New and efficient synthesis of solid-supported organotin reagents and their use in organic synthesis

Article abstract of DOI:10.1016/j.jorganchem.2005.11.031

Novel resin-bound organotin reagents have been prepared, including for the first time resin-bound dimethyl tin reagents. Mild methodology has also been developed for the very efficient synthesis of resin-bound distannanes. The resin-bound tin chloride rea

Full text of DOI:10.1016/j.jorganchem.2005.11.031

Journal of Organometallic Chemistry 691 (2006) 1466–1475
www.elsevier.com/locate/jorganchem
New and efficient synthesis of solid-supported organotin
reagents and their use in organic synthesis
*
´
Alejandro G. Hernan, Peter N. Horton, Michael B. Hursthouse, Jeremy D. Kilburn
School of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, UK
Received 10 November 2005; accepted 11 November 2005
Available online 22 December 2005
Abstract
Novel resin-bound organotin reagents have been prepared, including for the first time resin-bound dimethyl tin reagents. Mild meth-
odology has also been developed for the very efficient synthesis of resin-bound distannanes. The resin-bound tin chloride reagents have
been used in a catalytic Stille coupling cycle and the resin-bound distannanes have been used in atom transfer cyclisations and proved to
be much more effective than previously described resin-bound distannanes. As expected the use of resin-bound tin reagents facilitated
their easy removal at the end of the reaction, and consequently residual tin levels in the organic products were low or negligible. The
resin-bound distannanes could not, however, be successfully used for the palladium catalysed stannylation of a simple aryl iodide, which
would have provided a useful approach to radiolabeling of aromatic substrates. The reasons for the failure in the stannylation process is
unclear but crystal structure evidence indicates that there is a hypervalent interaction between the resin-bound tin atom and an adjacent
ether oxygen which may effect the reactivity of the tin intermediates in the stannylation sequence.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Organotin; Distannane; Solid-phase; Stille coupling; Atom transfer
1. Introduction
over the conventional Stille coupling of a vinyl stannane
and organic halide, which generates stoichiometric tin
byproducts [3]. Thus, in situ reduction of the trialkyltin
chloride to trialkyltin hydride and hydrostannylation of
an alkyne 1, was used to generate a vinyl stannane 2, which
was coupled to an organic halide 3 using palladium cataly-
sis. The catalytic cycle (Scheme 1) uses as little as 5 mol%
of tin reagent, although trimethyltin chloride performed
significantly better than tributyltin chloride. The protocol
raised the possibility of using a resin-bound tin reagent
for the in situ hydrostannylation of the alkyne component,
and hence easy removal of all tin contaminants from the
coupling reaction on completion.
Organotin compounds are widely used in organic syn-
thesis and are particularly useful in C–C bond forming
reactions as both reagents and as coupling partners. How-
ever, the toxicity and difficulty of removing organotin com-
pounds from final products does limit the use of these
methodologies, particularly in an industrial context. One
solution to this problem has been to use solid-supported
(resin-bound) tin reagents which can be removed at the
end of a reaction by simple filtration, as well as having
the potential for recycling, and a number of variants have
been described [1].
We were initially attracted to the use of resin-bound tin
reagents by recent work by Maleczka et al. [2], who devel-
oped a Stille coupling protocol that involved catalytic use
of trialkyltin chloride, providing a significant advantage
In order to investigate the possibility of using resin-
bound tin reagents in this way, we thus set out to prepare
both tributyl and trimethyl tin chlorides and the corre-
sponding hydrides. In the course of this work, we discov-
ered a novel and very efficient method for the preparation
of resin-bound distannanes, which we found could be used
very effectively in iodine atom-transfer cyclisations. These
*
Corresponding author. Tel.: +44 2380593596.
E-mail address: jdk1@soton.ac.uk (J.D. Kilburn).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.11.031

A.G. Herna´n et al. / Journal of Organometallic Chemistry 691 (2006) 1466–1475
1467
R2
prepared on a small scale by reduction of Me2SnCl2 using
Bu3SnH, followed by distillation, according to KuvilaÕs
method [6]. The Me2SnH2, which decomposes rapidly [7],
was mixed immediately with Me2SnCl2 to produce
Me2SnHCl, and used for the hydrostannylation of the allyl
ether. The excess of Me2SnHCl used in the synthesis of 6
decomposed during the reaction forming a grey precipitate
(presumably metallic tin) which was removed by an acidic
wash (1:1 1 N HCl/MeOH) to give resin 6 as a white solid.
Both polymers 5 and 6 were characterized by microanalysis
and by gel-phase 13C and 119Sn NMR. The NMR data
were consistent with the literature [1c,8].
