Pyrazole, Imidazole, and Isoindolone Dipyrrinone Analogs: pH

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Pyrazole, Imidazole, and Isoindolone Dipyrrinone Analogs: pH Dependent Fluorophores that Red-Shift Emission Frequencies in Basic Solution Nicole Benson, Olabisi Suleiman, Samuel O. Odoh, and Zachary Robert Woydziak J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b01708 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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The Journal of Organic Chemistry

Pyrazole, Imidazole, and Isoindolone Dipyrrinone Analogs: pH Dependent Fluorophores that Red-Shift Emission Frequencies in Basic Solution Nicole Benson,a Olabisi Suleiman,b Samuel O. Odoh,*b and Zachary R. Woydziak*a aDepartment bDepartment

of Physical and Life Sciences, Nevada State College, Henderson, NV 89002 of Chemistry, University of Nevada, Reno, NV 89557

Supporting Information Placeholder

Figure 1. Interconversion of Z and E isomers of dipyrrinones.

ABSTRACT: Dipyrrinones are non-fluorescent yellow-pigmented constituents of bilirubin that undergo Z to E isomerization when excited with UV/blue light. Mechanical restriction of the E/Z isomerization process results in highly fluorescent compounds such as N,N-methylene-bridged dipyrrinones and xanthoglows. This manuscript describes the first examples of dipyrrinone analogs, which exhibit fluorescence without covalently linking the pyrole-pyrrolidine nitrogen atoms. Instead these analogs restrict E/Z isomerization through intramolecular hydrogen bonding, resulting in mild to moderately fluorescent compounds (F = 0.01 – 0.30). Further, in basic solutions (pH > 12), the dipyrrinone analogs readily deprotonate and absorption/emission profiles of the fluorophores red-shifts by 10-49 nm. Ten analogs were prepared in 41-96% yields in one step directly from commercial materials. To estimate the capacity of which intramolecular hydrogen bonding has upon restricting the E/Z isomerization process, conformational energies of all analogs, in both the protonated and deprotonated species, were explored by using quantum-mechanical density functional theory (DFT) and time-dependent DFT calculations. The computed strengths of the intramolecular hydrogen bonds are related to the barriers of rotation about the 5-6 bond and both correlate with the experimentally measured fluorescence quantum yields.

radiative decay.1 Mechanically restricting the Z to E conversion, via bridging the lactam and pyrrole nitrogen atoms, results in dipyrrinones derivatives that still absorb 390-430 nm light, to generate a polar excited state, but relax via fluorescence.6-8 Lugtenburg et al., noted a fluorescent dipyrrinone derivative, 3H,5H-dipyrrolo[1,2-c:2',1'-f]pyrimidin-3-one (unsubstituted 2, Figure 2), in 1986 in which the lactam and pyrrole rings are bridged by a methylene group.6 Since this original discovery, methylene bridged dipyrrinones have been reported several times in literature and are generally characterized by 385-420 nm excitation wavelengths, with Stoke shifts of 60-90 nm, molar absorptivities of 12,000-28,000 M-1 cm-1 as well as high quantum yields (F > 0.7).9-11 As one might expect, extending the N,N-bridge chain length to ethylene or propylene reduces quantum yields substantially with each methylene insertion (F = 0.26 and 1.2 x 10-3 respectively) due to enhanced rotational freedom.11 A considerable downside to the production of methylene bridged dipyrrinones is that preparatory yields are often quite low (< 25%). For this reason, Lightner et al. prepared an array of N,N-carbonyl bridged dipyrrinone analogs (3, Figure 2), termed “xanthoglows,” which can be prepared in nearly

INTRODUCTION Dipyrrinones (1), similar to bilirubin,1, 2 undergo a Z to E isomerization typically when excited with light in the 390-430 nm range (Figure 1).3-5 The E isomer, once generated, converts back to the thermodynamically more stable Z isomer via non-

Figure 2. Common N,N-bridged analogs of dipyrrinones.

