Photochemical Ring-Opening in 2,3-Diphenyl Aziridines. Transient

Article pubs.acs.org/JPCA

Photochemical Ring-Opening in 2,3-Diphenyl Aziridines. TransientSpectral and Kinetic Behavior of Azomethine Ylides and Related Photointermediates Douglas Cyr, Sweta Shrestha, and Paritosh Das* Physical Sciences Department, Cameron University, Lawton, Oklahoma 73505United States ABSTRACT: By employing laser pulses at various wavelengths for nanosecond flash photolysis, a comprehensive time-resolved study has been performed on transient azomethine ylides photogenerated from several 2,3-diphenyl aziridines in fluid solutions under three different conditions, namely, by direct 266 nm excitation, under reversible electron-transfer sensitization by 1,4-dicyanonaphthalene singlet excited state, and via energy transfer from acetone triplet. Under each of the three conditions of photoexcitation, azomethine ylides are readily formed as transient species, characterized by broad, structureless absorption spectra with maxima at 470−500 nm and mostly complex decay kinetics in μs−ms time domain. Under acetone triplet sensitization, a second, shorter-lived transient species with absorption maximum at ∼360 nm is observed to grow and decay in the same time range as that of the growth of ylides. This species has been identified as the ring-opened precursor ylide triplet. The azomethine ylides are practically nonquenchable by oxygen, except that under acetone triplet sensitization in air-saturated acetonitrile, their decay is significantly enhanced. The latter is explained in terms of quenching through dipolarophilic reaction with singlet oxygen. A value of 1.6 × 109 M−1 s−1 has been estimated for the rate constant for reaction between singlet oxygen and ylide from trans-2,3-diphenylaziridine. We also report rate constants, in the range 2 × 103 to 4 × 109 M−1 s−1, for the quenching of azomethine ylides by two dipolarophiles, namely, maleic anhydride and dimethyl acetylene dicarboxylate. The dipolarophilic reactivity of ylides carrying bulky substituents on the N atom is relatively subdued. Acetic acid proved to be a modest quencher of ylides with rate constants close to 106 M−1 s−1.

1. INTRODUCTION 1,3-Dipolar intermediates such as carbonyl and azomethine ylides play key roles in thermal and photochemical 1,3cycloaddition reactions that are of considerable synthetic importance.1−17 In addition, these intermediates are implicated in a variety of other phenomena and reactions, some examples of which are as follows: thermochromism/photochromism,18 isomerization and photooxygenation of small-ring heterocycles,19−35 photoinduced electrocyclization of vinyl sulfides and enamines,36−38 reactions of carbenes with molecules containing multiply bonded heteroatoms,39−48 and ozonolysis of alkenes.49,50 Since 1,3-dipoles are of transitory nature under ambient reaction conditions, their involvement in a chemical reaction is established through chemical trapping with dipolarophiles (leading to cyclic adducts) and with alcohols and HX-type reagents (e.g., carboxylic acids). Time-resolved techniques, such as laser flash photolysis and pulse radiolysis allow real-time observation of these intermediates in fluid solutions.51 In continuation of our interest52−65 in transient-spectral properties and dipolarophilic reactivity of allyl-type 1,3-dipoles and related intermediates in fluid solutions under ambient conditions, we have carried out a nanosecond laser flash photolysis study of azomethine ylides photogenerated from a series of diphenyl aziridines (see Scheme 1). The ylides were produced by direct excitation, via energy transfer from acetone © 2013 American Chemical Society

Scheme 1. 2,3-Diphenyl Aziridines under Study

triplet, and under singlet-mediated reversible electron-transfer sensitization by 1,4-dicyanonaphthalene (DCN). In addition to the transient absorption spectra and decay kinetics of the ylides, we report in this paper data on their absolute reactivity with two dipolarophiles, namely, maleic anhydride (MA) and dimethyl acetylene dicarboxylate (DMAD), and acetic acid. Interestingly, the acetone triplet sensitization study allowed us Received: August 15, 2013 Revised: October 25, 2013 Published: October 28, 2013 12332

dx.doi.org/10.1021/jp408190s | J. Phys. Chem. A 2013, 117, 12332−12349

The Journal of Physical Chemistry A

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continuously fed with the solution under experiment from a reservoir. Kinetics measurements at specific monitoring wavelengths made use of static cells (quartz, path length usually 2 mm) containing 1−2 mL of the solutions that were shaken in between laser shots. Unless the effect of oxygen was meant to be studied, the solutions were deoxygenated by bubbling highpurity argon or nitrogen. All laser flash photolysis experiments were performed in fluid solutions at room temperature (25 °C). The ground-state absorbance of the solutions at laser excitation wavelengths was ≤0.5. A relatively recent batch of laser flash photolysis experiments were performed in updated laser flash photolysis systems (also at the Radiation Laboratory), consisting of Laser Photonics PRA/Model UV-24 nitrogen laser (337.1 nm, 4−6 mJ, ∼8 ns) and a PRO 230, Quanta-Ray Nd:YAG laser (operated for 266 nm, ∼11 ns, 10−12 mJ pulses). These systems, described in references 72−74, allowed observation of transients on a slightly longer time range, namely, ∼200 μs, than that of the one described in the previous paragraph. 2.3. Steady-State Photolysis and Spectral Measurements. The ground-state absorption spectra were recorded in a Cary 219 or a Shimadzu Multispec-1501 spectrophotometer. The steady-state fluorescence quenching experiments were carried out in an SLM photon-counting system (described in reference 75) or a Perkin-Elmer LS-5 spectrofluorimeter. For absorption-spectral measurements and steady-state photolysis at 77 K, long-stemmed rectangular quartz cells (path length 3 mm) containing solutions were immersed in liquid nitrogen in dewars equipped with two flat-faced quartz windows (opposite to each other). The excitation source for steady-state irradiation was a medium-pressure Hg lamp (Bausch and Lomb SP-200) coupled with a Bausch and Lomb 33−86−07 monochromator and Corning filters.

to determine the rate constant for the reaction of singlet oxygen with an azomethine ylide. Although the reactivity with singlet oxygen as a dipolarophile has been implicated22−25 in the mechanism for photooxygenation of aziridines and oxiranes, to our best knowledge, the rate constant for this reaction has not been previously reported in the literature. While the present work was in progress, Gaebert et al.66,67 reported on a study of transient-spectral behavior of azomethine ylides photogenerated from a number of phenyl aziridines by direct laser pulse excitation and their reactivity with methanol and a dipolarophile (acrylonitrile). While there is some overlap between the present work and that of Gaebert et al.,66,67 the aziridine systems studied in the two undertakings differ greatly in terms of substituents on the aziridine ring (particularly on the N atom). More importantly, in addition to direct-excitation laser photolysis, we have carried out investigations of the photogeneration of azomethine ylides via triplet energy transfer and under reversible electron-transfer sensitization, which in turn revealed several interesting aspects not observed in the course of direct photolysis. Also, the two dipolarophiles examined by us, maleic anhydride and dimethyl acetylene dicarboxylate, are different from the one (acrylonitrile) studied by Gaebert et al.66,67

2. EXPERIMENTAL SECTION 2.1. Materials. Three trans-diphenyl aziridines, 1a−3a, and four cis-diphenyl aziridines, 1b−4b, with structures shown in Scheme 1, were donated by Professor A. P. Schaap and were used as received. In addition, cis-1,2,3-triphenyl aziridine (5b in Scheme 1) was purchased from Aldrich and was used, without further purification, in a limited number of experiments. Dimethyl acetylene dicarboxylate (DMAD) and 2,5-dimethylhexa-2,4-diene (DMHD), both from Aldrich, were distilled under reduced pressure. Maleic anhydride (MA), also from Aldrich, was recrystallized from dichloromethane. 1,4-Dicyanonaphthalene (DCN) was synthesized from 1,4-dichloronaphthalene (Eastman) by a procedure used in the literature68 for 1cyanonaphthalene and was purified by treatment with charcoal followed by multiple recrystallization from toluene. Some experiments were also performed with a sample of DCN purchased from Alfa Aesar and recrystallized from toluene. Acetonitrile, Freon-112 (tetrachloro-1,2-difluoroethane), methanol, and acetone were of spectral grades. Diethyl ether, isopentane, and ethanol, used to make EPA by mixing the solvents in 5:5:2 (v/v) ratio, were of best grades commercially available. 2.2. Nanosecond Laser Flash Photolysis. For the bulk of the nanosecond laser flash photolysis experiments (performed at the Radiation Laboratory, University of Notre Dame), appropriately attenuated outputs from the following laser systems were used: UV-400 Molectron nitrogen (337.1 nm, 2− 3 mJ, ∼8 ns), Lambda-Physik EMG 101 excimer (Xe−Cl, 308 nm, ≤20 mJ, ∼20 ns), and Quanta-Ray Nd:YAG (266, 355, and 532 nm, ≤10 mJ, ∼6 ns). A description of the kinetic spectrophotometer and the data collection system is available in previous publications from the Radiation Laboratory.69−71 The time domains for observation of transient species were typically 100 ns to 100 μs. Usually, a front-face configuration was employed at an angle of ∼20° between the directions of laser pulses and the monitoring light. For wavelength-to-wavelength measurement of transient absorption spectra requiring a large number of laser shots, use was made of a flow system in which a rectangular quartz photolysis cell (path length ∼3 mm) was

