An unexpected Sn-Ph cleavage by mercaptoacetic acid

Article abstract of DOI:10.1016/j.inoche.2005.05.006

The reaction of triphenyltin chloride and mercaptoacetic acid under mild reaction conditions in the presence of an amine resulted in the cleavage of a Sn-Ph bond. This cleavage reaction was not observed when triphenyltin hydroxide or other triorganotin chlorides were employed. A possible pathway was proposed for the Sn-Ph cleavage. This is a first report of a mercaptoacetate ligand cleavaging Sn-Ph bonds under mild reaction conditions. It is also the first report of using ³J(¹¹⁹Sn-¹H) (-Sn-S-CH ₂) coupling constants to differentiate sulfur containing organotin compounds.

Full text of DOI:10.1016/j.inoche.2005.05.006

Inorganic Chemistry Communications 8 (2005) 725–728
www.elsevier.com/locate/inoche
An unexpected Sn–Ph cleavage by mercaptoacetic acid
a,
b
b
a
Xueqing Song *, Guiyun Zhong , Qinglan Xie , George Eng
a
Department of Chemistry, University of the District of Columbia, Washington, DC 20008, USA
b
Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, PR China
Received 17 February 2005; accepted 5 May 2005
Available online 16 June 2005
Abstract
The reaction of triphenyltin chloride and mercaptoacetic acid under mild reaction conditions in the presence of an amine resulted
in the cleavage of a Sn–Ph bond. This cleavage reaction was not observed when triphenyltin hydroxide or other triorganotin chlo-
rides were employed. A possible pathway was proposed for the Sn–Ph cleavage. This is a first report of a mercaptoacetate ligand
cleavaging Sn–Ph bonds under mild reaction conditions. It is also the first report of using 3J(119Sn–1H) (–Sn–S–CH2) coupling con-
stants to differentiate sulfur containing organotin compounds.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Sn–C cleavage; Mercaptoacetic acid; Crystal structure; Coupling constants
Tin–carbon bonds may be cleaved in both heterolytic
and homolytic processes. A wide range of tin–carbon
bonds are cleaved in heterolytic reactions by various
electrophilic reagents such as halogens (e.g., Br2 [1] or
I2 [2]), protic acids (e.g., hydrogen halides [3] and car-
boxylic acids [4]), metal halides (e.g., SnX4 [5], HgX2
[6], BX3 [7] and PdX3 [8]) and sulfur dioxide [9]. Hetero-
lytic cleavage has also been reported to occur with basic
nucleophilic reagents, such as alcoholic alkali [10], alk-
oxide [10], alkali metals [10] and alkyl- or phenyllithium
[11]. These cleavage reactions are used for both the syn-
theses of organotin compounds, RnSnY4 ꢀ n (especially
when Y = halide or carboxylate), as well as organic
compounds, R1–E, where E is an electrophile (Eq. (1)).
The order of cleavage by electrophilic reagents is:
allyl > phenyl > benzyl > vinyl > methyl > higher alkyl
[12]
or esters. A well-known example is the Stille reaction
[8]. It is still the method of choice for forming car-
bon–carbon bonds due to its mild characteristic and
the functional tolerability of both substrates and re-
agents. This is a useful protocol in various preparative
reactions as well as in the synthesis of bioactive sub-
stances [13].
Aryl cleavages by electrophiles are typical electro-
philic aromatic substitutions [14] as shown in Eq. (2).
However, a strong electrophilic reagent is normally
needed in such a cleavage reaction, e.g., FOSOꢀ4 Csþ,
[15] F2 [16], NO–Cl [17], etc. More electron-releazing
substituents (Z) lead to faster reactions [18]
E
SnR3
R3Sn
E
rate
determining
+ Y-
+ R3SnY
+ E
Y
+
R3Sn–R1 þ E–Y ! R3Sn–Y þ R1–E
ð1Þ
Z
Z
Z
Organotin compounds can also transfer organic
groups to electrophilic carbons in transition metal-
catalyzed cross-coupling reactions with organic halides
ð2Þ
The relative ease of cleavage can be predicted on the
basis of mechanistic studies such as kinetic data, substi-
tuent effects, solvent isotopic effects and stereochemistry
[18].
*
Corresponding author. Tel.: +1 202 274 7420; fax: +1 202 274 6237.
E-mail address: xsong@udc.edu (X. Song).
1387-7003/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2005.05.006

