Regio- and Stereoselective Homolytic Hydrostannylation of Propargyl Alcohols and Ethers with Dibutylchlorostannane

Article abstract of DOI:10.1021/jo035012s

The Et₃B-initiated reaction of γ-unsubstituted propargyl alcohols with dibutylchlorostannane (Bu₂SnClH) at low temperature gave (Z)-vinylstannanes with high regio- and stereoselectivity. The corresponding alkyl propargyl ethers also

Full text of DOI:10.1021/jo035012s

homolytic hydrostannylation of allyl and homoallyl al-
cohols.7 The chemoselectivity is attributable to the coor-
dination of the hydroxy group to the Lewis acidic tin
center in the â-stannylalkyl radical intermediate. This
deduction prompted us to utilize such an intramolecular
interaction for regio- and stereoselective hydrostannyla-
tion of alkynes bearing a polar functional group. In this
context, Davies and his co-workers have shown that the
reaction of 2-methyl-3-butyn-2-ol with Bu2SnClH affords
a 2:1 mixture of terminal and internal adducts, and the
terminal addition proceeds with high Z-selectivity.8 In
addition, Mitchell et al. have recently reported highly
regio- and stereoselective hydrostannylation of γ-unsub-
stituted propargyl ethers with Bu2SnXH (X ) Cl, Br).9
We herein provide some new and important information
on homolytic hydrostannylation of propargyl alcohols and
ethers with Bu2SnClH.10
Regio- a n d Ster eoselective Hom olytic
Hyd r osta n n yla tion of P r op a r gyl Alcoh ols
a n d Eth er s w ith Dibu tylch lor osta n n a n e
Katsukiyo Miura,* Di Wang, Yukihiro Matsumoto,
Naoki Fujisawa, and Akira Hosomi*
Department of Chemistry, 21st Century COE,
Graduate School of Pure and Applied Sciences, University of
Tsukuba, and CREST, J apan Science and Technology
Corporation (J ST), Tsukuba, Ibaraki 305-8571, J apan
miura@chem.tsukuba.ac.jp; hosomi@chem.tsukuba.ac.jp
Received J uly 13, 2003
Abstr a ct: The Et3B-initiated reaction of γ-unsubstituted
propargyl alcohols with dibutylchlorostannane (Bu2SnClH)
at low temperature gave (Z)-vinylstannanes with high regio-
and stereoselectivity. The corresponding alkyl propargyl
ethers also underwent regio- and stereoselective homolytic
hydrostannylation with Bu2SnClH; however, the regioselec-
tivity was not so high as that with the propargyl alcohols.
Initially, the reaction of 1-undecyn-3-ol (1a , R1
)
n-C8H17, R2 ) H) with Bu2SnClH was examined under
various reaction conditions (eq 2 and Table 1). In all
The addition of hydrostannanes to alkynes provides a
straightforward and efficient route to vinylstannanes,
which are important reagents working as vinyl anion
equivalents for carbon-carbon bond formation.1 The
hydrostannylation is induced by a radical initiator,2 a
transition metal catalyst,3 or a Lewis acid.4 The radical
chain process is considerably valuable for the preparation
of functionalized vinylstannanes (eq 1); however, it often
exhibits low stereoselectivity due to the stannyl radical-
induced isomerization of the products.5
cases, the hydrostannylation gave (Z)-vinylstannane 2a
as the major product along with its regioisomer 3a . As
expected from the previous report,8,9 Bu2SnClH sponta-
neously added to 1a in the absence of a radical initiator.
The isomeric ratio was dependent on the solvent used
(entries 2-4). Lowering the reaction temperature to 0
°C did not improve the regioselectivity (entry 5). Interest-
ingly, addition of Et3B as a radical initiator2b not only
accelerated the hydrostannylation but also raised the
ratio of (Z)-2a to 3a (entry 6). The Et3B-initiated reaction
at lower temperature achieved higher selectivity (entries
7-8). However, the reproducibility of the reaction ef-
ficiency at -78 °C was invariably poor, which may be
due to poor solubility of 1a at this temperature.
Previously, we have reported that dibutylchlorostan-
nane (Bu2SnClH)6 exhibits high chemoselectivity toward
(1) (a) Davies, A. G. Organotin Chemistry; VCH: Weinheim, Ger-
many, 1997. (b) Davies, A. G. In Comprehensive Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;
Pergamon Press: Oxford, UK, 1995; Vol. 2, pp 217-304. (c) Pereyre,
M.; Quintard, J .-P.; Rahm, A. Tin in Organic Synthesis; Butter-
worths: London, UK, 1987.
(2) AIBN: (a) Leusink, A. J .; Budding, H. A. J . Organomet. Chem.