X
cat. R3SnCl
aq. Na2CO3, PMHS
R1
R1
H
R2
cat. Pd2dba3,
trifurylphosphine
Et2O, reflux
R2
R1
X
3
SnR3
R1
+ Pd(0)
H
2
+ Pd(0)
1
R1
R2
Neumann has described the preparation of a polymer-
supported distannane 11 by reaction of dialkyl tin chloride,
linked to a macroporous polystyrene resin, with a soluble
reactive metal (e.g., lithium naphthalenide or magnesium/
anthracene complex) [9]. We found this methodology unre-
liable when applied to resins 5 and 6, and instead developed
an alternative approach for conversion of the dialkyltin
dichloride resins to the corresponding resin-bound distann-
anes. Thus, reduction of dimethyltin chloride resin 6 with
LiAlH4 [10] gave the corresponding tin hydride resin 8
(Scheme 2) in quantitative yield as determined by elemental
analysis, gel-phase 13C and 119Sn NMR. Addition of cata-
lytic (Pd(PPh3)4) (5 mol%) to resin 8, pre-swollen in tetra-
hydrofuran, resulted in immediate evolution of H2 gas
[11]. The IR spectrum of the resulting resin 10 showed
the total disappearance of the IR stretch for Sn–H
(ꢀ1800 cmÀ1). Magic angle spinning (MAS) 119Sn NMR
of resin 10 revealed a single peak at a very similar chemical
shift (À99.57 ppm relative to Me4Sn) compared to that
observed for tin hydride resin 6 (À98.22 ppm relative to
Me4Sn), but the peak for 10 was much broader than that
for 8 presumably as a consequence of the more restricted
mobility of the cross-linked ditin resin compared to the
tin hydride resin. A spectrum (MAS 119Sn NMR) of a
1:1 mixture of 10 and 8 and comparison with the spectra
for the separate resins confirmed the 119Sn peak assign-
ments and indicates that conversion of the tin hydride to
the ditin is essentially quantitative using this palladium
catalysed dehydrogenation methodology. An identical
sequence of reactions was used to convert dibutyltin chlo-
ride resin 5 to ditin resin 9. However, after reduction with
LiAlH4, 119Sn NMR analysis of 7 showed two signals at
similar shifts (À80.3 ppm (broad) and À86.2 ppm (sharp))
R3SnX
R3SnH
aq. Na2CO3, PMHS
Scheme 1.
resin-bound tin reagents were also investigated as potential
catch-and-release reagents for the radio isotopic labeling of
aromatic compounds. This paper describes in detail the
results of all of these studies [4].
2. Results and discussion
2.1. Synthesis of resin bound tin reagents
Several examples of resin-bound dibutyltin chloride and
hydride reagents have been described previously [1] and
have been investigated in tin hydride mediated radical reac-
tions and stoichiometric Stille couplings, but to our knowl-
edge the analogous resin-bound dimethyltin chloride and
hydride reagents have not been described. The dialkyltin
chloride resins 5 and 6 were prepared by hydrostannylation
of an allyl ether, which was prepared in turn from Merri-
field resin 4 (1.6 mmol/g, 1% DVB) [1f,1g,1i]. Hydrostan-
nylation of the alkene with Bu2SnHCl was accomplished
using NeumannÕs procedure [1a], which involves in situ
preparation of Bu2SnHCl from a 1:1 mixture of Bu2SnH2
[5] and Bu2SnCl2, and gave dibutyltin chloride resin 5 as
a white solid.
The dimethyltin chloride resin 6 was prepared by an
analogous procedure. Me2SnH2 was most conveniently
i)
HO
R
R
Sn
R
R
NaH, DMF
O
O
Sn R
Sn R
R
5 mol%
Cl
O
X
Sn R
Sn R
R
ii) R2SnCl2, R2SnH2,
AIBN, hν, toluene
Pd(PPh3)4
5 R = nBu, X = Cl
6 R = Me, X = Cl
4
9
R = nBu
10 R = Me
LiAlH4
7 R = nBu, X = H
8 R = Me, X = H
11 R = nBu
Scheme 2.

1468
A.G. Herna´n et al. / Journal of Organometallic Chemistry 691 (2006) 1466–1475
revealing a spontaneous partial dehydrogenation [12] to the
ditin polymer 9, which was competed by treatment with
catalytic (Pd(PPh3)4) (5 mol%).
After chromatographic purification the reaction prod-
ucts were obtained microanalytically pure and tin analy-
sis showed low or insignificant tin content (<5–60 ppm)
[14]. In comparison, using the same chromatographic
purification of reactions conducted using soluble trime-
thyltin chloride, gave products with tin impurities
>500 ppm.
2.2. Application of trialkyltin chloride resins in the catalytic
Stille reaction
The resin bound tin chloride resins 5 and 6 were tested in
the catalytic Stille coupling reaction protocol devised by
Maleczka et al. [2]. In a typical procedure the tin chloride
resin (6–100 mol%) was suspended in an aqueous
Na2CO3/THF mixture (1:20), with an alkyne 12, palladium
catalyst system (1 mol% Pd2dba3, 1 mol% PdCl2(PPh3)2
and 4 mol% tri-2-furylphosphine) and polymethyl-
hydrosiloxane (PMHS) (1.5 equiv.). Aryl iodide or bromide
13 (1.5 equiv.) was added over 15 h by syringe pump. Using
these optimised conditions, as reported by Maleczka with
trimethyltin chloride, the coupling of b-bromostyrene and
o-iodoanisole with various alkynols [13] proceeded in good
yield (Table 1) despite the multiple phases in which the cat-
alytic cycle has to take place (aqueous, organic and poly-
meric). Substoichiometric (30 mol%) amounts of the
dimethyltin chloride resin 6 could be used satisfactorily,
but lower amounts of resin (6 mol%) did give significantly
reduced yields. Clearly the reducing agent PHMS is able
to regenerate the polystyrene bound R(Me)2SnH in
contrast to its reported ineffectiveness for the regenera-
tion of R(Bu)2SnH in the polymer supported catalytic
Barton–McCombie deoxygenation of secondary alcohols
[1h].
