Lifetime Of Carbene Research Articles

Ultrafast time‐resolved studies of the photochemistry of diazo carbonyl compounds

AbstractThe rich photochemistry of diazo carbonyl compounds in solution has been extensively studied by ultrafast transient absorption spectroscopy with UV–Vis and mid‐IR detection. Both, stepwise (carbene mediated) and concerted Wolff rearrangement (WR) mechanisms have been confirmed for cyclic and acyclic diazo carbonyl precursors. Cyclic diazo‐carbonyl compounds are structurally locked in a syn conformation and mainly follow the concerted mechanism, however, a stepwise pathway does slightly contribute to the overall WR process, contrary to Kaplan's rule. With acyclic diazo ester carbonyl compounds, the WR process is dominated by the stepwise mechanism. A carbene–carbene interconversion process and singlet to triplet carbene intersystem crossing (ISC) processes are discussed in this review. The growth of secondary species (cations, ylides, OH insertion product) emerging from an interaction between a singlet carbene and solvent molecules has been characterized by picosecond time‐resolved UV–Vis transient absorption spectroscopy. Carbene reactivity results in shortening of the singlet carbene lifetime relative to more inert solvents. The quantum yield of diazo decomposition can be deduced from partial ground state bleach recovery using ultrafast time‐resolved IR spectroscopy. Copyright © 2009 John Wiley & Sons, Ltd.

Ultrafast Study of p-Biphenylyldiazomethane and p-Biphenylylcarbene

p-Biphenylyldiazomethane was excited by femtosecond pulses of UV light in acetonitrile, in cyclohexane, and in methanol. Ultrafast photolysis produces a singlet excited state of p-biphenylyldiazomethane with lambdamax = 490 nm, and lifetimes of less than 300 fs in acetonitrile, in cyclohexane, and in methanol. The decay of the excited state is accompanied by the growth of transient absorption with lambdamax = 360 nm. The carrier of this transient absorption is attributed to singlet p-biphenylylcarbene, a result that is consistent with the predictions of TD-DFT calculations. The singlet carbene lifetimes are 200 and 77 ps in acetonitrile and cyclohexane, respectively, and are controlled by intersystem crossing to the lower energy triplet state. The transient absorption does not decay to baseline in acetonitrile, because of the formation of nitrile ylide. The equilibrium mixture of singlet and triplet p-biphenylylcarbene reacts with acetonitrile to form a nitrile ylide (lambdamax = 370 nm), and with cyclohexane by C-H insertion 1-20 ns after the laser pulse. The singlet carbene lifetime is only 7.9 ps in methanol, owing to a rapid reaction with the solvent. Reaction with the solvent gives rise, in part, to a p-biphenylylbenzyl cation (lambdamax = 450 nm, tau = 6.3 ps) in methanol.

Matrix and Time-Resolved Infrared Spectroscopy of Chloro-p-nitrophenylcarbene and Related Species

Chloro-p-nitrophenyldiazirine (2) was deposited in an argon matrix. Photolysis (350 nm) releases chloro-pnitrophenylcarbene (1) as a persistent species. The IR and UV-vis spectra of carbene 1 were obtained, and the IR spectrum was adequately simulated by density functional theory (DFT) calculations. The carbene has an intense IR vibration at 1206 cm -1 involving the carbene carbon and aromatic ring carbon. This band was observed by TRIR spectroscopy upon laser flash photolysis (355 nm) of 2 in heptane at ambient temperature. The presence of benzene did not influence the frequency of this vibration. The carbene lifetime as measured by TRIR spectroscopy (400 ns) is consistent with previous LFP studies utilizing UV-vis detection. Upon LFP of 2 in CCl4 or CF2ClCFCl2 the photogenerated carbene abstracts chlorine atom from solvent to form R,R-dichloro-p-nitrobenzyl radical with a TRIR band at 1316 cm -1 . Upon LFP of 2 in acetonitrile, acetone, methyl acetate, pyridine, and tetrahydrofuran ylide species were produced. The ylides had similar TRIR spectra with bands at 1584, 1504, and 1312 cm -1 .

