Microwave-assisted Intramolecular Dehydrogenative Diels-Alder Reactions for the Synthesis of Functionalized Naphthalenes/Solvatochromic Dyes

Summary

Microwave-assisted intramolecular dehydrogenative Diels-Alder (DA) reactions provide concise access to functionalized cyclopenta[b]naphthalene building blocks. The utility of this methodology is demonstrated by one-step conversion of the dehydrogenative DA cycloadducts into novel solvatochromic fluorescent dyes via Buchwald-Hartwig palladium-catalyzed cross-coupling reactions.

Abstract

Functionalized naphthalenes have applications in a variety of research fields ranging from the synthesis of natural or biologically active molecules to the preparation of new organic dyes. Although numerous strategies have been reported to access naphthalene scaffolds, many procedures still present limitations in terms of incorporating functionality, which in turn narrows the range of available substrates. The development of versatile methods for direct access to substituted naphthalenes is therefore highly desirable.

The Diels-Alder (DA) cycloaddition reaction is a powerful and attractive method for the formation of saturated and unsaturated ring systems from readily available starting materials. A new microwave-assisted intramolecular dehydrogenative DA reaction of styrenyl derivatives described herein generates a variety of functionalized cyclopenta[b]naphthalenes that could not be prepared using existing synthetic methods. When compared to conventional heating, microwave irradiation accelerates reaction rates, enhances yields, and limits the formation of undesired byproducts.

The utility of this protocol is further demonstrated by the conversion of a DA cycloadduct into a novel solvatochromic fluorescent dye via a Buchwald-Hartwig palladium-catalyzed cross-coupling reaction. Fluorescence spectroscopy, as an informative and sensitive analytical technique, plays a key role in research fields including environmental science, medicine, pharmacology, and cellular biology. Access to a variety of new organic fluorophores provided by the microwave-assisted dehydrogenative DA reaction allows for further advancement in these fields.

Introduction

Small molecule design and synthesis is critical to the development of a range of scientific fields that includes pharmaceuticals, pesticides, organic dyes, and many more ¹. The Diels-Alder (DA) and dehydro-Diels-Alder (DDA) reactions are especially powerful tools in the synthesis of small cyclic and aromatic compounds ^2-4. Additionally, thermal dehydrogenative DA reactions of styrene dienes with alkyne dienophiles provide a potentially beneficial route to the synthesis of aromatic compounds by initially forming cycloadducts that can further aromatize under oxidative conditions ⁵. By employing a thermal intramolecular dehydrogenative DA reaction of styrene dienes with alkynes, the problems typically associated with utilizing styrene as a diene, such as undesired [2 + 2] cycloaddition ^5,6 and polymerization reactions ⁷ and poor regioselectivity, are alleviated and naphthalene compounds can be generated.

The thermal intramolecular dehydrogenative DA reaction of styrenes with alkynes is not without considerable issues. First, most reactions suffer from low yields, long reaction times, and high reaction temperatures ^8-11. Additionally, many reactions do not promote exclusive formation of the naphthalene product; both naphthalene and dihydronaphthalene are produced, often as inseparable mixtures by column chromatography ^11,12. The tethers of the precursor styrene-ynes are also restricted to include heteroatoms and/or carbonyl moieties. Only one example is reported for an all carbon-containing tether, requiring conditions of 250 °C neat for 48 hr in order to obtain naphthalene formation ¹⁰.

In addition to limited variety within the tethers of the starting materials, one of the most severe constraints of this methodology is the lack of functionality tolerated under the conventional thermal conditions. The alkyne terminus of the starting material is either unsubstituted or appended with a phenyl or trimethylsilyl (TMS) moiety ^8-13. In one instance, an ester at the alkyne terminus is shown to undergo the dehydrogenative DA reaction, but this results in a mixture of naphthalene and dihydronaphthalene products ¹¹. A later proposal suggests that a TMS group appended to the alkyne terminus is necessary to achieve exclusive naphthalene formation in high yields ¹⁰. The deficiency of diverse functionality reported for thermal dehydrogenative DA reactions severely limits the potential of this reaction toward the assembly of unique naphthalene structures.