3. RESULTS 3.1. Low-Temperature Photolysis: Absorption Spectra of Azomethine Ylides at 77 K. The present work is primarily aimed at characterization of azomethine ylide intermediates formed as a result of photocleavage of the C−C bond in the aziridines. The traditional method51 of observing ylide intermediates is to generate them as stable colored species under solid-state constraints and to record their absorption spectra. Upon steady-state lamp irradiation (254 nm) in EPA glass at 77 K, the aziridines under study develop colorations (pink to orange). The coloration in each case disappears upon melting the glass and is not regenerated when the glass is formed again by cooling the photolyzed solution. This established the transient nature of the species, very likely azomethine ylides, responsible for absorption in the visible. The absorption spectra of the photolyzed glasses at 360−600 nm are presented in Figure 1 and the corresponding wavelength maxima are compiled in Table 1. The ylide assignment of products of steady-state photolysis of oxiranes and aziridines in rigid matrices is well-documented in the literature.30,51,76,77 As evident from Figure 1, the ylide absorption spectra in the visible occupy the wavelength range 420−580 nm. All are broad and structureless; this feature is characteristic of carbonyl and azomethine ylides in general. The lack of resolution is not helpful to decide if the observed transients consist of a single form or multiple forms of ylides. Between the trans/cis pair of aziridines, 1a and 1b, the absorption maximum of the ylide from the cis isomer (1b) is distinctly red-shifted relative to that from the trans isomer (1a). The ylide absorptions in the case of 12333



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(discussed below). Figures 2 and 3 show the spectra at 0.6 μs following the laser pulse in acetonitrile and at four different times after the laser pulse in Freon-112. There were no significant changes in the spectral features in the course of the decay of the transients over ∼100 μs. The absorption maxima in acetonitrile and Freon-112 are given in Table 1. The transient absorption spectra in methanol are essentially identical to those in acetonitrile. Compared to the absorption maxima in EPA glass at 77 K (Figure 1), those in the solvents at room temperature are blue-shifted by 5−10 nm. The relative location of the maxima of the ylides from trans vs from cis aziridines (1a,b and 2a,b), as observed in EPA glass (described earlier), is also noticeable in the spectra in the solvents at room temperature (Table 1). Aziridine 3a stood apart from the other aziridines in two respects. First, the absorbance changes due to the photogenerated ylide(s) from it were significantly weaker than from other aziridines. This was true for its formation under all conditions of its excitation, namely, steady-state photolysis in EPA matrix at 77 K, direct 266 nm laser excitation in various solvents at room temperature, acetone triplet sensitization, and singlet DCN-mediated electron transfer sensitization. For example, when two acetonitrile solutions of 3a and 3b, optically matched with a ground-state absorbance of 0.165 for both at 266 nm in 2 mm cells, were separately subjected to laser flash photolysis with 266 nm laser pulses of equal intensity, the endof-pulse absorbance changes due to the long-lived ylide from 3a observed at 500 nm on a microsecond time scale was about one-fifth of that in the case of 3b. Second, the transient absorption decay traces from 3a at 480−540 nm monitored on a short time domain (∼150 ns) show the formation of a very short-lived transient species with lifetimes equal to or shorter than 9 ns in each of the three solvents examined. At 520 nm, this transient accounts for a substantial portion of the total endof-pulse absorbance (e.g., 66% in Freon-112). The characterization of the 9-ns transient was beyond the scope of this study, mainly because of its short lifetime and in part due to interference from short-lived emissions at 350−420 nm. 3.2.2. Decay Kinetics. Among the three solvents studied, namely, acetonitrile, Freon-112, and methanol, the transient ylides were, in general, longest-lived in acetonitrile. In this solvent, the decay of the transient absorption over ∼15 μs was less than 10% in most cases. Comparison of kinetic traces in N2-saturated and air-saturated solutions showed that oxygen at ∼2 mM had practically no effect on the decay kinetics. In the case of aziridine 5b, the ylide decay trace in N2-saturated

Figure 1. Absorption spectra of azomethine ylides photogenerated by steady-state Hg-lamp irradiation of diphenyl aziridines in EPA glass at 77 K.

isomeric aziridines 2a and 2b both exhibit maxima at wavelengths longer than those of ylides from 1a and 1b. Between 2a and 2b, the ylide from the cis isomer (2b) shows absorption maximum at a slightly shorter wavelength. The ylides from 3a,b and 4b show absorption maxima close to one another and red-shifted compared to those of ylides from 1a,b. In spite of a relatively long irradiation time used for 3a (∼2 times longer than those used for other aziridines), the absorption intensity of the ylide in this case was considerably weaker (Figure 1, bottom half). 3.2. Direct Laser Excitation at 266 nm in Fluid Solutions at Room Temperature. The diphenyl aziridines under study absorb in the UV region at wavelengths below 275 nm. The 0−0 band of the lowest-energy electronic transition is located at 269−273 nm (in EPA glass at 77 K). We studied the transient phenomena resulting from 266 nm laser pulse excitation of the aziridines in three solvents, namely, acetonitrile, Freon-112, and methanol. 3.2.1. Transient Absorption Spectra. The 266 nm laser pulse excitation of the aziridines in fluid solvents results in the formation of transient species characterized by broad, structureless absorption at 400−600 nm (λmax = 465−500 nm). The transients are formed within the laser pulse and decay over μs−ms time ranges, with solvent-dependent kinetics

Table 1. Absorption-Spectral Maxima of Azomethine Ylides Photogenerated from Aziridines under Various Conditions absorption maxima, nma b

Aziridine 1a 1b 2a 2b 3a 3b 4b 5b a

EPA , 77 K, steadystate photolysis (Hg lamp)

Freon-112 at 25 °C, direct laser excitation at 266 nm

Acetonitrile at 25 °C, direct laser excitation at 266 nm

475 485 510 500 505 500 500

465 485 495 490 500 500 495

465 470 505 490 500 500 495 500

Acetonitrile at 25 °C, acetone triplet sensitization, λex = 308 nm

Acetonitrile at 25 °C, DCN singlet sensitization, λex = 337 nm

470 475 495 495 490 500 495

465 470 500 490 490 490 495

±5 nm. bDiethyl ether:isopentane:ethanol 5:5:2 v/v. 12334



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Figure 2. Transient absorption spectra of azomethine ylides produced by 266 nm laser pulse excitation of diphenyl aziridines in acetonitrile at 25 °C. The spectra were monitored at 0.6 μs following the laser pulse.

decaying independently by first-order kinetics. The decay of ylide from 1b in Freon-112 could not be satisfactorily fitted into any of the three kinetic modes78−80 that were tried. In contrast to the decay of azomethine ylides in acetonitrile and Freon-112, that in methanol occurs by clean first-order kinetics. A representative trace (from 3a) in methanol is shown in Figure 4D. The enhanced decay and the first-order mode of decay kinetics are indicative of the alcohol reacting with ylides leading to O,N-acetals and, eventually, products of alcoholysis (i.e., benzadehyde acetals and N-benzyl aniline).81 The general reactivity of ylides with alcohols is well-documented in the literature.51 For example, at room temperature, carbonyl ylides photogenerated from 2,2-dicyano-3-(2-naphthyl)oxirane and aromatic keto-epoxides are quenched by methanol with rate constants of 105 − 107 M−1 s−1 in benzene and acetonitrile.54,57 Recently, Gaebert et al.66 showed that MeOH acts as a proton donor to the carbanionic site of azomethine ylides to form iminium ions with an isotope effect, kMeO‑H/kMeO−D, ranging from 5 to 7. Some representative kinetic data for azomethine ylides are presented in Tables 2 and 3. Experiments were conducted to see the effect of oxygen as a quencher by photogenerating the

acetonitrile was obtained at the longest time range (∼200 μs) available in this study. The decay profile, shown in Figure 4A, indicates complex kinetics. Attempts to fit it into second-order equal-concentration kinetics78 or into mixed first- and secondorder kinetics79 were not rewarding. However, the decay curve could be fit well into a two-component first-order decay (biphasic)80 with the fast-decaying, minor component having a decay rate constant (k1) 3.0 μs−1 and the slower major component, ∼0.1 ms−1. The ratio of the end-of-pulse absorbances of the major and minor components was 1.6. Compared to the kinetics of ylide decay in acetonitrile, that in Freon-112 was significantly faster. Two traces, given for ylides from 1a and 2a in Figure 4B and C, respectively, illustrate the difference. For ylides derived from 1a,b, the kinetic profiles could be fitted reasonably well into secondorder equal-concentration kinetics; that is, plots of ΔA−1 against time were linear. However, a slightly better fit of the experimental data with the computed curve was obtained in both cases in terms of second-order kinetics mixed with a slow first-order component. For ylide decay kinetics in the case of aziridines 3a and 2b−4b, best fits of the experimental decay traces in Freon-112 were obtained in terms of two components 12335



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Figure 3. Transient absorption spectra of azomethine ylides monitored at 4, 30, 45, and 65 μs following 266 nm laser pulse excitation of diphenyl aziridines in Freon-112 at 25 °C.

