Organometallics 2009, 28, 3957–3958 DOI: 10.1021/om900326g
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A Convenient Quick Synthesis of SnBu2RCl Derivatives Nora Carrera, M onica H. Perez-Temprano, Ana C. Albeniz,* Juan A. Casares,* and Pablo Espinet* IU CINQUIMA/Quı´mica Inorg anica, Universidad de Valladolid, 47071-Valladolid, Spain Received April 27, 2009 Summary: A convenient method for the quick preparation of SnBu2RX (Bu = n-Bu) has been developed using microwave irradiation and column chromatography in acidic alumina. The method can be a good alternative particularly for easy and quick preparation of small amounts for test purposes and, in general, for the synthesis of compounds that cannot be purified by fractional distillation due to thermal lability or low volatility.
Organotin derivatives find application in different fields, as antifouling paints, PVC stabilizers, cytotoxic agents, etc.1 They are also important synthetic reagents. Some examples are their use in Stille cross-coupling reactions2,3 and the application of organotin hydrides in a number of other organic syntheses.1,4 Some specific applications of organostannanes require compounds of the type Sn(alkyl)2RX. Recently, the synthesis of polymers containing -SnBu2R units has been reported as a strategy to improve the applicability of the Stille reaction.5 In contrast to SnR2X2 or SnR3X (X=halide, hydrocarbyl) compounds, many of which are commercially available, mixed hydrocarbyl SnR12R2X (X=halide, hydrocarbyl) compounds need to be prepared, but very few efficient synthetic methods for them are available.6 The most commonly used procedures are the slow addition of either bromine or a solution of hydrogen chloride in diethyl ether to SnR12R22, followed by distillation of the product mixture. This procedure often works well for multigram preparations of some compounds but, for some of the derivatives we report here, which were needed in small amounts, this procedure was not efficient, probably because of problems during distillation at low pressure. Indeed, the traditional procedure can be problematic when the thermal *To whom correspondence should be addressed. E-mail: albeniz@qi. uva.es (A.C.A.);
[email protected] (J.A.C.);
[email protected] (P.E.). (1) Davies, A. G. Organotin Chemistry; VCH: Weinheim, Germany, 2004. (2) Mitchell, T. N. Metal Catalyzed Cross-Coupling Reactions; Wiley-VCH: Weinheim, Germany, 2004. (3) Farina, V.; Krishanamurthy, V.; Scout, W. J. The Stille Reaction; Wiley: New York, 1998. (4) Davies, A. G. J. Chem. Res. Synop. 2006, 3, 141–148. (5) (a) Carrera, N.; Gutierrez, E.; Benavente, R.; Villavieja, M. M.; Albeniz, A. C.; Espinet, P. Chem. Eur. J. 2008, 14, 10141–10148. (b) Chretien, J. M.; Mallinger, A.; Zammattio, F.; Le Grognec, E.; Paris, M.; Montavon, G.; Quintard, J.-P. Tetrahedron Lett. 2007, 48, 1781–1785. (c) Nicolaou, K. C.; Winssinger, N.; Pastor, J.; Murphy, F. Angew. Chem. 1998, 110, 2677-2680; Angew. Chem., Int. Ed. 1998, 37, 2534-2537; (d) Kuhn, H.; Neumann, W. P. Synlett 1994, 123–124. (6) (a) Seyferth, D. J. Org. Chem. 1957, 22, 1599–1602. (b) Ingham, R. K.; Rosenberg, S. D.; Gilman, H. Chem. Rev. 1960, 60, 459–539. (c) Kuivila, H. G.; Sommer, R.; Green, D. C. J. Org. Chem. 1968, 33, 1119–1122. (d) Gitlitz, M. H. Main Group Met. Chem. 1999, 22, 641–644. (e) Apocada, P.; Cervantes-Lee, F.; Pannell, K. H. Main Group Met. Chem. 2001, 24, 597–601. (f) Thoonen, S. H. L.; Deelman, B.-J.; van Koten, G. J. Organomet. Chem. 2004, 689, 2145–2157. r 2009 American Chemical Society
labilty, low volatility, or closeness of boiling points of the compounds involved does not allow for separation by lowpressure fractional distillation. It would be interesting, particularly for exploratory test studies, to have a fast, flexible, and efficient alternative method for the preparation of small amounts of mixed organotins, which could afford easily a panoply of the desired reagents for test purposes prior to undertake multigram synthesis. This has been achieved using microwave irradiation to induce clean group redistribution in mixtures of SnBu2R2 and SnBu2Cl2 (excess). The conversion to SnBu2RCl (Bu = n-Bu) is quantitative in a short time, and the product can be easily separated from the excess of SnBu2Cl2 by simple filtration through a short column of acidic alumina. Moreover, the procedure can be also scaled to multigram preparations, avoiding heating in the separation and purification step.
