Femtosecond observation of benzyne intermediates in a molecular beam: Bergman rearrangement in the isolated molecule

Abstract

In this communication, we report our femtosecond real-time observation of the dynamics for the three didehydrobenzene molecules (p-, m-, and o-benzyne) generated from 1,4-, 1,3-, and 1,2-dibromobenzene, respectively, in a molecular beam, by using femtosecond time-resolved mass spectrometry. The time required for the first and the second C-Br bond breakage is less than 100 fs; the benzyne molecules are produced within 100 fs and then decay with a lifetime of 400 ps or more. Density functional theory and high-level ab initio calculations are also reported herein to elucidate the energetics along the reaction path. We discuss the dynamics and possible reaction mechanisms for the disappearance of benzyne intermediates. Our effort focuses on the isolated molecule dynamics of the three isomers on the femtosecond time scale.A rynes have presented chemists with structural and synthetic challenges for more than half a century. After the structure proposal for 1,2-didehydrobenzene by Wittig (1) , the challenge to understand the behavior and properties of these reactive species has attracted large numbers of chemists and spawned a whole area of inquiry within chemistry (3, 4). Although the greatest effort has been in the study of 1,2-dehydrobenzene, the behavior and properties of the 1,3-and 1,4-isomers have recently been the subject of many studies (refs. 5 and 6; and ref. 7 and references therein). The 1,3-isomer has been characterized recently (5). The 1,4-isomer has gained special prominence because of its propensity to undergo Bergman rearrangement (8) and its relationship to several currently exciting antitumor agents such as neocarzinostatin and dynemicin A. Materials and MethodsThe femtosecond laser and molecular beam apparatus is described in detail elsewhere (ref. 9 and references therein). Briefly, a femtosecond laser system was integrated to a molecular beam apparatus with the capability of measuring time-offlight mass spectra. The amplified pulses were typically Ϸ80 fs, and the energy input was Ϸ150 J⅐pulse Ϫ1 at 615 nm. For the pump, the 615-nm output was frequency doubled. The probe beam, which ionizes the transient species, was passed to a computer-controlled translation stage for the time delay. Both pump and probe beams were appropriately attenuated to minimize background signals; no ionization was observed when the pump pulse was blocked, and the transients are for the enhanced ionization when both pump and probe pulses are present. 1,4-Dibromobenzene (98%), 1,3-dibromobenzene (97%), and 1,2-dibromobenzene (98%), all from Aldrich, were used without further purification. The mass spectra show their characteristic patterns, as discussed below. The theoretical work reported herein involves density functional theory (DFT) and ab initio methods and will be detailed in the Discussion section. Fig. 1 shows the femtosecond mass spectra of 1,4-dibromobenzene (Fig. 1 A), 1,3-dibromobenzene (Fig. 1B), and 1,2-dibromobenzene (Fig. 1C). The mass spectra are similar. The parent signals are characterized by three pea...

The Expanding Role of Electrospray Ionization Mass Spectrometry for Probing Reactive Intermediates in Solution

“…(3) Typically short lifetime residence in the reaction mixture (not enough thermodynamic stability to be readily isolated and detected directly): reaction intermediates can vary widely in their chemical lifetime, from transient molecules with very short lifetime on the pico/nano scales (e.g., benzyne radicals [ 16 ], iminium ions in aqueous solution [ 48 , 49 ]), through semistable species on a lifetime scale of a few microseconds [ 10 ] or fractions of a second [ 8 , 50 ], to some long-lived intermediates within a duration of the order of seconds or minutes [ 51 ]; in a cell, the aqueous stabilities of reactive metabolites vary markedly between drugs [ 4 ], with apparent half-lives ( t 1/2 ) over a time frame normally ranging from fractions of a second to minutes or hours [ 31 ].…”

“…But for most of its history, MS has long served as a leading tool for gas-phase ion chemistry. By classic hard ionization techniques (such as electron impact, chemical ionization, atom bombardment) ions are created under vacuum, in which some highly reactive small (volatile) molecules could be generated and monitored in the gas phase, e.g., femtosecond real-time observation of benzyne intermediates in a molecular beam [ 16 ], neutralization-reionization mass spectrometry for neutral intermediates [ 1 , 17 , 18 ]. Indeed, a few “softer” vacuum ionization mass spectrometers, including secondary ion MS (SIMS) and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), could be used to produce intact molecular ions from condensed-phase samples.…”

Effects of the Substituents on the Energy Flow of Toluene Derivatives in Collisions with N2 and O2

“…The vibrational relaxation of the halogenated organic molecules has long been of interest . In particular, toluene derivatives such as chloro‐ and bromotoluene are important molecules in the chemical, pharmaceutical, and agricultural industries as well as safe preserving environment; thus, elucidating their reactivities and energy transfer characteristics are vital.…”

Real-time investigation of the photodissociation dynamics of p-chlorotoluene and p-dichlorobenzene

“…However, photodissociation of aryl halides generally occurs by a predissociative mechanism. The kinetic information can be obtained by the PTS [5][6][7] or femtosecond pump-probe experiments [8][9][10]. The dynamics of aryl halides is more complicated for the competition between intersystem crossing and internal conversion.…”