The dimethyltin chloride resin 6 also gave much better
results than the dibutyltin chloride resin 5 which is in good
agreement with the superiority of Me3SnCl in comparison
with Bu3SnCl in MaleczkaÕs cycle. Maleczka reported that
the catalytic cycle suffers from deactivation of the organo-
tin hydride intermediates by oxidative coupling to give dist-
annane species and this restricted the range of palladium
catalysts that could be used successfully in the catalytic
cycle. We also found that with the resin-bound tin cata-
lysts, a dramatic decrease in yield was observed when
changing to other solvents (DMF) or catalytic systems
(Pd(PhCN)2Cl2/Ph3As) routinely used in Stille couplings.
We had thought that the catalytic cycle might be more tol-
erant to various reaction conditions using polymer-
supported organotin reagents since the isolation of the
reactive sites by the solid support might result in the sup-
pression of the oxidative coupling – although, of course,
as we later discovered, in practice palladium mediated oxi-
dative coupling is an effective method to prepare the resin
bound distannane from the tinhydride resins 7 and 8 (vide
supra) [15]. Using resins with a lower loading, and hence
reduced potential for efficient oxidative coupling to give
the distannane, might nonetheless prove useful in this
Table 1
Yields of Stille coupling using catalytic resin bound tin chlorides
OH
Resin 5 or 6
HO
Me
Ar
+
Ar-X
Me
conditions
R
R
12
13
14
ArX
Alkyne
Resin
Resin equivs
Conditions
Yield of v (%)
4-Iodoanisole
4-Iodoanisole
4-Iodoanisole
4-Iodoanisole
4-Iodoanisole
4-Iodoanisole
4-Iodoanisole
4-Iodoanisole
4-Iodoanisole
R = Ph
R = Ph
R = Ph
R = Ph
R = Ph
R = Ph
R = Me
R = Me
R = C6H11
6
6
6
5
6
6
6
6
6
1.0
0.3
0.06
1.0
1.0
1.0
0.3
0.3
0.3
a
a
a
a
b
c
a
c
a
63
60
21
20
5
0
57
10
55
b-Bromostyrene
b-Bromostyrene
b-Bromostyrene
b-Bromostyrene
b-Bromostyrene
R = Ph
R = Ph
R = Me
R = Me
R = Ph
6
5
6
6
6
0.3
1.0
0.3
0.3
0.3
a
a
a
c
50
15
67
7
a
60
Bromobenzene
Bromobenzene
R = Ph
R = Ph
6
6
0.3
1.0
a
a
9
0
Conditions: (a) 1 mol% Pd2Cl2(PPh3)2, 1 mol% Pd2dba3, 4 mol% (2-furyl)3P, 1.5 equiv. PMHS, Na2CO3, H2O/THF, reflux; (b) 1 mol% Pd(PhCN)2, Ph3As,
1.5 equiv. PMHS, Na2CO3, H2O/THF, reflux; (c) 1 mol% Pd2Cl2(PPh3)2, 1 mol% Pd2dba3, 4 mol% (2-furyl)3P, 1.5 equiv. PMHS, Na2CO3, H2O/DMF,
reflux.

A.G. Herna´n et al. / Journal of Organometallic Chemistry 691 (2006) 1466–1475
1469
Table 2
context. Reaction optimisation using the less reactive Stille
Yields from cyclisation of iodide 15
coupling partner bromobenzene was also unsuccessful.
O
O
I
9 or 10
2.3. Application of the resin-bound distannanes in iodine
atom transfer cyclisations
N
N
C6H6, hn, reflux
Free radical reactions are an important tool in organic
synthesis and are routinely used in the preparation of even
the most complex target molecules [16]. Stannyl radicals
have played a central role in the development of this chem-
istry, and tributyltin hydride has been one of the most pop-
ular reagents for propagating radical reactions. Distannane
reagents have also been a popular source of stannyl radi-
cals since homolytic cleavage of tin–tin bonds takes place
under mild conditions [17]. Distannane reagents have been
widely used, in particular, for atom transfer cyclisations
[18] where premature termination of the radical chain by
a hydrogen atom donor is to be avoided. In order to avoid
problems associated with the toxicity and difficulty of
removing organotin compounds from final products there
have been several approaches to developing alternative
reagents for radical reactions [19], or modified tin reagents
which can be more easily removed at the end of the reac-
tion [20]. In the latter case there have been several studies
on polymer-supported tin hydride reagents but surprisingly
little has been reported on the preparation and utility of
polymer-supported distannanes. Neumann has described
the polymer-supported distannane 11, linked to a macro-
porous polystyrene resin, but utilisation of this resin in
atom transfer cyclisations led to significant amounts of
reduced products as well as the desired cyclised product
(vide infra) [9], presumably either as a consequence of trap-
ping of the radical intermediates by hydrogen atoms from
the polymer backbone [21], or because of tin hydride impu-
rities in the polymer-supported distannane reagent [22].
Both resins 9 and 10 were used in two atom transfer
reactions of iodides 15 and 17. In a typical experiment
the polymer-supported distannane was suspended in a
0.03 M solution of the iodide and the suspension heated
to ꢁ80C and irradiated by a 100-W UV Lamp. Reactions
were monitored by GC and final products were purified
by column chromatography to give isolated yields.
I
15
16
Entry Ditin reagent Equivs. ditin reagent Time (h) Yieldsa(%)
15 16
1
2
3
7
7
8
0.1
0.1
0.1
1
4
1
24 76 (67)
0
0
98 (90)
99 (93)
a
Yields determined by GC. Isolated yields are in parenthesis.
even 2 mol%) but yields of cyclised product were reduced
even after prolonged reaction (Table 3, entries 7–10).