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quantitative yields through treating dipyrrinones with DBU and CDI. Xanthoglows retain many of the physical characteristics of N,N-methylene bridged dipyrrinones such as similar absorbance/emission profiles and high quantum yields. Similarly, tricyclic structures of pyrroloindolizinediones analogs (4)12 and BODIPY analogs (5)13, 14 all bear similar excitation/emission wavelengths paired with high quantum yields, suggesting this particular 5.6.5 ring system is important for the mode of fluorescence. In our pursuit to generate novel tricyclic dipyrrinone derivatives, we synthesized a series of bicyclic dipyrrinone analogs that are fluorescent in an unbridged form (6-14, Figure 3). As far as we are aware, these series of compounds represent the only known versions of unbridged dipyrrinone analogs to date that possess any significant levels (F > 10-2) of fluorescence. Furthermore, 6-14 exhibit a 10-49 nm red-shifting effect for max em in basic solutions (pH >12). While pH dependent fluorescence has been described in several fluorophore systems,15 there are very few cases reported in which a red-shifting effect occurs in basic solutions (pH > 8) and no reports have been described for any dipyrrinone analogs known to literature.

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Table 1. Conditions and reaction yields for the synthesis of 6-14 and 22a

Entry

Pyrrolinone/ Isoindolone

1 2 3 4 5 6 7 8 9 10

15 15 15 15 16 16 16 17 17 17

Aldehyde 18 19 20 21 18 19 20 18 19 20

Yield (%)a

Time (h)

Product

80 41 79 61 49 49 94 96 70 66

24 24 24 24 30 27 24 6 24 24

6 7 8 22 9 10 11 12 13 14

a

Reactions performed on a 1 mmol scale in 5 mL of EtOH. bIsolated Yield.

Figure 3. Structures of fluorescent unbridged dipyrrinone analogs.

RESULTS AND DISCUSSION Dipyrrinone analogs 6-14 and 22 were synthesized using a base-catalyzed aldol condensation reaction, similar to traditional syntheses of dipyrrinones16-18 as illustrated in Table 1. The condensation required refluxing imidazole aldehyde (18/19) or pyrazole aldehyde (20/21) with pyrrolinone (15) or isoindole (16/17) for various reaction times (Table 1) in basic ethanolic conditions (KOH/EtOH). Products 7, 8, and 10-14 are highly crystalline in nature and were purified simply by filtration during the workup of the reaction to provide analytically pure material, only compounds 6, 9, and 22 were purified by chromatography (see Experimental Section). The reaction yields (Table 1), after purification, varied from moderate to high yield (41-94%) following the trends of typical dipyrrinone synthesis. The photophysical properties in neutral aqueous solution (pH 7.0 PBS buffer) and basic aqueous solution (1 M NaOH) and pKa of dipyrrinone analogs 6-14 and 22 are displayed in Table 2. The pKa values were measured for each of dipyrrinone analogs, using UV/Vis spectroscopy, and range from 12.0 to greater than 13.5. In neutral solutions, compounds 6-14 and

22 have max abs ranging from 324 nm to 365 nm, which is considerably blue-shifted from the typical max abs range for dipyrrinones (max abs 390-430 nm).3-5 In basic solutions, in which 6-14 and 22 are largely deprotonated, a red-shifting of max abs of at least 10 nm is observed for all compounds, with 11 possessing the largest shift of 37 nm. The molar absorptivities of dipyrrinone derivatives 6-14 and 22 range from roughly 15,000 to 30,000, and do not substantially deviate in neutral or basic aqueous media. While no detectable fluorescence was observed for 22, compounds 6-14 all displayed varying levels of fluorescence ranging from 410-455 nm in neutral solutions and 460-482 nm in basic solutions. To the naked eye, fluorescence generally appeared blue in neutral solutions and cyan-green in basic solutions (Figure 4). Similar trends regarding red-shifting of max abs in basic solutions were also observed for the max em of 6-14. Most notably, compounds 6, 9, 11, and 14 possess max em of 31 nm, 25 nm, 39 nm, and 34 nm respectively, between the protonated/deprotonated forms; the fluorescence curves for 6, 9, Table 2. Photophysical properties and pKa values of 6-14 and 22 in pH 7.0 PBS buffer and 1 M NaOH (given in parenthesis).

6 7 8 9

Abs. max (nm) 351 (384) 338 (380) 324 (349) 360 (373)

10 11 12 13 14 22

351 (373) 340 (357) 365 (378) 355 (380) 341 (363) 326 (358)

a

 (M-1 cm-1)

b

pKa

24500 (22800) 18600 (18600) 29800 (25700) 29000 (21300)

Fluor. max (nm) 451 (482) 442 (462) 455 (465) 449 (474)

0.30 (0.30) 0.01 (0.03) 0.01 (0.02) 0.25 (0.26)

12.7 12.8 13.0 12.0

17200 (19400) 20200 (23500) 15000 (15500) 15100 (16800) 19800 (23100) 29900 (21300)

432 (454) 410 (449) 457 (475) 409 (443) 427 (452) –a

0.07 (0.05) 0.02 (0.02) 0.22 (0.20) 0.03 (0.01) 0.02 (0.01) –a

12.8 >13.5 12.5 12.9 >13.5 12.9

Fluorescence was not detectable for 22. b Quinine (Q = 0.55)19 and Anthracene (Q = 0.27)19, 20 were used as standards.