Table 2. Data on Decay Kinetics of Azomethine Yildes Photogenerated by Direct 266 nm Laser Excitation in N2Saturated Freon-112 at 25 °C decay rate constantsa Aziridine

k2/(εl), μs−1

k1, ms−1

1a 1b 2a 2b 3a 3b 4b

0.44

0.15

0.60

0.36

k1, μs−1 b

0.060 0.16 0.093 0.10

k1′, ms−1

ΔA0′/ΔA0

b

b

∼0.01 3.1 ∼0.01 3.0

2.7 1.9 5.5 2.3

a Estimated errors, ±15%. bThe decay trace in the case of 1b could not be fitted well into either of the three kinetic modes attempted (see text).

Table 3. First-Order Decay Rate Constantsa of Azomethine Yildes Photogenerated by Direct 266 Nm Laser Excitation in Methanol at 25 °C Figure 4. Representative kinetic traces of ylide decay observed from 266 nm laser excitation of diphenyl aziridine substrates in N2-saturated solvents at 25 °C. The traces were monitored at or near ylide absorption maxima. The diphenyl aziridine substrates are: (A) 5b in acetonitrile (solid line: best fit based on biphasic decay, see text); (B) 1a in Freon-112 (solid line: best fit based on mixed first- and secondorder kinetics); (C) 2a in Freon-112 (solid line: best fit based on mixed first- and second-order kinetics); (D) 3a in methanol (solid line: best fit based first-order decay kinetics). a

12336

Aziridine

N2-saturated

O2-saturated

1a 1b 2a 2b 3a 3b 4b

0.036 0.047 0.080 0.0078 0.072 0.072 0.071

0.039 0.056 0.075 0.010 0.077 0.072 0.078

Estimated errors, ±15%. b[O2] = 0.0104 M.82



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ylides under direct 266 nm laser excitation in O2-saturated methanol ([O2] = 0.0104 M).82 Any slight enhancement in decay kinetics that was noted for ylides from some of the aziridines in O2-saturated solutions relative to corresponding deoxygenated solutions was within experimental errors (Table 3). In any case, the rate constants for the reactivity of azomethine ylides with O2 are estimated to be well below 1 × 106 M−1 s−1. It should be noted that the slightly enhanced kinetics at high [O2] in some cases could also be due to the reaction of the ylides with a trace amount of singlet oxygen produced from the oxygen quenching of aziridine excited states (vide inf ra). The end-of-pulse absorbance changes observed due to ylides in O2-saturated solutions were generally smaller than those in the same solutions under deoxygenated conditions, suggesting that the photoprecursors of the azomethine ylides are quenched by O2 to a certain extent. 3.3. Ylide Photogeneration by Reversible Electron Transfer to Singlet Excited State of 1,4-Dicyanonaphthalene. Our earlier work58,64,65 on phenyl oxiranes and Nbenzoyl aziridines has shown that the ring-opening in these systems occurs readily via reversible electron transfer interaction with lowest singlet excited state of 1,4-dicyanonaphthalene (DCN). As with phenyl oxiranes, the fluorescence of DCN in acetonitrile is readily quenched by diphenyl aziridines. For example, the Stern−Volmer plot of the steady-state fluorescence quenching of DCN by 5b at 0−7 mM in airsaturated acetonitrile was linear and had a slope (KSV′) of 140 M−1. A correction for the oxygen quenching of 1DCN* in airsaturated acetonitrile gave a value of 178 M−1 for KSV under oxygen-free conditions. Using KSV = kqS × τS, and a value of 9.5 ns83 for DCN fluorescence lifetime (τS), we obtained a value of 1.9 × 1010 M−1 s−1 for the singlet quenching rate constant (kqS). Not surprisingly, this value is larger than kqS’s with trans and cis 2,3-diphenyl oxiranes as quenchers (1.2 × 1010 and 9.9 × 109 M−1 s−1, respectively). From the 0−0 bands (269−273 nm) in the ground-state absorption spectra of diphenyl aziridines under study (in EPA at 77 K), their singlet energies (ES) are estimated to be 105−106 kcal/mol. From the 0−0 band at 342 nm of fluorescence in n-heptane, the ES of DCN has been located at 83.6 kcal/mol.75 Thus, the singlet−singlet energy transfer from 1 DCN* to aziridines would be highly endothermic and cannot account for the efficient fluorescence quenching observed. On the other hand, the diphenyl aziridines are expected to behave as excellent electron donors (in fact, better than diphenyl oxiranes) owing to the presence of the amino moiety. In all likelihood, the efficient quenching of DCN fluorescence by the aziridines is attributable to charge transfer interaction with the cyanoaromatic singlet excited state acting as an acceptor. Upon 337.1 nm laser pulse excitation of DCN in the presence of 5−20 mM diphenyl aziridines (except 5b) in deoxygenated acetonitrile, transient species spectrally and kinetically similar to those noted under direct 266 nm in the same solvent are observed to be formed within the laser pulse. At these aziridine concentrations, 1DCN* is quenched to the extent of 50−80%. Representative transient absorption spectra in two cases, namely, for aziridines 1a and 2a, are shown in Figure 5. Also, in the inset of Figure 5 (top part) is shown a kinetic trace over ∼8 μs at the maximum (475 nm) of the transient absorption spectrum in the case of 1a. The end-ofpulse absorbance change increased progressively in experiments with increasing aziridine concentrations. Analogous observations were made with other diphenyl aziridines under study

Figure 5. Transient absorption spectra at 0.5 and 6 μs following 337.1 nm laser pulse excitation of DCN in the presence of 10 mM diphenyl aziridine 1a (top) and 2a (bottom) in N2-saturated acetonitrile at 25 °C. Inset: a representative kinetic trace monitored at 475 nm in the case of 1a.

except 5b (see below). Based on the spectral and kinetic similarity with transient species formed under direct excitation as well as dipolarophilic reactivity (vide inf ra), we assign the transient species from 1DCN* sensitization as azomethine ylides also. In order to establish whether any radical ions are formed or not as a result of electron transfer from 1DCN* to aziridines, we examined the 380−400 nm region carefully. The radical anion of DCN in acetonitrile is known to have a sharp, intense maximum at 390 nm (εmax = 2.2 × 104 M−1 cm−1).84,85 It is readily quenched by oxygen and has a lifetime of 54 ns in airsaturated acetonitrile.84 The end-of-pulse transient absorbances at 390 nm observed upon 337.nm laser pulse excitation of DCN in the presence of ∼10 mM diphenyl aziridines (except 5b) were negligible, suggesting that almost no net electron transfer takes place in the course of the quenching of 1DCN* by the aziridines. That is, the interaction that is responsible for ring-opening to ylides is reversible charge transfer. The results with aziridine 5b under 1DCN* sensitization were very different from those with its counterparts under study. We subjected DCN to 337.1-nm laser flash photolysis in the presence of ∼30 mM 5b in both deoxygenated and airsaturated acetonitrile. At this relatively high concentration of 5b in the deoxygenated solution, 1DCN* is quenched to the extent of 85% by 5b. The resulting transient absorption spectra, presented in Figure 6A, showed the formation of DCN radical anion with a sharp absorption maximum at 390 nm. Additional significant transient absorptions were noticeable at 420−540 nm. The latter were assigned in part to the ring-closed radicalcation of 5b which is reported86,87 to have an intense band system with λmax at 441 nm in a Freon matrix at 77 K and in part to the DCN triplet which also absorbs in this spectral region (λmax = 455 nm).84 The assignments as DCN radicalanion and triplet were in agreement with the fact that the decay of both transient species was significantly enhanced by the presence of oxygen. The decay of transient absorbance due to DCN radical anion at 390 nm and those of mixed DCN triplet 12337



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Figure 6. (A) Transient absorption spectra at 0.2 μs following 337.1 nm laser pulse excitation of DCN in the presence of 30 mM diphenyl aziridine 5b in N2-saturated acetonitrile at 25 °C. Representative kinetic traces at 390 and 460 nm are shown in parts B and C, respectively.

