dione as a pyrrole precursor, through the C-3 alkylation of pentane-2,4-dione with ... phanes [8], 4-halo-3,5-dimethyl-1-nitro-1H-pyrazoles [9] and even with the ...
Progr Colloid Polym Sci (2004) 123 : 28–30 DOI 10.1007/b11616 Ó Springer-Verlag 2004
Abı´ lio J.F.N. Sobral Susana H. Lopes Anto´nio M. d’A. Rocha Gonsalves M. Ramos Silva A. Matos Beja J.A. Paixa˜o L. Alte da Veiga
Synthesis and crystal structure of new phase-transfer catalysts based on 1,8-diazabicyclo[5.4.0]undec-7-ene and 1,5-diazabicyclo[4.3.0]non-5-ene
Abı´ lio J.F.N. Sobral Æ Susana H. Lopes Anto´nio M. d’A. Rocha Gonsalves (&) Departamento de Quı´ mica, FCTUC, Universidade de Coimbra, 3049 Coimbra, Portugal M. Ramos Silva Æ A. Matos Beja J.A. Paixa˜o Æ L. Alte da Veiga CEMDRX, Departamento de Fı´ sica, FCTUC, Universidade de Coimbra, 3000 Coimbra, Portugal
Introduction The efficient N-alkylation of 1,8-diazabicyclo[5.4.0] undec-7-ene (DBU) and 1,5-diazabicyclo[4.3.0] non5-ene (DBN) with long-chain alkyl iodides opens the way to a new family of phase transfer catalysts. The use of organic hindered amines such as 1,8diazobicyclo[5.4.0]undec-7-ene (DBU) as catalysts when a strong non-nucleophilic base is required is a usual procedure. It is the case in the synthesis of 3-substituted pentane-2,4-diones [1]. The catalyst is particularly useful in the case of long-chain alkyl iodides due to its lower reactivity. In the course of our own studies on the synthesis of pyrroles and porphyrins for the production of LangmuirBlodgett films [2], we prepared 3-octadecylpentane-2,4dione as a pyrrole precursor, through the C-3 alkylation of pentane-2,4-dione with 1-octadecyl iodide. When DBU was used as catalyst in that synthesis, we unexpectedly isolated the DBU iodide salt 1 (Scheme 1) as a secondary product, a stable, sharp melting point crystalline solid, in 15% yield. Performing the reaction in the absence of pentane-2,4dione gives exclusively the iodide salt of the N-alkylated DBU 1, in 57% yield. An analogous result was obtained with 1,5-diazobicyclo[4.3.0]non-5-ene (DBN), furnishing in this case the iodide salt 2 (Scheme 1).
The new compounds 1 and 2 were characterised by H-NMR, FT-IR and elemental analysis, giving spectroscopic and physical characteristics for the iminium salts 1 and 2.1 The full characterisation of these interesting compounds was definitive when the structure of salt 1 was solved by single crystal X-ray diffraction (Scheme 2).2 Except for a paper of 1982 that refers to the synthesis of some N-alkyl derivatives [3] of DBU and DBN, these nitrogen bases are considered to be very hindered nonnucleophilic bases, and are usually used taking their nonnucleophilicity as granted. Actually there are scarce 1
Scheme 1
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Scheme 2 ORTEP diagram of the DBU salt 1. Displacements ellipsoids are drawn at the 50% probability level.