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The Journal of Organic Chemistry states) for brominated species. Finally, the pKa values of brominated species, in comparison to non-brominated counterparts, are all higher in value (this effect can be visualized for 14 vs. 11 in Figure S7 of the Supporting Information section). We rationalize the slightly higher pKa values are a result of electron-donation of the bromine atom into the -system of the ring and particularly the carbonyl situated at position 1.

Figure 4. Dipyrrinone Analogs 6-14 and 22 dissolved in DMSO or 10% aq. 1M NaOH in DMSO (in parenthesis) at 7.0 mM and irradiated with a 365 nm lamp source.

Figure 5. Normalized absorbance and emission spectra of 6, 9, and 14 in neutral (pH 7.0 PBS) and basic (1M NaOH) solvents. 11, and 14 (Figure 5) are shifted in a way that portions of the curves can be spectrally separated by filter sets commonly used by instrumentation to analyze fluorescence (such as in fluorescence microscopy). Stokes shifts are rather large (~75-110 nm) in both neutral and basic media for 6-14, and in most cases are near 100 nm. The quantum yields are relatively low, in comparison to N,N-bridged dipyrrinones, which could indicate a higher degree of rotational freedom between the adjacent ring systems. Nevertheless, quantum yields of 6, 9, and 12 fall within the range of a number of commonly used fluorophores in aqueous solvents such as rhodamine B (F = 0.23),19 acridine orange (F = 0.36),20 pyronin Y (F = 0.22),19 and most of the cyanine dye series (typically F = 0.12-0.28).21, 22 Finally, the quantum yield values and the molar absorptivities, appear to be conserved between protonated/deprotonated species of the same compound; molar absorptivities in the large majority of cases do not vary by more than 20% and quantum yields are within ±0.02 for all species at pH 7.0/14.0. These trends suggest that protonated and deprotonated forms of 6-14 likely absorb and emit light through a similar mechanism. In considering 9-11 and 12-14, comparisons of the photophysical properties and pKa can be made between non-brominated and brominated isoindole substituted dipyrrinone analogs (i.e. 9/12, 10/13, and 11/14). The bromine substituent does not appear to substantially alter the absorption wavelengths, as max values are all within 7 nm for analogous protonated and deprotonated states; however, brominated species are less efficient at absorbing light in aqueous solvents as molar absorptivities are all lower (Table 2), and in some cases by nearly half (i.e. protonated 9/12 and 10/13). There does not seem to be a consistent trend between analogous non-brominated and brominated isoindole substituted dipyrrinones in regards to max em, though quantum yield values tend to be lower (in both protonated and deprotonated

The mode of fluorescence for 6-14 is most likely a result of restricted Z to E rotation, similarly to the bridged-dipyrrinone analogs, but differing in that intramolecular hydrogen bonding, rather than covalent linkages, is inhibiting rotation. The idea of intramolecular hydrogen-bonding induced fluorescence is supported by the observation that 22, which is unable to generate an intramolecular hydrogen bond, is a completely non-fluorescent molecule both in the protonated (22a) and deprotonated (22b) states (Figure 6). Dipyrrinone derivatives 6-14 on the other hand, can form intramolecular hydrogen bonds in both the protonated (6a-12a) and deprotonated (6b-12b) states, due to tautomerization and/or rotation of the imidazole/pyrazole ring systems, therefore, both the protonated and deprotonated states result in observed fluorescence. Similar restriction of E/Z isomerization, via intramolecular hydrogen bonding, has been described for acyl hydrazones for non-fluorescent compounds23, 24 and intramolecular hydrogen-bonds, which produce constrained rotations, have been reported to enhance intensity of fluorescence, for a number of systems including: a 7-nitrobenz-2oxa-1,3-diazole system used to detect cysteine and homocysteine via restriction of C=N bond isomerization;25 6-propionyl-2-(dimethyamino)naphthalene derivatives in which the inability to form intramolecular hydrogen bonds substantially reduces quantum yields;26, 27 fluorinated pyrazoline analogs where C-H···F interactions impede molecular motion;28 and 2-quinolones that form rigid complexation in acidic solutions.29 However, dipyrrinone derivatives 6-14 are unique from previously described systems in that restricted isomerization serves as a mode of fluorescence in both protonated and deprotonated states of the molecule. The efficiency of fluorescence does seem to depend on the imidazole/pyrazole substitution pattern of the dipyrrinone analog system as compounds 6, 9, and 12 have considerably larger quantum yields in comparison to 7, 8, 10, 11, 13, and 14.