Photochemistry of 2,2,4,4-Tetramethyl-3-thietan-1-ylidene: A Heterocyclic Carbene with an Unusually Short Lifetime and Evidence for a Nonclassical Structure

Laser flash (308 nm) and steady-state photolysis (300 nm) of Δ3-1,3,4-oxadiazoline (3) produces a transient that can be trapped with pyridine to give an adduct with λmax = 350 nm, attributed to ylide 10. Laser flash photolysis (LFP) studies of the ylide indicate that the value of kpyrτ (where kpyr is the absolute rate constant of carbene reaction with pyridine and τ is the carbene lifetime) of the reactive intermediate is 25 times smaller than that deduced for cyclobutylidene. The sulfur-containing transient may have a much shorter lifetime than cyclobutylidene or a much smaller value of kpyr. These experimental results, along with ab initio calculations, suggest that the structure of 2,2,4,4-tetramethyl-3-thietan-1-ylidene is best represented as nonclassical, bisected structure 2 and not classical structure 1.

Geometrically Strained Carbenes: Interdependence among Geometry, Spin Multiplicity, and Reactivity.

The singlet and triplet states of cyclobutenylidene 1 and benzocyclobutenylidene 2 have been studied computationally (using ab initio and DFT methods) in order to assess the effect of angle strain on the S-T gap of vinyl- and phenylcarbenes. It is found that both carbenes have a singlet ground state. Cyclobutenylidene 1 hasa more significant singlet-triplet gap (-25 kcal mol-1) than benzocyclobutenylidene 2 (-14.5 kcal mol-1). The strong preference of 1 for a singlet ground state may be understood if it is viewed as bicyclobut-1-ene with a considerably puckered ring. Singlet benzocyclobutenylidene (12) is computed to have also a puckered cyclobutene ring albeit less pronounced. The lowest isomerization path available for singlet cyclobutenylidene (11) is the formation of vinylacetylene, which is predicted to have a barrier of around 9 kcal mol-1. Our calculations suggest that singlet benzocyclobutenylidene (12) lies in a rather deep potential well and should be observable under suitable experimental conditions. However, under low-temperature Ar matrix conditions no evidence for the formation of carbene 2 was obtained from the photolysis of precursor 15. On the other hand, laser flash photolysis (LFP) of precursor 16 enables us to observe benzocyclobutenylidene at room temperature. The lifetime of carbene 12 is determined to be 0.6-0.7, us in cyclohexane. The mCXY value for carbene 12 is determined to be 0.43, revealing a strong electrophilic nature and ranking 12 second among the electrophilic carbenes listed in the carbene philicity spectrum.

Study of trans-2-tert-Butylcyclopropylcarbene by Laser Flash Photolysis and Chemical Analysis1

Laser flash photolysis (LFP) of trans-2-tert-butylcyclopropyldiazirine (4) produces trans-2-tert-butylcyclopropylcarbene (5). Carbene 5 can be trapped with pyridine to form ylide 6. The rate of formation of ylide 6 was resolved and found to be linearly dependent on the concentration of pyridine. The lifetime of carbene 5 was determined to be 22 ns (pentane), 18 ns (acetonitrile and CF2ClCFCl2), 15 ns (cyclohexane), and 27 ns (cyclohexane-d12) at ambient temperature. The lifetime of carbene 5 in solution, at ambient temperature, is controlled substantially by reaction with solvent and only to a minor extent by rearrangement to cyclobutene 7. Photolysis of 4 (350 nm) in Freon-113 produces 3-tert-butylcyclobutene (7) along with products derived from attack of the carbene on solvent. Photolysis of 4 in the presence of trapping agents (tetramethylethylene, cyclopentene, propylamine, trifluoroethanol) produces adducts with little or no corresponding decrease in the yield of 7. The preferred interpretation is th...

