Tuning the reactivity of organotin(IV) by LiOH: Allylation and propargylation of epoxides via redox transmetalation

Article abstract of DOI:10.1016/j.jorganchem.2004.12.008

In presence of catalytic Pd(0) or Pt(II), a reagent combination of SnCl₂-LiOH promotes the reaction of organic halides and epoxides in dichloromethane leading to the regioselective formation of corresponding homoallyl and homopropargyl alcohols in good yields. This 2-carbon extension strategy adds to the repertoire of Barbier reactions via metal salts and reinforces views on the enhanced reactivity of organotin having -OH pendant.

Full text of DOI:10.1016/j.jorganchem.2004.12.008

Journal of Organometallic Chemistry 690 (2005) 1422–1428
www.elsevier.com/locate/jorganchem
Tuning the reactivity of organotin(IV) by LiOH: allylation and
propargylation of epoxides via redox transmetalation
*
Moloy Banerjee, Ujjal Kanti Roy, Pradipta Sinha, Sujit Roy
Organometallics and Catalysis Laboratory, Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India
Received 3 September 2004; accepted 10 December 2004
Available online 7 February 2005
Abstract
In presence of catalytic Pd(0) or Pt(II), a reagent combination of SnCl2–LiOH promotes the reaction of organic halides and epox-
ides in dichloromethane leading to the regioselective formation of corresponding homoallyl and homopropargyl alcohols in good
yields. This 2-carbon extension strategy adds to the repertoire of Barbier reactions via metal salts and reinforces views on the
enhanced reactivity of organotin having –OH pendant.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Organotin(IV); Allylation; Propargylation; Transmetalation
1. Introduction
ing to homoallyl and homopropargyl alcohols via
tandem rearrangement-carbonyl addition reaction
(Scheme 1). As elaborated later, the study also reinforces
earlier views on the enhanced reactivity of the potential
intermediate ‘‘R–Sn(OH)3’’.
Addition of allyl, propargyl or allenylstannanes to or-
ganic electrophiles like aldehydes, imines, and epoxides
is a well-known tool for carbon–carbon bond forming
strategy in organic chemistry [1–3]. Far-reaching utility
of these reagents have led to the development of Bar-
bier-like protocols wherein reactive organostannanes
are generated in-situ from tin(0/II/IV) partner. Along
with others, we have been exploring a bimetallic strategy
which involves oxidative-addition of an organic halide
(RX) across catalytic d8/d10 metal [M], followed by re-
dox-transmetalation of R–[M]–X to tin(II) to generate
R–Sn(IV) in-situ. Successful delineation of the strategy
for carbonyl allylation and propargylation [4] prompted
us to extend the same for the nucleophilic addition of
organotin(IV) to epoxide in one-pot. During the investi-
gation, we have observed a dramatic effect of lithium
hydroxide in promoting the reactivity of organotin lead-
2. Results and discussion
Reaction of styrene oxide 2a and 3-bromopropene 1a
in presence of stannous chloride and catalytic Pd2(dba)3
in dichloromethane under reflux led to the formation of
1-phenyl-pent-4-en-2-ol 3a and 1-phenyl-ethane-1,2-diol
in 15% and 70% yield, respectively. On the other hand,
reaction in absence of catalyst afforded unreacted epox-
ide 2a along with the diol. No further improvement was
possible by varying the solvent, catalyst, tin(II) precur-
sor, temperature, mode of addition or by adding water.
Since it is well established that the reagent combination
of Pd(0/II)/SnCl2/allyl bromide generates allyltrihalo
stannane [5], we ascribe the above failure due to the
poor nucleophilicity of the allylating agent. Concurrent
to this observation, we were enthused with the recent
*
Corresponding author. Tel.: +91 3222 283338; fax: +91 3222
282252.
E-mail address: sroy@chem.iitkgp.ernet.in (S. Roy).
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.12.008

