3 Bromobenzaldehyde Synthesis Essay

DOI: 10.1039/C4OB01583E (Paper) Org. Biomol. Chem., 2014, 12, 8019-8030

Synthesis of 3-benzylisoquinolines by domino imination/cycloisomerisation of 2-propargylbenzaldehydes†

Received 25th July 2014 , Accepted 2nd September 2014

First published on 3rd September 2014


An easy entry to uncommon 2-propargylbenzaldehydes was developed. 2-Propargylbenzaldehydes demonstrated to be suitable building blocks for the synthesis of 3-benzyl isoquinolines by microwave promoted domino imination/cycloisomerisation in the presence of ammonium acetate. A small library of 3-benzyl isoquinolines was obtained in good yields under mild reaction conditions. Two alternative plausible reaction mechanisms are proposed.


Introduction

During the past twenty years, many researchers have been fascinated by the development of new domino1 strategies for the synthesis of heterocyclic compounds,2 a field of research in constant evolution. A domino reaction is a process involving two or more bond-forming transformations taking place under the same reaction conditions without adding additional reagents and catalysts. The subsequent reactions are a consequence of the functionalities formed in the previous steps. The possibility of building up simple and complex heterocycles starting from easily achievable building blocks by a single sequential transformation is something deeply fascinating for all synthetic chemists.

For many years, we have been interested in the development of new sequential synthetic strategies for the construction of oxygen- or nitrogen-containing heterocycles starting from alkyne derivatives bearing a proximate carbonyl group. In particular, we have employed the addition/cycloisomerisation reactions of 2-alkynylbenzaldehydes (and their related heteroaromatic systems) in the presence of simple oxygen or nitrogen nucleophiles as useful tools to synthesise some interesting compounds such as isoquinolines,3 dihydroisobenzofurans,4 and isochromenes.5

Moreover, the domino approaches to dihydroisobenzofurans and isoquinolines have also been successfully transformed in two valuable Pd-catalysed multicomponent processes involving a one-pot coupling/addition/cyclisation sequence starting from simple building blocks, i.e., ortho-bromoarylaldehydes, terminal alkynes and a nucleophile, methanol6 or ammonia,7 respectively.

In order to extend the scope of our research, we have recently explored the reactivity of 2-alkynylacetophenones (keto-homologues of the 2-alkynylbenzaldehydes) and their pyridine analogues. We have found that the imination/annulation reactions of these less reactive substrates with ammonia needed a promoter, and silver triflate proved to be the preferred catalyst for these transformations, yielding the expected N-cyclisation products in addition to variable amounts of the isomeric carboannulation products.8

After having obtained these interesting results, we were intrigued by the idea of preparing some 2-propargylbenzaldehydes and to test their reactivity in the domino transformations described above. While the coupling of an alkynyl group in the ortho position of an ortho-haloarylaldehyde is a simple and well-known Pd-catalysed procedure, the introduction of a propargyl group in the same position is much more challenging, and only a few examples are reported in the literature.9 The most acknowledged approach was described by Eberbach and co-workers9a,b in 2000. The method includes five synthetic steps starting from the 2-bromobenzaldehyde. The first step of the approach involves the protection of the carbonyl group as a cyclic acetal, followed by a coupling reaction between the lithiated acetal derivative and the 3-bromo-1-(trimethylsilyl)-1-propyne. The triple bond is then deprotected by fluoride promoted desilylation, and functionalized by a Sonogashira coupling or by a base promoted nucleophilic substitution with methyl bromide. Finally, the acid promoted deprotection of the aldehyde gives the desired product. This is a useful and elegant method, and yet it presents some drawbacks: (1) the lithiation reaction is often a troublesome step requiring strictly controlled conditions; (2) the 3-bromo-1-(trimethylsilyl)-1-propyne is a rather expensive starting material; (3) the desilylation of the TMS-protected propargyl intermediate can experience scarce reproducibility (when fluoride promoted) or can give the isomeric allene (when base promoted).

Owing to the lack of general methods for the preparation of 2-propargyl arylaldehydes, the reactivity of these superior homologues of 2-alkynyl benzaldehydes has been less explored. For example, Eberbach and his co-workers have employed the 2-propargylbenzaldehydes as key-intermediates in the synthetic route to 2-propargylaryl nitrones, thus leading the way to the synthesis of the 1,2-dihydro[c]benzazepin-3-ones.9a,b On the other hand, Yamamoto's research group has documented the synthesis of important frameworks starting from these substrates. In 2010, they discovered an efficient metallic catalyst-free benzannulation with dialkylamines giving rise to various 2-dialkylaminonaphthalenes,10 and two years later they developed a smart synthetic approach to 2,3-dihydro-1H-inden-1-one derivatives through a Ni-catalysed intramolecular hydroacylation of 2-propargylbenzaldehydes.11

The 1-oxo-5-ynes are a class of related propargyl-based substrates, bearing a mandatory oxy group in the propargylic position. Liu and co-workers have described a highly stereoselective Au-catalysed synthesis of 9-oxabicyclo[3.3.1]nona-4,7-dienes from diverse 1-oxo-4-oxy-5-ynes.12 In 2013, a namesake of Liu (and his co-workers) proposed a new synthetic route to indeno[1,2-b]quinolones by reactions of 2-propargylbenzaldehydes with N-aryl amines based on an intramolecular aza-Diels–Alder (Povarov) reaction.13

In this work, we describe a general and effective approach to 2-propargylbenzaldehydes and their participation in microwave promoted domino addition/cycloisomerisation reactions in the presence of ammonia for the synthesis of 3-benzylisoquinolines (Scheme 1).


Scheme 1 Synthesis of 2-propargylbenzaldehydes and 3-benzylisoquinolines.

The isoquinoline nucleus is the core of various biologically active compounds, such as the alkaloid papaverine and the anaesthetic quinisocaine (Fig. 1). Saturated, functionalized and polycyclic isoquinolines show different important pharmacological properties;14 moreover, some simple 3-benzyl isoquinolines are also significantly active, and may be useful as lead compounds for developing potential chemotherapeutic agents. For example, some 1- and 3-benzylisoquinolines (e.g., 6,7-dimethoxy-3-veratryl-isoquinoline) and the corresponding quaternary salts have been tested for antimicrobial, antimalarial, cytotoxic, and anti-HIV activities.15 On top of this, the 3-benzylisoquinoline structure is the skeleton of some patented compounds, such as the PPAR-γ activity modulators16 proposed for the treatment of conditions such as type II diabetes and obesity, and the protein kinase inhibitors17 proposed for treating cancer, diabetes and Alzheimer's disease (Fig. 1).


