Indian Journal of Chemistry Vol. 40B. December 200 1 , pp. 1 258- 1 263

Note

Montmorillonite KIO clay catalysed Baeyer condensation of heterocyclic aldehydes with

N,N-dimethylaniline: Synthesis and photo irradiation studies of heteroaryldiaryl­

methanest

Ponnusamy Shanmuga* & Luxmi Varma Organic Chemistry Division, Regional Research

Laboratory(CSIR), Trivandrum 695 019, Kerala, India

Received 9 February 2000; accepted (revised) 6 June 2000

The montmorillonitemK I O clay catalysed Baeyer condensation of a number of aromatic heterocyclic aldehydes (1-7) with N,N­dimethylaniline affords novel triarylmethanes (8-14) in good yield. The triarylmethane 8 was irradiated at 325 nm for 1 hr under different solvent systems and the photoproducts were identified as 15 and 16.

The acid catalysed B aeyer condensation of aromatic aldehydes with N,N-dimethylaniline gives triarylmethane, commonly known as · leuco bases, which are important intermediates used in the synthesis of triarylmethane dyes I. There are number of general methods known2 for the synthesis of triarylmethanes. However, all these are acid catalysed condensation reactions, and the catalyst have to be used in high concentrations. The catalysts are non­recoverable, and entail problems of corrosion, work­up and effluent pollution. Recently, a wide range of organic reactions have been catalysed by clays which act as solid Lewis or Bronsted acids3• Montmorillonite clay is an ecofriendly, inexpensive, stable catalyst and has several advantages over conventional acids. It is strong acid but non-corrosive and the reaction conditions are often mild, and the work-up is easy and the products are obtained in high yields and selectivity in the product formation has been observed . 2 In many cases . I

In continuatio/ of our studies4 on clay catalysed organic reactions, we have explored the clay catalysed Baeyer condensation reaction of heterocyclic

tThis paper has been presented in the .. UGC-DRS National Symposium 011 Newer Vistas in Syllthetic Protocols and Structural Elucidatio/1 ill ChemistlY " held at Madurai Kamaraj University, Madurai 625 021 , India during April 22-24, 1 998.

aldehydes with N,N-dimethylaniline. The synthesis of heteroaryldiarymethane leuco bases and the photo irradiation studies of the leuco bases obtained are presented in this note. I t may be noted that recently the clay catalysed condensation reaction of a number of substituted benzaldehydes with N,N-dimethyl­aniline has appeared in the literatures. The schematic representation of our present study is shown in Scheme I.

Heating furfural 1 with N,N-dimethylaniline (DMA) in the presence of montmorillonite K I O clay (30% w/w) for 3 hr at 1 00 °C without any solvent afforded a colourless crystalline solid triarylmethane 8 (mp 90 °C, crystallised from ethanol) in 84% yield. The compound appears as an intensive blue colour on clay surfaces and also in protic solvents like ethanol. Its UV -Vis spectrum in 95% ethanol solution gave absorption maxima (Amax) at 2 1 3.7, 26 1 .5 and 297.8 nm. Compound 8 showed IR absorption bands (vrnax) at 2884, 2790, 1 6 1 7, 1 533, 953 and 8 1 8 and did not show any carbonyl absorption. The IH NMR (300 MHz) spectra showed characteristic peaks at 8 2 .8 (a singlet for methyl groups) and at 8 5 .2 (a singlet for methine proton). The l3C NMR spectra (75 .4 MHz) showed peaks at 8 49.0 (for tertiary methine carbon) and at 8 40. 1 (for methyl carbon). The GCMS showed a molecular ion peak at mlz 320(M+). The compound was also characterised by DEPT- 1 35 spectrum wherein it showed a characteristic CH-(methine) positive peak at 8 49.0. Interestingly, the reaction of 2-formylthiophene 2 with N,N-dimethylaniline gave its corresponding triaryl compound 9 which appears as an intensive green colour on the surface of the clay and in protic solvents. It is noteworthy that the thiophene leuco base 9 has an absorption maximum in the visible region (623 nm) when it was kept on the table for 3 hr.

It has been observed that the reaction of N,N­dimethylaniline with 2-formylpyrrole 3 is slower than with I -methyl-2-formylpyrrole 4. Similarly, 3-formylindole 5 reacted with N.N-dimethylaniline more slowly than I -methyl-3-formylindole 6.

