Accessing [g]-Face Ï-Expanded Fluorescent Coumarins by Scholl

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Accessing [g]-Face #-Expanded Fluorescent Coumarins by Scholl Cyclization Nitisha ., and Parthasarathy Venkatakrishnan J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b01223 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 11, 2019

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Accessing [g]-Face π-Expanded Fluorescent Coumarins by Scholl Cyclization Nitisha and Parthasarathy Venkatakrishnan* Department of Chemistry, Indian Institute of Technology Madras, Chennai - 600036, Tamil Nadu, India. [email protected]

Abstract EDG

O O

EDG

O

O

EDG

EDG

Emission Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Organic Chemistry

400

500

λ (nm)

600

700

[g]-Face π-expanded coumarins are synthesized employing Scholl cyclization method. These new arene-annulated dipolar coumarins display interesting absorption and fluorescent properties. The large Stokes-shifts, tuneable fluorescent quantum yields and high photostability reveal promise in bioimaging application.

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INTRODUCTION Coumarins1 are a celebrated class of heterocyclic compounds because of their renowned application as laser dyes,2 fluorescent probes,3 optical brighteners,4 biomedical imaging agents,5 and their potential use as light emitters in OLEDs,6 therapeutic agents,7 etc. To tap their potential, and extract beneficial properties to the fullest extent, various synthetic efforts are being consistently attempted on coumarins. The recent development by Gryko and others on the discovery of new π-expanded coumarin analogues8 has indeed opened-up a budding area of promising research for several research laboratories, for reasons due to the significant variations observed in their photophysical properties. For example, benzocoumarins (Figure 1) possessing elongated conjugation of π-molecular orbitals, with appropriate substituent, reveal several advantageous features, viz., red-shifted absorption, emission and improved fluorescence quantum yields, over simple coumarins.9 It is understood that the extended linear conjugation (rigid) and the substituent contributing to ICT (intramolecular charge transfer) are essentially the key factors for achieving coumarins with more promising characteristics; coumarin-314 is one such example. Extending the conjugation linearly, thereby ICT, in coumarins is known to shift the emission, but fails to improve the photoluminescence quantum yields (PLQE, Φf), due to their accompanied severe nonradiative decay pathways.10 In order to improve the PL characteristics, it is of course important to minimize the above-said pathway by functional rigidification. With this aim, we started out to synthesize a set of arene-annulated benzocoumarins (rigid conjugation, Figure 1) via π-expansion strategy, and examine the optical characteristics. Various

classical

methods

have

been

employed

on

to

1-/2-naphthols,11

1-/2-

hydroxyanthracenes,12 phenanthrenes,13 etc. to obtain various annulated π-expanded


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coumarins, but with harsh conditions, low yields and poor regioselectivity.14 On the other hand, photochemical oxidative cyclization have also been demonstrated as a powerful tool for

7

6 g 7

f 5

4 c

3

h

4

c

3

i h

O 2 O

8 Coumarin

O

j

O

O

O

Benzo[g]coumarin 12 Phenanthro[9,10-g]coumarin

Figure 1. Lateral growth of π-expanded coumarins.

expanding the π-system of coumarins against various faces (regioisomers).15 Along these lines, Scholl cyclization – a popular reaction utilized for the construction of small to large polycyclic aromatic hydrocarbons – has been scarcely employed to construct π-expanded coumarins.16 In 1942, Dilthey and coworkers reported the first synthesis of green emitting coumarins with 6fused benzene rings via dehydrogenative Scholl coupling.17 Recently, Gryko et al. synthesized blue-green emitting 1-oxaperylene-2-one by a two-step method involving the classical Pechmann reaction followed by Scholl reaction.18 Very recently, the same group expanded the above method further for the preparation of 1,7-dioxaperylene-2,8-diones (yellow emitter);19 the isolated π-expanded coumarins were not very fluorescent in solution.8 While all the above three approaches benefited from the aryl substituents at 3- or 4- or both positions, there are no reports employing Scholl methods on the π-expansion at the [g]-face of coumarin by utilizing the aryl substituents at 6- or (and) 7-positions (Figure 2). This could possibly be because of the strong dipolar nature of coumarins,20 which reduces the reactivity at the arene part of coumarin towards Scholl oxidative cyclodehydrogenation. In principle, annulation at the [g]-face must result in coumarins with larger transition dipole moments than the coumarins annulated at other faces.5a,8 3 ACS Paragon Plus Environment


6

4 3

O

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-expansion Earlier reports

Scholl

7

O Cyclization

O

O

-expansion This report

Figure 2. Strategies for the π-expansion of coumarins using Scholl cyclization.

In continuation of our on-going research on coumarins21 and Scholl oxidative cyclization reactions,21 we have attempted to π-expand coumarins at its 6,7-face ([g]face) exploiting intramolecular Scholl reaction, and evaluated their photophysical properties. Herein we report the syntheses, characterization and photophysical properties of various [g]-face arene-annulated coumarins. The [g]-face annulated coumarins demonstrate greater photostability, considerable improvement in their absorption and emission properties as well as appreciable fluorescence quantum yields as compared to benzocoumarins.

RESULTS AND DISCUSSION Initially, we endeavoured our synthesis starting with 7-bromo-4-methylcoumarin 1 (Scheme 1). The bromide 1 was converted using Miyaura borylation to the corresponding boronate ester 2, which upon Pd(II)-mediated Suzuki cross-coupling reaction with 2-bromobiphenyl (3) afforded 7-(biphen-2'-yl)-4-methylcoumarin 4. As may be seen, the biphenyl unit in 4 is appropriately situated to undergo oxidative cyclization reaction via C–C bond formation at either C-6 or C-8 positions of coumarin, involving the benzene ring, to offer either 12a or 5a, respectively. Thus, we next proceeded for the oxidative cyclodehydrogenation step with coumarin 4. Our initial efforts to oxidatively (I2/O2 and hv) photocyclize 4 was not fruitful. While various oxidative coupling reagents are available in the literature,16 we tried the most 4 ACS Paragon Plus Environment

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common ones, such as, FeCl3, AlCl3, DDQ/MeSO3H, etc. Unfortunately, none of the tested conditions yielded the expected cyclized product(s) (5a/12a, Scheme 1 and Table S1), and the starting material was recovered quantitatively. Similar results were also observed when 6(biphen-2'-yl)-4-methylcoumarin 4' was subjected to the above Scholl conditions; neither of the possible products 5b nor 12a was obtained. At this point, we hypothesized that the failure

Scheme 1. Scholl cyclization of 7-/6-(biphen-2-yl)coumarin 4/4'. 6 7

O

a

Br O

B

b

O

O

O

O

7-B-ester: 2 6-B-ester: 2'

7-Br: 1 6-Br: 1'

c

O

O

6

X

7

12a

O

O

7-BiPh: 4 6-BiPh: 4'

O

O

5a 5b

O

O

Reagents and conditions: a) Bis(pinacolatodiboron), PdCl2(PPh3)2, KOAc, toluene, 70 °C, 9 h, 82–83%. b) 2-Bromobiphenyl 3, Pd(PPh3)2Cl2, K2CO3, Toluene:EtOH (3:1), 90 °C, 12 h, 71– 98%. c) Oxidative Scholl cyclization or photocyclization conditions.

in the above cases (4 and 4') were due to the electron deficient benzene ring present in coumarin,22 which probably would have led to a highly unstable radical/arenium cation during oxidative Scholl cyclization.23 To verify this point, Scheme 2 was taken up. The idea was to first reduce the double bond at 3,4-positions of the pyranone ring of coumarin, and then to see if the cyclization could be achieved. Accordingly, the double bond of 6-/7-(biphen-2'-yl)-4methylcoumarin at 3-position was reduced using 10 wt% Pd/C. Further, the oxidative cyclization of 7-/6-(biphenyl-2'-yl)-4-methyl-3,4-dihydrochromen-2-one 6/6' were conducted 5 ACS Paragon Plus Environment


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under FeCl3 conditions, which provided compound 7 regioselectively. The formation of C–C bonds in 7 (as in Scheme 2), after successful oxidative cyclization of 6/6', not only verifies the electron deficient nature of the benzene ring present in coumarin, but also suggests that the electron deficient nature is essentially provided by the fused-pyranone unit.22 The compound 7 then upon oxidation led to the [g]-face annulated coumarin, 12a.