1968, 11, 533-539. Et3B: (b) Nozaki, K.; Oshima, K.; Utimoto, K. J .
Am. Chem. Soc. 1987, 109, 2547-2549. Photoinitiated reaction: (c)
Mitchell, T. N.; Amamria, A. J . Organomet. Chem. 1983, 252, 47-56.
Sonication: (d) Nakamura, E.; Imanishi, Y.; Machii, D. J . Org. Chem.
1994, 59, 8178-8186.
To isolate the hydrostannylation product by column
chromatography, the reaction mixture obtained from 1a
(3) (a) Ichinose, Y.; Oda, H.; Oshima, K.; Utimoto, K. Bull. Chem.
Soc. J pn. 1987, 60, 3468-3470. (b) Kikukawa, K.; Umekawa, H.; Wada,
F.; Matsuda, T. Chem. Lett. 1988, 881-884. (c) Zhang, H. X.; Guibe´,
F.; Balavoine, G. J . Org. Chem. 1990, 55, 1857-1867. (d) Betzer, J .-
F.; Delaloge, F.; Muller, B.; Pancrazi, A.; Prunet, J . J . Org. Chem. 1997,
62, 7768-7780. (e) Mitchell, T. N.; Moschref, S.-N. Synlett 1999, 1259-
1260. (f) Maleczka, R. E., J r.; Terrell, L. R.; Clark, D. H.; Whitehead,
S. L.; Gallagher, W. P.; Terstiege, I. J . Org. Chem. 1999, 64, 5958-
5965.
(4) (a) Asao, N.; Liu, J .-X.; Sudoh, T.; Yamamoto, Y. J . Chem. Soc.,
Chem. Commun. 1995, 2405-2406. (b) Gevorgyan, V.; Liu, J .-X.;
Yamamoto, Y. Chem. Commun. 1998, 37-38.
(5) (a) Leusink, A. J .; Budding, H. A.; Drenth, W. J . Organomet.
Chem. 1968, 11, 541-547. (b) Taniguchi, M.; Nozaki, K.; Miura, K.;
Oshima, K.; Utimoto, K. Bull. Chem. Soc. J pn. 1992, 65, 349-353.
(6) Preparation and reactions of Bu2SnClH: (a) Sawyer, A. K.;
Kuivila, H. G. Chem. Ind. 1961, 260. (b) Neumann, W. P.; Pedain, J .
Tetrahedron Lett. 1964, 2461-2465.
(7) Miura, K.; Saito, H.; Uchinokura, S.; Hosomi, A. Chem. Lett.
1999, 659-660.
(8) Davies, A. G.; Kinart, W. J .; Osei-Kissi, D. K. J . Organomet.
Chem. 1994, 474, C11-C13.
(9) Mitchell, T. N.; Moschref, S.-N. Chem. Commun. 1998, 1201-
1202.
(10) Recently, Baba et al. have reported that an ate complex derived
from Bu2SnIH is valuable for highly regioselective hydrostannylation
of simple aliphatic terminal alkynes forming R-alkyl-substituted
vinylstannanes. Shibata, I.; Suwa, T.; Ryu, K.; Baba, A. J . Am. Chem.
Soc. 2001, 123, 4101-4102.
10.1021/jo035012s CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/09/2003
8730
J . Org. Chem. 2003, 68, 8730-8732

and stereoselectivity.11 The Et3B-initiated reaction of 1g
with Bu2SnClH at -78 °C also gave 6g predominantly,
but with lower stereoselectivity (70%, (Z)-5g:(Z)-6g:(E)-
6g ) 2:51:47, eq 3). The use of the Lewis acidic hy-
drostannane did not affect the sense of regioselectivity.
TABLE 1. Hyd r osta n n yla tion of P r op a r gyl Alcoh ol 1a
w ith Bu 2Sn ClHa
time/ temp/
conv/
%
entry initiator solvent
h
°C
(Z)-2a :3a b
1
2
3
4
5
6
7
8
none
none
none
none
none
Et3B-O2 PhMe
Et3B-O2 PhMe
Et3B-O2 PhMe
none
PhMe
Et2O
MeOH
PhMe
3
3
3
3
3
3
6
6
rt
rt
rt
rt
0
0
-60
-78
100
91
97
100
75
91:9
91:9
81:19
91:9
91:9
93:7
c
100
97
c
98:2
>99:1
50-100d
c
a
All reactions were performed with Bu2SnClH (1.10 mmol) and
1a (1.00 mmol) in solvent (2.5 mL) or without solvent (entry 1).