M. Banerjee et al. / Journal of Organometallic Chemistry 690 (2005) 1422–1428
1423
The scope of the reaction has been successfully ex-
tended to the propargylation of epoxides in which
PtCl2(PPh3)2 was found to be most efficient among the
catalysts screened. Thus reaction of 3-bromo-but-1-yne
4b and styrene oxide 2a in presence of catalytic
PtCl2(PPh3)2 yielded 92% of 3-methyl-1-phenyl-pent-4-
yn-2-ol 5c (Table 3, entry 6). Similar reaction of propar-
gyl bromide and substituted propargyl bromides with
styrene oxide 2a and a-methyl styrene oxide 2b afforded
moderate to good yields of the corresponding homo-
propargyl alcohols (Table 4). Most strikingly no allenic
alcohol is obtained in any of the cases suggesting that
under the reaction condition metallotropic rearrange-
ment between the propargyltin(IV) and allenyltin(IV)
is completely inhibited.
The first issue concerning the mechanism of the reac-
tion relates to the formation of homoallyl and homo-
propargyl alcohols as end-organic product, which
clearly suggests that the reaction proceed via epoxide
rearrangement to give an aldehyde in-situ. Control
experiments with styrene oxide confirmed that under
our reaction condition such rearrangement indeed take
place by tin(II) but not the transition metal partner,
the order of efficiency being Sn(OTf)2 > SnCl2 [8d]. A
mechanism is proposed as shown in Scheme 2, which
is analogous to InCl3 catalyzed rearrangement of arylep-
oxide to carbonyl compounds [8d].
The second issue relates to the identity of the reactive
organotin(IV) species in the present case, which was
investigated for the allylation reaction. According to
the well-established redox-transmetalation sequence,
the reaction proceeds via a Pd(0)/Pd(II) catalytic cycle
with the initial formation of a p-allylpalladium(II)
intermediate [5]. Insertion of tin(II) followed by reduc-
tive transmetallation leads to allyltrichlorostannane
(Scheme 3).
Pronounced effect of lithium hydroxide in promoting
the reaction suggests that a more reactive intermediate
other than I is the actual reacting species. To test this
assumption, in-situ 1H NMR diagnosis was carried
out in which the chemical shift (d, ppm) and 2J117,119Sn–H
values of characteristic methylene doublet were in-
spected (Fig. 1). The spectrum of authentic allyl-SnCl3
(a) shows methylene peak at 3.15 (2JSn–H = 120 Hz,
Sn–CH2) along with signals at 5.35 (m, @CH2), and
5.92 (dtd, J = 1.8 Hz, J = 8.3 Hz, J = 18.5 Hz, @CH–).
Upon reacting allyl-SnCl3 with LiOH (3 equiv.) in
DCM at 40 °C for 5 h a residue was obtained after cen-
trifugation/concentration. NMR of the residue (b) con-
firmed the formation of a new r-allyltin(IV) species, in
which the methylene doublet shifts upfield to 2.50 ppm
(2JSn–H = 144 Hz, Sn–CH2), along with other signals at
4.9 ppm (m, @CH2), and 5.98 ppm (dtd, J = 1.7 Hz,
J = 8.4 Hz, J = 18.5 Hz, @CH–). The spectrum of allyl-
tin (c) generated in-situ from allyl bromide, anhydrous
SnCl2 and catalytic Pd2(dba)3 in CDCl3 after stirring
O
Ar
R
SnCl2 / 3LiOH
R4
R3
R5
Br
R1
Cat. Pt(II)
Cat. Sn(OTf)2
Cat. Pd(0)
Cat. Sn(OTf)2
Br
R2
Ar
OH R3
OH
Ar
R2
R4
R1
R
R
R5
Scheme 1. Tandem epoxide rearrangement
gylation.
– allylation/propar-
suggestion by Li and co-workers [6] that an alkoxy or
hydroxy pendant as in RSn(OH)3 and RSn(OR0)3 exerts
electron donation to the vacant d-orbital of tin, thereby
stabilizing the species and consequently enhancing its
reactivity. In view of this we desired to generate the po-
tential allylating agent C3H5Sn(OH)3 in non aqueous
medium. True to this expectation, addition of lithium
hydroxide (3 equiv.) led to dramatic improvement in
yield of 3a from 15% to 35% [7]. Further enhancement
of yield to 69% was achieved by using stannous triflate
as Lewis acid catalyst. Results from various optimiza-
tion studies are accrued in Table 1, from which we con-
clude that the best condition is as shown in entry 5.
The generality of the reaction was further tested with
substituted allyl bromides and epoxides (Table 2).
3-Substituted allyl bromides 1b–1e afforded exclusively
the c-regioselective products 3b–3e. Reaction of 2-bro-
momethyl(ethyl)acrylate 1g with 2b resulted in tandem
lactonization leading to 6-methyl-3-methylene-6-phe-
nyl-tetrahydropyran-2-one 3g. To test if steric crowding
is tolerated, a-methylstyrene epoxide 2b was reacted
with allyl bromides 1b and 1f. In each case the reactions
proceed smoothly to afford the products 3h and 3i in
moderate to good yields. However, epoxides bearing
only aliphatic substituents, for example cyclohexane
epoxide gave negligible yield of product.
Table 1
Reaction of styrene oxide 2a with allyl bromide 1a/SnCl2/LiOH: effect
of catalyst and Lewis acida
#
Catalyst (2 mol%)
LA (10 mol%)
Yield (%)
1
None
Sn(OTf)2
Sn(OTf)2
Sn(OTf)2
Sn(OTf)2
Sn(OTf)2
None
21
49
54
35
69
35
47
42
55
2
PtCl2(PPh3)2
PdCl2(PPh3)2
Pd(PPh3)4
Pd2dba3
3
4
5
6
Pd2dba3
Pd2dba3
7b
8b
9
Cu(OTf)2
Zn(OTf)2
Sc(OTf)2
Pd2dba3
Pd2dba3
a
Unless otherwise mentioned, all reactions were run for 9 h.
Reaction time 45 h.
b