726
X. Song et al. / Inorganic Chemistry Communications 8 (2005) 725–728
In an attempt to synthesize Ph3SnSCH2COOSnPh3
compounds, 1a–g, with the general formula
{[Ph2SnCl(l2-SCH2COO–)] [HNR3]} as given in Table
1. Their structures were deduced by multiple NMR
(1H, 13C, 119Sn), and the structure of 1g was also con-
firmed by X-ray crystallography (Fig. 2).
However, when triphenyltin hydroxide was used in-
stead of triphenyltin chloride, no Sn–C cleavage resulted
and a second series of triorganotin compound, 2a–f,
[Ph3Sn(l2-SCH2COO–)] [HNR3], resulted (Table 1).
Replacing the phenyl group with either cyclohexyl, butyl
or neophyl resulted in the formation of a third series of
compound, 3a–c, R3SnSCH2COOSnR3, again with no
Sn–C cleavage as shown in Table 1.
according to Scheme 1, an unexpected tin-phenyl cleav-
age reaction occurred and resulted in the formation
of the ionic tin complex {[Ph2SnCl(l2-SCH2COO)]
[HNEt3]} (Table 1; 1a). Its structure was confirmed by
1
X-ray crystallography (Fig. 1) as well as H, 13C and
119Sn NMR spectroscopies.
The tin-phenyl cleavage was also observed when dif-
ferent amines were used under the same reaction condi-
tions resulting in the formation of an analogous series of
The tin-phenyl cleavage reaction was then reinvesti-
gated under different experimental conditions to confirm
the validation of the reaction. The reaction conditions
and results are given in Table 2. Tin-phenyl cleavage
was observed in all cases, except in Experiment 8, when
no amine was added to the reaction.
Scheme 1.
The data indicated that the amine is necessary for the
cleavage reaction to occur. A possible pathway was pro-
posed for the Sn–Ph cleavage reaction as shown in
Scheme 2.
The reaction is believed to involve an intermediate
species, [Ph3SnSCH2COOH Æ NR3], which further reacts
with NR Æ HCl to give the Sn–Ph cleavage product. The
Table 1
Three series of organotin compounds with mercaptoacetate ligand
_
Cl
1a
1b
1c
1d
1e
1f
R3N@NEt3
R3N@HNEt2
Ph
Ph
O
S
+
O
HNR3
Sn
R3N@HN(CHMe2)2
R3N@HNPr2
1a–g
R3N@HNBun2
R3N@HNCy2
R3N@a-methyl pyridine
_
1g
Ph
Sn
Ph
Ph
O
S
+
O
HNR3
2a
2b
2c
2d
2e
2f
R3N@NEt3
R3N@HNEt2
R3N@HN(CHMe2)2
R3N@HNPr2
R3N@HNBun2
R3N@HNCy2
2a–f
3a
3b
3c
a
R = butyl
R = cyclohexyl
R = neophyla
R3SnSCH2COOSnR3 3a–c
Neophyl = 2-methyl-2-phenylpropyl.
˚
Fig. 2. Molecular structure of 1g selected bond lengths (A) and
˚
Fig. 1. Molecular structure of 1a. Selected bond lengths (A) and bond
angles (°): Sn–Cl = 2.470(2), Sn–S = 2.399(2), Sn–O = 2.228(4), Sn–
C(1) = 2.133(7), Sn–C(2) = 2.126(6) and C(1)–Sn–C(2) = 118.4(2), S–
Sn–O = 81.3(1).
bond angles (°): Sn–Cl = 2.102(7), 2.138(8); Sn–S = 2.396(6), 2.402(6);
Sn–O = 2.247(8), 2.181(9); Sn–C(1) = 2.120(7), 2.148(7); Sn–C(2) =
2.482(6), 2.459(7) and C(1)–Sn–C(2) = 117.4(4), 113.7(6); S–Sn–
O = 82.0(3), 79.4(3).