Figure 6. Potential of hydrogen bonding for protonated (a) and deprotonated (b) species of 6-14 and 22. To better rationalize the varying efficiency of fluorescence, quantum-mechanical calculations were performed for 6-14 and 22. The ground state geometries were optimized with the M06-2X30 exchange-correlation density functional while using 6-31+G* basis sets. Solvent (water) effects were accounted for with a conductor-like polarizable continuum



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Table 3. Calculated relative energies (kcal/mol) of the C1 and C2 conformations of 6-14 and 22 in the protonated states. The results for the deprotonated states are given in parenthesis.

Figure 7. Conformations studied via quantum-mechanical calculations for protonated and deprotonated states. C1 represents the lowest energy, hydrogen bonded conformation, whereas C2 represents conformation with 180º rotation about the 5-6 bond. (CPCM) model.31 We focused on computing the relative energies of conformations that result from rotation about the 56 bond (Figure 7) for 6-14 and 22 in the protonated and deprotonated states, in order to gauge relative strength of intramolecular hydrogen bonding. We also computed these relative energies for the first singlet excited state. For excited state properties, we used time-dependent density functional theory32 (TD-DFT) with the same functional and basis set. Our results are presented in Table 3. In the ground protonated state, the lowest energy conformers (C1) of 6-14 have hydrogen-bond interactions between the five-membered rings and are stabilized relative to conformations without hydrogen bonding, such as conformer C2 which results from a 180º rotation about the 5-6 bond. Interestingly, the hydrogen bond stabilization energies are highest for 6, 9 and 12, Table 3. Thermodynamically, rotation about the 5-6 bond is most disfavored for these species. By contrast, 22 which is incapable of forming intramolecular hydrogen bonds, possess conformers that are iso-energetic, indicative of free rotation. Similarly, for the deprotonated states of 6-14, the C1 structures are stabilized by hydrogen bonds which result from rotation or tautomerization (Table 3). Despite an alternative mode of forming intramolecular hydrogen bonds, we find that the greatest stabilizations are seen with 6, 9 and 12. These species still provide the strongest inhibition to rotation. For the deprotonated state of 22, there is still free rotation as the C1 and C2 structures are nearly iso-energetic. The relative

6 7 8 9 10 11 12 13 14 22

Ground State C1 C2 0.0 (0.0) 5.3 (7.7) 0.0 (0.0) 4.2 (3.8) 0.0 (0.0) 3.3 (3.0) 0.0 (0.0) 6.0 (7.4) 0.0 (0.0) 4.7 (3.7) 0.0 (0.0) 3.7 (2.9) 0.0 (0.0) 6.1 (7.2) 0.0 (0.0) 4.8 (3.6) 0.0 (0.0) 3.9 (2.8) 0.0 (0.0) 0.0 (0.6)

Excited Singlet State C1 C2 0.0 (0.0) 6.6 (6.7) 0.0 (0.0) 4.0 (4.6) 0.0 (0.0) 5.6 (1.8) 0.0 (0.0) 7.8 (6.9) 0.0 (0.0) 4.8 (4.6) 0.0 (0.0) 5.7 (1.8) 0.0 (0.0) 8.0 (6.8) 0.0 (0.0) 5.2 (4.4) 0.0 (0.0) 5.8 (1.9) 0.0 (0.0) 0.9 (1.4)

energies of the C1 and C2 structures obtained for the lowest singlet excited states of 6-14 and 22 closely mirror the ground state data (Table 3). Thus the thermodynamics of rotation about the 5-6 bond support intramolecular hydrogen bonding in the excited states of 6-14 and free rotation in 22. Indeed, a simple Boltzmann distribution predicts about 22% of the excited state of 22 as the higher-energy C2 conformer in the protonated state. In the deprotonated state, the higher-energy C2 conformer contributes about 9% of the total population. To obtain the barriers associated with 5-6 bond rotation, we scanned the dihedral angle between the imidazole/pyrazole ring systems. This was performed for the lowest excited singlet state. Our results are presented in Figure 8. For 6, 9 and 12, the barriers for rotation are larger than 18 kcal/mol for the protonated and deprotonated states. As such 5-6 bond rotation in these species is disfavored by thermodynamics (Table 3) as well as kinetics (Figure 8). The barriers for 7, 8, 10, 11, 13 and 14 are consistently lower than those of 6, 9 and 12 (Figure 8). This coupled with the smaller relative energies (Table 3) are consistent with the lower fluorescence quantum yields of 7, 8, 10, 11, 13 and 14 (Table 2). For 22, the barrier is below 18 kcal/mol for the protonated and deprotonated states, indicating that 5-6 bond rotation of the excited state surface can occur at room temperature. This finding is consistent with the non-fluorescent nature of 22.