Fig. 1 Some examples of biologically active isoquinolines.

Results and discussion

With Eberbach's seminal work in mind, we developed an alternative strategy, in order to simplify the preparation of the propargylic scaffolds. The peculiar feature of our approach is the preliminary preparation of properly substituted propargyl bromides 2: this implies avoiding the use of expensive (3-bromoprop-1-yn-1-yl)trimethylsilane and the subsequent desilylation step. Compounds 2a–h were prepared in two steps: first of all, the cheap and easily available propargyl alcohol was functionalized by reaction with various aryl iodides under standard Sonogashira cross-coupling conditions18 to give the substituted propargyl alcohols 1a–h, used in some cases for the subsequent step without the need of a chromatographic purification. The compounds 1a–h were then reacted with bromine and triphenylphosphine19 to give the desired substituted propargyl bromides 2a–h (Table 1).

Table 1Preparation of propargyl bromides



The Sonogashira coupling did not seem to be strongly influenced by the nature of substituents on the phenyl ring; in fact, the yields of the reactions were in general very good for both electron-rich and electron-poor aryl iodides (entries 1–7). The presence of a group in the ortho-position of the aryl moiety was well tolerated too (entries 6–8). Conversely, in the bromination step, the presence of stronger EWGs on the aryl moiety gave low yields (entries 4 and 5).

We then optimised the coupling step between the 2-bromobenzaldehyde partner and the substituted propargyl bromides through a Grignard reaction: this allowed us to skip the annoying lithiation step described in the Eberbach approach. We protected the 2-bromobenzaldehyde and the 5-fluoro-2-bromobenzaldehyde as cyclic acetals by treating them with ethylene glycol and p-toluenesulfonic acid in toluene at reflux. Then, a stirred solution of properly protected benzaldehyde in anhydrous tetrahydrofuran and magnesium turnings was heated at 80 °C under a protective nitrogen atmosphere. Once the magnesium was almost completely dissolved, we slowly added the suitable substituted propargyl bromides 2a–h, and the reaction mixture was heated at 80 °C to give the 2-propargylbenzaldehyde acetals 3a–i. These intermediates were then hydrolysed to give the corresponding 2-propargylbenzaldehydes 4a–i by treatment with p-toluenesulfonic acid in a mixture of water and acetone at reflux (Scheme 2).


Scheme 2 Optimised sequence for the synthesis of 2-propargylbenzaldehydes 4.

The key step of this procedure is the Grignard coupling reaction between 2-bromobenzaldehyde acetals and substituted propargyl bromides 2 (Table 2). The concentration of aryl bromide was found to be critical,20 and the best results were obtained using 1 M solution of the aryl bromide in tetrahydrofuran, while using a more diluted solution resulted in worse reproducibility and longer reaction times. The majority of the yields of the reactions ranged from fair to very good with all aryl propargyl bromides, while worse results were obtained in the presence of a methylsulfonyl group on arylalkyne terminus (entry 4), or of a fluorine atom on the benzaldehyde moiety (entry 9). In addition, the presence of a bulky group in the ortho position of the aryl iodide resulted in reduced yields (entry 8).

Table 2Grignard coupling reactions and deprotection of an aldehyde group



We also explored the Grignard coupling step under microwave heating.21 Arylmagnesium species can be efficiently generated from magnesium turnings and aryl bromides (or chlorides) under dielectric heating.20 The reactions performed under dielectric heating were faster and the yields of 3 were comparable to those observed under traditional heating (see footnote entries 1–3 and 5). In the end, however, as the microwave approach presented some reproducibility problems we decided to choose conventional heating as the standard procedure, because this enabled an easier control of the reaction progress. The dioxolane intermediates 3 were not so stable, in particular at room temperature and in the presence of a slightly acidic solvent (i.e., deuterochloroform); therefore, after a quick 1H NMR and MS characterization, they were directly hydrolysed into the corresponding aldehydes 4 with yields ranging from good to excellent.

Having optimized an effective entry to 2-propargylbenzaldehydes 4, we then explored the reactivity of these starting materials to synthesise 3-benzylisoquinolines. At first, we screened the optimal reaction conditions on the 2-propargylbenzaldehyde 4b as a model substrate in the presence of methanolic ammonia3 (Table 3).

Table 3Screening of reaction conditions with methanolic ammonia



We then ran the reaction under the conditions previously adopted for the addition/cycloisomerisation reactions of 2-alkynylbenzaldehydes (i.e., 20 equiv. of NH3).3 This uncatalysed reaction at 100 °C under microwave heating gave the corresponding isoquinoline 5b in low yield (entry 1). A tentative approach to promote the formation of the imine intermediate by the use of molecular sieves did not result in any improvement (entry 2). Based on our previous experience with 2-alkynylacetophenones,8 we planned to catalyse the reaction with AgOTf (10 mol%) both under dielectric and conventional heating conditions (entries 3–5). Under conventional heating the reaction yield raised to 51% in 6 h at 60 °C (entry 4), while under microwave heating at 100 °C the reaction gave almost the same yield in only 10 minutes (entry 5). Aiming to improve the yield, we tried some other metal catalysts potentially able to promote the reaction. Following the procedure previously optimised in our laboratory for the imination/annulation of 2-acetyl and 2-benzoyl N-propargylindoles22 and 2-acetyl-N-propargylpyrroles,3 we tried to catalyse the reaction with 0.5 equiv. of titanium tetrachloride, but these conditions gave poor yields (entry 6), while in the presence of a catalytic amount of TiCl2(indenyl)2 the desired product was obtained in 47% yield (entry 7). Using InCl3 as the catalyst gave comparable yield to the best results obtained in this screening (cf. entries 8, 4 and 5). We also tested some gold-based catalysts, well renowned as an alkynophilic Lewis acid, but the results were still unsatisfactory (entries 9–11).23

The homogeneous – but not completely satisfactory – results obtained under metal catalysis suggested that the tricky point of the approach was not the activation of the triple bond but, more probably, the nature of the nitrogen partner: this prompted us to try different ammonia sources.