Figure 7. Transient absorption spectra of benzophenone triplet monitored at 0.1, 0.5, 0.8, and 6 μs following 355 nm laser pulse excitation of benzophenone in the presence of 11 mM of each of 1a (top) and 2a (bottom) in acetonitrile at 25 °C.

and 5b radical-cation at 450−460 nm in deoxygenated solutions (Figure 6B,C) were complex and could not be fitted well into second-order or mixed first- and second-order kinetics. Interestingly, there was no indication of the formation of an azomethine ylide with transient absorption similar to that of one observed under direct 266 nm laser excitation of 5b (described in section 3.2). Compared to the intrinsic triplet yield of DCN in the absence of a quencher, the triplet yield 85% quenching by 5b under was pronounced, suggesting that the intersystem crossing in DCN is assisted by the electron transfer interaction. 3.4. Benzophenone and Acetone Triplet Sensitization. The triplet energies of diphenyl aziridines should be close to, or probably slightly lower than, the triplet energy of toluene (ET ∼83 kcal/mol).88 Thus, energy transfer from acetone triplet (ET = 79−82 kcal/mol)88,89 is expected to be nearly isoenergetic. In addition, there should be charge transfer interaction from the amino moieties of the aziridines as well as benzylic hydrogen abstraction from aziridines, both contributing to acetone triplet quenching. On the other hand, benzophenone triplet (ET = 69 kcal/mol)88 is not expected to be quenched by aziridines by energy transfer. In this case, any triplet quenching would arise from charge transfer and hydrogen abstraction from aziridines. We have studied the quenching of both acetone and benzophenone triplets by aziridines in acetonitrile at room temperature by using 308 and 355 nm laser pulse excitation for the two ketones, respectively. 3.4.1. Benzophenone Triplet Quenching. The 355 nm laser pulse excitation of benzophenone in the presence of ∼10 mM diphenyl aziridines in acetonitrile showed surprisingly enhanced decay of the ketone triplet (monitored at λmax ∼ 530 nm) by first-order kinetics. The time-resolved spectra along the ketone triplet decay in two representative cases are shown in Figure 7. As shown for 1a in Figure 7A, at the end of the triplet decay over ∼20 μs, very weak residual absorptions 470−500 nm were observed with some of the aziridines (namely, 1a, 2a, 2b, and 3b). These absorptions could be due to azomethine ylides, but were too weak for definite characterization. In contrast, with 3a, 1b, and 4b as quenchers, significant residual long-lived

absorptions with a maximum at 540−545 nm were noticed following benzophenone triplet decay (Figure 7B). Based on the similarity of the spectra with those reported in the literature90−92 for diphenyl hydroxyl methyl radical, these longlived absorptions are assigned to the ketyl radical formed as a result of hydrogen transfer/abstraction from the aziridines. From the dependence of benzophenone triplet decay rate constants on aziridine concentrations, the bimolecular triplet quenching rate constants (kqT) have been estimated. The kinetic data are given in Table 4. Estimates93 of ketyl radical yields (ϕketyl) are also included in Table 4. Table 4. Kinetic Data on Benzophenone Triplet Quenching by Aziridines

a

Aziridine

kq, 108 M−1 s−1,a

ϕketylb

1a 1b 2a 2b 3a 3b 4b

1.1 0.97 1.3 1.4 1.9 0.92 1.0

0.09 0.24 0.08 0.04 0.24 0.04 0.28

±15%. b±20%.

The lack of observation of azomethine ylides from benzophenone triplet quenching suggests that there is no energy transfer involved in the quenching process and that even if charge transfer contributes to the quenching, ring-opening does not occur via the resulting ion-pairs. 3.4.2. Acetone Triplet Sensitization. The 308 nm laser pulse excitation of 0.68 M acetone (absorbance = 0.45 in 1 mm cell) in the presence of 5−20 mM aziridines in acetonitrile resulted in interesting transient phenomena that revealed a great deal of information on ring-opening along the triplet excitation route. It should be noted that control experiments were performed to 12338



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maximum of the growth-decay profile of 360 nm species showed parallel, progressive increase with increasing [1a]. At high [1a] (i.e., 20 mM), the rate constants obtained from the best fit, into first-order kinetics, of the growth profile of 475 nm species and of the decay portion of 360 nm species were close to each other (∼2 × 106 s−1) . Combined together, these results suggest that the 360 nm species is a precursor intermediate in the triplet state for azomethine ylide formation along the triplet excitation route. This intermediate could possibly be ringclosed aziridine triplet (3Az*) or ring-opened ylide triplet (3Y*). Based on evidence from multiple observations described in the rest of this section, we propose the following mechanism for ring-opening of aziridines along the triplet sensitization route (Az, aziridine; Ac, acetone; Y, ylide):

show that at the highest concentrations used in in these experiments, none of the aziridines produced any transients upon 308 nm laser pulse excitation in the absence of acetone. Acetone-sensitized transient absorption spectra and kinetics with each of the aziridines over 10−20 μs following laser excitation have the following two prominent features. First, in the spectral region 400−650 nm, growths of long-lived transient absorbances, with λmax’s at 470−500 nm, were observed (see Figures 8 and 9A). The resulting transient

Ac + hν → 1Ac* → 3Ac*

(1)

3

Ac* → Ac

(2)

3

(3)

Ac* → Ac + hν′

3

Ac* + Az → Product(s) of H‐transfer, etc.

(4)

3

Ac* + Az → 1Ac + 3Az*

(5)

3

Az* → 3 Y *

(6)

3

(7)

Y * → 1Y

In these equations, Ac and Az represent acetone and aziridine in the singlet ground state, respectively. (b) In order to estimate the bimolecular rate constant for acetone triplet (3Ac*) quenching by 1a via steps 4 and 5 above, we plotted the reciprocal of ΔAmax of the ylide at 475 nm against 1/[Az]. The scheme based on steps 2−7 leads to the following equation:

Figure 8. Transient absorption spectra monitored at 0.1, 0.5, 0.8, and 6 μs following 308 nm laser pulse excitation of 0.68 mM acetone in the presence of 5 mM of each of 1a (top) and 1b (bottom) in acetonitrile at 25 °C.

ΔA max = εY l

spectra were similar to those observed from direct 266 nm excitation and under DCN singlet electron-transfer sensitization (vide supra). This, combined with the facile quenching of the decay kinetics of the 470−500 nm species by maleic anhydride (dipolarophile), led us to assign them as azomethine ylides. Second, at 350−370 nm in the same time range (10−20 μs), growths of transient absorptions (λmax ∼360 nm) followed by their decay to long-lived residual absorptions were noted (see Figures 8 and 9B). Assignment of this transient called for evidence from additional experiments (see below). 3.4.2.1. Assignment and Intermediacy of Ring-Opened Ylide Triplet. Using aziridine 1a as a representative case, we have performed a number of detailed experiments to establish the identity of the 360 nm species as the ylide triplet. The observations made with 1a were reproduced in a limited number of similar experiments with 1b and other diphenyl aziridines under study. The experiments/observations made with 1a as an acetone triplet quencher are as follows. (a) With [1a] increasing in the range 5−20 mM, the growth kinetics of the 475 nm transient species (azomethine ylide) became progressively enhanced; however, the growth profiles could not be fitted well into single first-order kinetics even at the highest [1a], i.e., 20 mM, used. Also, with increasing [1a], the growth component of the 360 nm species became more and more pronounced; this in turn made its follow-up decay portion seemingly faster. The absorbance change following the completion of growth of 475 nm species and that at the

k5[Az] [3Ac]0 k 0 + (k4 + k5)[Az]

(8)

In eq 8, εY is molar extinction coefficient of ground-state ylide, l is the photolysis cell path length, k4 and k5 are bimolecular rate constants associated with quenching steps 4 and 5, respectively, k0 is the sum of the first-order rate constants for steps 2 and 3 (i.e., reciprocal of acetone triplet lifetime in the absence of a quencher), and [3Ac*]0 is the end-of-pulse molar concentration of acetone triplet produced. Based on eq 8, a double-reciprocal plot of 1/ΔAmax against 1/[Az] would give a straight line, the intercept-to-slope ratio of which would be kqτT, where kq = k4 + k5 = overall quenching rate constant and τT is the acetone triplet lifetime. In Figure 10A is shown such a plot for 1a. From this plot we estimate kqτT to be 63 ± 9 M−1. (c) Another way to estimate the rate constants would be to fit the ylide growth profile to consecutive first-order kinetics with steps 2−5 constituting the first step and step 6 or 7, the second. It will be shown by 2,5-dimethylhexa-2,4-diene (DMHD) quenching, described in part (d) in this section, that step 7 is the slow one. The integrated equation for the growth kinetics based on two consecutive steps is as follows: ⎡ k e −k ′ t k′e−k 7t ⎤ ⎥ + ΔA = ΔA∞⎢1 − 7 ⎢⎣ k 7 − k′ k 7 − k′ ⎥⎦

(9)

where k′ = k0 + kq[Az]. Since ΔA∞, equal to ΔAmax, is known from the value of absorbance at the plateau of the growth, the fitting of the 12339



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Figure 9. Representative kinetic traces monitored under various conditions following 308 nm laser pulse excitation of 0.68 mM acetone in the presence of 20 mM 1a in acetonitrile at 25 °C. (A) λmon = 475 nm; (B) λmon = 360 nm; (C) λmon = 475 nm, [DMHD] = 10.5 mM; (D) λmon = 475 nm, [DMHD] = 10.5 mM, [O2] = 2.0 mM.