1424
M. Banerjee et al. / Journal of Organometallic Chemistry 690 (2005) 1422–1428
Table 2
Reaction of allyl bromides 1a–1g with epoxides 2a, 2b in presence of SnCl2/LiOH and catalytic Sn(OTf)2/Pd2(dba)3 at 40 °C
OH R3
R3
O
R1
Ar
Br
Ar
R
+
R2
R2
1
R1
3a - 3i
R
2
3
1a: R =R =R =H
2a: Ar=Ph, R=H
2b: Ar=Ph, R=Me
1
2
3
1b: R =Me, R =H, R =H
1
2
3
1c: R =Me, R =Me, R =H
1
n
2
3
1d: R = Pr, R =H, R =H
1
2
3
1e: R =Ph, R =H, R =H
1
2
3
1f: R =H, R =H, R =Me
1
2
3
1g: R =H, R =H, R =CO Et
2
#
1
2
Bromide
1a
Epoxide
Product
Pdt. No.
3a
Time
(h)
Yield
(%)
Syn:
Anti
OH
9
3
69
76
–
2a
2a
Ph
OH
Ph
Ph
42:58
1b
3b
Me
OH
–
3
4
5
3
3
4
56
56
12
1c
1d
1e
2a
2a
2a
3c
3d
3e
Me Me
OH
44:56
0:100
Ph
Ph
C3H7
OH
Ph
OH Me
Ph
Ph
6
7
3
3
52
45
1f
2a
2b
3f
–
–
Me
1g
3g
O
O
OH
Ph
Ph
8
9
2
2
80
52
69:31
38:62
1b
1f
2b
2b
3h
3i
Me Me
OH Me
Me
1
at 40 °C for 5 h shows methylene peak at 3.19 ppm
(2JSn–H = 120 Hz, Sn–CH2), which is similar to
(a) and therefore is most likely to be allyl-SnCl2Br.
Addition of 3 equivalent LiOH to the in-situ gener-
ated allyltin results in decrease in peak at 3.19 and
concomitant appearance of a new methylene peak at
2.78 ppm (2JSn–H = 118 Hz, Sn–CH2). Comparing
(b) and (d), we propose the new organotin species to
be a r-allyl(halo)(hydroxy)tin(IV) intermediate. Sev-
eral attempts to isolate the said intermediate remain
unsuccessful so far.
Using H and 119Sn NMR monitoring Blunden et al.
[9] have shown that base treatment of RSnCl3 (R = Me,
Bu) results in the formation of pH-dependent hydroxy
species, and at 1:6 ratio the formation of RSn(OH)3 is
suggested. Tagliavini and co-workers [10] proposed the
formation of hydrated organotin(IV) cations in aqueous
solution of organotin halides. Enhanced reactivity of
ArSn(OH)3, Ar3Sn(OMe), ArBi(OH)2 over the corre-
sponding halo derivatives in arylation reactions is fur-
ther noteworthy [6,11]. One may also note that while
existence of silanetriols RSi(OH)3 is fully established