In the existing literature, several ammonium salts are listed as alternative ammonia sources (e.g., (NH4)2CO3, NH4HCO3, HCO2NH4), and NH4OAc proved to be the most effective among them.24 Ammonium acetate is the salt of a weak acid (acetic acid) and a weak base (ammonia), and therefore it is easily decomposed by heat to AcOH and NH3. On top of this, ammonium acetate is a practical choice because it is an inexpensive and easy-to-handle solid. It has been employed only two times in the isoquinoline synthesis, that is in the cyclisation of 2-(1,1-difluoroalkenyl)-benzaldehyde25 and, more recently, in a Pd-catalysed multicomponent approach starting from 2-bromobenzaldehydes and terminal alkynes.26 However, the reactivity of the 2-propargylbenzaldehydes with ammonium acetate is unprecedented.

For this reason, we screened the optimal reaction conditions for the synthesis of 3-benzylisoquinolines with ammonium acetate as the ammonia source, and the results are shown in Table 4.

Table 4Screening of reaction conditions with ammonium acetate



It has been reported that a large excess of NH4OAc is necessary for an effective ammonia generation,27 so the first experiment was performed with 20 equiv. of NH4OAc in ethanol at 120 °C under microwave irradiation. After 30 minutes, the reaction gave the desired product 5b in a promising 68% yield (entry 1). Then, we tried the reaction in DMSO27b and we were pleased to observe a good rise in yield (entry 2). The reduction of the amount of ammonium acetate to 10 equiv. gave only a modest reduction of the reaction yield (entry 3). Conversely, an excellent rise in yields was observed when we lowered the temperature (entry 4). Aiming to make the approach more environmentally-friendly, we tried to use water as the solvent (entry 5), but all our attempts were unsuccessful, also in the presence of EtOH or DMSO as co-solvents (entries 6 and 7).

Working under the best available conditions, we investigated the scope and the limitation of the approach. The results are shown in Table 5.

Table 5Scope and limitation of the approach



This microwave-enhanced protocol proved to be a general route for the synthesis of an array of isoquinolines in very good yields (entries 1–9). Both electron-donating and electron-withdrawing substituents were allowed on the phenyl group at alkynyl terminus (entries 1–5). The presence of a bulky substituent in the ortho position of the arylalkyne moiety did not seem to affect the reaction course (entries 6–8). Moreover, the presence of a fluorine group on the benzaldehyde moiety was also well tolerated (entry 9). All the reactions were clean and complete within 30 minutes, and the final products were easily and quickly purified by flash column chromatography. All the products have been fully characterized by 1H NMR and 13C NMR spectroscopy, as well as by MS spectrometry.

In accordance with our previous findings,28 we proposed a tentative reaction mechanism in which two different pathways are feasible (Scheme 3). The first step, the in situ formation of an imine intermediate (I) by reaction of 2-propargylbenzaldehydes 4 with the ammonia obtained by thermal cleavage of ammonium acetate, is common to both paths. To explain the cycloisomerisation step there are two conceivable pathways: (a) the triple bond undergoes a 6-exo-dig cyclisation directly by the imine, and the subsequent isomerisation leads to the formation of the final product 5; (b) the reaction conditions (heat and ammonia) promote the isomerisation of the triple bond to allene,29 thus allowing the intramolecular attack of the nucleophile on the central carbon of the allene framework30 with direct formation of the isoquinoline 5. On top of this, the regiospecificity observed is probably due to the resonance stabilization of the six-membered cyclisation product with respect to the alterative seven-membered one. As proof of that, the 7-endo-dig cyclisation mode is quite uncommon in the literature (c).31


Scheme 3 Proposed reaction mechanism.

Conclusions

We have developed a new, easy and effective access to neglected 2-propargylbenzaldehydes. These building blocks proved to be suitable substrates for domino addition/cycloisomerisation reactions in the presence of a nitrogen nucleophile. The synthesis of the 3-benzylisoquinoline nucleus was achieved in an easy way and in very good yields by a microwave enhanced methodology. Ammonium acetate proved to be the most effective nitrogen source for this purpose. This approach represents a useful, simple and unprecedented alternative access to the 3-benzylisoquinoline skeleton, which is an important framework in a number of different molecules of biological interest.

Experimental

General experimental details

All the reactions that involve the use of reagents sensitive to oxygen or hydrolysis were carried out under an inert atmosphere. The glassware was previously dried in an oven at 110 °C and set with cycles of vacuum and nitrogen. Also syringes, used to transfer reagents and solvents, were previously set under a nitrogen atmosphere. Some solvents, used for reactions sensitive to oxygen and hydrolysis, were distilled and stored under a protected atmosphere of nitrogen, according to the following standard operations: dichloromethane: distilled on CaCl2 and placed on 4 Å sieves into a recycling appliance. Anhydrous THF, DMSO and ethanol are commercially available. The chromatographic column separations were conducted by the flash technique, using silica gel Davisil LC 60 Å (230–400 mesh). For thin-layer chromatography (TLC), silica gel 60 F254 FLUKA thin-layer plates were employed and the detection was performed by irradiation with UV light (λ = 254 nm and/or 366 nm). 1H NMR analyses were performed with a Varian-Gemini 200 at 200 MHz at room temperature. The coupling constants (J) are expressed in hertz (Hz) and the chemical shifts (δ) in ppm. The multiplicity of the proton spectra was described by the following abbreviations: s (singlet), d (doublet), t (triplet), q (quartet), dt (double triplet), dd (double doublet), ddd (double double doublet), m (multiplet). 13C NMR analyses were performed with the same instruments at 50.3 MHz; an APT sequence was used to distinguish the methine and methyl carbon signals from those arising from methylene and quaternary carbon atoms. Infrared spectra were recorded with a Perkin Elmer FT-IR 16 PC spectrometer, using discs of NaCl for liquid samples and KBr tablets for solid samples. The absorbance is reported in wavenumbers (cm−1) with values between 4000 and 400 cm−1. Low resolution MS spectra were recorded with a Fisons MD 800 spectrometer with an electron impact source and a Thermo-Finnigan LCQ-advantage AP electrospray/ion trap equipped instrument, using a syringe pump device to directly inject sample solutions. The values are reported as the mass–charge ratio and the relative intensities of the most significant peaks are shown in brackets. High resolution MS spectra were recorded with an instrument equipped with an electrospray source and an ICR-FTMS analyser. The melting points of the solid products were measured with a Stuart Scientific SMP3 apparatus and are uncorrected. Microwave promoted reactions were performed with a single-mode Personal Chemistry microwave synthesizer “Emrys Creator,” using sealed glass vessels. The temperature was detected with an infrared sensor.