The desire for variation in naphthalene structures stems from their function as small molecule building blocks in several scientific fields, especially organic fluorescent dyes ^14,15. The excellent spatial resolution and response-times of small organic dyes for monitoring real-time events ¹⁶ has led to the development of hundreds of commercially available fluorescent compounds. Many of these dyes are naphthalenes with discrete photophysical and chemical properties ¹⁵. Choosing fluorescent dyes with specific properties to monitor individual functions is challenging, which leads to an increasing need for new classes of fluorophores with more diverse photophysical properties. To this end, a thermal intramolecular dehydrogenative DA reaction of styrenes with alkynes that allows for diversification of a unique naphthalene scaffold would be potentially beneficial with application to developing new naphthalene-containing fluorescent dyes.

As an alternative to conventional heating, microwave-assisted chemistry is advantageous because it offers more uniform heating of the chemical sample, which leads to higher chemical yields, faster reaction rates, milder reaction conditions, and often different selectivity of products ¹⁷. Employing microwave-assisted versus conventional heating conditions for the intramolecular dehydrogenative DA reaction of styrenes serves to eliminate many of the problems associated with this methodology by reducing reaction time from days to minutes, increasing previously poor yields, lowering reaction temperatures, and offering more selective formation of the desired naphthalene product. Microwave-assisted reaction conditions may also be more likely to facilitate incorporation of a greater variety of functionality into the naphthalene products that was previously unattainable. Only one prior example has been reported utilizing microwave-assisted conditions for the dehydrogenative DA reaction in which a 90% yield of both naphthalene and dihydronaphthalene was obtained in as little as 15 min at 170 °C ¹².

Herein is reported a microwave-assisted intramolecular dehydrogenative DA reaction of styrenyl derivatives that leads to the exclusive formation of functionalized and diverse naphthalene products in as little as 30 min and in high to quantitative yields ¹⁸. The utility of this protocol is further demonstrated by the one-step conversion of a naphthalene product into a novel solvatochromic fluorescent dye with photophysical properties which rival that of the popular commercially available dye Prodan ¹⁹.

Protocol

1. Microwave-assisted Dehydrogenative DA Reaction

1. Add the para-chloro-styrene derivative (0.045 g, 0.18 mmol) and 1,2-dichloroethane (3 ml) to a 2-5 ml microwave irradiation vial equipped with a stir bar to create a 0.060 M solution. This concentration is used because higher concentrations lead to the formation of undesired products.
2. Cap the microwave irradiation vial and place it in the microwave synthesizer cavity.
3. Irradiate the solution at 180 °C for 200 min with stirring and with fixed hold time on. The hold time is how long irradiation will occur at the designated temperature. The reaction mixture will turn golden in color. Longer reaction times are not detrimental to the yield of the reaction.
4. Confirm the reaction is complete by thin layer chromatography (TLC) employing 5% ethyl acetate/hexane as the eluent. Visualize the TLC plate with UV light and potassium permanganate stain. The R_f of the reactant and product are 0.2 and 0.25, respectively.
5. Transfer the reaction to a scintillation vial using 1 ml of 1,2-dichloroethane to rinse the microwave reaction vial. This results in approximately 3 ml of solution in the scintillation vial.
6. Concentrate the contents of the scintillation vial under reduced pressure at 40 °C using a rotary evaporator (10-30 mmHg). Evaporation of the solvent will require 5-10 min, and 45 mg of a crude brown oil will be obtained. The crude oil is stable and can be stored indefinitely without decomposition.
7. Purify the crude oil by filtration through a pipette of silica gel with 5% ethyl acetate/hexanes as eluent to acquire 41 mg of naphthalene as a white solid.
8. Confirm the identity of the product by ¹H nuclear magnetic resonance (NMR) spectroscopy using deuterated chloroform (CDCl₃) as solvent. For a 300 MHz NMR spectrometer, the ¹H NMR spectrum of the naphthalene is as follows: 7.80 (d, J = 1.8 Hz, 1H), 7.72 (d, J = 9.0 Hz, 1H), 7.70 (s, 1H), 7.38 (dd, J = 1.8, 9.0 Hz, 1H), 3.07 (t, J = 7.1 Hz, 4H), 2.66 (s, 3H), 2.18 (p, J = 7.1 Hz, 2H) ppm.