Scheme 2. Scholl cyclization of 7-/6-(biphen-2'-yl)-4-methyl-3,4-dihydrocoumarin 6/6'.

a

b

6

c 12a

4/4' 7

O

O

O

6/6'

O

7

Reagents and conditions: a) H2, 10 wt% Pd/C, EtOAc, rt, 20 h, 88%. b) FeCl3, DCM, rt, 28 h, 28–50%. c) DDQ, benzene, reflux, 24 h, 99%. In order to accomplish the target molecule, i.e., [g]-face annulated coumarin, without involving the coumarin benzene ring, a new synthetic route as shown in Scheme 3 was adapted. Accordingly, 6,7-dihydroxy-4-methylcoumarin 8 was converted to its corresponding bis(triflate) 9, which was then subjected to Pd(II)-mediated two-fold Suzuki cross-coupling reaction with various arylboronic acids (10a-h) to afford the corresponding 6,7-diarylcoumarin derivatives (11a-h, Scheme 3 and Table 1). While photochemical oxidative cyclization was successful for 11f, it was unsuccessful for substrates 11a-e,g-h. Subsequently, in an attempt to realize the cyclodehydrogenated products (12a-h), Scholl oxidative cyclization reaction conditions were once again explored on 11a-h. The best optimized conditions and the suitable reagents for the oxidative cyclization of various diarylcoumarins are consolidated in Tables 1 and S2. As may be noticed, while the oxidative cyclodehydrogenation of coumarin 11g was accomplished only using DDQ/H+ as the oxidant, the other precursors 11a-f underwent smooth oxidative Scholl cyclization in 6 ACS Paragon Plus Environment

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presence of FeCl3 (2.0 equiv.) to afford the corresponding [g]-face arene-annulated products 12a-f in good to excellent yields (up to 91%), highlighting the effectiveness of FeCl3 as oxidant in Scholl cyclization. However, exchanging the reaction condition yielded discouraging results (see, Table S2). Notably, carbazole-incorporated coumarin 11h underwent cyclization effectively (ca. 1 min.) under DDQ/H+ conditions to furnish 12h in good (85%) yields.16c All the newly synthesized [g]-face annulated coumarins 12a-h are unprecedented, and their synthesis using von Pechmann reaction from corresponding phenols is not easy as the latter are not readily available. In view of this, the method demonstrated herein is advantageous in accessing myriad of π-expanded coumarins, and the synthetic attempts described herein would certainly pave the way for better understanding of the Scholl reaction. Beneficially, these coumarins are highly soluble in common organic solvents, such as, chloroform, dichloromethane, ethyl acetate, etc. Scheme 3. Scholl cyclization of 6,7-(diaryl)coumarins 11a-h. HO

a

HO

8

O

O

TfO TfO

9

R1

R4 R4

c

O R1

R3

O

b 10a-h

R2

R2 R3

O

O

12a-h

R3

R1

R4 R4

6 7

O

R1 11a-h

R3

R2

O

R

2

Reagents and conditions: a) Tf2O (2.0 equiv.), Et3N, DCM, rt, 3 h, 75%. b) arylboronic acids (10a-h), Pd(PPh3)2Cl2, K2CO3, Toluene:EtOH (3:1), 90 °C, 12 h, 74–96%. c) FeCl3 or DDQ/H+, DCM, 0 C to rt, 1 min–16 h.



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Table 1. Optimized Scholl cyclization condition for 11a-h to access 12a-h. SM 11a 11b 11c 11d 11e 11f 11g

Aryl Substituent R1 = R2 = R3 = R4 = H R2 = Me; R1 = R3 = R4 = H R1 = R2 = R4 = H; R3 = t-Bu R2 = OMe; R1 = R3 = R4 = H R2 = R3 = OMe; R1 = R4 = H R2 = R4 = OMe; R1 = R3 = H R1–R2 = CH=CH–CH=CH; R3–R4 = CH=CH–CH=CH

11h

R3 R2

R1,

R4

= H;

Condition FeCl3, DCM, rt, 16 h FeCl3, DCM, rt, 1.5 h FeCl3, DCM, rt, 10 h FeCl3, DCM, rt, 2 h FeCl3, DCM, rt, 1 h FeCl3, DCM, rt, 1 h DDQ/MeSO3H, DCM, 0 C to rt, 1 h DDQ/MeSO3H, DCM, 0 C to rt, 1 min

Product, % yield 12a, 40 12b, 50 12c, 47 12d, 73 12e, 91 12f, 91 12g, 78 12h, 85

N C6H13

Fortunately, the single crystals of 11d and 12d were grown by the slow evaporation of their solutions from chloroform and ethyl acetate, respectively. In 11d, the 3methoxyphenyl rings at 6- and 7-positions are tilted to 55.16 and 51.85, respectively, considering the mean plane of coumarin (Figure 3). After Scholl cyclization, 12d exhibited a near-planar structure, with a mild twist (ca. 3.05) at the bay regions of the pyranone-fused triphenylene (Figure 3). While the molecular packing diagram of 11d reveals a centrosymmetric π-π dimer (3.70 Å), 12d exhibits a strong π-π stacking interaction (d = 3.67–3.70 Å) between the neighbouring triphenylene units, as expected, in a non-centrosymmetric fashion (Figure S10). Triphenylenes are known to form onedimensional stacking arrangements due to favourable overlap of π-orbitals of the conjugated system — the behavior which is responsible for their popular liquid crystalline properties.24 Modified or annulated triphenylene-based liquid crystalline materials are recently gaining attention.24 Most excitingly, the solid-state arrangement of 12d reveals that the dipoles of π-expanded coumarins are aligned in the same direction (Figure S10) leading to a polar crystal (space group: Pna21). Molecules 8 ACS Paragon Plus Environment

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crystallize in polar space groups are shown to exhibit interesting pyro-/piezo/ferroelectric properties as reported in certain cases.25 Systems possessing polarized behaviour in the solid-state are attractive for their induced-changes associated with the dielectric properties;26 for example, ferroelectric liquid crystals gained importance due to their fast electro-optic response in modern photonics.26 Investigation of such properties is presently underway in our laboratories. OMe

12e

MeO

Me

O

12d

11d

MeO OMe

O

μg = 5.94 D μe = 8.31 D

Figure 3. X-ray molecular structures of 11d and 12d (thermal ellipsoids are drawn at 50% probablility level), and the ICT possible for 12e and the calculated dipole moments. These novel π-expanded coumarins are highly photostable; no photoreaction or decomposition was noticed upon photoirradiation at λexc = 350/254 nm under oxygen for more than 48 h. The effect of structural variation, i.e., π-expansion and substituents, on the photophysical properties of π-enlarged coumarins 12a-h were examined in dilute chloroform solutions (ca. 10-6 M, Table 2). The absorption bands for all the π-expanded coumarins were located between 300 and 450 nm (Figure 4a and SI). The strong intense band (high ε) in this region may be attributed to the typical π→π* transition.27 The accompanying less intense (low ε) weak band at longer wavelength region could be assigned probably to the n→π* transition.27 As expected, all the π-expanded coumarins 12a-h significantly displayed red-shifted, near-visible absorptions when compared to the corresponding uncyclized 11 or simple linear/bent benzocoumarins attesting to their π-extended nature.8 For example, absorption maximum of 12a is red-shifted



to ca. 32 nm (Table 2), when compared to parent benzo[g]coumarin (λmaxabs, CH3CN = 321 nm). The substituted coumarins 12b-h possess higher molar extinction coefficient (ε) than unsubstituted 12a. Particularly, the large molar extinction coefficient of 12g/12h is more likely due to its large π-system. The emission spectrum of 12a-e,g were featureless resembling other heteroarenes,28 and their emission maximum was found to be in the range 410 to 590 nm, which covers the blue to cyan to yellow emission region (Figure 4b and SI). The featured emission profile in 12f and 12h, despite having ICT, may presumably be attributed to the coupling of