General procedure for the synthesis of aryl propargyl alcohols (1a–h)

Under a nitrogen atmosphere, to a solution of the appropriate aryl iodide (6.00 mmol) in DEA (18 mL), propargyl alcohol (403.8 mg, 0.419 mL, 7.20 mmol) and trans-dichlorobis(triphenylphosphine)palladium(II) (84.3 mg, 0.120 mmol) were added. The reaction was stirred at room temperature for 15 min, and then CuI (12.0 mg, 0.063 mmol) was added. The reaction mixture was stirred at 50 °C until no starting product was detectable by TLC analysis (eluent: hexane–ethyl acetate = 95:5). After cooling to room temperature, the mixture was concentrated under reduced pressure, poured in a HCl 0.1 N aq. solution (60 mL) and extracted with diethyl ether (3 × 40 mL). The organic layers were dried over Na2SO4 and the solvent was removed under reduced pressure. Unless otherwise stated, the compounds were used without the need for chromatographic purification.

3-Phenylprop-2-yn-1-ol (1a). Reaction time: 1 h. Yield: 785 mg (99%). Brown oil. 1H NMR (CDCl3, 200 MHz): δ = 2.95 (bs, 1H, OH), 4.50 (s, 2H, CH2), 7.29–7.33 (m, 3H, HAr), 7.43–7.45 (m, 2H, HAr). These data are in good agreement with literature values.32

3-(4-Chlorophenyl)prop-2-yn-1-ol (1b). Reaction time: 1.5 h. Yield: 989 mg (99%). Brown solid. Mp 74–77 °C (lit. 76–78 °C). 1H NMR (CDCl3, 200 MHz): δ = 2.20 (bs, 1H, OH), 4.49 (s, 2H, CH2), 7.28 (d, J = 8.5 Hz, 2H, HAr), 7.36 (d, J = 8.5 Hz, 2H, HAr). These data are in good agreement with literature values.33

3-(p-Tolyl)prop-2-yn-1-ol (1c). Reaction time: 2 h. Yield: 868 mg (99%). Yellow wax. 1H NMR (200 MHz, CDCl3): δ = 2.50 (bs, 1H, OH), 2.34 (s, 3H, CH3), 4.48 (s, 2H, CH2), 7.10 (d, J = 8.1 Hz, 2H, HAr), 7.33 (d, J = 8.1 Hz, 2H, HAr). These data are in good agreement with literature values.34

3-(4-(Methylsulfonyl)phenyl)prop-2-yn-1-ol (1d). Reaction time: 3 h. Eluent for chromatography: hexane–EtOAc (5:5). Yield: 971 mg (77%). Light brown solid. Mp 81–82 °C. 1H NMR (200 MHz, CDCl3): δ = 2.41 (bs, 1H, OH), 3.04 (s, 3H, CH3), 4.49 (s, 2H, CH2), 7.53 (d, J = 8.1 Hz, 2H, HAr), 7.85 (d, J = 8.1 Hz, 2H, HAr). 13C NMR (50.3 MHz, CDCl3): δ = 44.6 (CH3), 51.5 (CH2), 83.9 (Csp), 91.9 (Csp), 127.5 (CHAr), 128.8 (Cq), 132.6 (CHAr), 140.0 (Cq). ESI-MS m/z (%): 211.2 (45) [M + H]+, 233.1 (60) [M + Na]+, 242 (100) [M + CH3OH + H]+. HRMS ESI [M + H]+ calcd for C10H11O3S 211.0423, found 211.424.

3-(3-(Trifluoromethyl)phenyl)prop-2-yn-1-ol (1e). Reaction time: 2 h. Yield: 1.19 g (99%). Light yellow oil. 1H NMR (CDCl3, 200 MHz): δ = 2.03 (s, 1H, OH), 4.51 (s, 2H, CH2), 7.39–7.46 (m, 1H, HAr), 7.52–7.59 (m, 2H, HAr), 7.68 (s, 1H, HAr). 13C NMR (50.3 MHz, CDCl3): δ = 51.7 (CH2), 84.4 (Csp), 89.1 (Csp), 123.8 (Cq), 123.9 (q, 1JC–F = 272.0 Hz, CF3), 125.2 (q, 3JC–F = 3.8 Hz, CHAr), 128.7 (q, 3JC–F = 3.8 Hz, CHAr), 129.0 (CHAr), 131.2 (q, 2JC–F = 33.0 Hz, Cq), 134.9 (CHAr). ESI-MS m/z (%): 223.1 (100) [M + Na]+. HRMS ESI [M + H]+ calcd for C10H8F3O 201.0522, found 201.0520.