The results are significantly better than those described
previously by Neumann and co-workers [9], who reported,
for example, that cyclisation of the same substrate 17,
using macroporous ditin resin 11, led to mixtures of
reduced starting material 20 and cyclised products 18
and 19 (Table 3, entries 17 and 18). The significantly better
performance of 9 and 10 can probably be attributed to the
greater flexibility and accessibility of the polystyrene resin
used here, and probably to the purity of the distannane
reagent. The results are also on a par with those achieved
using R6Sn2 reagents in solution. Clearly premature
quenching of radical intermediates by hydrogen atoms
abstracted from the solid support is not an issue in this
system.
As anticipated the polymer-supported ditin reagent can
be easily removed from the reaction by filtration and anal-
ysis of isolated products revealed very low residual tin lev-
els [10] (<5–34 ppm) for the various samples tested. The
polymer-supported distannane can also be recycled for fur-
ther use (Table 3, entries 11–16). Resin 10 was recovered
from an initial iodine-transfer cyclisation of 17 (Table 3,
entry 6). The resin was dried under vacuum and then
reused for the same reaction giving a 25% yield (GC) of
18 after 16 h (Table 3, entry 13). However, after reduction
with LiAlH4 and subsequent reaction with Pd(PPh3)4, the
same sample of resin then gave a 79% yield (GC) of 18 after
16 h (Table 3, entry 16).
Cyclisation of diallyl amide 15 (Table 2) was carried out
with 10 mol% of ditin reagent and very high yields of cyc-
lised product 16 were obtained with no reduced byproducts
being detected. The reaction was essentially complete in 1 h
using the methyl substituted ditin resin 8, but was some-
what slower using the sterically more encumbered butyl
substituted ditin resin 7.
2.4. Application of resin-bound tin reagents for the
preparation of isotopically labeled aromatics
The more demanding cyclisation of iodoester 17 was
studied in greater detail. Good yields of the cyclised prod-
uct 18 were again obtained using 10 mol% of ditin reagent
but the reaction time required was ꢀ16 h for completion
(Table 3, entries 1–6). Again the sterically more accessible
ditin polymer 10 appeared to perform somewhat better
than 9. The cyclisation could also be carried out using
lower amounts of polymer-supported ditin (5 mol% and
The halodestannylation of arylstannanes is an efficient
method for the mild and selective introduction of a halogen
atom in an aromatic ring [23]. This methodology has been
used to for the production of radiolabelled (iodinated and
fluorinated) aromatic substrates [24] and is particularly
useful since the short half-life of the fluorine and iodine iso-
topes precludes the storage of the radioactive compounds,

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A.G. Herna´n et al. / Journal of Organometallic Chemistry 691 (2006) 1466–1475
Table 3
Yields from cyclisation of iodide 17
O
O
O
I
O
O
O
9 or 10
+
C6H6, h , reflux
X
17
18 X = I
19 X = H
20
Entry
Ditin reagent
Equivs. ditin reagent
Time (h)
Yieldsa(%)
17
18
19
20
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
a
9
9
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.05
0.05
0.02
0.1
0.1
0.1
0.1
0.1
0.1
0.5
0.1
1
4
16
1
4
16
1
70
58
14
60
14
0
75
67
55
65
100
90
61
90
69
9
30
34
80
40
83 (78)
92 (90)
16
22
33
30
0
5
25
8
26
79
–
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
35
5
0
5
6
0
0
8
2
2
2
5
0
3
10
2
3
7
61
50
9
10
10
10
10
10
10
4
16
16
1
4
16
1
4
16
–
10b
10b
10b
10c
10c
10c
11d
11d
–
36
Yields determined by GC except for those in parenthesis which are isolated yields.
Recovered directly from previous reaction (see text).
Recycled resin (treated with LiAlH4, then Pd(0)) from previous reaction (see text).
Published results, see Ref. [9].
b
c
d
necessitating the rapid synthesis and purification of these
and are easily removed by filtration. This approach has
been investigated, particularly by Hunter [25], who found,
for example, that resin-bound tin chlorides could be treated
with aryl lithium reagents to give resin-bound aryl stann-
anes. Subsequent treatment with iodine gave the desired
aryl iodide in good yield and easy removal of the resin-
bound tin by-products (Scheme 3).
This approach is restricted, however, by the need to gen-
erate the aryl metal reagent which inevitably means that it
is incompatible with some functional groups and hence
with some substituted aromatics that might be of interest
compounds in situ. However, the tin by-products have to
be removed due to the low tolerance of most radiopharma-
ceutical applications to this kind of compound. Conse-
quently, there is interest in the development of solid-
phase techniques that facilitate the synthesis and the fast
purification of such radiopharmaceuticals [25]. Thus,
attachment of the arylstannane precursor to an insoluble
solid support followed by halodestannylation leads to
release of the halogenated compound into solution while
the tin byproducts remain attached to the solid support
R
ArLi
Sn Cl
R
R
R
R
*I2
Sn Ar
R
+ Ar*I
Sn *I
R
Sn R
Sn R
R
ArX, Pd(0)
X = I, Br, OTf
Scheme 3.

A.G. Herna´n et al. / Journal of Organometallic Chemistry 691 (2006) 1466–1475
1471
as radiopharmaceuticals. An alternative approach for the
formation of arylstannanes involves the relatively mild pal-
ladium mediated coupling of an aryl iodide (or bromide,
triflate, etc.) with a hexaalkyldistannane [26]. The availabil-
ity of resin-bound distannanes 9 and 10 prompted us to
investigate the possibility of developing an analogous
solid-phase based methodology and a novel catch-and-
release approach to radiolabeled aryl halides (Scheme 3).