2. Buchwald-Hartwig Palladium-catalyzed Cross-coupling Reaction

1. Add RuPhos palladacycle (3 mg, 0.004 mmol) to an oven-dried 0.5-2 ml Biotage microwave irradiation vial equipped with a stir bar and cap the vial.
2. Evacuate and refill the vial with nitrogen three times by piercing the septum of the cap with a small gauge needle. Once purging of the vial is complete, remove the needle. The microwave irradiation vial will act as a sealed tube during the reaction, and the best results are obtained when minimal air is present in the reaction vessel.
3. Through the septum, add lithium bis(trimethylsilyl)amide (0.32 ml of a 1.0 M solution in THF, 0.32 mmol) via syringe with stirring. The solution will turn red.
4. After stirring for 2-10 min, add naphthalene (0.038 g, 0.16 mmol) in 0.3 ml anhydrous tetrahydrofuran (THF) via syringe. Additional THF (up to 0.2 ml) can be used to fully dissolve the naphthalene.
5. After 2-10 min of stirring, add dimethylamine (0.12 ml of a 2.0 M solution in THF, 0.24 mmol) via syringe and lower the reaction vessel into a preheated 85 °C oil bath.
6. Heat the reaction mixture for 3 hr at 85 °C, or until the reaction is complete by TLC. The reaction mixture will be dark brown in color. For TLC, utilize 20% ethyl acetate/hexanes as the eluent, and visualize the resulting plate with UV light and potassium permanganate stain. The R_f of the reactant and product are 0.5 and 0.4, respectively.
7. Cool the reaction to room temperature, remove the vial cap, and quench the reaction with saturated aqueous ammonium chloride solution (10 ml).
8. Using a 60 ml separatory funnel, separate the aqueous layer from the organic layer. Extract the aqueous layer three times with ethyl acetate (12 ml).
9. Combine the organic layers in the separatory funnel and wash once with brine (15 ml).
10. Dry the combined organic layers over sodium sulfate for 10 min, and then remove the sodium sulfate by gravity filtration.
11. Using a rotary evaporator, concentrate the resulting solution under reduced pressure at 30 °C (10-30 mmHg). Evaporation of the solvent will require 5-10 min, and a crude brown oil will be obtained.
12. Purify the crude product by silica gel column chromatography with a 1.5 cm chromatography column and 5% ethyl acetate/hexanes as eluent. The dye will be obtained as 27 mg of a yellow solid.
13. Confirm the identity of the product by ¹H NMR spectroscopy using CDCl₃ as solvent. For a 400 MHz NMR spectrometer, the ¹H NMR spectrum for the dye is as follow: 7.64 (d, J = 9.0 Hz, 1H), 7.56 (s, 1H), 7.11 (dd, J = 2.5, 9.0 Hz, 1H), 6.87 (d, J =2.5 Hz, 1H), 3.02 (s, 6H), 3.02 - 2.87 (m, 4H), 2.65 (s, 3H), 2.12 (p, J = 7.3 Hz, 2H) ppm.

3. Preparation of Dye Solution for Photophysical Studies

1. Transfer 1 mg of the dye into a clean, dry 10 ml volumetric flask and dilute to volume with dichloromethane (DCM) to obtain a 0.4 x 10^-3 M stock solution of the dye.
2. Transfer 253 μl of the stock solution to a second 10 ml volumetric flask and dilute to volume with DCM to prepare a 1 x 10^-5 M solution of the dye. This solution will be used to collect both the UV-Vis and the fluorescence data for the dye.

4. UV-Visible Absorption Spectroscopy

1. Fill two quartz spectrophotometer cells with DCM. These are the blank samples. Place them into the UV-Vis spectrophotometer cavity. Never touch the optical surfaces of the cell. Handle the cells at the top of the side plates that do not face the optical axis.
2. Set the instrumental parameters to a slit width of 2 and an acquisition rate of 480 nm/min. Choose a name for the sample and select an acquisition range from 600 to 200 nm.
3. Collect the background spectrum, remove the sample cell from the instrument, empty it, and rinse with several portions of the 1 x 10^-5 M dye solution before filling. Avoid overfilling the cell. Before inserting the sample cell back into the holder, carefully wipe the cell windows with a clean lens tissue.
4. Collect the absorption spectrum of the sample. The absorption maximum is observed at 377 nm.
5. Carefully clean the quartz spectrophotometer cells with water, acetone, and ethanol before running UV-Vis absorption analyses on other samples.
6. Use Excel or Origin software to plot and analyze the collected data.