1.0

12a 12b 12c 12d 12e 12f 12g 12h

a)

0.5

0.0

300

400 Wavelength (nm)

Normalized Emission Intensity

locally excited (LE) states with the CT states.29 Noticeably, 12g displays two emission bands Normalized Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0

12a 12b 12c 12d 12e 12f 12g 12h

b)

0.5

0.0

400

500 600 Wavelength (nm)

Figure 4. Absorption (a) and emission spectrum (b) of the coumarins (12a-h) in CHCl3. Inset shows the photographs of 12a-h (left to right) in CHCl3 under UV irradiation (λex = 365 nm). Table 2. Optical properties of coumarins 12a-h. Entry 12a 12b 12c 12d 12e 12f 12g 12h a c

λmax(abs)a (nm) 310, 338, 362, 380 (sh) 313, 327, 347, 386 (sh) 314, 344, 387 (sh) 318, 333, 365 (sh) 322, 333, 368 (sh) 329, 344, 390 (sh) 332, 343, 393(sh) 310, 334, 386 (sh)

εa (M-1cm-1) 13810 14380 14160 14520 20580 15740 42490 64380

λmax(em)b (nm) 415 422 423 475 491 538, 585 (sh) 415, 478 536, 583 (sh)

∆ʋ (cm-1) 3500 5100 5400 9000 9600 10500 8200 11300

Φfc (%) 12 11 14 17 8 10 12 10

Measured in CHCl3 solutions (ca. 10−6 M) at room temperature. b Recorded at λex = 333 nm. Measured using quinine sulfate in H2SO4 (0.1 M) as a standard.30 10 ACS Paragon Plus Environment

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at 415 and 478 nm (Figure 4b), originating probably from π-π* (LE) and CT transitions (vide infra), respectively, besides aggregation (due to its large π-system) at concentrations higher than 10-4 M in CHCl3 (1H NMR spectrum, Figure S2). As may be inferred from Tables 2 and S3, the absorption and emission maximum of coumarins 12d-f bearing electron donating groups (–OMe) are relatively red-shifted (as compared to 12a), which is in analogy to the common trend observed with similarly substituted coumarins/benzocoumarins, due to the activation of ICT (as shown in Figure 3).31 Incidentally, the extent of ICT provided by two methoxy groups in 12f is found to be nearly equivalent to that by one carbazole nitrogen atom in 12h. The stronger the ICT, the larger is the red-shift in the absorption and emission maxima. This fact is further supported by the interesting positive emission solvatochromism (less pronounced in absorption) in 12e (Figure 5a) and subsequent reduction in its PLQE (Φf CHCl3 to MeOH = 0.08 to 0.02), which otherwise are less significant in 12a. This result, along with the DFT calculated μg = 5.94 and μe = 8.31 D for 12e, hint to the increase of dipole moment (∆μ, ca. 2.37 D) upon photoexcitation. Thus, these coumarins reveal large Stokes-shifts (up to ca. 11,300 cm-1) in solvents of varying polarity. In addition, the fluorescence quantum yields for 12a-h in CHCl3 were determined to be in the range 0.06 to 0.17, which is relatively higher than that of the simple coumarins (Φf = 0.002) or close to linearly π-expanded coumarins, such as, 8-methoxy- or 8-hydroxy-benzo[g]coumarin (Φf = 0.12–0.07), reported earlier (for comparison, see Table S3).8 In principle, such advantageous properties (long wavelength emission, efficient ICT, good Φf) associated with these skeletons can be further enhanced by grafting strong donor groups (such as, –NEt2) at suitable positions and acceptor units at C3 position, as has been demonstrated with 7-diethylaminocoumarin-3-carboxylate (λmaxem, CH2Cl2 = 529 nm, Φf = 0.81) or 8-dimethylamino-3-cyanobenzo[g]coumarin (λmaxem, EtOH = 607 nm, Φf = 0.67) in the literature.8 Indeed, dipolar coumarin dyes with large Stokes-shifts and improved emission characteristics can be excited at longer wavelengths under two-photon



conditions (as autofluorescence from biological samples is reduced), enabling deep-tissue imaging.6 These aspects are presently being investigated. The fluorescent lifetime measurements (Figure 5b) reveal that the coumarin derivatives 12a-c follow monoexponential, and the other coumarins 12d-h display bi-exponential decays. The bi-exponential decay unveils the existence of two emissive excited states; probably the locally excited state (LE) and the charge transfer states (CT), as described earlier.32 The emission spectrum of 12e in solvents of varying polarity is also suggestive of this behaviour. The derived lifetimes (ns, in all cases) indicate the origin of emission is S1→S0.32

1.0

10000

a)

MeOH DMSO CHCl3

b)

prompt

12 a 12 b 12 c 12 d 12 e 12 f 12 g

1000

THF Toluene

Counts

Normalized Emission Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.5

100 10

0.0

1

400

500 600 Wavelength (nm)

10

20 30 Time (ns)

40

Figure 5. a) Emission solvatochromism for coumarin 12e (λexc = 330 nm), b) fluorescence lifetime decay profiles of coumarins 12a–g (λexc = 295 nm). CONCLUSIONS In summary, it is shown that the Scholl cyclization methods can be accomplished successfully on 6,7-diarylcoumarins to achieve [g]-face annulated coumarins, whose absorption and emission characteristics are better than those of simple coumarins or benzocoumarins reported in the literature. These dipolar coumarins exhibit substituent- as well as solvent-dependent emission behaviour with very large Stokes shifts proving such coumarins when appropriately designed may serve as potential candidates for two-photon bioimaging applications.


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EXPERIMENTAL SECTION

General Information. All the reagents were commercially purchased and used without further purification. Spectroscopic grade solvents were used for recording UV-vis absorption and emission spectra. The compound 4-methyl-6,7-dihydroxycoumarin is commercially purchased and the compounds 6- or 7-bromo-4-methylcoumarin,33 and 4-methyl-6,7-coumarin ditriflate34 were synthesized by following the literature procedures.

General procedure for the synthesis of 2/2': To a 50 mL reaction tube, 18 mL dry toluene was added and purged with nitrogen gas for 20 minutes. To this, compound 1/1' (500 mg, 2.1 mmol, 1.0 eq.), bis(pinacolato)diboron (639 mg, 2.5 mmol, 1.2 eq.), PdCl2(PPh3)2 (3 mol %) and potassium acetate (309 mg, 3.15 mmol, 1.5 eq.) were added, and the mixture was heated in an oil bath at 70 oC for 9 h. After completion of the reaction, water (10 mL) was added and the organic contents were extracted with ethyl acetate (10 mL × 3). The combined organic portions were dried over anhydrous sodium sulphate, and the solvent was evaporated to dryness under rotary evaporator. The crude compound thus obtained was further purified by a short pad silica-gel column chromatography using hexane/ethyl acetate (8:2) mixtures to afford a white solid with 82–83% yield.