X. Song et al. / Inorganic Chemistry Communications 8 (2005) 725–728
727
Table 2
Sn–Ph cleavage reactions conducted in various experimental conditions
Entry
Molar ratio of reactants
Solvent
Reaction time (h)
Yield (%)
Et3N
HSCH2COOH
Ph3SnCl
1
2
3
4
5
6
7
8
2
2
1
2
1
2
1
–
1
1
1
1
1
1
1
1
2
1
1
1
1
1
1
1
Toluene
Toluene
Toluene
Acetone
Acetone
Acetone
Acetone
Toluene
2
2
80
88
2
85
83
4
4
81
62
12
12
4
46
Polymer
[Ph3SnSCH2COO- HNR3] + Cl HNR3
-
+
+
NR3
_
It is believed that the sulfur containing ligands play a
major role in the weakening of the Sn–C bonds in these
reactions. It has been reported that the presence of a sul-
fur atom in a group attached to the tin atom weakens
the corresponding tin–carbon bond [19]. Cleavage by a
weak electrophilic reagent, such as ammonium chloride,
is possible. Similar Sn–C cleavage reactions have been
reported in the literature [20].
Ph3SnCl + HSCH2CO2H
Ph
_
Cl
+
Ph
Ph
+
Ph
O
O
S
O
O
+
HNR3 Ph H
HNR3
Sn
Sn
Ph
S
Scheme 2.
This is a first report of the cleavage of a Sn–Ph bond
by a mercaptoacetate ligand under mild reaction condi-
intermediates have been successfully isolated and identi-
fied when Ph3SnOH was used instead of Ph3SnCl in the
presence of various amines. The reaction of compound
2a–f with an appropriate NR3 Æ HCl resulted in Sn–Ph
cleavage products supporting the proposed pathway in
Scheme 2.
3
tions. It is also the first report of using J(119Sn–1H)
(–Sn–S–CH2) coupling constants to differentiate sulfur
containing organotin compounds.
Characteristic coupling constants 3J(119Sn–1H) (from
Acknowledgments
1
–Sn–S–CH2) were observed in the HNMR spectra for
all three series of compounds. The order of the coupling
119Sn–1H) >
Financial supports from the National Natural Sci-
ence Foundation of China and the State Commission
of Education of China as well as support for Dr. X.
Song from the National Institutes of Health, Minority
Biomedical Research Support Programs MBRS/Scores
GM08005, are gratefully acknowledged.
3
constants decreases in the order: J1a–g
(
3J2a–f 119Sn–1H) > 3J3a-c 119Sn–1H). This can be ascribed
( (
to the decrease of electron density around the tin atoms
3
in the three series. The coupling constant J(119Sn–1H)
can be used to distinguish the three series of compounds
as can be seen in Table 3.
Appendix A. Supplementary data
Table 3
Important 1H NMR data for compounds 1a–g, 2a–f and 3a–c
Crystallographic data for the structures reported in
this paper have been deposited with the Cambridge Crys-
tallographic Data Center and allocated the deposition
number CCDC 246685 for 1a and 246684 for 1g. Copies
of this information may be obtained free of charge from
the Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: +44 1223 336033; e-mail: deposit@ccdc.
cam.ac.uk or www: http://www.ccdc.cam.ac.uk). Supple-
mentary data associated with this article can be found, in
the online version, at doi:10.1016/j.inoche.2005.05.006.
d for SCH2 (ppm)
3J(119Sn–1H) (Hz)
1a
1b
1c
1d
1e
1f
3.48
3.46
3.47
3.44
3.46
3.43
3.59
47.5
44.6
46.6
45.6
46.0
48.0
48.0
1g
2a
2b
2c
2d
2e
2f
3.30
3.34
3.30
2.98
3.28
3.02
35.2
36.6
38.8
32.0
35.6
34.4
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3a
3b
3c
3.57
3.30
3.47
24.2
22.0
23.6
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