Results and Discussion Compounds SnBu2R2 (1) were prepared by addition of 2 equiv of RMgX or LiR to SnBu2X2 (Table 1).1 The products were characterized by 1H, 13C, and 119Sn NMR and mass spectrometry (see the Supporting Information). The reaction of SnBu2R2 (1) and SnBu2Cl2 (2) in the absence of solvent or catalyst, to give SnBu2RCl (3) according to eq 1, was initially carried out by heating the mixture in an oil bath at temperatures between 150 and 200 °C for 10-48 h. This procedure did not produce a clean rearrangement despite long reaction times. MW
SnBu2 R2 þ SnBu2 Cl2 f 2SnBu2 RCl 1
2
ð1Þ
3
The use of a microwave oven allowed for shorter reaction times and higher conversions.7 Thus, the reaction of the stoichiometric amounts of reagents (1:2 = 1:1) afforded 3, with moderate conversions (in the range 55-85%), mixed with unreacted 1 and 2. It is not easy to isolate 3 from this mixture, either by distillation or by column chromatography. The remaining SnBu2R2 is particularly problematic and very difficult to separate from the target product SnBu2RCl. Fortunately, the problem of separation was very much alleviated using 2 equiv of 2 (1:2=1:2), which led to 100% conversion of 1 to 3 (Figure 1). Then, only the excess of 2 needed to be separated from the desired product 3. Column chromatography using silica gel was still troublesome and led to complete retention and eventually decomposition of (7) The advantages of using microwave irradiation have been shown in many organic and organometallic transformations: (a) Kappe, C. O. Angew. Chem., Int. Ed. 2004, 43, 6250–6284. (b) Loupy, A. Microwaves in Organic Synthesis; Wiley-VCH: Weinheim, Germany, 2006.
Published on Web 06/15/2009
pubs.acs.org/Organometallics
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Organometallics, Vol. 28, No. 13, 2009
Carrera et al.
Table 1. Synthesis of SnBu2R2 (1)a compd
R
reagent/solvent
isolated yield (%)
δ(119Sn)b
1a Me LiR/Et2O 85 -2.60 94 -72.46 1b Ph BrMgR/Et2O BrMgR/Et2O 97 -67.96 1c C6H4-F-p BrMgR/THF 78 -69.23 1d C6H4-(OMe)-p 92 -72.28 1e p-tolyl BrMgR/Et2O 76 -68.30 1f o-tolyl BrMgR/Et2O 57 -89.44 1g mesityl BrMgR/Et2O 77 -42.01 1h cyclohexyl BrMgR/Et2O a Bu=n-Bu. b Data obtained at 293 K, in CDCl3, referenced to SnMe4.
of 1h with 2 leads to an equimolar mixture of SnBuCy2Cl and SnBu3Cl. Some examples of the difficult exchange of cyclohexyl groups in tin chemistry can be found in the literature.6d,9
Conclusion In summary, the method reported here can be convenient when series of SnBu2RCl (3) compounds are needed for research purposes, as their preparation by this MW/ chromatography protocol is fast and easy and can provide freshly prepared samples of compounds that could, otherwise, rearrange when stored. The reactions can be scaled up. The method can also be a good alternative to the current methods when purification by distillation methods is problematic.
Experimental Section
Figure 1. 119Sn NMR spectra of the reaction 1a + 2a (1:2), yielding 3a + 2a (1:1): (top) starting reactants; (bottom) reaction products. Table 2. Synthesis of SnBu2RCl (3)a compd
R
reacn conditions
isolated yield (%)
δ(119Sn)b
3a Me 90 min, 100 °C 70 123.62 3b Ph 30 min, 150 °C 76 82.56 75 min, 100 °C 65 84.83 3c C6H4-F-p 60 min, 100 °C 60 87.61 3d C6H4-(OMe)-p 3e p-tolyl 60 min, 125 °C 63 85.46 3f o-tolyl 60 min, 160 °C 69 90.12 3g mesityl 30 min, 150 °C 52 79.46 a 100% conversion measured by 1H NMR; Bu=n-Bu. b Data obtained at 293 K, in CDCl3, referenced to SnMe4.