Thus, using iodoanisole as a model compound, the pal-
ladium mediated coupling of the iodoanisole with resin-
bound distannanes 9 and 10 was attempted under a range
of conditions (5 mol% Pd(PPh3)4 in refluxing dioxane,
DMF, THF, toluene). However, although the complete
consumption of the iodoanisole could be observed by mon-
itoring the coupling reaction by hplc, no satisfactory anal-
ysis could be obtained for the product resin and indeed
subsequent treatment of the product resin with iodine did
not lead to release of any iodoanisole. Treatment of the
tin chloride resin 6 with the aryl lithium derived from iod-
oanisole, on the other hand, did give the desired resin-
bound arylstannane which on treatment with iodine led
to release of significant quantities of iodoanisole.
The failure of the resin-bound distannanes to perform as
anticipated in the palladium mediated coupling with a sim-
ple aryl iodide was disappointing and somewhat perplex-
ing. In HunterÕs work [25], the resin-bound tin chlorides
were prepared using rigid macroporous resins rather than
the more flexible Merrifield resins used for the preparation
of 9 and 10. With this in mind we successfully prepared the
corresponding distannane resins on commercial Argo-pore
resin, but these were no more successful in attempted cou-
pling with iodoanisole. It is likely that the ether linkage
used in the preparation of our organotin resins influences
the reactivity of the distannanes and indeed we were able
Fig. 1. Crystal structures of (a) compound 23 and (b) compound 24.

1472
A.G. Herna´n et al. / Journal of Organometallic Chemistry 691 (2006) 1466–1475
Cl
O
Sn
R
R2SnCl2, R2SnH2,
LiAlH4
Et2O
5 mol%
Sn
H
Sn
Sn
R
O
AIBN, hν, toluene
Pd(PPh3)4
R
R
R
27 R = C6H5O(CH2)3
28 R = HOC6H4O(CH2)3
25 R = C6H5O(CH2)3
26 R = HOC6H4O(CH2)3
21 R = H
22 R = OH
23 R = H
24 R = OH
Scheme 4.
bound dimethyl tin reagents. Mild methodology was also
developed for the very efficient synthesis of resin-bound dist-
annanes. The resin-bound tin chloride reagents were success-
fully used in a catalytic Stille coupling cycle and the resin-
bound distannanes proved to be much more effective in atom
transfer cyclisations than previously described resin-bound
distannanes, which had been prepared on macroporous solid
supports. As expected the use of resin-bound tin reagents
facilitated their easy removal at the end of the reaction (Stille
coupling or atom transfer cyclisation), and consequently
residual tin levels in the organic products were low or negli-
gible. In cases where residual tin was detected this is likely to
be a consequence of mechanical, rather than chemical, deg-
radation of the resin on stirring, which could be addressed
with greater attention to the stirring devices used. The
resin-bound distannanes could not, however, be successfully
used for the palladium catalysed stannylation of a simple
aryl iodide, which would have provided a useful approach
to radiolabeling of aromatic substrates. Given the success
of the resin-bound reagents in the Stille coupling and atom
transfer cyclisations the reasons for the failure in the stanny-
lation process is unclear but crystal structure evidence indi-
cates that a hypervalent interaction between the ether
oxygen and the tin atom may well effect the reactivity of
the tin intermediates in the stannylation sequence, and anal-
ogous solution versions of the various ether substituted tin
reagents also failed to undergo the stannylation reaction
under conditions which were successful using Me6Sn2
instead. This suggests that using an alternative linker to the
ether used here might prove more successful, but an investi-
gation of this possibility is beyond the scope of this study.
Table 4
˚
Bond lengths (A) and angles (ꢁ) for compounds 23 and 24
˚
Bond lengths (A)
and angles (ꢁ)
23
24
Molecule 1
Molecule 2
Sn–O
Sn–Cl
Sn–C (chain)
Sn–C (Me1)
Sn–C (Me2)
2.773(2)
2.4211(18)
2.145(3)
2.130(3)
2.132(3)
2.536(9)
2.482(3)
2.162(13)
2.127(11)
2.109(10)
2.507(7)
2.498(3)
2.157(11)
2.133(11)
2.089(14)
Cl–Sn–O
169.07(5)
170.4(2)
170.0(2)
Cl–Sn–C (chain)
Cl–Sn–C (Me1)
Cl–Sn–C (Me2)
97.72(9)
100.34(10)
100.81(10)
95.2(4)
97.9(4)
95.8(4)
95.3(3)
97.5(4)
96.0(3)
O–Sn–C (chain)
O–Sn–C (Me1)
O–Sn–C (Me2)
71.40(10)
84.87(11)
85.44(11)
75.3(4)
88.3(4)
87.4(4)
75.0(4)
85.5(4)
90.8(4)
C (chain)–Sn–C (Me1)
Cl (chain)–Sn–C (Me2)
C (Me1)–Sn–C (Me2)
120.69(15)
116.19(14)
114.95(16)
122.2(5)
114.8(5)
119.3(5)
119.3(5)
118.2(5)
118.9(6)
to obtain crystal structures (Fig. 1) [27] of the aryloxy-
ethyl(dimethyl)tin chlorides 23 and 24, prepared from the
corresponding allyl ethers (Scheme 4) which reveal a strong
hypervalent interaction of the ether oxygen and the tin [28]
in the solid state.