4-Methyl-6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2H-chromen-2-one, 2’

O

O B O

O

2'

Yield: 495 mg, 82%; Mp: 102–108 ºC; Rf = 0.53 (30% ethyl acetate in hexane); 1H NMR (400 MHz, CDCl3) δ 8.0 (d, J = 1.2 Hz, 1H), 7.93 ( dd, J1 = 8.4 Hz, J2 = 1.2 Hz, 1H), 7.29 (d, J = 13 ACS Paragon Plus Environment


8.4 Hz, 1H), 6.27 (d, J = 1.6 Hz, 1H), 2.47 (d, J = 1.6 Hz, 3H), 1.34 (s, 12H); 13C{1H} NMR (100 MHz, CDCl3) δ 160.7, 155.7, 152.9, 138.2, 137.7, 119.5, 116.5, 115.0, 84.3, 24.9, 18.9; IR (KBr, cm-1): 2979, 2926, 1737, 1604, 1455, 1367, 1340, 1273, 1144, 1093, 853, 677; HRMS (ESI–TOF) m/z: [M + H]+ Calcd. for C16H20BO4, 287.1455; found, 287.1468.

4-Methyl-7-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2H-chromen-2-one, 2

O

B O

O

O

2

Yield: 520 mg, 83%; Mp: 159–161 ºC; Rf = 0.53 (30% ethyl acetate in hexane); 1H NMR (400 MHz, CDCl3) δ 7.73 (d, J = 1.2 Hz, 1H), 7.68 (d, J1 = 8.0 Hz, J2 = 1.2 Hz, 1H), 7.57 (d, J = 8.0 Hz, 1H), 6.32 (d, J = 1.2 Hz, 1H), 2.44 (d, J = 1.2 Hz, 3H), 1.36 (s, 12H); 13C{1H} NMR (100 MHz, CDCl3) δ 160.9, 153.0, 152.2, 130.1, 123.8, 123.2, 122.1, 116.2, 84.5, 25.0, 18.7; IR (KBr, cm-1): 2978, 2930, 1725, 1619, 1508, 1402, 1394, 1239, 1143, 970, 854, 686; HRMS (ESI–TOF) m/z: [M + H]+ Calcd. for C16H20BO4, 287.1455; found, 287.1479.

General procedure for the synthesis of 4/4': To a 50 mL schlenk tube, solution of 2/2’ (100 mg, 0.35 mmol) in toluene:ethanol (3:1), was added 2-bromobiphenyl (122 mg, 0.52 mmol), potassium carbonate (121 mg, 0.87 mmol). The solution was purged with nitrogen gas for a period of 20 mins, and then Pd(PPh3)2Cl2 (101 mg, 7 mol%) was added into the solution. The reaction mixture was kept for stirring and heated in an oil bath at 80 °C for overnight under nitrogen gas atmosphere. The reaction was monitored by TLC. After the completion of the reaction, the reaction mixture was diluted with water and extracted with ethyl acetate (3 × 15 mL). The combined organic layers were dried over anhydrous sodium sulphate, filtered and the filtrate was concentrated in vacuo. The crude 14 ACS Paragon Plus Environment

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product thus obtained was purified by silica-gel column chromatography using hexane-ethyl acetate mixtures (10% ethyl acetate in hexanes) as the eluent.

7-([1,1'-Biphenyl]-2-yl)-4-methyl-2H-chromen-2-one, 4

O

O

4

Yield: 78 mg, 71%; white solid; Mp: 126–128 °C; Rf = 0.31 (20% ethyl acetate in hexanes); 1H NMR (400 MHz, CDCl3) δ 7.43–7.47 (m, 4H), 7.41 (d, J = 8.4 Hz, 1H), 7.21–7.24 (m, 3H), 7.18 (d, J = 1.6 Hz, 1H), 7.13–7.15 (m, 2H), 7.03 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 1H), 6.25 (d, 1H, J = 1.2 Hz), 2.39 (d, J = 1.2 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 161.1, 153.4, 152.3, 145.9, 140.9, 140.8, 138.7, 131.0, 130.6, 129.9, 128.5, 128.3, 127.8, 127.0, 126.2, 124.0, 118.4, 118.2, 114.8, 18.7; IR (KBr, cm-1): 3059, 1725, 1616, 1442, 1394, 1260, 1223, 1162, 947, 862, 830; HRMS (ESI–TOF) m/z: [M + H]+ Calcd. for C22H17O2, 313.1229; found, 313.1240.

6-([1,1'-Biphenyl]-2-yl)-4-methyl-2H-chromen-2-one, 4’

O

O

4'

Yield: 107 mg, 98%; white solid; Mp: 143–145 °C; Rf = 0.37 (20% ethyl acetate in hexanes); 1H

NMR (400 MHz, CDCl3): δ 7.47 (bs, 4H), 7.37 (d, J = 8.4 Hz, 1H), 7.18–7.29 (m, 5H),

7.10–7.16 (m, 2H), 6.23 (s, 1H), 2.15 (s, 3H); 13C{1H} NMR (125 MHz, CDCl3):δ 160.9, 152.5, 152.3, 150.5, 141.3, 138.9, 137.5, 133.3, 130.9, 130.4, 130.0, 128.3, 128.2, 128.0, 126.9, 126.3, 119.5, 116.8, 115.2, 18.5; IR (KBr, cm-1): 3055, 3016, 1724, 1619, 1570, 1480, 1443, 1422,



1373, 1265, 1178, 925; HRMS (ESI–TOF) m/z: [M + H]+ Calcd. for C22H17O2, 313.1229; found, 313.1239.

General procedure for the synthesis of 6/6': To a solution of 4/4’ (50 mg, 0.32 mmol) in a two-necked 25 mL round bottom flask equipped with hydrogen gas bladder, was added dry ethyl acetate (3 mL), hydrogen gas was flushed followed by the addition of 10 wt% Pd/C. The reaction mixture was stirred at room temperature in the presence of hydrogen gas and the progress of the reaction was monitored by TLC. After 20 h, the reaction mixture was poured into the celite pad to remove the Pd/C and filterate was evaporated to dryness to obtain the crude product which was further purified by silica-gel column chromatography using hexane-ethyl acetate mixtures (10% ethyl acetate in hexane).

7-([1,1'-Biphenyl]-2-yl)-4-methyl-3,4-dihydro-2H-chromen-2-one, 6

O

O

6

Yield: 88 mg, 88%; white gummy solid; Rf = 0.41 (20% ethyl acetate in hexanes); 1H NMR (500 MHz, CDCl3): δ 7.40–7.43 (m, 4H), 7.22 (m, 3H), 7.13–7.15 (m, 2H), 7.04–7.07 (m, 1H), 6.86–6.88 (m, 2H), 3.10–3.20 (m, 1H), 2.79–2.86 (m, 1H), 2.51–2.61 (m, 1H), 1.32 (d, J = 6.8 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 168.6, 151.0, 142.2, 141.2, 140.7, 139.2, 130.6, 129.9, 128.1, 128.0, 127.7, 126.8, 126.3, 126.0, 125.9, 118.3, 37.0, 29.2, 19.8; IR (KBr, cm-1): 1767, 1616, 1571, 1474, 1410, 1235, 1151, 1077, 1005, 832; HRMS (ESI–TOF) m/z: [M + H]+ Calcd. for C22H19O2, 315.1385; found, 315.1381.


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6-([1,1'-Biphenyl]-2-yl)-4-methyl-3,4-dihydro-2H-chromen-2-one, 6’

O

O

6'

Yield: 88 mg, 88%; white solid; Mp: 149–151°C; Rf = 0.47 (20% ethyl acetate in hexanes); IR (KBr, cm-1): 1765, 1495, 1482, 1469, 1218, 1152, 902, 753; 1H NMR (400 MHz, CDCl3): δ 7.43 (bs, 4H), 7.22 (m, 3H), 7.11–7.13 (m, 3H), 6.95 (d, J = 8.0 Hz, 1H), 6.86 (s, 1H), 2.91– 3.01 (m, 1 H), 2.72–2.80 (m, 1H), 2.45–2.52 (m, 1H), 1.07 (d, J = 7.2 Hz, 3H);

13C{1H}

NMR

(100 MHz, CDCl3): δ 168.4, 150.0, 141.6, 140.8, 139.5, 137.9, 130.8, 130.3, 130.0, 129.6, 128.5, 128.1, 127.83, 127.80, 127.2, 126.7, 116.7, 36.9, 29.4, 19.8. HRMS (ESI–TOF) m/z: [M + H]+ Calcd. for C22H19O2, 315.1385; found, 315.1376.