Unlikely, this may be due to the free -NH group competing in the condensation reaction in these cases. Under similar conditions, the reaction of furfural 1 with 1 .2 equivalent of phenol and 1 .2 equivalent of

NOTES 1 259

0-" + o-� NMe2 Mo ntmorillonite K1 0. 1 00:,c. f \ 3-22h. 45-92%. neat X C H O -

X= O. S, N H , N-Me 8-14

Scheme I

n + o-� NMe2 M:mtmorillonite K1Q. 100 �,

�o"CHO 6h, 50%, neat

Unreacted phenol Scheme II

hv, 325 nm, 1 h, CHCI3 .. Me2+ N=<J=ON+Me2

15 + 8 Scheme III 0-o C�H

DMA furnished only compound 8 in 50% yield along with the unreacted phenol (Scheme II), This reaction shows the reactivity of N,N-dimethylaniline towards Bayer condensation is more than that of phenols.

The reaction of pyridine-3-carboxaldehyde 7 with DMA also yields its corresponding triaryl compound 14 and thus, it may be applicable for other six membered heterocylic aldehydes. The clay recovered from the reaction mixture by filtration can be recycled three times without losing its activity on heating the clay at 1 00 °C for 3 hr. The results of the condensation reaction of variety of hetero aldehydes with N,N-dimethylaniline is summarised in Table I. All the compounds reported herein gave satisfactory spectral data (UV-Vis, IR, J H NMR, J 3C NMR, DEPT- 1 35 and mass). We have also observed that by increasing 1 0% w/w of montmorillonite clay, the reaction period is reduced considerably and the yields are improved around 5- 10% in all the cases. When exposed to light all pure colourless crystalline

1 6

compounds change their colours (cI Table I) and persist the colour as long as they are exposed to light.

Irradiation of triaryl leuco base 8 at 325 nm (Scheme III) under different solvent systems (chloroform, acetonitrile, and methanol with and. without degassing) shows shift of absorption from UV-region to Visible region (Figure I) . This photo transformation was found irreversible in nature when it was irradiated at 630 nm for 1 hr. The irradiated products were identified as 15 and 16 based on NMR data.

The crude J H NMR (300 MHz) spectrum of the irradiated compound (see Experimental) showed complete disappearance of aromatic peaks at 8 6.5 and 7 .0 and appearance of carboxylic acid proton at 8

. 1 3 .2 along with [uran ring protons <\t8 5 .8, 6.3 and 7.2 (Figure 2). Currently we are adopting this methodology for the synthesis of poly aromatic systems containing two and three methine carbons by condensing di- and tri-formyl substituted benzene,

1 260 INDIAN 1 CHEM. SEC B. DECEMBER 2001

Table I-Baeyer condensation of heteroaldehydes with N.N-dimethylaniline catalysed by montmorillonite KIO a.h.c

Entry Heteroaldehyde Reaction Colour on clay Product d m.p Yield! period surface ( 0c)

(hr)

Furfural 3 Blue R 89-90 84

2 2-Formylthiophene 6 Green 9 92-93 62

3 2-FormylJpyrrole 1 9 Blue lOe 1 67 45

4 N-Methyl-2- 1 3 Blue I I 1 7 1 52 formylpyrrole

5 3-Formylindole 22 Pink 1 2e 126 59

6 N-Methyl-3-formyl 1 8 pink 1 3 1 35 63 indole

7 Pyridine-3- 22 Green 14 1 10 92 carboxaldehyde

a.The clay was dried at 85 °C for 2h before each reaction. b. 30% (w/w)montmorilJonite(Aldrich) clay was used for each case. c. For conditions see typical experimental procedure. d. The homogeneity of the compounds was established by column chromatographic techniques. e. 5- 10% of the monoarylated compounds also isolated !. Isolated after column purification .

..... � C)

C) -o g 250 . . . . "'.,--.,.-.!-.� 300 400 500

, "I

600 700

- C) 750

o g

Figure l-UV-irradiation spectrum of compound 8 in degassed acetonitrile.

biphenyl and naphthyl systems with N.N­dimethylaniline. These compounds are well known examples for stabilised radical substrates at room temperature.

In conclusion, a number of triaryl leuco bases were prepared by montmorillonite K I O clay catalysed Baeyer condensation of heterocyclic aldehydes with N.N-dimethylaniline and the photo irradiation studies

, ppa

a

J i

7;5 7.0

NOTES

J I ........ 6;:5 �,;O' 5:.5 5'.;0:

� �' � r1 ( l r . , " -. , " . . ' ', ' . -

;ffi" , :i! ' " f!i 'g: I - �' j :- :, � ,: �

, B.O 7.5 7.0 6�(l 5.S

Figure 2---{a) 'HNMR spectrum of compound 8.(b) IHNMR spectrum of compound 8 after irradiation at 325 nm in degassed acetonitrile

Experimental Section

1261

of the Leuco bases were carried out. The photo irradiated compounds were isolated and characterised. Further studies in this direction are underway.