For compound 23 the crystal structure (Fig. 1) has one
molecule in the asymmetric unit, but for 24 there are two
distinct molecules in the asymmetric unit. In the crystal
˚
structure of 23 the Sn–O bond length is 2.77 A, whereas
in 24 the Sn–O bond length is shortened to 2.54 or
˚
2.51 A because of the electron donating effect of the para
methoxy group. As a consequence the Sn–Cl bond is
slightly longer in 24 than in 23 (see Table 4).
4. Experimental
The tin chlorides 23 and 24 could be reduced to the cor-
responding tin hydrides 25 and 26 (LiAlH4, Et2O) and con-
verted to the distannanes 27 and 28, in a sequence identical
to that used for the solid-phase synthesis. The soluble dist-
annanes, 27 and 28, however, proved to be rather unstable
and decomposed on standing in CDCl3 solution. Further-
more, attempted coupling of the distannanes to iodoanisole
using 5 mol Pd(PPh3)4 in dioxane was not successful,
although the same conditions were successfully used for
the coupling of anisole using Me6Sn2.
All manipulations and handling were performed under
an inert Ar atmosphere using standard Schlenk techniques.
Dry solvents were obtained by distillation from calcium
hydride. Et2O and THF were distilled under an Ar atmo-
sphere from a purple Na/benzophenone mixture. TLC on
SiO2 plates with UV detection (220 and 330 nm) and phos-
phomolybdic acid as visualising reagent. GC: column Hew-
lett–Packard HP5; temperature gradient of 80–250 ꢁC at
25 ꢁC/min. Fourier transform infrared spectra taken with
a Thermo-Mattison spectrometer equipped with a SPE-
CAC ATR unit. 1H and 13C NMR experiments were
performed on a Bruker AM300 NMR spectrometer. 119Sn
3. Conclusions
Novel resin-bound organotin reagents were prepared on
standard Merrified resin, including for the first time resin-
NMR measurements were performed on
a Bruker
DPC400 NMR. Magic-Angle Spinning 119Sn NMR mea-

A.G. Herna´n et al. / Journal of Organometallic Chemistry 691 (2006) 1466–1475
1473
surements were performed by the EPSRC solid-state NMR
4.4. Dimethyltindihydride (Me2SnH2)
service at the University of Durham Industrial Research
Laboratories. The 119Sn chemical shifts are referenced to
Sn(CH3)4. Tin analysis was carried out by digesting the
sample in sulfuric acid followed by inductively coupled
plasma (ICP) analysis and was performed by Medac Ltd.
This product was obtained by modification of KuivilaÕs
procedure [6]. A more convenient method for a large-scale
synthesis is described elsewhere [29]. Me2SnCl2 (14 g;
64 mmol) was charged in a 250 mL two-necked flask under
Ar. The flask was equipped with an addition funnel and to
two consecutive traps at À78 ꢁC via glass connections.
After purging with Ar the system was evacuated with a
water pump (15 mmHg). Bu3SnH (66 mL; 245 mmol) was
slowly added under stirring. The flask was heated to
60 ꢁC and the pressure adjusted compatible with the evolu-
tion of gas. After approximately 90 min and cessation of
gas evolution, the system was purged with Ar and 6 mL
Me2SnH2 (92% yield) were obtained in the À78 ꢁC ice trap.
The necessary amount was used immediately for the next
hydrostannylation step. The excess of unused dimethyltin-
dihydride was destroyed with dilute aq HCl at À78 ꢁC
(warning: H2 evolution).
4.1. Poly(4-allyloxymethyl)styrene [1f]
Allyl alcohol (6.1 mL; 70 mmol) was slowly added over
45 min to a mixture of NaH 95% (2.55 g; 106 mmol) in dry
DMF (40 mL) at 0 ꢁC. The solution was stirred for 2 h at
room temperature and transferred via cannula to a suspen-
sion of Merrifield resin 4 (5 g; 1.6 mmol/g; 8 mmol) in dry
DMF (20 mL). The mixture was gently stirred for 20 h at
50 ꢁC. The excess of sodium hydride was destroyed by a
slow addition of EtOH 95% at 0 ꢁC and the polymeric
material collected by filtration and washed with ethanol,
water, acetone, DCM and ether (2 · 50 mL each, stirring
for 10 min). After drying under vacuum at 40 ꢁC for 2 days,
a cream coloured polymer was obtained (5.1 g, 99%); mmax
(ATR, cmÀ1) 1620 (C@C), 917, 1000 (–CH@CH2); 13C
NMR (gel-phase in C6D6, 75.5 MHz): d 134.88, 116.91,
71.95, 70.85, 40. Microanalysis: Cl <0.1%.
4.5. Poly[(4-(3-dimethylchlorostannyl)-
propyloxymethyl)styrene] (6)
Poly(4-allyloxymethyl)styrene (5 g; 8 mmol), Me2SnCl2
(4.4 g; 20 mmol) and AIBN (100 mg; 0.6 · 10À3 mmol)
were suspended in 60 mL benzene. The reaction was kept
at 20 ꢁC in a circulating water bath. Me2SnH2 (2.1 mL;
20 mmol) was cannulated into the reaction vessel and the
mixture irradiated with UV light (220–240 V, 50 Hz, 2 A)
for 16 h. Another portion of AIBN (100 mg;
0.6 · 10À3 mmol) was added after 8 h. The polymeric mate-
rial was collected by filtration and washed with toluene,
ethanol, acetone, toluene, acetone and dichloromethane
(2 · 50 mL each and shaken for 10 min before filtering).