In entries 2-8, the resultant mixture was treated with galvinoxyl
(0.05 mmol), evaporated, and subjected to 1H NMR analysis.
b
Determined by 270-MHz 1H NMR analysis. c 1 M Et3B in hexane
d
(0.10 mmol) and dry air (10 mL) were introduced. The reproduc-
ibility of the conversion was poor.
To examine the applicability of the present method,
methyl propargyl ether 8a was selected as a substrate.
The Et3B-initiated reaction with Bu2SnClH (1.1 equiv)
at -60 °C for 6 h led to a mixture of adduct 9a , its
regioisomer 10a , and bisstannylated product 11a (eq 4,
TABLE 2. Et3B-In itia ted Hyd r osta n n yla tion of
P r op a r gyl Alcoh ols 1 w ith Bu 2Sn ClH F ollow ed by
Bu tyla tion a
substrate
yield
entry
R1
R2
no. temp/°C ((Z)-5+6)/% (Z)-5:6b
1
2
3
4
5
6
n-C8H17
c-C6H11
Ph
H
H
H
1a
1b
1c
-60
-78
-78
-78
-78
-78
81c
98:2
99:1
99:1
96:4
97:3
85c
72c
Me
Me 1d
96d
(CH2)5
1e
1f
92d
H
H
71c (81)e
>99:1
a
All reactions were performed with Bu2SnClH (1.10 mmol), 1
(1.00 mmol), Et3B (0.10 mmol), and dry air (10 mL) in toluene
(2.5 mL). The mixture was stirred at -78 or -60 °C for 6 h and 0
°C for 30 min. The resultant mixture was diluted with Et2O (4
mL) and treated with BuLi (2.5 mmol) at 0 °C for 30 min. Before
an aqueous workup, CCl4 (1 mL) was added to the reaction mixture
b
to suppress the stannyl radical-induced isomerization. The iso-
meric ratio was determined by 270-MHz 1H NMR analysis (entries
87% conversion, (Z)-9a :(E)-9a :10a :11a ) 80:1:10:9). In-
creasing the reaction temperature to 0 °C gave a similar
regioselectivity (6 h, 100% conversion, (Z)-9a :(E)-
9a :10a :11a ) 85:<1:11:4). Interestingly, the AIBN-
initiated reaction with Bu2SnClH (1.05 equiv) at 60 °C
showed higher regioselectivity (0.5 h, 100% conversion,
(Z)-9a :(E)-9a :10a :11a ) 91:<1:7:2).12 Mitchell et al. have
reported that the spontaneous reaction of γ-unsubstituted
propargyl ethers with Bu2SnClH at room temperature
forms only (Z)-vinylstannanes.9 However, the hydrostan-
nylation of 8a under the same conditions resulted in only
modest regioselectivity (12 h, 98% conversion, (Z)-9a :(E)-
9a :10a :11a ) 87:<1:12:1).
The AIBN-initiated hydrostannylation of 8a (R ) Me,
R1 ) n-C8H17) at 60 °C for 30 min (method A) and the
subsequent butylation gave (Z)-12a in a good yield with
high stereo- and regioselectivity (eq 4 and entry 1 in
Table 3). The reactions of benzyl and methoxymethyl
ethers, 8b and 8c, showed lower Z-selectivity (entries 2
and 4). The stereoselectivity could be improved by the
Et3B-initiated reaction at 0 °C (method B), in which the
regioselectivity slightly decreased (entries 3 and 5). In
contrast to the results with 8a -c, the use of TBDMS
ether 8d afforded (E)-12d predominantly (entry 6). The
1-3 and 6) or isolated yields of (Z)-5 and 6 (entries 4 and 5).
d
c Bisstannylated product 7 was isolated in 1-5% yield. (Z)-5d ,e
were isolated in 93% and 89% yields, respectively. e Instead of
BuLi, BuMgBr was used.
and Bu2SnClH was treated with BuLi. The Et3B-initiated
hydrostannylation (-60 °C, 6 h, then 0 °C, 30 min) and
the subsequent butylation (0 °C, 30 min) afforded a 98:2
mixture of (Z)-5a and 6a in 81% yield. Bisstannylated
product 7a also was isolated in a small quantity (5%).
The results with several propargyl alcohols 1a -f are
summarized in Table 2. In all entries, (Z)-vinylstannanes
5 were obtained with high regio- and stereoselectivity.