Figure 8. Potential energy surfaces for excited state 5-6 bond rotation for 6-14 and 22 in the protonated (left) and deprotonated states (right).


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The Journal of Organic Chemistry In Figure 9, we present the electrostatic surface potential (ESP) maps for the lowest singlet excited states of protonated 6 and 22. ESP maps provide insights into the nature of intermolecular association between the compounds of interest and the surrounding solvent. The purple regions indicate sites with significant negative potentials. For 6, 5-6 bond rotations will open up the ring and lead to greater hydrogen bonding with the solvent. However, this process is thermodynamically and kinetically inhibited (Table 3 and Figure 8). Thus, 6 relaxes mostly by fluorescence, rather than via intermolecular quenching. In comparison, the free rotation of 22 shifts the nucleophilic site for interaction with the aqueous solvent. It is likely that this alters the solvent coordination environment around the molecule and thus the excited state of 22 can relax via intermolecular energy transfer, without fluorescence.

Figure 9. Electrostatic surface potential maps of the C1 and C2 conformers of 6 and 22 in the lowest singlet excited protonated states.

CONCLUSIONS This study describes an efficient and moderate/high yielding synthetic route to a range of fluorescent dipyrrinone derivatives 6-14. As far as we are aware, 6-14 are the only cases of dipyrrinone analogs to exhibit fluorescent properties without covalently linking the pyrrole/lactam nitrogen atoms. Additionally, 6-14 are fluorescent in both protonated and deprotonate states and possess a mode of fluorescence which depends upon the restriction of Z to E isomerization, via intramolecular hydrogen bonding. Quantum-mechanical density functional theory (DFT) and time-dependent DFT calculations are consistent with experimental observations, suggesting that 5-6 bond rotational barriers are larger than 18 kcal/mol for the protonated and deprotonated species of only 6, 9 and 12, which are the compounds with the largest observed quantum yields. These relatively high rotational barriers likely restrict the isomerization process leading to a mode of fluorescence similar that that observed for N,N-methylenebridged dipyrrinones and xanthoglows. Potentially, these fluorophore systems could be used as ratiometric pH probes in highly basic (pH > 12) aqueous environments.

EXPERIMENTAL SECTION General. 1H (400 MHz) and 13C (101 MHz) NMR spectra were acquired on a Varian 400 MHz instrument at the University of Las Vegas, Nevada. Chemical shifts are reported in ppm () and are referenced to DMSO-d6 (2.50 ppm for 1H and 39.5 ppm for 13C) or CDCl3 (7.27 ppm for 1H and 77.0 ppm for 13C). Coupling constants JHH are in hertz and are reported as follows: chemical shift, multiplicity (br = broad, s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, dd = doublet of doublets, dt = doublet of triplets), coupling constant, and integration. Melting points were acquired using a