Procedure for the oxidative cyclization of 6 to yield 7:

O

O

7

To a solution of 6 (50 mg, 0.16 mmol) in dry dichloromethane (10 mL), FeCl3 (129 mg, 0.80 mmol) was added under nitrogen gas atmosphere and the reaction was kept for stirring at room temperature. The progress of the reaction was monitored by TLC. After 7.5 h of the reaction, another portion of FeCl3 (0.80 mmol) was added to the reaction mixture. After 28 h, the reaction was quenched by addition of water. The organic contents were extracted using chloroform (3  5 mL). The organic layer was combined, washed with brine, dried over anhydrous Na2SO4, filtered and evaporated to dryness. The obtained crude product was purified by silica-gel column chromatography using hexane-ethyl acetate mixtures (10% ethyl acetate in hexane) as the eluent. A white solid in 28% yield (14 mg) was isolated. Characterization details of 7 are given below. 17 ACS Paragon Plus Environment


Procedure for the oxidative cyclization of 6' to yield 7:

O

O

7

To a solution of 6’ (50 mg, 0.16 mmol) in dry dichloromethane (5 mL), FeCl3 (52 mg, 0.32 mmol) was added under nitrogen gas atmosphere and the reaction was allowed to stir at room temperature. The progress of the reaction was monitored by TLC. After 24 h of stirring, the reaction was quenched by addition of water. The extraction of the organic contents was done by using chloroform (3  5 mL). The organic layer was combined, washed with brine, dried over anhydrous Na2SO4, filtered and evaporated to dryness. The crude product thus obtained was purified by silica-gel column chromatography using hexane-ethyl acetate mixtures (10% ethyl acetate in hexane) as the eluent to obtain a white solid in 50% yield (25 mg).

3,4-Dihydro-4-methyl-phenanthro[9,10-g]-2H-chromen-2-one, 7: White solid; Mp: 128– 130 °C; Rf = 0.39 (20% ethyl acetate in hexanes); IR (KBr, cm-1): 1767, 1731, 1634, 1149, 756, 719; 1H NMR (400 MHz, CDCl3): δ 8.65 (m, 2H), 8.60 (m, 1H), 8.49–8.52 (m, 2H), 8.27 (s, 1H), 7.67–7.68 (m, 4H), 3.43–3.47 (m, 1H), 2.96–3.02 (m, 1H), 2.70–2.76 (m, 1H), 1.26 (d, J = 7.2 Hz, 3H); 13C{1H} NMR (125 MHz, CDCl3): δ 168.4, 150.8, 130.5, 130.1, 129.7, 129.3, 129.1, 127.93, 127.89, 127.6, 127.5, 127.3, 126.8, 123.63, 123.57, 123.5, 123.2, 121.6, 110.8, 37.2, 30.1, 20.3; HRMS (ESI–TOF) m/z: [M + H]+ Calcd. for C22H17O2, 313.1229; found, 313.1236.

Procedure for the synthesis of 12a from 7: DDQ (2.9 mg, 0.013 mmol) was added into a solution of 7 (4 mg, 0.013 mmol) in benzene (1.0 mL) and kept for stirring at 80 ˚C in an oil bath for 24 h. Upon completion, the reaction was


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quenched by NaHCO3 solution. The product was extracted into ethyl acetate (2 × 3 mL), washed with brine solution. The organic layers were combined together and dried over anhydrous Na2SO4, filtered and evaporated to dryness. The crude product was passed through a short-pad filter column using hexane-ethyl acetate mixtures (10% ethyl acetate in hexane) as the eluent and the product was isolated in quantitative yield. Please see below for the characterization details.

General procedure for the preparation of 6,7-bis(aryl)-4-methyl-2H-chromen-2-ones 11a-h: To a toluene:ethanol (3:1) solution of bis(triflate) analogue 5 (500 mg, 1.09 mmol) and the corresponding arylboronic acid (3.29 mmol) was added K2CO3 (603 mg, 4.36 mmol). The nitrogen gas was purged into the solution for 20 min and Pd(PPh3)2Cl2 (61 mg, 8 mol%) was added to the solution. The mixture was heated in an oil bath at 80 °C under nitrogen gas atmosphere and kept for stirring overnight. The reaction was monitored by TLC. After the completion of the reaction, the reaction mixture was diluted with water and extracted with ethyl acetate (3 × 25 mL). The combined organic layers were dried over anhydrous sodium sulphate, filtered and the filtrate was concentrated in vacuo. The crude product thus obtained was purified by silica-gel column chromatography using hexane-ethyl acetate mixtures (10% ethyl acetate in hexane).

4-Methyl-6,7-diphenyl-2H-chromen-2-one, 11a

O

O

11a



Yield: 279 mg, 82%; white solid; Mp: 168–170 °C [lit., 163-165 °C]34; Rf = 0.65 (20% ethyl acetate in hexane); IR (KBr, cm-1): 3058, 3025, 1727, 1618, 1545, 1480, 1443, 1406, 1383, 1292, 1271, 1223, 1164, 1075; 1H NMR (400 MHz, CDCl3) δ 7.61 (s, 1H), 7.41 (s, 1H), 7.23– 7.26 (m, 6H), 7.12–7.16 (m, 4H), 6.34 (d, J = 1.2 Hz, 1H), 2.48 (d, J = 1.2 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3) δ 161.0, 152.8, 152.3, 144.6, 140.3, 139.8, 137.3, 129.97, 129.8, 128.3, 127.6, 127.2, 126.6, 119.2, 118.7, 115.4,18.8; HRMS (ESI–TOF) m/z: [M + H]+ Calcd. for C22H17O2, 313.1229; found, 313.1212.

6,7-Bis(3-methylphenyl)-4-methyl-2H-chromen-2-one, 11b

O

O

11b

Yield: 356 mg, 96%; white solid; Mp: 140–142 °C; Rf = 0.83 (20% ethyl acetate in hexane); IR (KBr, cm-1): 3019, 2921, 2862, 1724, 1618, 1546, 1435, 1402, 1382, 1294, 1272, 1220, 1163; 1H NMR (400 MHz, CDCl3) δ 7.59 (s, 1H), 7.39 (s, 1H), 7.04–7.14 (m, 4H), 7.01 (s, 2H), 6.88–6.89 (m, 2H), 6.33 (d, J= 1.2 Hz, 1H), 2.48 (d, J= 1.2 Hz, 3H), 2.29 (s, 3H), 2.27 (s, 3H); 13C{1H} NMR(100 MHz, CDCl3) δ 161.1, 152.7, 152.4, 144.8, 140.3, 139.7, 137.9, 137.4, 130.6, 130.4, 128.3, 128.0, 127.8, 127.1, 127.0, 126.5, 119.0, 118.5, 115.2, 21.5, 18.8; HRMS (ESI–TOF) m/z: [M + H]+ Calcd. for C24H21O2, 341.1542; found, 341.1527.

6,7-Bis(4-tert-butylphenyl)-4-methyl-2H-chromen-2-one, 11c


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O

O

11c

Yield: 389 mg, 84%; white solid; Mp: 133–136 °C; Rf = 0.72 (20% ethyl acetate in hexane); IR (KBr, cm-1): 3019, 2962, 2905, 2869, 1729, 1617, 1541, 1515, 1482, 1413, 1396, 1385, 1363, 1293, 1269, 1221, 1165, 1113, 1074, 1011; 1H NMR (500 MHz, CDCl3): δ 7.60 (s, 1H), 7.40 (s, 1H), 7.23–7.26 (m, 4H), 7.06 (m, 4H), 6.32 (s, 1H), 2.46 (s, 1H), 1.31 (s, 9H), 1.30 (s, 9H);

13C{1H}

NMR (125 MHz, CDCl3): δ 161.2, 152.8, 152.4, 150.6, 150.1, 144.8, 137.5,

137.3, 136.9, 129.6, 129.4, 126.5, 125.1, 118.9, 118.5, 115.1, 114.9, 34.6, 31.7, 31.46, 31.4, 18.8; HRMS (ESI–TOF) m/z: [M + Na]+ Calcd. for C30H32NaO2, 447.2300; found, 447.2284.