3-(2-Ethylphenyl)prop-2-yn-1-ol (1f). Reaction time: 3 h. Eluent for chromatography: hexane–EtOAc (9:1). Yield: 836 mg (87%). Light yellow oil. 1H NMR (200 MHz, CDCl3): δ = 1.24 (t, J = 7.5 Hz, 3H, CH3), 1.81 (bs, 1H, OH), 2.80 (q, J = 7.5 Hz, 2H, CH2), 4.54 (s, 2H, CH2), 7.09–7.31 (m, 3H, HAr), 7.41 (d, J = 7.5 Hz, 1H, HAr). 13C NMR (50.3 MHz, CDCl3): δ = 15.0 (CH3), 27.8 (CH2), 52.0 (CH2), 84.6 (Csp), 90.8 (Csp), 121.8 (Cq), 125.8 (CHAr), 128.1 (CHAr), 128.9 (CHAr), 132.6 (CHAr), 146.5 (Cq). ESI-MS m/z (%): 301 (52) [dimer − H2 − H2O + H]+, 303 (32) [dimer − H2O + H]+, 339 (100) [dimer + H2O + H]+. HRMS ESI [M + H]+ calcd for C11H13O 161.0961, found 161.0959.
3-(3-Chloro-2-methylphenyl)prop-2-yn-1-ol (1g). Reaction time: 3 h. Eluent for chromatography: hexane–EtOAc (9:1). Yield: 1.07 g (99%). Light brown solid. Mp: 54–59 °C. 1H NMR (200 MHz, CDCl3): δ = 1.71 (bs, 1H, OH), 2.50 (s, 3H, CH3), 4.54 (s, 2H, CH2), 7.06 (t, J = 7.9 Hz, 1H, HAr), 7.32 (d, J = 7.9 Hz, 2H, HAr). 13C NMR (50.3 MHz, CDCl3): δ = 18.5 (CH3), 51.8 (CH2), 84.2 (Csp), 91.9 (Csp), 124.5 (Cq), 126.7 (CHAr), 129.7 (CHAr), 130.9 (CHAr), 134.9 (Cq), 138.3 (Cq). ESI-MS m/z (%): 145.1 (50) [M − Cl]+, 164.8 (100) [M − H2O + H]+, 180.9 (75) [M + H]+. HRMS ESI [M + H]+ calcd for C10H10ClO 181.0415, found 181.0413.
3-(2-Isopropylphenyl)prop-2-yn-1-ol (1h). Reaction time: 2 h. Eluent for chromatography: hexane–EtOAc (9:1). Yield: 994 mg (95%). Brown oil. 1H NMR (200 MHz, CDCl3): δ = 1.25 (d, J = 7.0 Hz, 6H, CH3), 3.44 (sept, J = 7.0 Hz, 1H, CH), 4.54 (s, 2H, CH2), 7.08–7.17 (m, 1H, HAr), 7.26–7.30 (m, 2H, HAr), 7.41 (d, J = 7.3 Hz, 1H, HAr). 13C NMR (50.3 MHz, CDCl3): δ = 23.4 (CH3), 31.7 (CH), 51.9 (CH2), 84.6 (Csp), 91.1 (Csp), 121.5 (Cq), 125.2 (CHAr), 125.7 (CHAr), 129.1 (CHAr), 132.8 (CHAr), 150.8 (Cq). ESI-MS m/z (%): 174.2 (10) [M]+, 234.4 (100) [M + AcOH]+. HRMS ESI [M + H]+ calcd for C12H15O 175.1117, found 175.1119.

General procedure for the synthesis of aryl propargyl bromides (2a–h)

Triphenylphosphine (3.93 g, 15.0 mmol) was dissolved in dichloromethane (40 mL). Bromine (2.40 g, 15.0 mmol) was then added dropwise at 0 °C and stirred for 30 min. The appropriate functionalized propargyl alcohols 1a–h (12.5 mmol) were added at 0 °C and the reaction mixture was stirred until no starting product was detectable by TLC analysis (eluent: hexane–ethyl acetate = 9:1). Hexane (130 mL) was added to precipitate the phosphine oxide and the white suspension was passed through a short silica pad (3 cm diameter × 2 cm height) and washed with hexane. The crude product was freed from solvents under reduced pressure. Unless otherwise stated, the compounds were used without the need of chromatographic purification.

(3-Bromoprop-1-yn-1-yl)benzene (2a). Reaction time: 2 h. Yield: 1.97 g (81%). Light yellow oil. 1H NMR (200 MHz, CDCl3): δ = 4.17 (s, 2H, CH2), 7.31–7.36 (m, 3H, HAr), 7.43–7.48 (m, 2H, HAr). These data are in good agreement with literature values.35

1-(3-Bromoprop-1-yn-1-yl)-4-chlorobenzene (2b). Reaction time: 2 h. Yield: 2.58 g (90%). Colourless oil. 1H NMR (200 MHz, CDCl3): δ = 4.14 (s, 2H, CH2), 7.29 (d, J = 8.6 Hz, 2H, HAr), 7.37 (d, J = 8.6 Hz, 2H, HAr). These data are in good agreement with literature values.36

1-(3-Bromoprop-1-yn-1-yl)-4-methylbenzene (2c). Reaction time: 2 h. Yield: 2.59 g (99%). Light yellow oil. 1H NMR (200 MHz, CDCl3): δ = 2.35 (s, 3H, CH3), 4.17 (s, 2H, CH2), 7.12 (d, J = 7.8 Hz, 2H, HAr) 7.34 (d, J = 7.8 Hz, 2H, HAr). These data are in good agreement with literature values.37

1-(3-Bromoprop-1-yn-1-yl)-4-methylsulfonylbenzene (2d). Reaction time: 2 h. Eluent for chromatography: hexane–EtOAc (6:4). Yield: 1.94 g (57%). Brown solid. Mp: 98–102 °C. 1H NMR (200 MHz, CDCl3): δ = 3.05 (s, 3H, CH3), 4.15 (s, 2H, CH2), 7.60 (d, J = 8.6 Hz, 2H, HAr), 7.88 (d, J = 8.6 Hz, 2H, HAr). 13C NMR (50.3 MHz, CDCl3): δ = 14.4 (CH2), 44.6 (CH3), 84.8 (Csp), 88.4 (Csp), 127.6 (CHAr), 128.2 (Cq), 132.8 (CHAr), 140.6 (Cq). ESI-MS m/z (%): 217.3 (100) [M − Br + Na]+, 273.2/275.1 (18) [M + H]+, 295.0/297.1 (31) [M + Na]+. HRMS ESI [M + H]+ calcd for C10H10BrO2S 272.9579, found 272.9581.