literature references to the nucleophilicity of DBU and DBN although they are always considered as unexpected. To explain those products, the delocalised positive iminium salts were suggested as intermediates for the reaction of DBU and/or DBN in the esterification of carboxylic acids with alkyl halides [4], on the reaction with bicyclic bromoketones [5], 1-halocyclopropane-1, 2-diesters [6], 1-bromo-4-benzoyloxyimino-1,2,3,4-tetrahydrophenanthrene [7] and more recently with phosphanes [8], 4-halo-3,5-dimethyl-1-nitro-1H-pyrazoles [9]
1 a) Synthesis of the DBU salt 1 (1-octadecyl-2,3,4,6,7,8,9,10octahydropyrimido[1,2-a]azepin-1-ium; iodide): a mixture of octadecyl iodide (2 g, 5 mmol) and 1,8-diazobiciclo[5.4.0] undec-7-ene (DBU) (0,76 ml, 5 mmol) in 60 mL of dry acetone is placed in a 100-mL round-bottomed flask fitted with a reflux condenser and a silica guard tube. The mixture is stirred and heated under reflux for 5 hours. The required compound is extracted into dichloromethane/water and the organic phase is dried with anhydrous MgSO4. Some remaining octadecyl iodide is removed by dissolution with ethyl ether and the desired product, which is insoluble in this solvent, is filtered off. The desired product undergoes crystallisation by slow evaporation of the solvent and is obtained with 57% yield. Melting point: 111–112 °C. 1H-NMR (solvent: CDCl3; internal reference: TMS): d ¼ 0.88 (3H, t, J ¼ 6.7 Hz, CH3-(CH2)n), 1.25 (30H, m, CH3-(CH2)15-CH2), 1.64 (2H, s (broad), N-CH2-CH2), 1.84 (6H, s (broad), NCH2-(CH2)3-CH2), 2.18 (2H, m, NCH2-CH2(CH2)15), 2.89 (2H, d, J ¼ 6.1 Hz, C-CH2-CH2), 3.51 (2H, t, J ¼ 8.0 Hz, N-CH2-CH2), 3.70 (6H, m, N-(CH2)3-N). Elemental analysis for C27H53IN2: Required (C 60.88; H 10.03; N 5.26); Found: (C 60.38; H 10.05; N 5.27). FT-IR in KBr (cm)1;% T; group): (722.10, 73.64, c(CH2)); (1465.83, 60.91, d(CH2)); (1626.55, 43.06, m(C ¼ N)); (2849.08, 39.82, m(C-H)); (2954.04, 52.49, m(C-H)). b) Synthesis of the DBN salt 2 (1-octadecyl-2,3,4,6,7,8-hexahydropyrrolo[1,2-a]pyrimidin-1-ium; iodide): a mixture of octadecyl iodide (2 g, 5 mmol) and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) (0.65 mL, 5 mmol) in 60 mL of dry acetone is placed in a 100-mL round-bottomed flask fitted with a reflux condenser and a silica guard tube. The synthesis and isolation are as reported above for salt 1, giving the desired product with 74% yield. Melting point: 76–78 °C. 1H-NMR (solvent: CDCl3; internal reference: TMS): d ¼ 0.88 (3H, t, J ¼ 6.5 Hz, CH3-(CH2)n), 1.25 (28H, m, CH2(CH2)14-CH2), 2.10 (2H, m, NCH2-CH2-CH2N), 2.25 (4H, m, CH2(CH2)2-(CH2)14), 3.21 (2H, t, J ¼ 7.9 Hz, N-CH2-CH2), 3.41 (2H, t, J ¼ 7.7 Hz, NCH2-CH2-CH2), 3.54 (4H, m, N-(CH2)2-CH2), 3.85 (2H, t, J ¼ 7.4 Hz, N-CH2-CH2). Elemental analysis for C25H49IN2: required (C 59.59; H 9.79; N 5.55); Found: (C 59.19; H 9.78; N 5.68). FT-IR in KBr (cm)1;% T; group): (747.92, 74.15, c(CH2)); (765.60, 74.16, c(CH2)); (1505.55, 69.23, d(CH2)); (1732.34, 68.08, m(C ¼ N)); (3057.11, 60.84, m(C-H)); (3075.91, 60.79, m(C-H)).