6,7-Bis(3-methoxyphenyl)-4-methyl-2H-chromen-2-one, 11d OMe

O OMe

O

11d

Yield: 300 mg, 74%; white solid; Mp: 138–140 °C; Rf = 0.8 (20% ethyl acetate in hexane); IR (KBr, cm-1): 3008, 2954, 2834, 2361, 2337, 1726, 1603, 1472, 1430, 1402, 1383, 1284, 1213, 1169, 1039, 1012; 1H NMR (400 MHz, CDCl3): δ 7.62 (s, 1H), 7.41(s, 1H), 7.15–7.20 (m, 2H), 6.75–6.81 (m, 4H), 6.67 (s, 2H), 6.33 (d, J = 1.2 Hz, 1H), 3.65 (s, 3H), 3.63 (s, 3H), 2.47 (d, J= 1.2 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 161.0, 159.4, 159.3, 152.8, 152.3, 144.4, 141.7, 141.1, 137.1, 129.3, 126.4, 122.3, 122.1, 119.2, 118.5, 115.5, 115.4, 115.0, 113.7, 113.0, 55.30, 55.29, 18.8; HRMS (ESI–TOF) m/z: [M + Na]+ Calcd. for C24H20NaO4, 395.1259; found, 395.1265. 21 ACS Paragon Plus Environment


6,7-Bis(3,4-dimethoxyphenyl)-4-methyl-2H-chromen-2-one, 11e O O

O O

O

11e

O

Yield: 396 mg, 84%; white solid; Mp: 116–119 °C; Rf = 0.42 (20% ethyl acetate in hexane); IR (KBr, cm-1): 3015, 2933, 2837, 1726, 1609, 1585, 1518, 1486, 1462, 1406, 1358, 1323, 1251, 1164, 1141, 1025; 1H NMR (500 MHz, CDCl3): δ 7.59 (s, 1H), 7.41 (s, 1H), 6.80–6.82 (m, 4H), 6.60 (s, 1H), 6.58 (s, 1H), 6.33 (s, 1H), 3.88 (s, 3H), 3.87 (s, 3H), 3.62 (s, 3H), 3.61 (s, 3H), 2.48 (s, 1H); 13C{1H} NMR (125 MHz, CDCl3): δ 160.9, 152.8, 152.2, 148.8, 148.5, 144.3, 136.9, 133.3, 132.5, 126.3, 122.1, 122.0, 118.9, 118.3, 115.2, 113.7, 113.4, 111.3, 111.2, 56.1, 56.0, 55.97, 55.9, 18.7; HRMS (ESI–TOF) m/z: [M + H]+ Calcd. for C26H25O6, 433.1651; found, 433.1681.

6,7-Bis(3,5-dimethoxyphenyl)-4-methyl-2H-chromen-2-one, 11f OMe

MeO MeO

O

O

11f

OMe

Yield: 396 mg, 84%; white solid; Mp: 172–174 °C; Rf = 0.42 (20% ethyl acetate in hexane); IR (KBr, cm-1): 3000, 2951, 2839, 1726, 1595, 1548, 1457, 1425, 1403, 1382, 1362, 1206, 1154, 1060, 1017; 1H NMR (400 MHz, CDCl3): δ 7.61 (s, 1H), 7.42 (s, 1H), 6.36–6.38 (m, 2H), 6.32–6.35 (m, 5H), 3.65 (s, 6H,), 3.64 (s, 6H), 2.47 (d, J= 1.2 Hz, 3H); 13C{1H} NMR (125 MHz, CDCl3): δ 161.0, 160.6, 160.5, 152.8, 152.3, 144.4, 142.3, 141.7, 137.1, 126.2,


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119.2, 118.4, 115.4, 108.1, 107.8, 100.1, 99.3, 55.5, 18.8; HRMS (ESI–TOF) m/z: [M + H]+ Calcd. for C26H25O6, 433.1651; found, 433.1662.

4-Methyl-6,7-di(phenanthren-9-yl)-2H-chromen-2-one, 11g

O

O

11g

Yield: 449 mg, 79%; white solid; Mp: >260 °C; Rf = 0.53 (20% ethyl acetate in hexane); IR (KBr, cm-1): 3061, 3016, 1725, 1620, 1491, 1448, 1406, 1378, 1292, 1267, 1219, 1178, 1159, 1054 cm-1;1H NMR (400 MHz, CDCl3): δ 8.54–8.59 (m, 1H), 8.44–8.50 (m, 3H), 7.69–7.92 (m, 4H), 7.58–7.66 (m, 3.4H), 7.55 (s, 0.6H), 7.40–7.54 (m, 4H), 7.32–7.40 (m, 4H), 6.42-6.43 (m, 1H), 2.48 (d, J = 1.2 Hz, 1.2H), 2.46 (d, J = 1.2 Hz, 1.8H);

13C{1H}

NMR (100 MHz,

CDCl3): δ 161.0, 152.6, 152.4, 144.9, 137.2, 136.0, 132.0, 130.9, 130.3, 130.0, 129.6, 129.4, 128.71, 128.67, 128.6, 128.0, 127.9, 127.6, 127.3, 127.0, 126.9, 126.8, 126.74, 126.69, 126.6, 126.54, 126.48, 126.4, 126.3, 126.2, 122.99, 122.95, 122.8, 122.5, 122.4, 120.2, 119.9, 119.4, 115.7, 18.9; HRMS (ESI–TOF) m/z: [M + Na]+ Calcd. for C38H24NaO2, 535.1674; found, 535.1665.



6,7-Bis(9-hexyl-9H-carbazol-2-yl)-4-methyl-2H-chromen-2-one, 11h N

C6H13

O N

O

11h C6H13

Yield: 128 mg, 89%; white solid; Mp: 158–161 °C; Rf = 0.69 (20% ethyl acetate in hexane); IR (KBr, cm-1): 3057, 2930, 2864, 1726, 1610, 1459, 1333, 1231, 1168, 1004, 902, 863; 1H NMR (400 MHz, CDCl3): δ 8.02 (d, J = 7.6 Hz, 1H), 8.01 (d, J = 7.2 Hz, 1H), 7.99 (d, J = 6.8 Hz, 1H), 7.95 (d, J = 8.0 Hz, 1H), 7.79 (s, 1H), 7.61 (s, 1H), 7.40 (d, J = 7.6 Hz, 1H), 7.38 (d, J = 7.6 Hz, 1H), 7.22–7.26 (m, 2H), 7.14–7.22 (m, 4H), 7.11 (s, 1H), 7.04 (s, 1H), 6.37 (s, 1H), 3.97 (t, J = 6.8 Hz, 2H), 3.95 (t, J = 6.8 Hz, 2H), 2.54 (s, 3H), 1.25–1.33 (m, 4H), 0.86–1.11 (m, 12 H), 0.76 (t, J = 7.2 Hz, 3H), 0.72 (t, J = 7.2 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 161.0, 151.9, 151.5, 142.4, 142.1, 139.7, 139.5, 133.7, 127.0, 126.7, 126.5, 125.8, 124.8, 124.3, 123.8, 123.4, 122.8, 122.7, 121.1, 121.0, 119.1, 118.3, 114.6, 114.0, 113.8, 109.8, 108.9, 108.7, 101.7, 100.5, 43.0, 42.7, 31.7, 31.6, 28.9, 28.7, 27.2, 27.0, 22.73, 22.71, 18.5,14.2; HRMS (ESI-TOF) m/z: [M + H]+ Calcd. for C46H47N2O2, 659.3638; found, 659.3628.