growth of 475 nm transient absorbance could be fitted well into single first-order kinetics (see Figure 9C), giving a value of 4.3 μs−1 for the corresponding rate constant. This value remained essentially unchanged at high [DMHD]’s. The decay of the 360 nm species at high [DMHD]’s also followed first-order kinetics with a rate constant equal to that of the growth kinetics at 475 nm. These results strongly suggest that the 360 nm species, nonquenchable by DMHD, acts as a precursor of the ylide. Based on eq 10 below, a Stern−Volmer plot was made for the quenching of the plateau absorbance change at 475 nm at varying concentrations of DMHD, with [1a] maintained constant at 0.020 M.

growth kinetics data into eq 9 involves adjustment of only two independent parameters, namely, k′ and k7. The best fit of the growth profile at 475 nm with [1a] = 0.020 M, shown in Figure 10B, gave values of 2.8 μs−1 and 4.2 μs−1 for k′ and k7, respectively. Combining the two results, namely, k′ = k0 + kq[Az] = k0 + 0.020kq = 2.8 μs−1 and kqτT = kq/k0 = 63 M −1, we obtain k0 = 1.2 μs−1 and kq = 78 M−1 μs−1, i.e., 7.8 × 107 M−1 s−1. This value, well below the limit of diffusion control, is in keeping with endothermic triplet energy transfer between acetone triplet and 1a. The value of 4.2 μs−1 for k7 gave an estimate of the lifetime of the ylide triplet of 1a at 240 ns. (d) Quenching studies with 2,5-dimethyl-hexa-2,4-diene (DMHD) provided convincing evidence in support of the assignment of the 360 nm transient as short-lived ylide triplet. DMHD, with its triplet energy (ET) at 58.7 kcal/mol,88 is expected to quench both acetone and ring-closed aziridine triplets with rate constants in the limit of diffusion control. From the onset of absorption spectra of azomethine ylides below 540 nm (see Figure 1), their lowest singlet energies (ES) are estimated to be below 53 kcal/mol. The corresponding lowest triplet energies for these strong π→π* transitions would be considerably lower. Thus, one would not expect a ringopened ylide triplet to be quenched by DMHD. The 308 nm laser pulse excitation of acetone (0.48 M) in the presence of a given millimolar concentration of aziridine 1a and varying concentrations of DMHD in the range 0−11 mM showed a progressive decrease in the transient absorbance at both 475 and 360 nm. In addition, the presence of increasing concentrations of DMHD enhanced the kinetics of the growth of transient absorbance at 475 nm and of the growth part of transient absorbance at 360 nm. At [DMHD] ≥ 11 mM, the

ΔA 0,max ΔA max

= 1 + kq , Dτ′[DMHD]

(10)

where kq,D = quenching rate constant for 3AC* quenching by DMHD τ′ = k′−1 = (k0 + kq[Az])−1 ΔA0,max = Plateau absorbance at 475 nm with [DMHD] =0 ΔAmax = Plateau absorbance at 475 nm at a given [DMHD] The Stern−Volmer plot, presented in Figure 10C, was linear with a slope of 2.15 × 103 M−1. Using k′ = 2.8 μs−1 found earlier for [1a] = 0.020 M, we obtain a value of 6.0 × 109 M−1 s−1 for kq,D. This value is very reasonable for the quenching of acetone triplet under conditions where the energy transfer from it is highly exothermic. 12340



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species are both considerably enhanced in the presence of oxygen (compare Figure 9C and D). The oxygen sensitivity of decay of the ylide triplet may arise from not only triplet excitation transfer to 3O2 (forming singlet oxygen) but also from the possibility that 3O2 reacts with ylide triplet as a dipolarophile. It should be noted that the photochemical ring-opening in aziridines can also occur through homolytic bond cleavage.94 In particular, along the triplet route, the cleavage of the C−N bond of the aziridine ring may occur, leading to a triplet nitrene via a benzyl-aminyl type radical.95 Based on the spectral and time ranges of our observation, we could neither rule out nor establish the formation of such transient species. However, the kinetic data presented above concerning the 360 nm species strongly favor its assignment as the ring-opened triplet precursor of ylide, rather than a transient species related to C−N bond cleavage. 3.4.2.2. Reactivity of Azomethine Ylide with Singlet Oxygen. An interesting outcome of the study of acetone triplet sensitization has been that we found strong evidence for the reactivity of azomethine ylides with singlet oxygen (1O2*, 1 Δg). As Figure 9D shows, the decay of the ylide in the airsaturated solution under acetone triplet sensitization is pronounced compared to that in deoxygenated solutions in acetonitrile. The enhanced decay is more clearly shown in Figure 11. As mentioned earlier in section 3.2.2, the decay Figure 10. Plots with data from experiments with 308 nm laser pulse excitation of 0.68 mM acetone in the presence of 20 mM 1a in acetonitrile at 25 °C. (A) Double-reciprocal plot of 1/ΔAmax due to ylide at 470 nm vs 1/[Az], based on eq 8; (B) Growth of ΔA at 470 nm and kinetic fit (solid line) based on two-stage first-order growth (eq 9); (C) Stern−Volmer plot based on eq 10 for DMHD quenching of ΔAmax due to ylide at 470 nm.

DMHD quenching has also been studied for acetone triplet sensitization of other aziridines. The results are similar to those in the case of 1a. The lifetime data obtained for ylide triplets of several aziridines, obtained under extensive DMHD quenching, is given in Table 5. Table 5. Lifetime Data on 360 nm Species Formed in the Course of Acetone Triplet Quenching by Aziridines in Acetonitrile at 25 °C (Assigned as Azomethine Ylide Triplets)

a

Aziridine

τT,Y, nsa

1a 1b 2a 2b 3b 4b

220 194 110 104 ≤50 ∼90

Figure 11. Kinetic trace monitored at 470 nm following 308 nm laser pulse excitation of 0.68 mM acetone in the presence of 5.0 mM 1a and 2.0 mM O2 in acetonitrile at 25 °C. Solid line: best numerical fit based on eqs 11−14

kinetics of ylides produced by direct excitation is practically unaffected in air-saturated solutions (compared to deoxygenated ones). Only saturation with pure oxygen (in methanol) showed a small effect in some cases (kq,3O2 < 106 M−1 s−1). Thus, the enhanced decay of the ylide in air-saturated solution under acetone triplet sensitization cannot be explained by quenching by triplet oxygen in the ground state. Noting the fact that singlet oxygen (1O2*) is formed at substantial concentrations as a result of the oxygen quenching of the acetone triplet and also possibly ylide triplet, it is very likely that the enhancement of the ylide decay is due to reaction with singlet oxygen. In fact, Schaap et al.,22−25 implicated dipolarophilic addition of singlet oxygen to ylides in the

±15%.

(e) Oxygen quenching supports the assignment of the 360 nm species as ylide triplet. The ΔAmax values at both 475 and 360 nm are significantly reduced in air-saturated solutions (compared to deoxygenated solutions). That this reduction is due to oxygen quenching of both acetone and ylide triplet is suggested by the fact that the growth kinetics of the ylide absorbance (475 nm) and the decay kinetics of the 360 nm 12341



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3.5. Reactivity of Azomethine Ylides with Dipolarophiles and Acetic Acid. We have studied acetic acid and two dipolarophiles, namely, maleic anhydride (MA) and dimethyl acetylene dicarboxylate (DMAD), for their effects on the decay kinetics of azomethine ylides. The reactivity, if any, of these substances with ylides was manifested in the enhancement of ylide decay in their presence at millimolar concentrations and the ylide decay was first-order with the reagents in the concentration ranges studied. Figure 12 shows

mechanism of photooxygenation of oxiranes and aziridines under electron-transfer photosensitization. Also, in an earlier report61 on azomethine ylides from N-benzoylaziridines, the role of singlet oxygen as a potential reactant for ylides has been suggested. Under the conditions of our experiments, about 20 mJ of a 308 nm laser pulse was focused onto an area of ∼0.5 cm2 of the photolysis cell of 0.2 cm path length. Upon absorption of the photons by acetone at ground-state absorbance of 0.45, the end-of-pulse concentration of 3Ac* produced is estimated to be 0.32 mM (assuming the intersystem crossing yield of acetone to be unity). Under oxygen quenching in air-saturated acetone solution with [1a] = 5.0 mM (Figure 11), the maximum absorbance due to ylide at 470 nm was only 6% of that observed in the same solution under N2-saturation. Thus, the 94% reduction of the ylide yield occurred from the oxygen quenching of 3Ac* and possibly, of the ylide triplet. If the formation of 1O2* from this quenching is close to quantitative, the initial [1O2*] produced in the photolysis cell as a result of laser pulse excitation would be ∼0.3 mM and [ylide] would be about 1/16 of this value. Interestingly, the initial part of the decay curve in Figure 11 could be fitted reasonably well into first-order kinetics, giving a value of 0.36 μs−1 for the corresponding rate constant. Assuming 1O2* to be a very long-lived quencher over the short time range (∼2 μs) under consideration and at a concentration of ∼0.3 mM (estimated above), we obtain a value of the bimolecular quenching rate constant (kq,1O2*Y) at ≥1.2 × 109 M−1 s−1. However, to fit the data properly, singlet oxygen should be treated as a transient species that is consumed in the course of its reaction with ylide. The reported values96−98 for the rate constant for deactivation of singlet oxygen in acetonitrile at room temperature are 0.033 μs−1 and 0.018 μs−1. Using Euler’s method and time segments of 0.01 μs, we solved the coupled differential eqs 13 and 14 for steps 11 and 12 below. We fitted the data from the kinetic trace in Figure 11 by adjusting three parameters, namely, the rate constant (kq,1O2*Y) for step 12, initial concentration of 1O2*, and the ratio of the initial concentrations of 1O2* and the ylide. 1