1-(3-Bromoprop-1-yn-1-yl)-3-(trifluoromethyl)benzene (2e). Reaction time: 2.5 h. Yield: 1.94 g (59%). Dark yellow oil. 1H NMR (200 MHz, CDCl3): δ = 4.15 (s, 2H, CH2), 7.39–7.50 (m, 1H, HAr), 7.54–7.65 (m, 2H, HAr), 7.70 (s, 1H, HAr). 13C NMR (50.3 MHz, CDCl3): δ = 14.6 (CH2), 85.2 (Csp), 86.0 (Csp), 123.4 (Cq), 123.8 (q, 1JC–F = 272.8 Hz, CF3), 125.6 (q, 3JC–F = 3.8 Hz, CHAr), 128.9 (q, 3JC–F = 3.8 Hz, CHAr), 129.1 (CHAr), 131.3 (q, 2JC–F = 33.0 Hz, Cq), 135.1 (CHAr). ESI-MS m/z (%): 206.4 (100) [M − Br + Na]+. HRMS ESI [M + H]+ calcd for C10H7BrF3 262.9678, found 262.9681.

1-(3-Bromoprop-1-yn-1-yl)-2-ethylbenzene (2f). Reaction time: 2.5 h. Eluent for chromatography: hexane–EtOAc (9:1). Yield: 1.90 g (68%). Light yellow oil. 1H NMR (200 MHz, CDCl3): δ = 1.25 (t, J = 7.6 Hz, 3H, CH3), 2.80 (q, J = 7.6 Hz, 2H, CH2), 4.21 (s, 2H, CH2), 7.07–7.34 (m, 3H, HAr), 7.41 (dd, J = 7.4, 1.1 Hz, 1H, HAr). 13C NMR (50.3 MHz, CDCl3): δ = 15.1 (CH3), 15.7 (CH2), 27.8 (CH2), 86.0 (Csp), 87.7 (Csp), 121.5 (Cq), 125.8 (CHAr), 128.2 (CHAr), 129.3 (CHAr), 132.7 (CHAr), 147.0 (Cq). ESI-MS m/z (%): 143.1 (75) [M − Br]+, 447 (100) [dimer + H]+. HRMS ESI [M + H]+ calcd for C11H12Br 223.0117, found 223.0120.

1-(3-Bromoprop-1-yn-1-yl)-3-chloro-2-methylbenzene (2g). Reaction time: 2.5 h. Yield: 2.47 g (81%). Yellow solid. Mp: 60–65 °C. 1H NMR (200 MHz, CDCl3): δ = 2.50 (s, 3H, CH3), 4.19 (s, 2H, CH2), 7.07 (t, J = 7.6 Hz, 1H, HAr), 7.32 (d, J = 7.6 Hz, 2H, HAr). 13C NMR (50.3 MHz, CDCl3): δ = 15.1 (CH2), 18.4 (CH3), 85.3 (Csp), 88.7 (Csp), 124.1 (Cq), 126.7 (CHAr), 130.0 (CHAr), 130.9 (CHAr), 135.0 (Cq), 138.7 (Cq). ESI-MS m/z (%): 649.5 (100) [trimer − Br + H]+. HRMS ESI [M + H]+ calcd for C10H9BrCl 242.9571, found 242.9576.

1-(3-Bromoprop-1-yn-1-yl)-2-isopropylbenzene (2h). Reaction time: 2 h. Eluent for chromatography: hexane–EtOAc (9:1). Yield: 2.52 g (85%). Light brown oil. 1H NMR (200 MHz, CDCl3): δ = 1.26 (d, J = 7.0 Hz, 6H, CH3), 3.41 (sept, J = 7.0 Hz, 1H, CH), 4.21 (s, 2H, CH2), 7.09–7.17 (m, 1H, HAr), 7.26–7.32 (m, 2H, HAr), 7.40 (d, J = 7.3 Hz, 1H, HAr).13C NMR (50.3 MHz, CDCl3): δ = 15.8 (CH2), 23.3 (CH3), 31.8 (CH), 86.0 (Csp), 87.9 (Csp), 121.1 (Cq), 125.2 (CHAr), 125.7 (CHAr), 129.4 (CHAr), 132.8 (CHAr), 151.3 (Cq). ESI-MS m/z (%): 157.4 (100) [M − Br]+. HRMS ESI [M + H]+ calcd for C12H14Br 237.0273, found 237.0277.

General procedure for the synthesis of 2-(2-bromophenyl)-1,3-dioxolanes

Ethylene glycol (1.80 g, 1.62 mL, 29.0 mmol) and p-toluenesulfonic acid·H2O (50 mg, 0.26 mmol) were added to a solution of 2-bromobenzaldehyde or 2-bromo-5-fluorobenzaldehyde (14.5 mmol) in toluene (40 mL). The mixture was heated at reflux in a Dean–Stark apparatus for 3 hours. After cooling to room temperature, the mixture was washed with a satd aq. NaHCO3 solution (40 mL) and with brine (40 mL). The organic phase was dried over Na2SO4 and the solvent was removed at reduced pressure. The residue was purified by flash column chromatography, affording the desired dioxolane derivatives.
2-(2-Bromophenyl)-1,3-dioxolane. Eluent for chromatography: hexane–EtOAc (95:5). Yield: 3.02 g (91%). Colourless oil. 1H NMR (200 MHz, CDCl3): δ = 4.05–4.10 (m, 2H, O-CH2), 4.13–4.18 (m, 2H, O-CH2), 6.10 (s, 1H, CH), 7.20 (dt, J = 7.3, 1.8 Hz, 1H, HAr), 7.34 (dt, J = 7.5, 1.4 Hz, 1H, HAr), 7.58 (dt, J = 7.5, 1.6 Hz, 2H, HAr). These data are in good agreement with literature values.38
2-(2-Bromo-5-fluorophenyl)-1,3-dioxolane. Eluent for chromatography: hexane–EtOAc (9:1). Yield: 2.83 g (79%). Colourless oil. 1H NMR (200 MHz, CDCl3): δ = 4.03–4.10 (m, 2H, O-CH2), 4.13–4.20 (m, 2H, O-CH2), 6.04 (s, 1H, CH), 6.95 (tdd, J = 8.5, 3.1, 0.6 Hz, 1H, HAr), 7.33 (dd, J = 3.1, 9.3 Hz, 1H, HAr) 7.52 (dd, J = 5.1, 8.7 Hz, 1H, HAr). 13C NMR (50.3 MHz, CDCl3): δ = 65.8 (CH2), 102.2 (d, 4JC–F = 1.1 Hz, CH), 115.3 (d, 2JC–F = 24.4 Hz, CHAr), 117.0 (d, 4JC–F = 3.0 Hz, Cq), 117.9 (d,2JC–F = 22.9 Hz, CHAr), 134.5 (d, 3JC–F = 7.6 Hz, CHAr), 139.2 (d, 3JC–F = 6.9 Hz, Cq), 162.2 (d, 1JC–F = 247 Hz, C–F). ESI-MS m/z (%): 247.2/245.2 (100) [M]+. HRMS ESI [M + H]+ calcd for C9H9BrFO2 246.9764, found 246.9761.