Stuart SMP10 Digital Melting Point Apparatus and are uncorrected. Infrared spectra (IR) were recorded with a Shimadzu IRAffinity-1 FT-IR spectrophotometer. UV/Vis measurements employed a Beckman Coulter DU-800 spectrophotometer. Fluorescence measurements used a Horiba Scientific FluoroMax-4 spectrofluorometer. High-resolution mass spectra were obtained at the Mass Spectrometry Facility at the University of California, Irvine. Thin layer chromatography (TLC) used aluminum-backed silica plates (0.20 mm, 60 F-254). Column chromatography and silica plugs were performed using Silicycle Siliaflash P60 silica. Plates were visualized by UV light. Commercial reagents were used as received unless otherwise noted. Yields are reported based on isolated material. General Procedure. Pyrrolinone/isoindolone (15 [125 mg], 16 [133 mg], or 17 [212 mg]; 1.00 mmol) and the corresponding pyrazole/imidazole aldehyde (18, 19, 20, or 21; 96.1 mg, 1.00 mmol) were dissolved in 5 mL of ethanol and aqueous KOH (10 M, 24.0 mmol) was added in one portion. The resulting mixture was refluxed and monitored for completion by TLC, cooled, and the volatiles were removed under reduced pressure. The aqueous oily mixture was neutralized with acetic acid (1.70 mL, 30.0 mmol), cooled to 0 ºC, and the product was further purified by filtration (if crystalline) or by extraction/chromatography as described for each of the following compounds. (Z)-5-((1H-imidazol-2-yl)methylene)-3-ethyl-4-methyl-1,5dihydro-2H-pyrrol-2-one (6). Using the General Procedure, 6 was extracted with 3 x 5 mL of CH2Cl2 and the combined organic extracts, were directly loaded onto a column of silica gel and eluted with 10% MeOH in CH2Cl2. A total of 162 mg (80%) of yellow powder was collected after removal of the solvent under reduced pressure. Decomposes at 160 ºC; 1H NMR (400 MHz, DMSO-d6) δ 12.3 (brs, 1H), 9.87 (s, 1H), 7.13 (apps, 2H), 5.93 (s, 1H), 2.23 (q, J = 7.5 Hz, 2H), 2.00 (s, 3H), 0.98 (t, J = 7.5 Hz, 3H); 13C{1H} NMR (101 MHz, DMSO- d6) δ 170.7, 144.8, 140.0, 139.6, 133.9, 130.2, 117.6, 94.1, 16.7, 13.6, 9.33; IR (thin film) 3742, 3148, 3063, 2924, 2353, 1651, 1543, 1450, 771, 717 cm-1; HRMS (ESI-TOF) m/z: [M+Na]+ Calcd for C11H13N3ONa 226.0956, Found 226.0956. (Z)-5-((1H-imidazol-5-yl)methylene)-3-ethyl-4-methyl-1,5dihydro-2H-pyrrol-2-one (7). Using the General Procedure, 7 was directly isolated by vacuum filtration as fine yellow crystals (96 mg, 41%). Decomposes at 161 ºC; 1H NMR (400 MHz, DMSO-d6) δ 7.79 (s, 1H), 7.35 (s, 1H), 6.10 (s, 1H), 2.23 (q, J = 7.5 Hz, 2H), 2.01 (s, 3H), 0.99 (t, J = 7.5 Hz, 3H); 13C{1H} NMR (101 MHz, DMSO-d ) δ 170.5, 140.1, 137.6, 6 137.3, 136.0, 132.3, 118.6, 100.0, 16.7, 13.8, 9.46; IR (thin film) 3334, 3102, 2963, 2924, 2870, 2646, 2584, 2361, 1643, 1574, 1443, 1381, 1311, 1242, 833, 756, 610 cm-1; HRMS (ESI-TOF) m/z: [M+H]+ Calcd for C11H14N3O 204.1137, Found 204.1138. (Z)-5-((1H-pyrazol-5-yl)methylene)-3-ethyl-4-methyl-1,5dihydro-2H-pyrrol-2-one (8). Using the General Procedure, 8 was directly isolated by vacuum filtration as fine yellow crystals (159 mg, 79%). mp 185-187 ºC; 1H NMR (400 MHz, DMSO-d6) δ 13.09 (s, 1H), 9.29 (s, 1H), 7.73 (s, 1H), 6.48 (s, 1H), 6.09 (s, 1H), 2.23 (q, J = 7.6 Hz, 2H), 2.01 (s, 3H), 0.98 (t, J = 7.5, 3H); 13C{1H} NMR (101 MHz, DMSO-d6) δ

171.2, 147.8, 140.9, 137.7, 133.3, 130.0, 106.3, 99.4, 16.7, 13.7, 9.51; IR (thin film) 3742, 3348, 3170, 3048, 2962, 1674,



