Synthesis and Spectroscopic Characterisation of N-Alkyl Quaternary Ammonium Salts Typical Precursors of Cyanines

The synthesis and spectroscopic characterisation of some representative N-alkyl-substituted quaternary ammonium salts derived from benzothiazole, benzoxazole, benzo-selenazole, indole and quinoline are described. These heterocyclic salts, bearing an activated methyl group in the 2-position in relation to the nitrogen atom and N-methyl, -pentyl, -hexyl and -decyl chains, are typical precursors of cyanine dyes.


Introduction
Despite the fact that several N-alkyl quaternary ammonium salts bearing a 2-methylbenzothiazole, -benzoselenazole, -benzoxazole, -indole and -quinoline nuclei are easily found in the literature, mainly as unisolated precursors of cyanine dyes [1][2][3], their full physical and spectroscopic characterisation is not usually described. To the best of our knowledge, for some of those most commonly used for the synthesis of cyanines, namely the N-ethyl, -pentyl, -hexyl and -decyl substituted ones, only the 1 H-NMR spectral data for the N-ethyl quaternary salts [4,5] and the 13 C-NMR spectral data for the Nethylquinolinium salts [5] are available.
The 2-methylheterocyclic ammonium salts are generally synthesised by heating the corresponding heteroaromatic base with a molar equivalent or an excess of an alkylating agent such as an alkyl iodide, bromide, sulphate or tosylate, among others [1]. Alternatively, the condensation can be carried out in a polar aprotic solvent as acetonitrile [6] or N,N-dimethylformamide (DMF) [7,8].
a: R = CH 2 CH 3 ; b: R = (CH 2 ) 4 CH 3 ; c: R = (CH 2 ) 5 CH 3 ; d: R = (CH 2 ) 9 CH 3 The quaternary iodides were generally obtained in rather good yields, usually decreasing as the length of the N-alkyl chain increases (Table 1, see Experimental). The corresponding melting point (m.p.), Fourier Transform Infrared Spectra (FTIR) and High Resolution Fast Atom Bombardment Mass Spectra (HR FAB-MS) data are also collated in Table 1.
The most intense and/or characteristic bands of the FTIR spectra appeared around 3000 cm -1 (C-H from Ar-H stretch), 2826-2994 cm -1 (C-H from CH 2 stretch), 1635-1578 (C=C), 1509-1524 cm -1 (C=N), 1428-1467 cm -1 (CH 3 asymmetric deformation and CH 2 deformation) and 757-781 cm -1 (skeletal "out of plane" vibration). These last two bands are generally the most intense ones found in the spectra of the benzoazole and indole salts. For each series of compounds, the intensity of the aliphatic C-H absorption bands within the 2826-2994 cm -1 region was observed to increase with the length of the N-alkyl chain.
The 1 H-and 13 C-NMR spectral data of all the heteroaromatic salts synthesised are collated in Tables 2-5. The NMR spectra were recorded in DMSO-d 6 solutions except those of the benzoxazolium salts 8a-d, which were recorded in CDCl 3 solution, since in the latter solvent an unexpected transformation was observed. This result is currently the subject of a separate study, the results of which shall be published elsewhere.
The assignment of the aromatic 1 H-and 13 C-NMR signals was based on their chemical shifts and multiplicity (for the 1 H signals) and was unambiguously established with the aid of HMQC (Heteronuclear Multiple Quantum Coherence), HMBQ (Heteronuclear Multiple Bond Correlation) and COSY (Correlated Spectroscopy) experiments. The attributions of the aromatic 1 H-NMR chemical shifts are in full agreement with those previously reported for the N-ethyl quaternary salts derived from the same heteroaromatic bases employed herein [4,5]. The assignments of the aromatic 13 C-signals are also consistent with the spectral data of salts similar to 6-9 [9][10][11][12] and with that described for N-ethyl-2-methylquinolinium iodide (10a) [5]. The assignment of the aliphatic 1 H-and 13 C-NMR chemical shifts was accomplished based on the deshielding effect displayed by the neighbouring electronegative nitrogen atoms [13].
The 1 H-NMR absorptions of the 2-CH 3 groups typically range from 2.84 to 3.36 ppm. The salts derived from 2,3,3-trimethylindole (4) invariably display the lowest chemical shift, coincidentally the same for compounds 9a-d, once the deshielding effect of the heteroatoms of benzoazoles 6-8 and that of the sp 2 carbon in the quinolinium salt 10 are absent.
The 13 C-NMR chemical shifts of 2-C fall within the 13.7-19.9 ppm region. Accordingly to the reasons afore mentioned, the lower field absorptions are ascribed to the substituted indolium salts 9a-d.
The order of the aromatic 13 C chemical shifts show a clear dependence on the nature of the heterocyclic moiety of the salt, being in almost every cases different from one salt to another. Only the 2-C absorption is, for all compounds, the one at lower field as a consequence of being, simultaneously, sp 2 hybridised and adjacent to a positively charged nitrogen (Table 5). On the contrary, the order of the chemical shifts of the aromatic protons is the same for compounds 6-9, when their relative position with respect to the nitrogen atom of the heterocyclic moiety is considered. An exception to this trend is observed for the quinolinium salts 10a-d where the absorption of the 5-H appears at higher field than that of 8-H.

Conclusions
The synthesis and spectroscopic characterisation of some representative ammonium quaternary salts, bearing benzothiazole, benzoxazole, benzoselenazole, indole and quinoline nuclei, possessing a methyl group in the 2-position in relation to the ammonium group and methyl, pentyl, hexyl and decyl N-alkyl chains was described.

Acknowledgements
Thanks are due to FCT, Lisbon, POCTI and FEDER for their financial support for this work (Project POCTI/32915/QUI/00).

General
All reagents and chemicals were obtained from Sigma-Aldrich. Solvents were of analytical grade. Reactions were monitored by thin-layer chromatography using 0.20 mm aluminium-backed silica-gel plates (Merck GF 254 ). The plates were eluted with dichloromethane/methanol (5-10%) and the spots examined under 254, 312 and 365 nm UV light. Melting points were measured in open capillary tubes in a Büchi 530 apparatus and are uncorrected. FTIR spectra were recorded on a Mattson 5000 spectrophotometer. All samples were prepared by mixing FTIR-grade KBr (Sigma-Aldrich) with 1% (w/w) salt, and grinding to a fine powder. Spectra were recorded over the 400-4000 cm -1 range without baseline corrections. Characteristic absorptions are given in cm -1 . 1 H-and 13 C-NMR spectra were recorded in DMSO-d 6