and even with the large macrocycle of methyl pheophorbide a [10]. Although these results showed the nucleophilic character of DBU and DBN, the foregoing reactions have still been considered unexpected and the existence of a covalent bond between the DBU or DBN to the carbon N-substituents was only now confirmed by X-ray crystallography. In salt 1 the bond distances between carbon C9 and nitrogen atoms are N1-C9 1.305(15) A˚ and N2-C9 1.330(14) A˚, showing a delocalised character of the double bond and confirming the existence of a large delocalised iminium cation. The amphiphilic nature of these salts prompted us to check their performance as phase-transfer agents. For these preliminary studies we chose the transfer of
Fig. 1 Percentage of KMnO4 transferred to benzene after extraction from water, with salts 1, 2 and tetrabutylammonium iodide, presents as phase-transfer agents
Crystal data: C27H53N2I, M ¼ 532.6, monoclinic a ¼ 6.9488(5) A˚, b ¼ 63.300(5) A˚, c ¼ 7.0619(17) A˚, b ¼ 108.751(14)°, V ¼ 2941.38(8) A˚3, T ¼ 293(2) K, space group P21/n (No. 14), )1 Z ¼ 4, l(CuKa) ¼ 8.64 mm , 2860 reflections measured, 2596 unique (Rint ¼ 0.051) which were used in the full matrix leastsquares refinement. The final R(F2) was 0.079 (for I>2 r(I)) and wR(F2) was 0.17 (for all reflections). Full crystal data has been deposited at the Cambridge Crystallographic Data Centre and allocated the deposition number CCDC 184814. 2
30
KMnO4 from water to benzene [13], a standard system to evaluate the efficacy of phase-transfer agents. The solubilisation of KMnO4 in organic solvents, aided by some crown ethers [11] and quaternary ammonium salts [12, 13], is crucial for the efficient oxidation of several substrates. These results show that the iminium salts 1 and 2 are very promising materials. In Fig. 1 we see that the transfer of KMnO4 from a water solution (0.05 mmol of KMnO4 in 20 mL of water) to benzene (20 mL) is much faster with our salts than with tetrabutylammonium iodide, a classical phase-transfer agent. The transfer of KMnO4 to the organic layer is almost complete when we use the phase-transfer agent in an equimolar ratio to the inorganic salt, in opposition with
the tetrabutylammonium iodide where a much higher ratio is required. Whether or not this good behaviour is related to the delocalised nature of the salts is a matter for future studies to fully interpret the phase transfer mechanism of these new compounds. Studies are also under way to extend this N-alkylation reaction to other hindered nitrogen bases of the DBU family, to produce new nitrogen amphiphilic compounds. Acknowledgements The authors would like to thank Prof. Hugh D. Burrows from the University of Coimbra for the useful discussions on the phase-transfer studies. Financial assistance from FCT (Sapiens POCTI/QUI/42536) and Chymiotechnon, Portugal, is also acknowledged.
References 1. Price R, Johnson AW, Markham E (1962) Org Synth 42:75; Clark JH, Miler JM (1977) J Chem Soc Perkin Trans I1743; Raban M, Yamamoto G (1977) J Org Chem 42:2549 2. Richardson T, Smith VC, Johnstone RAW, Sobral AJFN, d’A. Rocha Gonsalves AM (1998) Thin Solid Films 327–329:315; Ramos Silva M, Matos Beja A, Paixa˜o JA, Alte da Veiga L, Sobral AJFN, d’A. Rocha Gonsalves AM (2000) Acta Cryst C56:1263
3. Alder RW, Sessions RB (1982) Tetrahedron Lett 23:1121 4. Ono N, Yamada T, Saito T, Tanaka K, Kaji A (1978) Bull Chem Soc Jpn 51:2401 5. House HO, DeTar MB, Vanderveer D (1979) J Org Chem 44:3793 6. McCoy LL, Mal D (1981) J Org Chem 46:1016 7. Juneja TR, Garg DK, Schafer W (1982) Tetrahedron 38:551 8. Reed R, Reau R, Dahan F, Bertrand B (1993) Angew Chem Int Ed Engl 32:399
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