1543, 1458, 1350, 1281, 764 cm-1; HRMS (ESI-TOF) m/z: [M+Na]+ Calcd for C11H13N3ONa 226.0956, Found 226.0961. (Z)-3-((1H-imidazol-2-yl)methylene)isoindolin-1-one (9). Using the General Procedure, 9 was extracted with 3 x 5 mL of CH2Cl2 and the combined organic extracts, were directly loaded onto a column of silica gel and eluted with 10% MeOH in CH2Cl2. A total of 103 mg (49%) of yellow solid was collected after removal of the solvent under reduced pressure. Decomposes at 228 ºC; 1H NMR (400 MHz, DMSO-d6) δ 12.34 (s, 1H), 10.74 (s, 1H), 7.90 (dd, J = 12.7, 7.5 Hz, 1H), 7.74 (dd, J = 7.6, 4.1 Hz, 1H), 7.63 (dt, J = 14.5, 7.5 Hz, 1H), 7.50 (dt, J = 14.7, 7.3 Hz, 1H), 7.17 (dd, J = 14.7, 7.5 Hz, 2H), 6.46 (q, J = 3.0 Hz, 1H); 13C{1H} NMR (101 MHz, DMSOd6) δ 167.3, 144.9, 137.3, 135.1, 132.8, 129.9, 129.3, 123.6, 121.0, 117.6, 92.9; IR (thin film) 3741, 3201, 3086, 2361, 2322, 1682, 1543, 1520, 1119, 748, 687 cm-1; HRMS (ESITOF) m/z: [M+Na]+ Calcd for C12H9N3ONa 234.0643, Found 234.0641. (Z)-3-((1H-imidazol-5-yl)methylene)isoindolin-1-one (10). Using the General Procedure, 10 was directly isolated by vacuum filtration as fine yellow powder (105 mg, 49%). Decomposes at 214 ºC; 1H NMR (400 MHz, DMSO-d6) δ 7.91 (dt, J = 7.7, 0.9 Hz, 1H), 7.87 (d, J = 1.0 Hz, 1H), 7.72 (dt, J = 7.6, 1.0 Hz, 1H), 7.62 (td, J = 7.5, 1.0 Hz, 1H), 7.47 (td, J = 7.4, 0.8 Hz, 1H), 7.39 (d, J = 1.1 Hz, 1H), 6.64 (s, 1H); 13C{1H} NMR (101 MHz, DMSO-d6) δ 166.9, 138.0, 137.6, 137.2, 132.4, 131.1, 128.9, 128.9, 123.3, 120.6, 117.8, 98.6; IR (thin film) 3109, 2986, 2878, 2669, 2608, 2361, 1659, 1612, 1443, 756, 686, 609 cm-1; HRMS (ESI-TOF) m/z: [M+Na]+ Calcd for C12H9N3ONa 234.0643, Found 234.0638. (Z)-3-((1H-pyrazol-5-yl)methylene)isoindolin-1-one (11). Using the General Procedure, 11 was directly isolated by vacuum filtration as fine yellow crystals (198 mg, 94%). Decomposes at 214 ºC; 1H NMR (400 MHz, DMSO-d6) δ 11.51 (brs, 2H), 7.96 (dt, J = 7.8, 1.0 Hz, 1H), 7.76 (d, J = 2.3 Hz, 1H), 7.73 (dt, J = 7.5, 0.9 Hz, 1H), 7.65 (td, J = 7.6, 1.2 Hz, 1H), 7.50 (td, J = 7.5, 1.0 Hz, 1H), 6.65 (s, 1H), 6.55 (d, J = 2.4 Hz, 1H); 13C{1H} NMR (101 MHz, DMSO-d6) δ 167.8, 138.1, 132.9, 132.8, 131.2, 129.5, 129.1, 123.4, 120.8, 120.3, 106.0, 97.5; IR (thin film) 3302, 3171, 3048, 2970, 2361, 1697, 1612, 1474, 1350, 764, 687, 640 cm-1; HRMS (ESI-TOF) m/z: [M+Na]+ Calcd for C12H9N3ONa 234.0643, Found 234.0647. (Z)-3-((1H-imidazol-2-yl)methylene)-5-bromoisoindolin1-one (12). Using the General Procedure, 12 was directly isolated by vacuum filtration as fine yellow crystals (278 mg, 96%). Decomposes at 213 ºC; 1H NMR (400 MHz, DMSOd6) δ 7.97 (s, 1H), 7.59 (d, J = 7.8 Hz, 1H), 7.48 (d, J = 8.0, 1H), 7.06 (s, 2H), 6.57 (s, 1H); 13C{1H} NMR (101 MHz, DMSO-d6) δ 167.7, 146.6, 140.2, 134.5, 131.7, 129.1, 125.8, 125.03, 124.99, 123.6, 96.5; IR (thin film) 3742, 3240, 2314, 1682, 1543, 1520, 1435, 1312, 1080, 826, 694 cm-1; HRMS (ESI-TOF) m/z: [M+Na]+ Calcd for C12H8BrN3ONa 311.9749, Found 311.9752. (Z)-3-((1H-imidazol-5-yl)methylene)-5-bromoisoindolin1-one (13). Using the General Procedure, 13 was directly isolated by vacuum filtration as fine yellow crystals (203 mg, 96%). Decomposes at 161 ºC; 1H NMR (400 MHz, DMSOd6) δ 8.23 (d, J = 1.5, 1H), 7.86 (s, 1H), 7.72 – 7.57 (m, 2H), 7.39 (d, J = 1.5, 1H), 6.77 (s, 1H); 13C{1H} NMR (101 MHz, DMSO-d6) δ 166.0, 140.0, 137.5, 137.2, 131.6, 129.7, 127.9, 126.2, 125.2, 123.7, 118.9, 100.4; IR (thin film) 3302, 3117, 2994, 2932, 2886, 2361, 1690, 1582, 840 cm -1; HRMS (ESI-