5. Fluorescence Emission Spectroscopy

1. Fill a quartz fluorometer cell with the 1 x 10^-5 M dye solution and place it into the spectrofluorometer. Avoid skin contact with the optical surfaces of the cell.
2. Set the instrumental parameters: excitation wavelength at 334 nm, slit width of 2, acquisition rate of 0.1 nm/sec, acquisition range from 390 to 750 nm. A 390 nm cut-on filter is needed to remove scattered light from the emission source.
3. Collect the fluorescence emission spectrum of the sample. The fluorescence emission maximum is observed at 510 nm.
4. Clean the quartz fluorometer cell with water, acetone, and ethanol before running fluorescence analyses on other samples.
5. Use Excel or Origin software to plot and analyze the collected data.

Results

Microwave irradiation (MWI) of styrenyl derivatives at 180 °C results in complete cyclopenta[b]naphthalene formation in as little as 30 min and in high to quantitative yields (Figure 1) ¹⁸. No dihydronaphthalene byproduct is observed, and by ¹H NMR spectroscopy the products appear pure without the need for additional purification after irradiation (Figure 2). Various changes to the naphthalene framework are well tolerated utilizing these the...

Discussion

Microwave-Assisted Dehydrogenative DA Reaction

The intramolecular dehydrogenative DA reaction of styrenyl precursors by microwave irradiation (MWI) produces diverse naphthalene structures in high yields of 71-100% and short reaction times, requiring as little as 30 min (Figure 1) ¹⁸. The most difficult aspect of performing the dehydrogenative DA reaction is solvent selection, which is often complicated because a variety of solvent characteristics need to be considere...

Materials

Name	Company	Catalog Number	Comments
Reagent/Material
1,2-Dichlor–thane, ACS reagent ≥99.0%	Sigma-Aldrich	319929
SiliaPlate G TLC - glass-backed, 250 μm	Silicycle	TLG-R10011B-323
Ethyl acetate, certified ACS ≥99.5%	Fisher Scientific	E14520
Hexanes, certified ACS ≥98.5%	Fisher Scientific	H29220
Silica gel, standard grade	Sorbent Technologies	30930M	60 A, 40-63 μM (230 x 400 mesh)
RuPhos palladacycle	Strem	46-0266
Nitrogen gas	Matheson TRIGAS	NI304	Nitrogen 304cf, industrial
Lithium bis(trimethylsilyl) amide solution	Sigma-Aldrich	225770	1.0 M solution in THF
Tetrahydrofuran anhydrous ≥99.9%	Sigma-Aldrich	401757	Inhibitor-free
Dimethylamine solution	Sigma-Aldrich	391956	2.0 M solution in THF
Ammonium chloride	Fisher Scientific	A661-500
Sodium sulfate, anhydrous (granular)	Fisher Scientific	S421-500
Chromatography column	Chemglass	CG-1188-04	½ in ID x 18in E.L.
Cyclohexane, ≥99.0%	Fisher Scientific	C556-1
Toluene anhydrous, 99.8%	Sigma-Aldrich	24451
1,4-Dioxane anhydrous, 99.8%	Sigma-Aldrich	296309
Tetrahydrofuran anhydrous, ≥99.9%	Sigma-Aldrich	186562	250 ppm BHT as inhibitor
Dichloromethane	Sigma-Aldrich	650463	Chromasolv Plus
Chloroform, ≥99.8%	Fisher Scientific	C298-1
Acetonitrile anhydrous, 99.8%	Sigma-Aldrich	271004
Dimethyl sulfoxide, ≥99.9%	Fisher Scientific	D128
Ethyl alcohol	Pharmco-AAPER	11ACS200	Absolute
Equipment
Microwave Synthesizer	Biotage	Biotage Initiator Exp
Microwave Vial	Biotage	352016	0.5 – 2 ml
Microwave Vial	Biotage	351521	2 – 5 ml
Microwave Vial Cap	Biotage	352298
Microwave Synthesizer	Anton Paar	Monowave 300
Microwave Vial G4	Anton Paar	99135
Microwave Vial Cap	Anton Paar	88882
NMR Spectrometer	Bruker	Avance	300 or 400 MHz
UV-Visible Spectrometer	PerkinElmer	Lamda 9
Spectrophotometer cell	Starna Cells	29B-Q-10	Spectrosil quartz, path length 10 mm, semi-micro, black wall
Spectrofluorometer	HORIBA Jobin Yvon	FluoroMax-3 S4
Fluorometer cell	Starna Cells	29F-Q-10	Spectrosil quartz, path length 10 mm, semi-micro