Rearrangement of Dimethylcarbene to Propene: Study by Laser Flash Photolysis and ab Initio Molecular Orbital Theory

Laser flash photolysis (Nd:YAG laser, 355 nm, 35 mJ, 150 ps) of dimethyldiazirine and dimethyldiazirine-d6 produces dimethylcarbene (DMC) and dimethylcarbene-d6 (DMC-d6), respectively. The carbenes were trapped with pyridine to form ylides which absorb around 364 nm. It was possible to resolve the growth of the ylides as a function of pyridine concentration in Freon-113, α,α,α-trifluoromethylbenzene, and perfluorohexane as a function of temperature. The observed rate constant (kobs) of ylide formation was linearly dependent on the concentration of pyridine in all solvents and at all temperatures. From plots of kobs versus [pyridine] it was possible to extract values of kpyr (the absolute rate constant of reaction of the carbene with pyridine) and τ, the carbene lifetime in the absence of pyridine, and their associated Arrhenius parameters. In Freon-113 and α,α,α-trifluoromethylbenzene the carbenes decay both by rearrangement and by reaction with solvent. In perfluorohexane the carbene decay appears to be predominantly unimolecular. The experimental results are compared with ab initio molecular orbital calculations. The experimentally determined barrier to disappearance of DMC in perfluorohexane (2.56 ± 0.05 kcal/mol) is much smaller than that calculated (7.4 ± 2 kcal/mol) using ab initio molecular orbital theory. The Arrhenius parameters and isotope effects indicate that the rearrangement of DMC in perfluorohexane has a large component of quantum mechanical tunneling. The activation energy for the disappearance of DMC-d6 in perfluorohexane (5.63 ± 0.03 kcal/mol) is consistent with calculations which indicate that QMT makes only a minor contribution to the deuterated system under the conditions of this study.

LASER FLASH PHOTOLYSIS STUDY OF PHENYLCARBENE,o-TOLYLCARBENE AND MESITYLCARBENE

Laser flash photolysis (LFP, XeCl, 308 nm, 20 ns) of phenyldiazomethane (PDM), o-tolydiazomethane (TDM) and mesitydiazomethane (MDM) produces phenylcarbene (PC), o-tolycarbene (TC) and mesitylcarbene (MSC), respectively. Transient spectra of PC and TC could not be detected in pentane at ambient temperature; however, these carbenes could be trapped with pyridine to form UV–Vis-active ylides. The rate of formation of these ylides was resolved in CF2ClCFCl2 (Freon-113) and yields, after analysis, values of kpyr and τ, where kpyr is the absolute rate constant of the reaction of the spin equilibrated carbene with pyridine and τ is the carbene lifetime in Freon-113 in the absence of pyridine. LFP of MDM produces the transient spectra of both triplet MSC and 3,5-dimethyl-1,2-benzoquinodimethane (3,5-BQM). The lifetime of 3MSC in pentane is ca 500 ns. 3MSC reacts with pyridine with an apparent rate constant of kpyr=1·35×107 M−1 s−1. The decay of 3MSC in a variety of solvents does not lead to increased absorption of 3,5-BQM in solution, nor does the presence of a carbene scavenger (methanol) lead to a reduction in yield of 3,5-dimethyl-1,2-benzoquinodimethane upon LFP of precursor. Thus, the decay of 3MSC in solution does not lead to the formation of quinodimethane. This species (3,5-BQM) is formed, instead, by a hydrogen atom transfer in the excited state of the precursor. Product analysis reveals that the excited state rearrangement which forms xylylene is a minor process. Photolysis of MDM in a nitrogen matrix at 10 K leads to the UV–Vis spectra of matrix-isolated triplet mesitylcarbene and 3,5-BQM. The carbene rearranges slowly to 3,5-BQM in the cryogenic matrix, in contrast to its behavior in solution. © 1997 John Wiley & Sons, Ltd.