General procedure for the Scholl oxidative cyclization General procedure for the oxidative cyclization of 11a-f,h under FeCl3 To a solution of 11a-f (100 mg, 1.0 equiv.) in 20 mL dichloromethane, was added FeCl3 (2.0 equiv.). The reaction mixture was stirred at room temperature from 1-16 h. TLC was used to analyze the progress of the reaction. After completion, the reaction was quenched with methanol. The organic contents were extracted using chloroform, washed with water followed by brine solution. The combined organic layers were dried over anhydrous sodium sulfate, 24 ACS Paragon Plus Environment

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filtered and the solvent was evaporated in vacuo. The crude product thus obtained was purified using silica gel column chromatography using hexane-ethyl acetate mixtures.

4-Methyl-phenanthro[9,10-g]-2H-chromen-2-one, 12a

O

O

12a

Yield: 40 mg, 90%; white solid; Mp: 250 °C; Rf = 0.56 (2:8, ethyl acetate:hexane); IR (KBr, cm-1): 2939, 2900, 1713, 1622, 1540, 1412, 1381, 1307, 1227, 1178;1H NMR (400 MHz, CDCl3): δ 8.69 (s, 1H), 8.60–8.63 (m, 2H), 8.53–8.55 (m, 1H), 8.50 (d, J= 6.4 Hz, 1H), 8.38 (s, 1H), 7.66–7.73 (m, 4H), 6.37 (s, 1H), 2.61 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.9, 152.1, 133.1, 130.7, 129.7, 129.1, 128.8, 128.6, 127.8, 126.4, 124.1, 123.7, 123.6, 123.1, 119.7, 119.5, 115.8, 110.4, 18.8; HRMS (ESI-TOF) m/z: [M + H]+ Calcd. for C22H14O2, 311.1072; found, 311.1091.

4,7,12-Trimethyl-phenanthro[9,10-g]-2H-chromen-2-one, 12b

O

O

12b

Yield: 49 mg, 50%; white solid; Mp: >240 °C; Rf = 0.51 (2:8, ethyl acetate:hexane); IR (KBr, cm-1): 3015, 2965, 1722, 1622, 1415, 1380, 1260, 1176, 1078, 1024; 1H NMR (500 MHz, CDCl3): δ 8.63 (s, 1H), 8.45 (d, J1 = 8.5 Hz, 1H), 8.44 (d, J1 = 8.5 Hz, 1H), 8.26 (s, 1H), 8.25 (s, 1H), 7.49 (dd, J1 = 8.5 Hz, J2 = 1.0 Hz, 1H), 7.47 (dd, J1 = 8.5 Hz, J2 = 1.0 Hz, 1H), 6.35 (d, J = 1.0 Hz, 1H), 2.62 (s, 3H), 2.60 (s, 6H); 13C{1H} NMR (125 MHz, CDCl3) δ 161.0, 152.1, 25 ACS Paragon Plus Environment


152.0, 137.1, 133.2, 130.2, 129.1, 128.7, 128.5, 128.2, 127.5, 126.4, 124.1, 123.4, 123.3, 123.0, 119.5, 119.3, 115.6, 110.3, 22.0, 21.9, 18.9; HRMS (ESI-TOF) m/z: [M + H]+ Calcd. for C24H19O2, 339.1385; found, 339.1360.

8,11-Di-tert-butyl-4-methyl-phenanthro[9,10-g]-2H-chromen-2-one, 12c

O

O

12c

Yield: 47 mg, 47%; white solid; Mp: 226–229 °C; Rf = 0.43 (10% ethyl acetate in hexanes); IR (KBr, cm-1): 3014, 2961, 2905, 2869, 1727, 1622, 1476, 1421, 1384, 1365, 1311, 1265, 1226, 1178, 1074; 1H NMR (500 MHz, CDCl3): δ 8.77 (s, 1H), 8.66 (s, 2H), 8.55 (d, J = 8.5 Hz, 1H), 8.51 (d, J = 8.5 Hz, 1H), 8.45 (s, 1H), 7.74–7.78 (m, 2H), 6.39 (d, J = 1.0 Hz, 1H), 2.66 (d, J = 1.0 Hz, 3H), 1.57 (s, 18H); 13C{1H} NMR (125 MHz, CDCl3): δ 161.0, 152.2, 152.0, 151.7, 150.5, 133.1, 130.7, 129.6, 127.0, 126.6, 126.3, 125.7, 125.6, 124.1, 123.1, 119.4, 119.3, 119.2, 115.5, 110.2, 35.4, 35.2, 31.6, 31.5, 18.8; HRMS (ESI-TOF) m/z: [M + Na]+ Calcd. for C30H30NaO2, 445.2143; found, 445.2159.

7,12-Dimethoxy-4-methyl-phenanthro[9,10-g]-2H-chromen-2-one, 12d OMe

O

O

12d

OMe


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Yield: 73 mg, 73%; white solid; Mp: 230–232 °C; Rf = 0.56 (20% ethyl acetate in hexane); IR (KBr, cm-1): 2908, 2857, 1707, 1611, 1458, 1411, 1378, 1235, 1174, 1034; 1H NMR (400 MHz, CDCl3): δ 8.64 (s, 1H), 8.45 (d, J = 8.8 Hz, 1H), 8.43 (d, J = 8.8 Hz, 1H), 8.35 (s, 1H), 7.95 (d, J = 2.8 Hz, 1H), 7.87 (d, J = 2.8 Hz, 1H), 7.31 (d, J1 = 9.6 Hz, J2 = 2.6 Hz, 1H), 7.28 (d, J1 = 9.6 Hz, J2 = 2.6 Hz, 1H), 7.27–7.32 (m, 2H), 6.39 (d, J = 1.2 Hz, 1H), 4.04 (s, 3H), 4.03 (s, 3H), 2.63 (d, J = 1.2 Hz, 1H); 13C{1H} NMR (125 MHz, CDCl3): δ 160.9, 158.6, 158.5, 152.1, 152.0, 133.2, 129.6, 129.3, 128.9, 126.4, 124.8, 124.7, 123.8, 119.8, 119.4, 117.8, 115.7, 115.1, 110.4, 106.9, 106.1, 55.7, 55.6, 18.8; HRMS (ESI-TOF) m/z: [M + H]+ Calcd. for C24H18O4, 371.1283; found, 371.1301.

7,8,11,12-Tetramethoxy-4-methyl-phenanthro[9,10-g]-2H-chromen-2-one, 12e O O

O O

O

12e O

Yield: 90 mg, 91%; pale yellow solid; Mp: 236–238 °C; Rf = 0.39 (50% ethyl acetate in hexanes); IR (KBr, cm-1): 2923, 2852, 1719, 1617, 1540, 1514, 1459, 1422, 1389, 1264, 1217, 1196, 1152, 1045; 1H NMR (500 MHz, CDCl3): δ 8.22 (s, 1H), 7.83 (s, 1H), 7.67 (s, 1H), 7.59 (s, 1H), 7.54 (s, 1H), 7.48 (s, 1H), 6.28 (s, 1H), 4.11 (s, 3H), 4.09 (s, 6H), 4.00 (s, 3H), 2.51 (s, 3H);

13C{1H}

NMR (125 MHz, CDCl3): δ 160.9, 152.1, 151.0, 150.5, 149.8, 149.2, 149.1,

131.8, 125.2, 124.9, 123.8, 122.7, 122.1, 118.6, 118.3, 115.1, 109.2, 104.99, 104.7, 104.6, 104.1, 56.3, 56.2, 56.18, 56.0, 18.6; HRMS (ESI-TOF) m/z: [M + H]+ Calcd. for C26H23O6, 431.1495; found, 431.1476.



