis, complex non-nitrogenous precursors for dimethylcarbene have been synthesized .…”

Section: Introductionmentioning

confidence: 99%

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Dimethylcarbene versus Direct Propene Formation in Dimethylketene Photodissociation

Datta

Davis

2021

J. Phys. Chem. A

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Highly reactive carbenes are usually produced by photolysis of ketenes, diazoalkanes, or diazirines. Sequential kinetic pathways for deactivation of nascent carbenes usually involve bimolecular reactions in competition with isomerization producing stable products such as alkenes. However, the direct photolytic production of stable products, effectively bypassing formation of free carbenes, has been postulated for over 50 years but remains very poorly understood. Often termed “rearrangement in the excited state” (RIES), examples include 1,2-hydrogen migration within photoexcited carbene precursors yielding alkenes and the Wolff rearrangement in photogenerated carbonyl-substituted carbenes producing ketenes. In this study, the two competing CO elimination channels from photoexcited gaseous dimethylketene, producing dimethylcarbene and propene, were studied as a function of electronic excitation energy, under collision-free conditions, by using photofragment translational energy spectroscopy with vacuum ultraviolet photoionization of the products. A significant fraction of the dimethylcarbene → propene isomerization exothermicity (∼300 kJ/mol) was released as propene + CO translational energy, indicating that propene is formed prior to or concurrent with CO elimination. An increase in the propene yield with increasing excitation energy suggests that the effective potential energy barrier for this channel lies ∼24 kJ/mol above the energetic threshold for dimethylcarbene formation via CC bond fission. Possible mechanisms for direct propene elimination are discussed in light of the observed energy dependence for the competing pathways.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Dimethylcarbene versus Direct Propene Formation in Dimethylketene Photodissociation

Datta

Davis

2021

J. Phys. Chem. A

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“…[64] On one hand, the carbene 62a could easily be transformed into the chlorostyrene 63a through an intramolecular hydride shift. On the other hand, the diazoalkane 61a could also form 63a through acid-catalyzed decomposition.…”

Section: Halodiazirinesmentioning

confidence: 99%

Valence Isomerization between Diazo Compounds and Diazirines

Korneev

2011

Eur J Org Chem

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Diazo compounds and diazirines have been separately studied in detail and are widely used in research and for applied purposes. Although the mutual isomerization of these compounds was discovered 50 years ago, it has still been only modestly investigated. Meanwhile, this type of mutual transformation has to be taken into account on every occasion when one of these compounds is used or investigated. This review discusses all main cases of mutual isomerization between diazo compounds and diazirines with the goal of providing an impulse for its rational application, as well as describing the isomerism.

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“…If cycloreversion proceeds via such an intermediate (see eq. [5]), then one would expect to observe scrambling of deuterium between positions 3 and 4 of the cyclobutenyl ring, since carbenes of this type are known to undergo ring expansion to the corresponding cyclobutene in addition to fragmentation to alkene and alkyne (49)(50)(51). Examination of the 'H NMR spectrum of an irradiated 0.02 M solution of 2c-d, after ca.…”

Section: Discussionmentioning

confidence: 99%

Cyclobutene photochemistry. Substituent effects on the photochemistry of 1-phenylcyclobutene

Leigh¹,

Postigo²

1995

Can. J. Chem.

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The photochemistry and photophysics of I-phenylcyclobutene and five aryl-substituted derivatives have been studied in various solvents at room temperature. All six compounds fluoresce with quantum yields in the 0.2-0.3 range in cyclohexane and acetonitrile solution. I-Phenylcyclobutene undergoes [2+2]-cycloreversion ( 4 = 0.09) to yield phenylacetylene upon photolysis in either hydrocarbon or acetonitrile solution, and undergoes (Markovnikov) solvent addition upon irradiation in methanol solution ( 4 = 0.08) in addition to cycloreversion. Triplet sensitization and quenching experiments indicate that cycloreversion and methanol addition are both excited singlet state processes. None of the six compounds studied undergo ring opening to the corresponding 2-aryl-1.3-butadiene in detectable yield. Quantum yields for cycloreversion in cyclohexane, acetonitrile, and methanol solution and methanol addition have been determined for the six compounds, along with excited singlet state lifetimes. The quantum yields and rate constants for cycloreversion and methanol addition are both enhanced by substitution with electron-donating groups. ' The variation in the rate constant for [2+2]-cycloreversion with substituent indicates that there is substantial dipolar character developed in the cyclobutenyl a-bond framework during the reaction, in almost exact correspondence with that developed in the .rr system during photoprotonation. No deuterium scrambling is observed in I-phenylcyclobutene-2,4,4-4 after photolysis in pentane solution to ca. 80% conversion, indicating that skeletal rearrangements leading to cyclopropyl carbenes do not occur in the direct photolysis of arylcyclobutene derivatives. A pericyclic mechanism for the photocycloreversion reaction is suggested. Triplet-triplet absorption spectra and triplet lifetimes of I-phenyl-, 1-(porn-methylpheny1)-, and I-fj1arc2-trifluoromethyIphenyl)cyclobutene in hydrocarbon solution are also reported.Key words: photochemistry, cyclobutene, fluorescence, [2+2]-cycloreversion, substituent effects, nanosecond laser flash photolysis, lifetime, triplet state, styrene, photoaddition Resume : On a CtudiC, dans differents solvants et 6 diffkrentes temperatures, la photochimie et la photophysique du I-phenylcyclobutkne et de cinq derives aryles substitues. Tous ces six composes entrent en fluorescence avec des rende~nents quantiques de l'ordre de 0.2 6 0,3 en solution dans le cyclohexane et dans I'acCtonitrile. Le I -phCnylcyclobutkne subit une cycloCIimination [2+2] ( 4 = 0,09) pour conduire au phCnylacCtylkne par photolyse en solution dans un hydrocarbure ou dans I'ac6tonitrile. avec en plus une addition de solvant (de type Markovnikov) par irradiation dans une solution de methanol ( 4 = 0,08). Les experiences de sensibilisation du triplet et de dksactivation indiquent que la cycloClimination et I'addition de methanol sont toutes deux des Ctats singulets excites. Aucun des six composCs CtudiCs ne conduit par ouverture du cycle au 2-aryl-1,3-butadikne en quantite dCcelable. On ...

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Concurrent hydrogen migration and nitrogen extrusion in the excited states of alkylchlorodiazirines

Cited by 70 publications

References 0 publications

Dimethylcarbene versus Direct Propene Formation in Dimethylketene Photodissociation

Dimethylcarbene versus Direct Propene Formation in Dimethylketene Photodissociation

Valence Isomerization between Diazo Compounds and Diazirines

Cyclobutene photochemistry. Substituent effects on the photochemistry of 1-phenylcyclobutene

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