The grey polymer obtained was suspended for 2 h in a
1:1 mixture of 1 N HCl in methanol (50 mL). After wash-
ing with methanol, water, ether (2 · 50 mL each and sha-
ken for 10 min before filtering) the resin was dried under
vacuum at 40 ꢁC for 2 days and a white polymer obtained
(6.2 g, 96%). 13C NMR (gel-phase in C6D6, 75.5 MHz): d
73.2, 71.1, 40, 25.9, 16.3, À0.1. 119Sn MAS NMR (C6D6;
112 MHz): d 59. Microanalysis: Sn 14.9% (1.3 mmol/g);
Cl 4.81% (1.4 mmol/g).
4.2. Dibutyltindihydride (Bu2SnH2) [1d]
NaBH4 (6 g; 160 mmol) was dissolved in 100 mL water
at 0 ꢁC and deoxygenated by bubbling Ar for 30 min. A
solution of Bu2SnCl2 (9 g; 30 mmol) in 100 mL Et2O was
slowly added over a period of 45 min and the solution stir-
red for another 15 min. The organic layer was separated,
washed with water (2 · 25 mL), dried over MgSO4 and
concentrated under vacuum to give Bu2SnH2 as a colour-
less oil (6 g, 85%). mmax (neat, cmÀ1) 1810 (Sn–H).
4.3. 3-(Dibutylchlorostannyl)-propyloxymethyl-polystyrene
(5)
The method was adapted from a procedure described by
Enholm and Schulte [1f]. To a suspension of poly(4-allyl-
oxymethyl)styrene (2 g; 3.1 mmol), nBu2SnCl2 (2.92 g;
9.6 mmol), AIBN (60 mg; 0.46 mmol) in dry toluene
(50 mL) nBu2SnH2 was slowly added (2.25 g; 9.5 mmol).
The reaction flask was placed in a water bath to maintain
the temperature below 20 ꢁC and irradiated (220–240 V,
50 Hz, 2 A). Portions of AIBN (60 mg each) were added
every 12 h. After a total reaction time of 31 h the polymeric
material was collected by filtration and washed with tolu-
ene, ethanol, acetone, toluene, acetone and dichlorometh-
ane (2 · 50 mL each and stirred for 10 min before
filtering). The resin was dried under vacuum at 40 ꢁC for
2 days to give a white polymer (2.56 g, 65%). 13C NMR
(gel-phase in C6D6, 75.5 MHz): d 73.1(br), 71, 40, 27.8,
26.7, 26, 18.4, 15, 13.7. 119Sn MAS NMR (gel-phase in
4.6. 3-(Dibutylhydrostannyl)-propyloxymethyl-polystyrene
(7) and 3-(dimehtylhydrostannyl)-propyloxymethyl-
polystyrene (8)
The reduction of tin chloride resins 5 and 6 was per-
formed according to the procedure described by Wong
and co-workers [10]. A slurry of the resin 5 or 6 (3 g;
5 = 3.1 mmol, 6 = 3.9 mmol) in 45 mL anhydrous tetrahy-
drofuran was cooled to À55 ꢁC under an Ar atmosphere
and treated with an excess of a lithium aluminium hydride
(12 mL of a 1 M solution in THF, 12 mmol). After the
addition was completed the mixture was allowed to warm
C6D6; 112 MHz):
d
63.8. Microanalysis: Sn 13%
(1.1 mmol/g), Cl 3.9% (1.1 mmol/g).

1474
A.G. Herna´n et al. / Journal of Organometallic Chemistry 691 (2006) 1466–1475
to room temperature and stirring was continued for
another hour. The resulting beads were collected by filtra-
tion under argon and washed rapidly with anhydrous THF
and Et2O (5 · 50 mL each and 10 min shaking before filtra-
tion) and finally evacuated in a dessicator at room temper-
ature for 24 h to give 2.9 g 7 and 2.8 g 8. Polymer 7: mmax
(ATR, cmÀ1) 1803 (Sn–H). 13C NMR (gel-phase in C6D6,
75.5 MHz): d 74 (br), 40, 30.3, 29.4, 26.7 (br), 13.3, 9.7,
7.8, 5.9. 119Sn NMR (gel-phase in C6D6; 112 MHz): d
À80.3 (small, br), À86.2. Microanalysis: Sn 12.5%
1822 (Sn–H). 13C NMR (gel-phase in C6D6, 75.5 MHz): d
73.1 (br), 45.6 (br), 40 (br), 26.8, 5.3, À12.9 119Sn MAS
NMR (C6D6; 112 MHz): d À98.22. Microanalysis: Sn
14.8% (1.25 mmol/g), Cl <0.1%.
4.9. Standard procedure for atom transfer cyclization
procedure: 4-iodo-cis-hexahydro-2-coumarone (18)
The procedure for the radical 5-exo cyclization was
adapted from that described by Curran and Tamine [30].
In a typical experiment the polymer-supported distannane
7 or 8 was suspended in 25 mL of a 0.03 M solution of
the cyclohexen-2-en-1-yl iodoacetate 17 (or iodide 15) in
benzene and the suspension heated to 80 ꢁC. The mixture
was irradiated with a 100-W UV Lamp (Blak Rayꢂ, 220–
240 10% V, 50 Hz, 2.0 A) positioned above and to the
side of the heating bath at a distance of 14 cm. Aliquots
of 100 ll were taken to monitor the reaction by TLC and
GC. When an isolated yield is reported, the resin was fil-
tered off and washed with benzene (3 · 30 mL, 10 min
shaking), the combined filtrates concentrated and the crude
solid product 18 purified by flash chromatography (hex-
anes/ethyl acetate, 3/1, v/v). Analytical data of isolated
18 (1H; 13C; EIMS) was consistent with that reported in
the literature [30].