3

O2 * → O2

(11)

Y +1 O2 * → product(s)

(12)

−

d[1O*2 ] = k11[1O*2 ] + kqY,1O*[Y ][1O*2 ] 2 dt

−

d[Y ] = kqY,1O*[Y ][1O*2 ] 2 dt

Figure 12. Representative kinetic traces for ylide decay monitored at or near absorption maxima in the presence of quenchers. (A) 1a, 266 nm excitation, 1.52 mM MA; (B) 1a, 266 nm excitation, 83.1 mM acetic acid; (C) 2a, DCN sensitization, 1.50 mM MA; (D) 2a, DCN sensitization, 8.13 mM DMAD. The solvent is acetonitrile in each case.

some representative kinetic traces of ylides in the presence of the quenching agents. In some cases, particularly with some ylides photogenerated under electron-transfer sensitization by 1 DCN*, small long-lived residual absorptions were noted at the end of the enhanced decay of ylides in the presence of dipolarophiles (MA or DMAD) see Figure 12C and D. These residual absorptions are probably due to some minor forms of the ylides that are very sluggish in their reactivity with dipolarophiles. Checking the reactivity of these weak components with dipolarophiles on a long time scale (i.e., > ms) was beyond the scope of this study. With ylides photogenerated from 1b by direct 266 nm laser excitation, their quenching by MA at relatively low concentrations (0−2.5 mM) revealed a second prominent, longer-lived ylide component, the reactivity of which with MA was studied using the dipolarophile at 10 times higher concentrations. With acetic acid as quencher of ylides photogenerated by direct 266 nm laser excitation, no significant residual absorptions were noted at the end of the enhanced decay under quenching. Figures 13 and 14 show representative linear plots based on eq 15, with Q representing acetic acid, MA, or DMAD.

(13)

(14) 97,98

The value of k11 was taken as that in the literature, i.e., 0.018 μs−1 or 0.033 μs−1. The solid line in Figure 11 represents the best fit based on k11 = 0.018 μs−1, kq,1O2*Y = 1.56 mM−1 μs−1, [1O2*]0 = 0.273 mM, and [1O2*]0/[Y]0 = 15.7. Using k11 = 0.033 μs−1, we obtained slightly different values for the parameters, but the fit with the experimental data was not as good as with k11 = 0.018 μs−1. Note that the value kq,1O2*Y = 1.56 × 109 M−1 s−1 is in agreement with the value ≥1.2 × 109 M−1 s−1, estimated from a first-order fit of the initial portion of the decay curve in Figure 11. Interestingly, the numerical fit of the data led to an estimate of the molar extinction coefficient at 3 × 103 M−1 cm−1 at 470 nm for triplet-mediated azomethine ylide from 1a.

kobs = τY −1 + kqY [Q] 12342

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Table 6. Rate Constantsb for Reactivity of Azomethine Ylides with Dipolarophiles and Acetic Acida kq, M−1 s−1 c

Aziridine

MA

1a 1b

3.8 × 10 3.1 × 109

2a 2b 3a 3b 4b

4.8 3.0 ∼1 1.1 4.7

9

× × × × ×

108 108 107 105 105

DMADc

AcOHd

1.2 × 10 1.1 × 108

1.4 × 10 1.5 × 106

8

2.8 2.8 ∼2 2.4 3.7

× × × × ×

107 107 105 103 103

MAd 6

3.3 3.8 2.0 1.5 1.6

× × × × ×

106 105 106 106 106

3.6 3.1 1.5 4.7 1.3 ∼4 9.1 2.9

× × × × × × × ×

109 109 107 108 105 105 104 105

a Solvent: acetonitrile at 25 °C. bEstimated errors, ±15%. cWith azomethine ylides formed under DCN singlet electron transfer sensitization. dWith azomethine ylides generated by direct 266 nm laser pulse excitation.

with kYq ’s observed for ylides produced under direct 266 nm excitation and 1DCN* sensitization (Table 6). 3.6. Two-Color Laser Flash Photolysis. Carbonyl ylides from phenyl oxiranes are known100,101 to be photobleachable. This photobleaching has been attributed to ring closure. We have done an experiment in which the azomethine ylide was photogenerated from 1a via triplet acetone sensitization using a 308 nm laser pulse to photoexcite acetone in the presence of 20 mM 1a. This ylide was subjected to photoexcitation by a 532 nm laser pulse at ∼3.5 μs after the first laser pulse (308 nm). The transient absorption spectra 0.2 μs before and 1 μs after the photolysis laser pulse (532 nm) are presented in Figure 15A. It is evident from these spectra that as a result of 532 nm photolysis, a substantial decrease in absorption occurs at 480−

Figure 13. Representative plots based on eq 15 for the quenching of ylides produced by direct 266 nm laser excitation in acetonitrile. Quencher: (A) MA; (B) acetic acid.

Figure 14. Representative plots based on eq 15 for the quenching of ylides produced under DCN sensitization in acetonitrile. Quencher: (A) MA; (B) DMAD.

Note that in eq 15, τY denotes a first-order decay of the ylide in the absence of a quencher (reactant), but in practice this decay is not first-order in most cases. However, its contribution to kobs becomes negligible even at the lowest [Q] we have used. The rate constants (kYq ) for reactivity of azomethine ylides with acetic acid and two dipolarophiles, MA and DMAD, are compiled in Table 6. The dipolarophilic reactivity of ylides photogenerated from 1a under acetone triplet sensitization in the presence of 10 mM [DMHD] was verified with MA at 1.0 mM as the reactant (quencher). We estimated99 a value of >3.1 × 109 M−1 s−1 for kYq from the single-concentration measurement, which agreed

Figure 15. (A) Transient absorption spectra at 0.2 μs before and 2 μs after 532 nm laser flash photolysis of ylide produced from 1a under 308 nm laser pulse excitation of acetone in acetonitrile. Representative kinetic traces of ylide monitored at (B) 455 nm and (C) 520 nm showing increase and decrease in absorbance, respectively, as a result of 532 nm photoexcitation. 12343



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in the case of 2b points to the formation of two kinetically distinct forms, with the major longer-lived component’s end-ofpulse absorbance being about 3 times larger than that of the minor, shorter-lived component. In methanol, the decay kinetics of the ylide(s) from 2b is considerably slower than that of the ylide from 2a in the same solvent. The formation of spectrally and kinetically distinct forms of ylides in the case of 2a,b also suggests that the principal mode of ring-opening is still disrotatory for this pair. The observed blue shift of the absorption maximum of the major form of the ylide from 2b, relative to that from 2a, could be due to the structural distortion of the endo,endo form from steric interactions among the 1,2-phenyl substituents and the diphenylmethyl group on the N atom. For the isomeric pair, 3a,b, the absorption maxima of the ylides from direct excitation are essentially identical and so are their first-order decay rate constants in methanol. The decay kinetics in Freon-112 suggests the formation of two ylide forms in the case of both aziridines. It is possible that the symmetry rule for photochemical ring-opening is significantly relaxed in this case owing to the steric interactions from the bulky tert-butyl group on the N-atom. Minor exceptions to the symmetry rule have also been documented in several cases of oxiranes in the literature.29,33−35 The formation of azomethine ylides within the laser pulse from 337.1 nm excitation of DCN in the presence of millimolar concentrations of diphenyl aziridines (except 5b) in acetonitrile is explainable in terms of back electron transfer within the photogenerated ion-pair in which the radical cation from an aziridine is in the ring-opened form. Similar observation of azomethine ylide formation under electron transfer sensitization from DCN singlet has been made in an earlier study64 on N-benzoyl aziridines. Ring-opening of phenyl oxiranes to carbonyl ylides via reversible electron transfer under cyanoaromatic sensitization has also been proposed and studied in earlier reports.58,65,107 In recent studies,86,87 Gaebert and coworkers have shown that if the N atom carries an alkyl substituent while a phenyl ring is attached to a C-atom of the aziridine ring, spontaneous ring-opening occurs in the aziridinederived radical cations to yield azomethine ylide radical cations. On the other hand, if the N atom carries a phenyl ring, the aziridine ring appears to retain its structure after oxidation. Our results regarding ready formation of azomethine ylides under DCN sensitization of diphenyl aziridines other than 5b as well as the lack of observation of ylide with 5b under the same mode of electron-transfer photosensitization are in agreement with those reported by Gaebert et al.86,87 The differences in the broad and structureless transient absorption spectra of ylides from isomeric pairs of aziridines under electron-transfer sensitization in acetonitrile were not significant enough to permit assignment of them to any specific ylide forms (i.e., endo,endo or exo,endo). Also, we did not study the kinetics of such ylides on a >ms time scale to see if multiple species participate in the decay. However, following quenching by dipolarophiles (MA and DMAD) in many cases, we observed longer-lived residual absorptions that appeared to be very sluggish in their reactivity toward the dipolarophiles. These results suggest that more than multiple forms of ringopened radical cations might be formed in the photogenerated ion-pair leading to multiple forms of ylides following back electron transfer. In two earlier papers,61,64 we reported on transient-spectral behaviors of azomethine ylides photogenerated from a number of C-benzoylaziridines and N-benzoylaziridines. In terms of