M. Banerjee et al. / Journal of Organometallic Chemistry 690 (2005) 1422–1428
1425
Table 3
Reaction of styrene oxide 2a with propargyl bromide 4b/SnCl2/LiOH:
Scheme 3, (b) reaction of lithium hydroxide with I to
generate RSnXn(OH)3 ꢀ n II in solution, (c) tandem rear-
rangement of arylepoxide to benzylic aldehyde III by
tin(II), and (d) carbonyl addition via SE20 attack of
organotin(IV) intermediate II to aldehyde III to furnish
the end-organic product.
In conclusion, we demonstrated here the allylation
and propargylation of aryl substituted epoxides under
Barbier-like protocol to afford corresponding homoallyl
and homopropargyl alcohols with two carbon extension,
and with 100% c-regioselectivity in all cases. Further
light on the enhanced organometallic reactivity of
hydroxytin(IV) species is demonstrated.
effect of catalysta
#
Catalyst (2 mol%)
L.A. (10 mol%)
Yield (%)
1
2
3
4
5
6
7
8
9
None
None
15
28
36
21
43
92
57
73
61
[RhCl(COD)]2
[IrCl(COD)]2
NiCl2(PPh3)2
PtCl2(PPh3)2
PtCl2(PPh3)2
PdCl2(PPh3)2
Pd2(dba)3
Sn(OTf)2
Sn(OTf)2
Sn(OTf)2
None
Sn(OTf)2
Sn(OTf)2
Sn(OTf)2
Sn(OTf)2
Pd(PPh3)4
a
All reactions were run for 15 h.
(vide isolation and X-ray structure), the analogous stan-
nanetriols still await such confirmation [12].
A plausible mechanism (Scheme 4) of the present
reaction involves (a) formation of R–SnXCl2 I vide
3. Experimental
1H (200 MHz) NMR spectra were recorded on a
BRUKER-AC 200 MHz. Spectrometer. Chemical shifts
Table 4
Reaction of propargyl bromides 4a–4d with epoxides 2a, 2b in presence of SnCl2/LiOH and catalytic Sn(OTf)2/PtCl2(PPh3)2 at 40 °C
OH
R4
R5
Br
Ar
O
Ar
R
+
R4
R5
R
4
5
5a - 5g
4a: R = R = H
2a: Ar=Ph, R=H
2b: Ar=Ph, R=Me
4
5
4b: R = H ,R = Me
4
5
4c: R = R = Me
4
5
4d: R = Me, R = Et
#
1
Bromide
Epoxide
Product
Pdt.
No.
Time
(h)
Yield
(%)
Syn:
Anti
OH
Ph
18
51
–
4a
2a
5a
OH
Ph
2
3
4
5
6
7
22
43
45:55
4a
4b
4b
4c
4c
4d
2b
2a
2b
2a
2b
2a
5b
Me
OH
Ph
15
14
18
16
20
92
78
73
48
55
72:28
100:0
–
5c
5d
5e
5f
Me
OH
Ph
Me Me
OH
Ph
Me Me
OH
Ph
55:45
54:46
Me Me
Me
OH
5g
Ph
Me
Et

1426
M. Banerjee et al. / Journal of Organometallic Chemistry 690 (2005) 1422–1428
-
SnY2
O
SnY2
O
O
H
+
H
- SnY2
H
SnY2
H
Ar
Ar
(Y = Cl, OTf)
Ar
H
Ar
H
O
Scheme 2. Plausible route to benzylic aldehyde from arylepoxide via catalytic Sn(II).
LiOH
Sn(II)
X
SnCl2
-Pd0
R-SnXCl2 (I)
R-SnXn(OH)3-n
Pd0
(II)
PdII
PdII
X
SnCl2X
O
(I)
SnCl2X
ArCH2CHO
(III)
Ar
Scheme 3. Redox transmetalation from allyl-Pd(II) to allyl-Sn(IV).
(II)
OH
Ar
are reported in ppm from tetramethylsilane with the sol-
vent resonance as the internal standard (deuterochloro-
form: d 7.27 ppm). Data are reported as follows:
R
Scheme 4. Tandem rearrangement of epoxide to aldehyde and
carbonyl addition of organotin(IV) in presence of LiOH.
chemical shifts, multiplicity (s = singlet, d = doublet,
t = triplet, q = quartet, br = broad, m = multiplet), cou-
pling constant (Hz). 13C (54.6 MHz) NMR spectra were
recorded on a BRUKER-AC 200 MHz. Spectrometer
with complete proton decoupling. Chemical shifts are
reported in ppm from tetramethylsilane with the solvent
resonance as the internal standard (deuterochloroform:
d 77.0 ppm). EIMS (70 ev) spectra and HRMS were ta-
ken using a VG Autospec M and Waters LCT mass
spectrometer. Elemental analyses were carried out using
a CHNS/O Analyzer Perkin Elmer 2400 Series II
instrument.
All reactions were carried out under an argon atmo-
sphere in flame dried glassware using Schlenk tech-
niques. Chromatographic purifications were done with
either 60–120 or 100–200 mesh silica gel (SRL). For
reaction monitoring, precoated silica gel 60 F254 TLC
sheets (Merck) were used. Petroleum ether refers to the
fraction boiling in the range 60–80 °C.
3.1. General procedure
The procedure given below was followed in all cases.
All products showed satisfactory spectral and analytical
data.
3.2. Allylation of epoxide
1-Bromo-3-methyl-but-2-ene 1c (224 mg, 1.5 mmol)
was added dropwise to a mixture of anhydrous SnCl2
(189 mg, 1 mmol) and Pd2(dba)3 (9 mg, 0.01 mmol)
in dry DCM (3 ml) under argon atmosphere at a bath
temperature of 40 °C, and was allowed to stir (5 h).
LiOH (75 mg, 3 mmol) was added to the mixture
and further stirred for 1 h. Finally, a solution of sty-
rene oxide 2a (0.06 ml, 0.5 mmol) in dry DCM
Fig. 1. NMR profile of intermediates in the chemical shift region of
the methylene doublet. a: authentic allyltrichlorostannane in CDCl3; b:
product from reaction of a with 3 equiv. LiOH in DMSO-d6; c:
allyltrihalostannane generated in-situ from allyl bromide/SnCl2/Pd(0)
in CDCl3; d: after addition of 3 equiv. LiOH to c; ¼ indicates peaks
due to 2JSn–H and denotes peak due to residual proton in DMSO-d6.
*

