A convenient quick synthesis of SnBu2RCl derivatives

Article abstract of DOI:10.1021/om900326g

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

Full text of DOI:10.1021/om900326g

Organometallics 2009, 28, 3957–3958 3957
DOI: 10.1021/om900326g
A Convenient Quick Synthesis of SnBu2RCl Derivatives
ꢀ
Nora Carrera, Monica H. Perez-Temprano, Ana C. Albeniz,* Juan A. Casares,* and
ꢀ
ꢀ
Pablo Espinet*
´
ꢀ
IU CINQUIMA/Quımica Inorganica, Universidad de Valladolid, 47071-Valladolid, Spain
Received April 27, 2009
Summary: A convenient method for the quick preparation of
labilty, low volatility, or closeness of boiling points of the
compounds involved does not allow for separation by low-
pressure fractional distillation. It would be interesting, par-
ticularly 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.
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.
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. More-
over, the procedure can be also scaled to multigram prepara-
tions, avoiding heating in the separation and purification step.
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 organos-
tannanes 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
Results and Discussion
Compounds SnBu2R2 (1) were prepared by addition of 2
equiv of RMgX or LiR to SnBu2X2 (Table 1).1 The products
1
were characterized by H, 13C, and 119Sn NMR and mass
spectrometry (see the Supporting Information).
SnR1 R2X (X=halide, hydrocarbyl) compounds need to be
2
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 SnR1 R2 , followed by distillation
The reaction of SnBu2R2 (1) and SnBu2Cl2 (2) in the
absence of solvent or catalyst, to give SnBu2RCl (3) accord-
ing 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 rearrange-
ment despite long reaction times.
2
2
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
MW
SnBu R þ SnBu Cl ꢀꢀ 2SnBu2RCl
ð1Þ
f
2
2
2
2
1
2
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 chromatogra-
phy. 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
*To whom correspondence should be addressed. E-mail: albeniz@qi.
uva.es (A.C.A.); casares@qi.uva.es (J.A.C.); espinet@qi.uva.es (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.
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in many organic and organometallic transformations: (a) Kappe, C. O.
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in Organic Synthesis; Wiley-VCH: Weinheim, Germany, 2006.
r
2009 American Chemical Society
Published on Web 06/15/2009
pubs.acs.org/Organometallics

3958 Organometallics, Vol. 28, No. 13, 2009
Carrera et al.
Table 1. Synthesis of SnBu2R2 (1)a
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
isolated
yield (%)
compd
R
reagent/solvent
δ(119Sn)b
1a
1b
1c
1d
1e
1f
Me
Ph
LiR/Et2O
85
94
97
78
92
76
57
77
-2.60
-72.46
-67.96
-69.23
-72.28
-68.30
-89.44
-42.01
Conclusion
BrMgR/Et2O
BrMgR/Et2O
BrMgR/THF
BrMgR/Et2O
BrMgR/Et2O
BrMgR/Et2O
BrMgR/Et2O
C6H4-F-p
C6H4-(OMe)-p
p-tolyl
o-tolyl
mesityl
cyclohexyl
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.
1g
1h
a Bu=n-Bu. b Data obtained at 293 K, in CDCl3, referenced to SnMe4.
Experimental Section
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. Micro-
wave-promoted experiments were carried out with a CEM
Discover 300W single-mode microwave instrument, with simul-
taneous cooling with compressed air. The reaction mixtures
were prepared in 10 mL special glass reaction tubes with self-
sealing 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.
Figure 1. 119Sn NMR spectra of the reaction 1a + 2a (1:2),
yielding 3a + 2a (1:1): (top) starting reactants; (bottom) reac-
tion products.
Table 2. Synthesis of SnBu2RCl (3)a
isolated
reacn conditions yield (%) δ(119Sn)b
compd
R
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 1,2-
dibromoethane, 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 distilla-
tion 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.
3a
3b
3c
3d
3e
3f
Me
Ph
90 min, 100 °C
30 min, 150 °C
75 min, 100 °C
60 min, 100 °C
60 min, 125 °C
60 min, 160 °C
30 min, 150 °C
70
76
65
60
63
69
52
123.62
82.56
84.83
87.61
85.46
90.12
79.46
C6H4-F-p
C6H4-(OMe)-p
p-tolyl
o-tolyl
mesityl
3g
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 com-
pounds 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. Con-
sequently, long columns should be avoided. The products
were characterized by H, 13C, and 119Sn NMR and mass
spectrometry (see the Supporting Information).
1
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 condi-
tions 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. How-
ever, SnBu2(Cy)Cl could not be prepared, since the redistribu-
tion of the cyclohexyl groups is disfavored. Thus, the reaction
Acknowledgment. We gratefully acknowledge finan-
cial 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 Leon (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.
(8) (a) Plazzogna, G.; Bresadola, S.; Tagliavini, G. Inorg. Chim. Acta
1968, 2 (3), 333–336.
(9) Jousseaume, B.; Riague, H.; Toupance, T.; Lahcini, M.; Mount-
ford, P.; Tyrrell, B. R. Organometallics 2002, 21, 4590–4594.