560 nm, while an increase in absorption occurs at 420−470 nm. In addition, relatively small a decrease in absorption is noticed at the short wavelength region, 320−400 nm. In Figure 15B and C are two representative kinetic traces showing the increase in transient absorption at 455 nm and the decrease at 520 nm from 532 nm photolysis of the ylide. While the decrease in transient absorptions at 480−560 nm and 320−400 nm is attributable to photobleaching from ring closure, the most reasonable way to explain the increase in transient absorption at 420−470 nm would be in terms of photoisomerization of the ylide from one form to another. Interestingly, no increase or growth of transient absorption is observed at 320−340 nm, suggesting that the azomethine ylide does not undergo photocleavage to phenyl carbene which would react with acetonitrile to produce a long-lived nitrile ylide (λmax = 335 nm)55 absorbing in the short-wavelength spectral region. The formation of nitrile ylides from facile reaction of carbenes with acetonitrile is well-recognized in the literature.45,46,102−105

4. DISCUSSION 4.1. Assignment of Azomethine Ylides. Under direct excitation in both EPA glass at 77 K and in solutions at ambient temperatures, the isomeric aziridines 1a and 1b form azomethine ylides with differing absorption maxima (λmax). The λmax of the ylide from the trans isomer (1a) is blue-shifted relative to that from the cis isomer (1b). In this respect, the behavior of the two isomeric aziridines is similar to that of cis/ trans 1,2-diphenyl oxiranes opening to carbonyl ylides under direct photoexcitation. Analysis of the kinetic data for decay of the azomethine ylides from 1a in Freon-112 and methanol suggests that a single ylide form is produced in these solvents and that this form from 1a is kinetically distinct from the ylide or ylides from 1b. For 1b, the decay kinetics in Freon-112 are complex and do not allow us to decide if the ring-opening results in multiple forms or a single form. However, in methanol, 1b forms predominantly one ylide form with a decay rate constant distinct from that of the ylide from 1a. The formation of spectrally and kinetically distinct azomethine ylides from 1a and 1b is explainable in terms of symmetryallowed photochemical disrotatory ring-opening106 leading to the exo,endo form from 1a and endo,endo or exo,exo form from 1b (see Scheme 2). Between the endo,endo and exo,exo Scheme 2. Three Possible Forms of Ylide Structures from Ring-Opening in Diphenyl Aziridines

forms from 1b, the latter can be ruled out on steric grounds. Without evidence from structural studies, such assignments should be considered tentative. The situations with the other aziridines are complex relative to 1a,b. For example, unlike in the case of 1a,b, the absorption spectral maximum of the ylide (or ylides) from the trans aziridine 2a is red-shifted relative to that of ylide(s) from its cis counterpart, 2b. While the decay kinetics in Freon-112 in the case of 2a suggests the formation of a single form of ylide, that 12344



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almost an order of magnitude. For ylides from 2a and 2b, the reactivity with MA also shows a similar difference. The reactivity of the ylides from isomeric aziridines 1a,b with each of the three quenchers are close to each other, except for the reaction of MA with ylide from 1b under direct excitation where a slower-reacting component was observed. 4.4. Ylide Reactivity with Singlet Oxygen. Our incidental observation of significantly enhanced decay of the ylide from 1a under acetone triplet sensitization in air-saturated acetonitrile is best explained in terms of quenching by singlet oxygen produced at a relatively high concentration under the conditions of the experiment. The numerical fit of the decay profile gave reasonable values for the parameters involved, namely, initial concentrations of 1O2* and ylide, and the rate constant (kq,1O2*Y) for ylide quenching by 1O2*. The value, 1.56 × 109 M−1 s−1, of kq,1O2*Y is significantly large and is in the limit of diffusion control. It suggests that the diffusional reaction of ylides with singlet oxygen leading to photooxygenation can be important in the cases where the ylide is relatively long-lived and 1O2*is available in high concentrations.

location of absorption-spectral maxima and decay kinetics, azomethine ylides from the diphenyl aziridines in the present study are very similar to those from C-benzoylaziridines. The ylides from N-benzoyl aziridines show dual maxima at 350−750 nm, the longer-wavelength band-systems (λmax = 580−610 nm) being very weak in low-temperature matrices. As with diphenyl aziridines, the lifetimes of ylides from N-benzoylaziridines are shorter in methanol than in acetonitrile. The ylides from both classes of benzoylaziridines are very reluctant in their reactivity toward DMAD (relative to those from most of the diphenyl aziridines in the present study). 4.2. Assignment of Azomethine Ylide Triplet. The second major transient species observed in the photogeneration of ylides from aziridines via energy transfer from acetone triplet in acetonitrile is the one at ∼360 nm growing and decaying in the same microsecond time range over which the two-step growth of the ylides becomes complete. We have assigned the 360 nm transient as the ring-opened triplet precursor of ylides on the basis of not only its kinetic correspondence with the formation of ground-state ylides, but also its quenching behavior toward DMHD and oxygen. While the diene (DMHD) has the same reducing effect on the yield of the 360 nm transient as on the ylides (owing to acetone triplet quenching), it has no appreciable effect on the decay kinetics of the 360 nm transient in the limit of high [DMHD]. Oxygen quenches both the yield and the decay kinetics of the 360 nm transient. An alternative assignment of the 360 nm transient as a product of acetone triplet quenching by pathways other than energy transfer (e.g., hydrogen and electron transfer) is not supported by the lack of observation of this species in the course of benzophenone triplet quenching by diphenyl aziridines. Two other observations regarding the 360 nm species, assigned as the ring-opened ylide triplet, deserve further consideration. First, the decay lifetimes of this species (Table 5) from isomeric pairs of aziridines, 1a,b or 2a,b, are very close to each other (that is, these agree with each other within experimental errors). Second, the transient absorption spectra of the triplet-mediated ylides from the isomeric aziridines resemble each other very noticeably (see Figure 8 for 1a,b). It is tempting to interpret these observations in terms of a common, structurally identical form of the ylide triplet produced from the isomeric aziridines. 4.3. Reactivity with Dipolarophiles and Acetic Acid. Several points of note regarding the kinetic data in Table 6 are as follows. The rate constants in some cases are those for the major ylide species, the enhanced decay of which in the presence of quenchers could be reliably studied in the time ranges 0.1−100 μs. The sluggish reactivity of the longer-lived minor species in these cases was not investigated. In the case of 1b, under direct 266 nm photoexcitation in acetonitrile, there was definitive evidence of the formation of two forms of ylides quenchable by MA with rate constants that differ from each other by 2 orders of magnitude (Table 6). With dipolarophiles, MA and DMAD, as quenchers, the reactivity of ylides drops significantly on going from aziridines (1a,b) unsubstituted at the N atom to those (e.g., 2a,b) substituted by bulky groups at this position. This suggests a role of the N-substituents, probably steric, in the 1,3-addition of dipolarophiles. The rate constants for reaction with acetic acid, however, are close to one another for the ylides from the various aziridines (except 2b). It is not obvious why the acetic acid quenching rate constant in the case of 2b is lower than that in the case of 2a by