TOF) m/z: [M+Na]+ Calcd for C12H8BrN3ONa 311.9749, Found 311.9749. (Z)-3-((1H-pyrazol-5-yl)methylene)-5-bromoisoindolin-1one (14). Using the General Procedure, 14 was directly isolated by vacuum filtration as fine yellow crystals (191 mg, 66%). Decomposes at 293 ºC; 1H NMR (400 MHz, DMSOd6) δ 8.30 (dd, J = 1.4, 0.9 Hz, 1H), 7.79 (d, J = 2.3 Hz, 1H), 7.67 (m, 2H), 6.80 (s, 1H), 6.55 (d, J = 2.4 Hz, 1H); 13C{1H} NMR (101 MHz, DMSO-d6) δ 166.9, 140.1, 137.7, 132.3, 131.6, 131.0, 128.1, 126.6, 125.3, 124.1, 106.3, 99.2; IR (thin film) 3318, 3194, 3048, 2970, 1713, 1605, 1435, 1350, 1072, 864, 840, 779, 694, 633 cm-1; HRMS (ESI-TOF) m/z: [M+K]+ Calcd for C12H8BrN3OK 327.9488, Found 327.9496. (Z)-5-((1H-pyrazol-4-yl)methylene)-3-ethyl-4-methyl-1,5dihydro-2H-pyrrol-2-one (22). Using the General Procedure, 6 was extracted with 3 x 5 mL of CH2Cl2 and the combined organic extracts, were directly loaded onto a column of silica gel and eluted with 7% MeOH in CH2Cl2. A total of 133 mg (66%) of yellow powder was collected after removal of the solvent under reduced pressure. Decomposes at 202 ºC; 1H NMR (400 MHz, 20% CD3OD in CDCl3) δ 1H NMR (400 MHz, Chloroform-d) δ 7.74 (apps, 2H), 6.01 (s, 1H), 2.27 (q, J = 7.4 Hz, 2H), 2.02 (s, 3H), 1.02 (q, J = 7.4 Hz, 3H); 13C{1H} NMR (101 MHz, CDCl3) δ 173.7, 141.9, 136.0, 133.0, 116.1, 105.0, 100.8, 16.9, 13.4, 9.61; IR (thin film) 3163, 3117, 3048, 2963, 2362, 1674, 1558, 1512, 1396, 1257, 1157, 948, 871, 794, 702 cm-1; HRMS (ESI-TOF) m/z: [M+Na]+ Calcd for C11H13N3ONa 226.0956, Found 226.0955.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. 1H NMR, 13C{1H} NMR, UV/Vis absorption spectra, pKa plots, molar absorptivity plots, quantum yield plots (PDF), and details (ground state and excited state methods, total energies, structures as well as potential energy surface character) for the computational work

AUTHOR INFORMATION Corresponding Authors *Molecular Modeling Studies, E-mail: [email protected]. Phone: (775)-784-6804. Fax: (775)-784-6173. *Synthesis and Chemical Characterization, E-mail: [email protected]. Phone: (702)-992-2656. Fax: (702)992-2601.

Author Contributions N.B. and Z.R.W contributed to the synthesis and chemical characterization portions of the study. O.S. and S.O.O. contributed to the quantum-mechanical calculations in the study.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Z.R.W. and N.B. thank the NIH (2P20 GM103440-14A1) for their generous funding as well as Jungjae Koh and the University of Nevada, Las Vegas for their assistance in acquiring 1H and 13C{1H} NMR. Work by O.S. and S.O.O were supported by the University of Nevada, Reno.


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The Journal of Organic Chemistry

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Pyrazole, Imidazole, and Isoindolone Dipyrrinone Analogs: pH

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