General procedure for the Grignard coupling reaction (3a–i)

Mg turnings were activated by trituration in a mortar. Mg turnings (53 mg, 2.2 mmol) were charged in a reaction flask under a nitrogen atmosphere. An appropriate amount of 1,3-dioxolane derivative (2.0 mmol) was dissolved in THF (2 mL) and added to Mg turnings. The mixture was stirred for 1 h at 80 °C under a nitrogen atmosphere until the Mg was almost completely dissolved. Then the propargyl bromides (2a–h) (2.0 mmol) were added and the reaction was stirred at 80 °C until no starting product was detectable by TLC analysis. After cooling to rt, the reaction mixture was poured in a saturated ammonium chloride solution (30 mL) and extracted with diethyl ether (3 × 20 mL). The organic layers were washed with brine (40 mL) and dried over Na2SO4. The solvent was evaporated under reduced pressure and the crude was quickly purified by flash chromatography on a short silica gel column. The dioxolane intermediates were not very stable (in particular at rt or in a slightly acidic solvent such as deuterochloroform), so after 1H NMR and MS characterization they were immediately converted into the corresponding aldehydes.
2-(2-(3-Phenylprop-2-yn-1-yl)phenyl)-1,3-dioxolane (3a). Reaction time: 3 h. Eluent for chromatography: hexane–EtOAc (99:1). Yield: 0.32 g (61%). Light brown oil. 1H NMR (200 MHz, CDCl3): δ = 4.01 (s, 2H, CH2), 4.04–4.10 (m, 2H, O-CH2), 4.11–4.17 (m, 2H, O-CH2), 6.07 (s, 1H, CH), 7.22–7.50 (m, 7H, HAr), 7.53–7.71 (m, 2H, HAr). These data are in good agreement with literature values.9a
2-(2-(3-(4-Chlorophenyl)prop-2-yn-1-yl)phenyl)-1,3-dioxolane (3b). Reaction time: 3 h. Eluent for chromatography: hexane–EtOAc (99:1). Yield: 0.48 g (81%). Light yellow oil. 1H NMR (200 MHz, CDCl3): δ = 3.99 (s, 2H, CH2), 4.05–4.09 (m, 2H, O-CH2), 4.12–4.16 (m, 2H, O-CH2), 6.05 (s, 1H, CH), 7.24–7.38 (m, 6H, HAr), 7.61 (m, 2H, HAr). ESI-MS m/z (%): 299.2 (100) [M + H]+.
2-(2-(3-(p-Tolyl)prop-2-yn-1-yl)phenyl)-1,3-dioxolane (3c). Reaction time: 3 h. Eluent for chromatography: hexane–EtOAc (99:1). Yield: 0.39 g (68%). Dark yellow oil. 1H NMR (200 MHz, CDCl3): δ = 2.34 (s, 3H, CH3), 3.99 (s, 2H, CH2), 4.04–4.10 (m, 2H, O-CH2), 4.17–4.10 (m, 2H, O-CH2), 6.07 (s, 1H, CH), 7.06–7.15 (m, 2H, HAr), 7.27–7.43 (m, 4H, HAr), 7.52–7.69 (m, 2H, HAr). ESI-MS m/z (%): 279.3 (100) [M + H]+. These data are in good agreement with literature values.9a

Entry1, 2Rt (h) step 11 Yielda (%)t (h) step 22 Yielda (%)
1aH1.099b2.081d
2b4-Cl1.599b2.090d
3c4-CH32.099b2.099d
4d4-SO2CH33.077c2.057c
5e3-CF32.099b2.559d
6f2-C2H53.087c2.568c
7g2-CH3, 3-Cl3.099c2.581d
8h2-(i-Pr)2.095c2.085c
Entry3, 4R1R2t (h) step 13 Yielda (%)t (h) step 24 Yieldf (%)
1aHH3.061b2.085
2bH4-Cl3.081c2.094
3cH4-CH33.068d2.096
4dH4-SO2CH34.0322.082
5eH3-CF34.076e2.078
6fH2-C2H54.0792.089
7gH2-CH3, 3-Cl4.0773.089
8hH2-(i-Pr)3.0382.582
9iF4-Cl5.5543.078

EntryCatalyst (mol%)Energy – T (°C)t (min)5b Yield (%)
1μW – 1001036a
24 Å mol. sievesμW – 1002014a
3AgOTf (10)μW – 1001041b
4AgOTf (10)Oil bath – 6036051b
5AgOTf (10)μW – 1001048a
6TiCl4 (50)μW – 1001034a
7TiCl2(Indenyl)2 (5)μW – 1001047a
8InCl3 (10)μW – 1002049a
9NaAuCl4 (5)μW – 804036a
10AuCl3 (5)μW – 1005046a
11Au(PPh3)NTf2 (3)μW – 1001042b

Entry4, 5R1Ar5 Yielda (%)
1aHPh-78
2bH4-Cl-Ph-93
3cH4-CH3-Ph-74
4dH4-SO2CH3-Ph-67
5eH3-CF3-Ph-81
6fH2-C2H5-Ph-71
7gH(2-CH3, 3-Cl)-Ph-83
8hH2-(i-Pr)-Ph-76
9iF4-Cl-Ph-71

1. Introduction

Schiff bases as nitrogen unsaturated organic compounds are versatile ligands which have received a great deal of attention in many area of research [1,2], as corrosion inhibitors, pigments and dyes, and small-molecule catalysts activators and in biological activities [2,3,4,5,6]. The Schiff base, structurally known as azomethine or imine, is a nitrogen analogue of ketone or aldehyde in which the carbonyl group has been replaced by an azomethine group after water molecule elimination [3]. Several azomethines possess remarkable antibacterial, antifungal, anticancer and diuretic activities. The lone pair of electrons belongs to the nitrogen atom of azomethine which is expected to be involved in the formation of an H-bond with the active centers of the target cell constituents and interferes in normal cell function [4].