5. CONCLUSIONS The major findings from this time-resolved study of ringopening in seven 2,3-diphenylaziridines to azomethine ylides as a result of photoexcitation under different conditions are as follows. (a) Azomethine ylides, characterized by absorption-spectral maxima at 475−500 nm, are formed as stable, colored species under direct steady-state irradiation (Hg-lamp) in EPA glass at 77 K. The location of the maxima varies slightly among different aziridines as substrates. (b) In fluid solutions in acetonitrile, Freon-112, and methanol at room temperature, direct 266-nm laser pulse excitation leads to formation, within the laser pulse, of transient ylide species with absorption maxima at 465−500 nm and decay kinetics in the μs−ms time range. The lifetimes of first-order decay of ylides in methanol are in the range 13−28 μs; these remain practically unaffected under oxygen saturation. Analyses of decay kinetics in Freon-112 suggest the formation of more than one form of the ylide from some of the aziridines. (c) Under electron-transfer sensitization of 1,2-diphenyl aziridines (except 5b) by 1,4-dicyanonaphthalene (DCN) singlet excited state (λex = 337.1 nm) in acetonitrile, transient ylides with absorption spectra and decay kinetics similar to those found under direct 266 nm excitation are formed within the laser pulse. Since no separated radical ions are observed, the ring-opening is attributed to back electron transfer from DCN radicalanion to ring-opened ylide radical-cations in the electrontransfer-derived ion-pair. With the N-phenyl-substituted aziridine (5b) under DCN sensitization, no azomethine ylide is observed, suggesting that the ring-opening does not take place in the radical-cation of 5b. In this case, some charge separation takes place leading to the formation of transient DCN radical-anion (λmax = 390 nm) and ring-closed radical-cation of 5b (λmax = 450− 460 nm). (d) The diphenyl aziridines readily quench benzophenone triplet with rate constants close to 108 M−1 s−1. However, essentially no ylides are formed as a result of the 12345



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Dame Radiation Laboratory (NDRL). We are grateful to Drs. Gordon Hug, Ian Carmichael (Director, NDRL), and G. N. R. Tripathi for help and hospitality during part of this work.

quenching. Diphenyl hydroxymethyl radical in low yields results from the quenching in some cases. Under photoexcitation of acetone with 308 nm laser pulses in the presence of diphenyl aziridines at millimolar concentrations in acetonitrile, two transients are observed in each case. One of them, characterized by two-step growth kinetics over 5−10 μs and by absorption spectra similar to those found for transient species produced under direct excitation and under DCN sensitization, is assigned as azomethine ylides. The other one appears at 360 nm with growth followed by decay on the same time scale as that of growth of ylide absorptions. On the basis of kinetic analyses and quenching effects from DMHD and oxygen, the 360 nm transient is assigned as a triplet ylide precursor formed as a result of fast ring-opening in aziridine triplets. The lifetimes of the ylide triplets from various aziridines are in the range 50−220 ns. The enhancement of decay kinetics of acetone-tripletsensitized ylide from aziridine 1a in air-saturated acetonitrile has been explained in terms of quenching by singlet oxygen formed primarily from the O2quenching of precursor acetone triplet. A numerical fit of the enhanced ylide decay to relevant coupled differential equations led to an estimate of the rate constant for ylide quenching by 1O2* at 1.6 × 109 M−1 s−1. The dipolarophilic reactivity of azomethine ylides from aziridines with bulky substituents on the N atom of the ring is considerably less than that of the ones from aziridines without such substituents. As a result of 532 nm laser-pulse excitation of acetonetriplet-sensitized ylide transient from aziridine 1a, prominent depletion of transient absorption occurs on the long wavelength side of the ylide spectrum and increase in absorption occurs on the short wavelength side. While the decrease in transient absorptions at long wavelengths is attributable in part to photobleaching due to ring closure, the increase in transient absorption on the short wavelength side is indicative of photoisomerization of the ylide from one form to another.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

Douglas Cyr, Chevron Products Company, Richmond, California 84801, United States. Sweta Shrestha, Chemistry Department, Ohio State University, Columbus, Ohio, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge generous financial support from the Harvard L. and Judith D. Tomlinson of Duncan Endowed Lectureship in Physical Sciences at Cameron University. We are thankful to Professor A. P. Schaap for kind gifts of aziridine samples and to Dr. K. Bhattacharyya for assistance with measurements of low-temperature absorption spectra. The bulk of the experimental work in this paper made use of laser flash photolysis systems and other instrumental facilities at the Notre 12346



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ϕketyl =

ΔAketyl εT ΔA T0 εketyl

kd kd − 1/τT

where ΔAketyl is the absorbance change due to diphenyl hydroxymethyl radical at 540 nm following the decay of benzophenone triplet, ΔAT0 is the end-of-pulse absorbance change due to benzophenone triplet at 525 nm, εketyl and εT are the corresponding molar extinction coefficients (namely, 6.5 × 103 and 3.2 × 103 M−1 cm−1, respectively),91,92 kd is the pseudo-first-order decay rate constant of benzophenone triplet under quenching by aziridines and τT is benzophenone triplet lifetime. Under the conditions of our measurement, kd was considerably larger than 1/τT and hence the kinetic correction factor, kd/(kd − τT−1) was close to 1. (94) Padwa, A.; Hamilton, L. Mechanism of the Photodeamination of 2-Benzoylaziridines. J. Am. Chem. Soc. 1967, 89, 102−112. (95) We are thankful to one of the reviewers for pointing out the possibility of triplet-mediated C-N bond cleavage. (96) Wilkinson, F.; Brummer, J. G. Rate Constants for the Decay and Reactions of the Lowest Electronically Singlet State of Molecular Oxygen in Solution. J. Phys. Chem. Ref. Data 1981, 10, 809−999. (97) Merkel, P. B.; Kearns, D. R. Radiationless Decay of Singlet Oxygen in Solution. An Experimental and Theoretical Study of Electronic-to-Vibrational Energy Transfer. J. Am. Chem. Soc. 1972, 94, 7244−53. (98) Young, R. H.; Brewer, D. R. The Mechanism of Quenching of Singlet Oxygen. In Singlet Oxygen Reactions with Organic Compounds and Polymers, Ranby, B., Rabek, J. F., Eds.; Wiley: New York, 1976; pp 27−35. (99) The fact that the precursor ylide triplet with a lifetime of ∼250 ns was involved complicated the measurement of enhanced ylide decay in the presence of MA. (100) Becker, R. S.; Bost, R. O.; Kolc, J.; Bertoniere, N. R.; Smith, R. L.; Griffin, G. W. Spectroscopy and Photochemistry of Aryloxiranes. J. Am. Chem. Soc. 1970, 92, 1302−1311.

1 1 = + k 2′t ΔA ΔA 0 where ΔA0 = end-of-pulse absorbance change (i.e., at t = 0), k2′ = k2/ (εl), k2 being the second-order decay rate constant, ε = molar extinction coefficient of ylide at the monitoring wavelength, and l = photolysis cell path length. (79) Mixed first order and equal-concentration second-order kinetics based on a single decaying species was tested by fitting the transient absorbance (ΔA) data to the integrated equation:

k′ 1 ek1t = + 2 (ek1t − 1) ΔA ΔA 0 k1 where the notations mean the same parameters as in reference 78. In this case, optimization of two parameters (i.e., k1 and k2′) and sometimes ΔA0 is involved. (80) Biphasic decay of first order decay of two independent components is tested in terms of:

ΔA = (ΔA 0 − ΔA′0 )e−k1t + ΔA′0 e−k ′1 t where k1 and k1′ are the rate constants of the two first-order decay components and ΔA0′ is the end-of-pulse absorbance change of the second component. In this optimization three parameters, namely, k1, k1′, and ΔA0′, are involved. (81) Nozaki, H.; Fujita, S.; Noyori, R. Photochemistry of 1,2,3Triphenylaziridine. Tetrahedron 1968, 24, 2193−2198. 12348



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(101) Becker, R. S.; Kolc, J.; Bost, R. O.; Kietrich, H.; Petrellis, P.; Griffin, G. Spectroscopy and Photochemistry of Phenyloxiranes. J. Am. Chem. Soc. 1968, 90, 3292−3293. (102) Griller, D.; Montgomery, C. R.; Scaiano, J. C.; Platz, M. S.; Hadel, L. A Critical Examination of Transient Assignments in the Laser Flash Photolysis of 9-Diazofluorene. J. Am. Chem. Soc. 1982, 104, 6813−6814. (103) Brauer, B.-E.; Grasse, P. B.; Kaufman, K. J.; Schuster, G. B. Irradiation of Diazofluorene on a Picosecond Time Scale and at Very Low Temperature. J. Am. Chem. Soc. 1982, 104, 6814−6816. (104) Grasse, P. B.; Brauer, B.-E.; Zupancic, J. J.; Kaufman, K. J.; Schuster, G. B. Chemical and Physical Properties of Fluorenylidene: Equilibration of the Singlet and Triplet Carbenes. J. Am. Chem. Soc. 1983, 105, 6833−6845. (105) Barcus, R. L.; Wright, B. B.; Platz, M. S.; Scaiano, J. C. Chemical, Kinetic, and Spectroscopic Evidence for the Reaction of 1Naphthylcarbene with Acetonitrile to Form a Nitrile Ylide. Tetrahedron Lett. 1983, 24, 3955−3958. (106) Woodward, R. B.; Hoffman, R. The Conservation of Orbital Symmetry; Verlag Chemie: Weinheim, Germany, 1970. (107) Albini, A.; Arnold, D. R. Radical Ions in Photochemistry. 6. The Photosensitized (Electron Transfer) Ring Opening of Aryloxiranes. Can. J. Chem. 1978, 56, 2985−2993.

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