3, but a simple filtration through a short column of acidic alumina, using diethyl ether as eluent provided, clean compounds 3, which were obtained as liquids by evaporation of the eluent. The reaction conditions and isolated yields are collected in Table 2. In spite of the complete conversion for all the compounds, their isolated yields are different, which is due to some degree of decomposition of 3 in the column. Consequently, long columns should be avoided. The products were characterized by 1H, 13C, and 119Sn NMR and mass spectrometry (see the Supporting Information). The success of a clean redistribution of groups relies on a sufficient difference in bond energies and reactivities for the R groups involved (in our examples, Bu vs R), so that the target product 3 will be preferred. Thus, under the conditions of Table 2 the compounds SnBu2(aryl)Cl (3b-g) are obtained pure. It has been shown that the reactivity of the Sn-alkyl bond follows the trend Me > Bu > iPr.8 Thus, the cleavage of the Sn-Me instead of the Sn-Bu bond is preferred, and the compound SnBu2MeCl (3a) is obtained pure. However, SnBu2(Cy)Cl could not be prepared, since the redistribution of the cyclohexyl groups is disfavored. Thus, the reaction (8) (a) Plazzogna, G.; Bresadola, S.; Tagliavini, G. Inorg. Chim. Acta 1968, 2 (3), 333–336.
General Considerations. Solvents were dried over CaH2 or Na, distilled, and deoxygenated prior to use. SnBu2Cl2 and the organic halides were purchased from Aldrich or Acros. Microwave-promoted experiments were carried out with a CEM Discover 300W single-mode microwave instrument, with simultaneous cooling with compressed air. The reaction mixtures were prepared in 10 mL special glass reaction tubes with selfsealing septa that control the pressure with a pressure sensor on top of the vial. The temperature was monitored through a noncontact infrared sensor centrally located beneath the cavity floor. Magnetic stirring was provided to ensure complete mixing of the reaction mixture. The power applied was 300 W with a ramp time of 1 min. Complete characterization data of the compounds by multinuclear NMR and MS are collected in the Supporting Information. General Procedure for the Synthesis of SnBu2R2 (1). A solution of RBr in dry THF or Et2O was added to an equimolecular amount of magnesium turnings, activated with I2 or 1,2dibromoethane, and boiled under reflux until the magnesium turnings disappeared. The reaction mixture was cooled to room temperature, and SnBu2Cl2 was added. After the mixture was stirred for 24 h at room temperature, water was added and the aqueous layer was extracted twice with Et2O. The combined organic solutions were washed with a saturated water solution of NH4Cl, dried over MgSO4, and filtered, and the filtrate was concentrated under vacuum to yield compounds 1 as colorless liquids. Some of the SnBu2R2 species were purified by distillation under reduced pressure. General Procedure for the Synthesis of SnBu2RX (3). The corresponding organotin compounds SnBu2R2 and SnBu2Cl2 were placed in a 10 mL microwave reaction vessel in a 1:2 molar ratio. The reaction conditions for each compound are given in Table 2. After the time indicated, the product SnBu2RX (3) was easily separated from the remaining SnBu2Cl2 in excess by chromatographic purification through an acidic aluminum oxide column, using ether as eluent.
Acknowledgment. We gratefully acknowledge financial support from the Spanish MEC (DGI, Grant CTQ2007-67411/BQU; Consolider Ingenio 2010, Grant INTECAT, CSD2006-0003; FPU fellowships to N.C. and M.H.P.-T.) and the Junta de Castilla y Le on (Projects VA044A07 and GR169). Supporting Information Available: Text giving complete characterization data of the compounds 1 and 3 by multinuclear NMR and MS. This material is available free of charge via the Internet at http://pubs.acs.org. (9) Jousseaume, B.; Riague, H.; Toupance, T.; Lahcini, M.; Mountford, P.; Tyrrell, B. R. Organometallics 2002, 21, 4590–4594.