(1.06 mmol/g) Cl <0.1%. Polymer 8: mmax (ATR, cmÀ1
)
4.7. Crosslinked bis-3-(methylpolystyryl)-propyloxy-
tetrabutyl distannane (9) and crosslinked bis-3-
(methylpolystyryl)-propyloxy-tetramethyl distannane (10)
Polymer-supported tin hydride
7
or
8
(1 g;
7 = 1.06 mmol, 8 = 1.25 mmol) was suspended in dry
THF (20 mL). The suspension was deoxygenated by bub-
bling with Ar for 15 min. Pd(PPh3)4 (5 mol%) was added
under a positive pressure of Ar and the suspension heavily
stirred to allow dissolution of the catalyst. After evolution
of gas (H2) the mixture was very slowly stirred for 4 h. The
resin was filtered and washed under Ar with dry THF
(3 · 30 mL, 10 min shaking before each filtration). No
weight difference was observed for either polymer 9 or
10. Polymer 9: 13C NMR (gel-phase in C6D6, 75.5 MHz):
d 71.5 (br), 41.6, 40 (br), 30.1 (br), 26.7 (br), 16.2, 15.4,
13.2, 9.6, 8.2, 5.3. 119Sn NMR (gel-phase in C6D6;
112 MHz): d À82.3 (br). Microanalysis: Sn = 14.5% (1.23
mmol/g). Polymer 10: 13C NMR (gel-phase in C6D6,
75.5 MHz): d 73 (b), 45.3, 40 (b), 28.1, 7.2, À7.3. 119Sn
MAS NMR (C6D6; 112 MHz): d À99.57. Microanalysis:
Sn 14.8% (1.25 mmol/g).
Crystal structures of 23 and 24
Compound
23
24
Empirical formula
Formula weight
Crystal system
Space group
Unit cell
C11H17ClOSn
319.39
Orthorhombic
P212121
C11H17ClO2Sn
335.39
Triclinic
ꢀ
P1
dimensions
˚
a (A)
6.8719(1)
7.9780(1)
24.0387(5)
10.7920(3)
11.0704(3)
11.8391(4)
87.995(1)
89.886(1)
68.912(2)
1318.82(7)
4
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
1317.90(4)
4
Z
Crystal size
Shape; colour
h Range for data
collection (ꢁ)
Reflections
collected
Independent
reflections
(Rint)
0.26 · 0.24 · 0.08 0.35 · 0.30 · 0.03
4.8. Standard procedure for catalytic Stille coupling
Slab; colourless
3.06–27.47
Plate; colourless
5.12–27.49
The procedure was adapted from that described by
Maleczka. In a typical procedure the tin chloride resin 5
8476
6062
or
6 (6–100 mol%) was suspended in an aqueous
Na2CO3/THF mixture (5 mL; 1:20), with an alkyne 12
(1 mmol), palladium catalyst system (1 mol% Pd2dba3,
1 mol% PdCl2(PPh3)2 and 4 mol% tri-2-furylphosphine)
and PMHS (1.5 equiv.). Aryl iodide or bromide 13 (1.5
equiv.) was added over 15 h by syringe pump. The resin
was removed by filtration and washed with THF and then
Et2O (3 · 30 mL each and 10 min shaking before each fil-
tration), the combined filtrates concentrated and the crude
solid product purified by flash chromatography (hexanes/
ethyl acetate, 3/1, v/v). Analytical data of isolated coupling
products 14 (1H; 13C; EIMS) was consistent with that
reported in the literature [2].
2986 (0.0562)
2340 (0.0512)
Goodness-of-fit
on F2
1.039
1.177
Data/restraints/
parameters
Final R indices
[F2 > 2r(F2)]
R indices
2986/0/130
2340/0/278
R1 = 0.0249,
wR2 = 0.0604
R1 = 0.0261,
wR2 = 0.0610
R1 = 0.0249,
wR2 = 0.0604
R1 = 0.0249,
wR2 = 0.0604
(all data)

A.G. Herna´n et al. / Journal of Organometallic Chemistry 691 (2006) 1466–1475
1475
[16] (a) P. Renaud, M.P. Sibi (Eds.), Radicals in Organic Synthesis,
Wiley–VCH, Weinheim, 2001;
Acknowledgements
(b) A.F. Parsons, An Introduction to Free Radical Chemistry,
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188;
We thank Dr. David Apperley, NMR Service Manager
at the Industrial Research Laboratories, University of Dur-
ham, UK for carrying out the MAS 119Sn NMR
experiments.
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˚
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Data collection software, R. Hooft and B.V. Nonius (1998)) and
DENZO (Z. Otwinowski and W. Minor, Methods in Enzymology, 276
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(G.M. Sheldrick (1997), University of Go¨ttingen, Germany). Crys-
tallographic data (excluding structure factors) for the structures in
this paper, has been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication numbers 287876 and
287877. Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
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[14] Tin analysis was carried out by digesting the sample in sulphuric acid
followed by ICP (Inductively Coupled Plasma) analysis, and was
performed by Medac Ltd.
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organotin reagents has also been noted in several instances using
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