Direct Observation of an Alkylidenecarbene by Ultrafast Transient Absorption Spectroscopy

Du, Lili; Lan, Xin; Phillips, David Lee; Coldren, William H.; Hadad, Christopher M.; Yang, Xi; Thamattoor, Dasan M.

doi:10.1021/acs.jpca.8b06118

Cited by 12 publications

(17 citation statements)

References 23 publications

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“… 11 Besides computational data, 12 , 13 very few data on singlet vinylidenes exist, which are based on matrix isolation 14 , 15 or gas phase spectroscopy. 16 − 18 Singlet H 2 C=C: has been proposed in combustion processes and rearranges on the picosecond time scale to acetylene. 19 In addition to the singlet state, vinylidenes also feature two symmetry-distinct triplet states of similar energy, which are typically high-lying electronically excited states.…”

Section: Introductionmentioning

confidence: 99%

Characterization of a Triplet Vinylidene

et al. 2021

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Singlet vinylidenes (R 2 C=C:) are proposed as intermediates in a series of organic reactions, and very few have been studied by matrix isolation or gas-phase spectroscopy. Triplet vinylidenes, however, featuring two unpaired electrons at a monosubstituted carbon atom are thus far only predicted as electronically excited-state species and represent an unexplored class of carbon-centered diradicals. We report the photochemical generation and low-temperature EPR/ENDOR characterization of the first ground-state high-spin (triplet) vinylidene. The zero-field splitting parameters ( D = 0.377 cm –1 and | E| / D = 0.028) were determined, and the 13 C hyperfine coupling tensor was obtained by 13 C-ENDOR measurements. Most strikingly, the isotropic 13 C hyperfine coupling constant (50 MHz) is far smaller than the characteristic values of triplet carbenes, demonstrating a unique electronic structure which is supported by quantum chemical calculations.

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Section: Introductionmentioning

confidence: 99%

Characterization of a Triplet Vinylidene

et al. 2021

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“…The direct observation of carbene reaction pathways has been the subject of several previous studies. 53 − 61 By monitoring the C=O stretching modes using time-resolved infrared (TRIR) spectroscopy, 54 Xue et al. demonstrated that a variety of ylides could be directly observed from the reaction of the singlet α-carbonyl carbene (2) with nucleophilic solvents such as THF, ACN, and deuterated methanol (MeOD).…”

Section: Introductionmentioning

confidence: 99%

Direct Observation of the Dynamics of Ylide Solvation by Hydrogen-bond Donors Using Time-Resolved Infrared Spectroscopy

Phelps

Orr-Ewing

2022

J. Am. Chem. Soc.

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The photoexcitation of α-diazocarbonyl compounds produces singlet carbene intermediates that react with nucleophilic solvent molecules to form ylides. The zwitterionic nature of these newly formed ylides induces rapid changes in their interactions with the surrounding solvent. Here, ultrafast time-resolved infrared absorption spectroscopy is used to study the ylide-forming reactions of singlet carbene intermediates from the 270 nm photoexcitation of ethyl diazoacetate in various solvents and the changes in the subsequent ylide–solvent interactions. The results provide direct spectroscopic observation of the competition between ylide formation and C–H insertion in reactions of the singlet carbene with nucleophilic solvent molecules. We further report the specific solvation dynamics of the tetrahydrofuran (THF)-derived ylide (with a characteristic IR absorption band at 1636 cm –1 ) by various hydrogen-bond donors and the coordination by lithium cations. Hydrogen-bonded ylide bands shift to a lower wavenumber by −19 cm –1 for interactions with ethanol, −14 cm –1 for chloroform, −10 cm –1 for dichloromethane, −9 cm –1 for acetonitrile or cyclohexane, and −16 cm –1 for Li + coordination, allowing the time evolution of the ylide–solvent interactions to be tracked. The hydrogen-bonded ylide bands grow with rate coefficients that are close to the diffusional limit. We further characterize the specific interactions of ethanol with the THF-derived ylide using quantum chemical (MP2) calculations and DFT-based atom-centered density matrix propagation trajectories, which show preferential coordination to the α-carbonyl group. This coordination alters the hybridization character of the ylidic carbon atom, with the greatest change toward sp 2 character found for lithium-ion coordination.

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“…2 Measurement of absolute rates of 1,-2-migration requires direct detection of the alkylidene carbene, with Phillips, Hadad and Thamattoor reporting the first observation in solution (PhMeC=C:) by femtosecond spectroscopy, just two years ago. 15 It has long been recognized that some alkylidene β-substituents (RX, RY) have a much greater propensity for 1,2-migration (e.g. H, Aryl, Alkynyl) than others (e.g., Alkyl), [6][7][8] yet only a few studies have sought to explicitly assess this, using isotopic 13 C/ 14 C labelling.…”

Section: Introductionmentioning

confidence: 99%

Systematic Evaluation of 1,2-Migratory Aptitude in Alkylidene Carbenes

et al. 2021

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Alkylidene carbenes undergo rapid inter-and intra-molecular reactions and rearrangements, including 1,2-migrations of β-substituents to generate alkynes. Their propensity for substituent migration exerts profound influence over the broader utility of alkylidene carbene intermediates, yet prior efforts to categorize 1,2-migratory aptitude in these elusive species have been hampered by disparate modes of carbene generation, ultrashort carbene lifetimes, mechanistic ambiguities, and the need to individually prepare a series of 13 C-labelled precursors. Herein we report on the rearrangement of 13 C-alkylidene carbenes generated in situ by the homologation of carbonyl compounds with [ 13 C]-Li-TMS-diazomethane, an approach that obviates the need for isotopically labelled substrates and has expedited a systematic investigation ( 13 C{ 1 H} NMR, DLPNO-CCSD(T)) of migratory aptitudes in an unprecedented range of more than 30 alkylidene carbenes. Hammett analyses of the reactions of 26 differentially substituted benzophenones reveal several counterintuitive features of 1,2-migration in alkylidene carbenes that may prove of utility in the study and synthetic application of unsaturated carbenes more generally.

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Direct Observation of an Alkylidenecarbene by Ultrafast Transient Absorption Spectroscopy

Cited by 12 publications

References 23 publications

Characterization of a Triplet Vinylidene

Characterization of a Triplet Vinylidene

Direct Observation of the Dynamics of Ylide Solvation by Hydrogen-bond Donors Using Time-Resolved Infrared Spectroscopy

Systematic Evaluation of 1,2-Migratory Aptitude in Alkylidene Carbenes

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