7,9,10,12-Tetramethoxy-4-methyl-phenanthro[9,10-g]-2H-chromen-2-one, 12f OMe

MeO MeO

O

O

12f OMe

Yield: 90 mg, 90%; white solid; Mp: 206–208 °C; Rf = 0.40 (30% ethyl acetate in hexanes); IR (KBr, cm-1): 3003, 2939, 2835, 1725, 1605, 1571, 1459, 1425, 1401, 1383, 1365, 1328, 1303, 1280, 1231, 1203, 1182, 1163, 1118, 1065, 1037, 1019; 1H NMR (400 MHz, CDCl3): δ 8.56 (s, 1H), 8.28 (s, 1H), 7.50 (s, 1H), 7.44 (s, 1H), 6.76 (s, 1H), 6.74 (s, 1H), 6.37 (s, 1H), 4.02 (s, 3H), 4.01 (s, 3H), 4.00 (s, 3H), 3.99 (s, 3H), 2.62 (s, 1H); 13C{1H} NMR (125 MHz, CDCl3): δ 160.9, 159.2, 152.23, 152.18, 131.6, 127.2, 119.9, 119.5, 115.6, 110.7, 100.0, 98.0, 97.9, 97.4, 77.4, 77.1, 76.9, 56.0, 55.8, 55.7, 18.8; HRMS (ESI-TOF) m/z: [M + H]+ Calcd. for C26H23O6, 431.1495; found, 431.1462. This compound was also obtained by photochemical means whose procedure is provided below.

1’,1’’-Dihexyl-1’H,1’’H-indolo[2’,3’:2,3;2”,3”:6,7]-4-methyl-phenanthro[9,10-g]-2Hchromen-2-one, 12h N

C6H13

O N

O

12h C6H13

Yield: 54 mg, 54%; yellow solid; Mp: 242–244 °C; Rf = 0.56 (20% ethyl acetate in hexane); IR (KBr, cm-1): 3023, 2942, 2871, 1724, 1618, 1432, 1372, 1221, 1042, 932; 1H NMR (400 MHz, CDCl3): δ 9.27 (s, 1H), 9.25 (s, 1H), 8.60 (s, 1H), 8.41 (s,1H), 8.36 (d, J = 7.2 Hz), 8.35


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(d, J = 7.2 Hz, 1H), 8.19 (s, 1H), 8.17 (s, 1H), 7.60 (t, J = 8.0 Hz, 1H), 7.58 (t, J = 7.6 Hz, 1H), 7.45 (d, J = 8.0 Hz, 1H), 7.40 (d, J = 8.0 Hz, 1H), 7.36 (t, J = 7.2 Hz, 2H), 6.22 (s, 1H), 4.30 (t, J = 7.2 Hz, 2H), 4.28 (t, J = 7.6 Hz, 2H), 2.59 (s, 3H), 1.90–1.99 (m, 4H), 1.44–1.49 (m, 4 H), 1.30–1.39 (m, 8 H), 0.88 (t, J = 6.8 Hz, 6H); 13C{1H} NMR (125 MHz, CDCl3): δ 161.0, 152.0, 151.8, 142.6, 142.3, 139.9, 139.7, 134.0, 127.1, 127.0, 126.8, 126.0, 124.5, 124.0, 123.6, 122.9, 122.8, 121.1, 121.0, 119.3, 119.2, 114.9, 114.2, 114.1, 110.1, 109.0, 108.8, 101.9, 100.7, 43.2, 42.9, 31.7, 31.6, 29.8, 29.0, 28.8, 27.2, 27.1, 22.7, 18.6, 14.2; HRMS (ESI-TOF) m/z: [M + H]+ Calcd. for C46H45N2O2, 657.3481; found, 657.3467. This compound 12h was also synthesized in 85% yields by following the DDQ/H+ conditions described below.

General procedure for the Scholl oxidative cyclization of 11g under DDQ/H+ conditions A representative procedure for the synthesis of 12g from 11g is given below. To a solution of 11g (100 mg, 0.19 mmol, 1.0 equiv.) in 20 mL dichloromethane, was added methanesulphonic acid (20.0 equiv.) dropwise at 0 °C followed by the addition of DDQ (2.0 equiv.). The reaction was kept at room temperature for stirring. The reaction progress was monitored by TLC. The reaction was quenched by NaHCO3 solution. The organic components of the reaction were extracted into chloroform, washed with water and washed with brine solution. The combined organic layers were dried over anhydrous sodium sulfate, filtered and the solvent was evaporated under vacuo. The crude product thus obtained was purified by silica-gel column chromatography using hexane-ethyl acetate mixtures as eluent.



4-Methyl-tetrabenzo[1,2:3,4:5,6:7,8]phenanthro[9,10-g]-2H-chromen-2-one, 12g

O

O

12g

Yield: 78 mg, 78%; yellow solid; Mp: 238–240 °C; Rf = 0.51 (20% ethyl acetate in hexane); IR (KBr, cm-1): 2922, 2852, 1722, 1619, 1429, 1377, 1264, 1207, 1175, 1055; 1H NMR (400 MHz, CDCl3): δ 9.13 (s, 1H), 8.76–8.86 (m, 5H), 8.63 (t, 2H), 7.97 (t, 2H), 7.73–7.82 (m, 4H), 7.58 (t, 2H), 7.21–7.23 (m, 2H), 6.40 (s, 1H), 2.59 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 161.1, 131.6, 130.7, 130.6, 130.3, 129.9, 128.5, 128.4, 128.3, 127.5, 127.48, 127.3, 127.1, 126.7, 125.7, 125.6, 125.58, 123.7, 123.5, 123.4, 123.2, 121.0, 115.7, 114.8, 111.4, 18.8; HRMS (ESI-TOF) m/z: [M + H]+ Calcd. for C38H23O2, 511.1698; found, 511.1700.

General procedure for the photochemical oxidative cyclization of 11f A representative photochemical procedure for the synthesis of 12f from 11f is given below. A solution (ca. 0.4 M) of 11f (10 mg, 0.02 mmol) and iodine (2.9 mg, 0.02 mmol, 1.0 equiv.) in dry dichloromethane (60 mL) was photoirradiated under 350 nm lamps using LuzChem photochemical reactor for a period of 14h under optimal conditions. After complete consumption of the starting material (as identified by TLC), the photoirradiation was stopped and the photolysate was washed with hypo solution, followed by brine. The organic extracts were combined. The crude solid obtained after evaporation of the solvent was subjected to a short-pad silica-gel column chromatography using ethyl acetate-hexane mixtures as the eluent to obtain a pale yellow solid (12f, 9.5 mg) in 95% yield.


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AUTHOR INFORMATION Corresponding Author Email: [email protected] ORCID P. Venkatakrishnan: 0000-0002-0895-2861 Notes The authors declare no competing financial interest. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthesis optimization conditions, 1H and

13C

NMR scans of intermediates and π-expanded

coumarins 12a-h, UV-vis absorption and emission spectra of 12a-h, absorption and emission solvatochromism, time-resolved fluorescence measurements of 12a-h, concentration dependent 1H NMR spectrum, dipole moment calculation details of 12e by DFT, and single crystal X-ray diffraction details of 11d (CCDC 1900606)/12d (CCDC 1901484) and their ORTEP drawings. ACKNOWLEDGMENTS P.V.K is grateful to DST-SERB SB/S1/OC-47/2014, New Delhi, India for generous funding. Nitisha thanks CSIR-UGC for a SRF fellowship. Department of Chemistry, IIT Madras is acknowledged for the facilities. Mr. Ramkumar, Department of Chemistry, IIT Madras is acknowledged for the help with X-ray structures. Mr. Rajamani, Department of Chemistry, IIT Madras is thanked for help with DFT calculations.



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