All the melting points and boiling points are uncorrected. UV - Vis spectra were recorded on a

1 262 INDIAN J CHEM. SEC B, DECEMBER 2001

Shimadzu UV-2 1 00 spectrophotometer and irradiation study was performed on a ORIEL photochemical reactor. IR spectra were recorded on a Nicolet Impact 4000 spectrophotometer; IH NMR spectra on Bruker A vance DPX 300 spectro meter (chemical shifts in 8, .ppm ) with TMS as standard; and mass spectra on a Fisons GC 8000 series mass spectrometer. Thin layer chromatograms (TLC) were developed on glass plates coated with silica gel-G (ACME) of 0.25 mm thickness and visualised with iodine. Column chromatography was carried out with alumina (ACME, 1 00-200 mesh) using hexane-ethyl acetate mixture as eluent. The organic extracts of crude products were dried over anhydrous Na2S04. Solvents used were of reagent grade and were purified according to literature procedure6.

Typical Experimental Procedure A mixture of the heterocyclic aldehyde (5 mmoles),

N,N-dimethylanil ine ( 1 5 mmoles) and montmorillo­nite K- l O (30% w/w of the aldehyde) was heated at 100 °C with constant stirring and the reaction was monitored by TLC. The mixture was cooled to room temperature and treated with methylene chloride (20 mL). Montmorillonite K- lO clay was recovered by filtration and washed with methylene chloride (2x5 mL). The solvent was removed under reduced pressure to afford the crude material containing the main product along with some unreacted aldehyde and N,N-dimethylaniline. The crude reaction mixture was chromatographed over neutral alumina column using petroleum ether and ethyl acetate-petroleum ether( l :3) mixture. The fractions containing the required product were concentrated to afford the expected product. The product was further purified by crystallisation from ethanol for spectral analysis.

Spectral data for some selected compounds Compound 8: UV-Vis (95% ethanol) : 2 1 3 .78,

2 1 6.56, 297.88 nm ; IR(CCI4): 2824, 2790, 1 6 1 7, 1 533, 953, and 8 1 8 cm· l ; I H NMR (300 MHz, CDCliTMS): 8 2.8(s, 1 2H, 4 CHj), 5 .2(s, I H, -CH), 5 .8(d, I H. J=2.9 Hz, furan-H), 6.2(m, I H, furan-H), 6.6(d, 4H, J=8.7 Hz, Ar-H), 6.6(d, 4H, J=8.6 Hz, Ar­H), 7 .3(s, I H, furan-H) ; 13C NMR (75.4 MHz, CDCI3ITMS): 8. 1 58.3, 149.2, 1 4 1 .4, 1 33 .7, 1 29 .2, 1 1 2.6, 109.8, 1 07.5 , 49.0, 40. 1 ; GCMS: rn/z 320 (M+).

Compound 14: UV -Vis (95% ethanol): 208,264 and 3 1 2 nm ; IR(CCI4) : 2880, 2795, 1 6 1 0, 1 140, 953, and 8 1 8 cm- l ; IH NMR (300 MHz, CDCh ITMS): 8 2.9(s, 1 2H, 4 CHj), 5 .4(s, I H, -CH), 6.5(d, J=8.5 Hz, 4H), 6.8(d, J=8.5 Hz, 4H), 7 . 1 (m, I H), 7 .2(d, J=7.7 Hz, I H), 8 .2(m, 2H) ; l 3c NMR (75 .4 MHz, CDChrrMS): 8 1 50.8, 1 49. 1 , 1 47 . 1 , 1 40.8, 1 36.5, 1 3 1 .4, 1 29.7, 1 23 .0, 1 1 2 .5, 52.5, 40.6 ; GCMS: rn/z 232(M+).

Irradiation studies UV -Visible: UV Spectrum was recorded in every

two minute interval for the compound 8 in degassed acetonitrile after irradiating at a photochemical reactor for 30 sec.

IH NMR: 5 mg of the compound 8 in CHCI3/CDCh( 1 : 1) was taken in a clean NMR tube and irradiated at 325 nm for 1 hr. The NMR spectrum was recorded after irradiation and products were identified based on spectrum analysis.

Acknowledgement The authors thank Dr G Vijay Nair, Director of this

institute for his interest in this work and Dr Mangalam S Nair for useful discussions. One of the authors (P S) also thanks Dr K R Gopidas and Dr Manoj, Photochemistry Research Unit for UV-Vis spectra and irradiation studies and Mrs S Vij i for recording IR spectra.

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NOTES 1 263

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