Due to the presence of >C=N-, an electron-rich functional group (π and N-free lone pair electrons), Schiff bases, as ligands, have played a very important role in the development or building of new metal ion complexes [7,8,9]. Despite the huge number of organic compounds, there is always an urgent need to develop new chelated ligands and enhance their properties as well as their applications.

In connection with previous research that belongs to our group on the synthesis, characterization, complexation and biological application of mono- or polydentate Schiff bases [6,7,8,9,10,11], in this work N-[(1E)-(3-bromophenyl)methylene]-N-(2-piperidin-1-ylethyl)amine was synthesized and characterized by various spectral techniques. The reaction was monitored by IR. The entitled compound showed promising antibacterial activities, and further future works will be carried out.

2. Results and Discussion

N-[(1E)-(3-bromophenyl)methylene]-N-(2-piperidin-1-ylethyl)amine was synthesized by condensing equimolar amounts of 2-(piperidin-1-yl)ethanamine and 3-bromobenzaldehyde in absolute ethanol under reflux conditions for 4 h, as shown in Scheme 1. The product is colorless, naturally oily, completely soluble in dichloromethane and partially soluble in alcohols, and insoluble in water and non-polar solvents such as n-hexane.

EI-MS of the compound is in good agreement with the assigned structure and showed the experimental molecular ion [M+] m/z = 295.0 (295.2 theoretical).

The thermal properties of the TG/DTA of the title compound were investigated under an open atmosphere in the temperature range of 0 °C–250 °C and a heating rate of 10 °C/min. Figure 1 showed that one broad step of typical decomposition started from 98 °C and ended at 135 °C, with exothermic DTA = 128 °C signal and weight loss of ∼99% (no intermediate decomposition steps).

We have monitored the reaction by using FT-IR spectroscopy. The starting materials were followed up before and after its condensation to produce the desired compound by IR measurement, as seen in Figure 2. The formation of the product was confirmed as follows: the aldehyde C=O (at 1690 cm−1) shifted to C=N (at 1645 cm−1) as seen in Figure 2a,c. The disappearance of the N-H amine and C-H aldehyde of stretching vibrations at 3266 and 3110 cm−1, respectively, supported the success of the condensation of the starting materials to form the desired product, as seen in Figure 2.

The electronic absorption spectrum of the prepared compound was acquired in CH2Cl2; Figure 3 shows the electronic absorption as expected (in the UV region only) at λmax = 215 and 250 nm which are assigned mainly to intra-ligand π-π* transitions.

The 1H-NMR spectrum data is in a good agreement with its assigned structure; the signals of the aromatic, aliphatic and amide function groups are cited to their positions. The 1H-NMR spectrum of the compound is shown in Figure 4. The 1H-NMR spectrum showed multiple singlets at δ 1.21, 2.65, 2.76, 2.90 and δ 3.76 ppm due to aliphatic protons. The four aromatic protons resonated as a multiplet in the region of δ 7.2–7.9 ppm, and the azomethine proton resonated as a singlet at δ 8.28 ppm.

The 13C-NMR spectrum revealed several sets of signals: six signals were cited to aromatic carbons in the region of ~120–140 ppm, five types of aliphatic carbons were cited to the region of ~45–90 ppm and the azomethine carbon was cited to 161.3 ppm, as seen in Figure 5. Figure 5 showed the chemical shifts of all the carbons together with their numbers.

3. Experimental Section

The UV-visible spectrum was measured on TU-1901 double-beam UV-visible spectrophotometer. The IR spectra for samples were recorded by PerkinElmer Spectrum 1000 FT-IR Spectrometer (PerkinElmer Inc., Waltham, MA, USA). High-resolution NMR was recorded on Bruker DRX 500 spectrometer (Bruker, Mainz, Germany) (1H, 500 MHz frequency, 13C, 125 MHz) at 298 K. EI-MS data was obtained on a Finnigan 711A (8 kV) (PerkinElmer Inc.). TG spectrum was measured by using a TGA-7 PerkinElmer thermogravimetric analyzer (PerkinElmer Inc.).

A solution of 3-bromobenzaldehyde (2 mmol) in EtOH (30 mL) was mixed with 2-(piperidin-1-yl)ethanamine (2.1 mmol) and allowed to reflux under stirring for 4 h. The resulting mixture was concentrated under reduced pressure and the title compound was precipitated as oil upon addition of 100 mL of n-hexane. The oily product was washed three times with (80 mL) of distilled water.

The elemental analysis of the compound is consistent with the proposed molecular formula (Calcd. for C14H19BrN2: C, 56.96; H, 6.49; Br, 27.07; N, 9.49. Found: C, 56.85; H, 6.31; Br, 26.88; N, 9.45).

Yield 85%, at RT the product is colorless oil; Molecular formula C14H19BrN2; IR: 3020 cm−1C–H Ph, 2970–2780 cm−1C-H aliphatic, 1645 cm−1C=N. 1H-NMR (500 MHz, CDCl3): δ (ppm) 1.21 (m, 2H, NCH2CH2CH2), 2.65 (m, 4H, NCH2CH2CH2), 2.76 (2m, 4H, NCH2CH2CH2), 2.90 (m, 2H, =NCH2CH2N), 3.76 (m, 2H, =NCH2CH2N), 7.0–8.0 (4m, 4H, Ph), 8.28 (s, 1H, HC=N). 13C-NMR (125 MHz, CDCl3): δ (ppm) 46.1 (CH2, 1C, C3), 53.2 (CH2, 2C, C2), 56.1 (CH2, 2C, C1), 59.1 (CH2N, 1C, C4), 87.2 (CH2N, 1C, C5), 122.1 (CHBr, 1C, C11), 126.4 (CH, 1C, C9), 129.2 (CH, 1C, C12), 130.5 (CH, 1C, C8), 46.1 (CH, 1C, C10), 46.1 (C, 1C, C7), 161.3 (CHN, 1C, C6). [M+] = 295.0 m/z.

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