M. Banerjee et al. / Journal of Organometallic Chemistry 690 (2005) 1422–1428
1427
(1 ml) was added followed by Sn(OTf)2 (21 mg, 0.05
mmol). Upon completion (TLC monitoring silica gel,
eluent: n-hexane-EtOAc 9:1), the reaction was
quenched with 2(N) HCl (5 ml), and aq. NH4F (10%,
5 ml). Organic product was extracted with diethyl
ether, washed with water, brine, and dried over anhy-
drous magnesium sulfate. Solvent removal under re-
duced pressure followed by column chromatography
over silica gel 60–120 (gradient elution with EtOAc-
hexane 2–10%) afforded 3,3-dimethyl-1-phenylpent-4-
en-2-ol 3c (54 mg, 56% with respect to epoxide). 1H
NMR (200 MHz, CDCl3): d 1.03 (s, 6H), 1.53 (brs,
1H), 2.31–2.43 (m, 1H), 2.77–2.85 (dd, 1H, J = 11, 2
Hz), 3.39–3.35 (dd, 1H, J = 11, 11 Hz), 4.97–5.06 (m,
2H), 5.78–5.92 (m, 1H), 7.16–7.26 (m, 5H). 13C
NMR (50.3 MHz, CDCl3): d 22.80, 38.39, 41.42,
79.31, 112.96, 126.21, 128.25, 129.23, 139.83, 145.25.
EIMS m/z (rel. abundance): 190 (MÅ+, 2), 173 [(M–
OH)+, 8], 157 (60), 131 (46), 121 (42), 117 (30), 91
(100), 77 (38), 69 (52), 65 (22). Anal. (C13H18O) calcd.
C: 82.06, H: 9.53; found, C: 81.77, H: 9.89.
with EtOAc-hexane 1:99) afforded phenylacetaldehyde
(36 mg, 60% with respect to epoxide), which was com-
pared and found to identical with an authentic sample.
Under identical condition as above, but with SnCl2 as
catalyst (9.5 mg, 0.05 mmol, 10%), the yield of phenyl-
acetaldehyde was 18 mg (30% with respect to epoxide).
Under identical condition as above, but with catalytic
Pd2(dba)3 (46 mg, 0.05 mmol, 10%), the yield of phenyl-
acetaldehyde was <2% (vide NMR).
Acknowledgement
We thank CSIR for financial support. Research fel-
lowship to MB (CSIR-SRF), PS (UGC-SRF), and
UKR (UGC-JRF) is acknowledged.
Appendix A. Supplementary data
General procedure, spectral and analytical data of
products. Supplementary data associated with this arti-
cle can be found, in the online version, at doi:10.1016/
j.jorganchem.2004.12.008.
3.3. Propargylation of epoxide
Following identical method as above using the re-
agents 3-bromo-3-methyl-but-1-yne 4c (221 mg, 1.5
mmol), anhydrous SnCl2 (228 mg, 1.2 mmol),
PtCl2(PPh)2 (8 mg, 0.01 mmol), LiOH (86 mg, 3.6
mmol), styrene oxide 2a (0.06 ml, 0.5 mmol), and
Sn(OTf)2 (21 mg, 0.05 mmol) afforded 3,3-dimethyl-1-
phenyl-pent-4-yn-2-ol 5e (69 mg, 73% with respect to
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