In the reaction of 2-methyl-3-butyn-2-ol (1d ), the present
method achieved high regioselectivity unlike the result
reported by Davies et al. (entry 4 of Table 2).8
The Et3B-initiated addition of Bu3SnH to 1a in toluene,
which was much slower than that of Bu2SnClH, showed
low Z-selectivity (rt, 6 h, 49% conversion, (Z)-5a :(E)-5a :
6a ) 66:26:8). The hydrostannylation of 1-dodecyne with
Bu2SnClH (0 °C, 6 h) and the subsequent butylation
provided 1-tributylstannyl-1-dodecene with E-selectivity
(95%, Z:E ) 13:87). These observations suggest that the
coordination of the hydroxy oxygen to the Lewis acidic
tin center is operative for the Z-selective addition of Bu2-
SnClH to 1 (vide infra).
(11) Ensley, H. E.; Buescher, R. R.; Lee, K. J . Org. Chem. 1982, 47,
404-408.
(12) Without AIBN, the hydrostannylation of 8a at 60 °C showed
slightly lower regioselectivity (0.5 h, 89% conversion, (Z)-9a :(E)-9a :
10a :11a ) 89:1:9:1).
Homolytic hydrostannylation of 2-butyn-1-ol (1g) with
Bu3SnH has been reported to give (Z)-6g with high regio-
J . Org. Chem, Vol. 68, No. 22, 2003 8731

TABLE 3. Hyd r osta n n yla tion of P r op a r gyl Eth er s 8
w ith Bu 2Sn ClH F ollow ed by Bu tyla tion a
SCHEME 1
substrate
yield
yield (13+14)/%
entry
R
no. methoda (12)/% (Z:E)b
(13:14)b
1
2
3
4
5
6
Me
Bn
Bn
MOM
MOM
8a
8b
8b
8c
8c
A
A
B
A
B
A
86 (>99:1)c
75 (89:11)c
72 (95:5)c
87 (88:12)d
75 (96:4)d
74 (7:93)d
9 (90:10)
8 (>99:1)
12 (>99:1)
6 (>97:3)
8 (88:12)
TBDMS 8d
12 (>99:1)
a
All reactions were performed with Bu2SnClH (1.05 mmol) and
8 (1.00 mmol) in toluene (2.0 mL). Method A: AIBN (0.05 mmol),
60 °C, 30 min. Method B: Et3B (1 M in hexane, 0.10 mmol), dry
air (10 mL), 0 °C, 6 h. The reaction mixture was diluted with Et2O
b
the vinyl radical intermediates by the Sn-O coordinate
bond may also take part in the stereocontrol.
(5 mL) and treated with BuLi (1.20 mmol) at -78 °C for 1 h. The
Z:E ratio and the ratio of 13 to 14 were determined by 1H NMR
d
1
analysis. c Isolated yield. The yield was determined by H NMR
In conclusion, we have demonstrated that propargyl
alcohols as well as their alkyl ethers undergo regio- and
stereoselective hydrostannylation with Bu2SnClH. The
present results disclose that the coordination of the
oxygen functionality to the Lewis acidic tin center plays
a crucial role for the stereocontrol.
analysis of a mixture of 12-14.
inverse selectivity is probably because the silyl group
hinders the coordination of the ether oxygen by its
bulkiness and π-electron-withdrawing ability.13
We further examined the reactions of homo- and
bishomo-propargyl alcohols, 15a and 15b, with Bu2-
SnClH. The hydrostannylation followed by butylation
gave only terminal addition products 16; however, the
stereoselectivity is rather low (eq 5). Interestingly, the
Z-selectivity decreased as the methylene tether elon-
gated.
Exp er im en ta l Section
Et3B-In itia ted Hyd r osta n n yla tion F ollow ed by Bu tyla -
tion (Typ ica l P r oced u r e). Bu2SnH2 (129 mg, 0.55 mmol) was
added to a solution of Bu2SnCl2 (167 mg, 0.55 mmol) in toluene
(1 mL) at 0 °C. After being stirred for 10 min, the mixture was
cooled to -60 °C. A toluene (1 mL) solution of propargyl alcohol
1a (168 mg, 1.00 mmol) was added to the mixture. After 10 min
Et3B (1.0 M in hexane, 0.10 mL, 0.10 mmol) and dry air (10 mL)
were added. The mixture was stirred at -60 °C for 6 h, then at
0 °C for 30 min. After addition of Et2O (4 mL), the resultant
mixture was treated with BuLi (1.65 M in hexane, 1.5 mL, 2.5
mmol) and stirred for 30 min. After addition of CCl4 (1 mL), the
mixture was poured into aqueous NH4Cl (10 mL). The extract
with t-BuOMe was dried over Na2SO4 and evaporated. Purifica-
tion of the crude product by silica gel column chromatography
gave a mixture of (Z)-1-tributylstannyl-1-undecen-3-ol ((Z)-5a )
and 2-tributylstannyl-1-undecen-3-ol (6a ) (395 mg, (Z)-5a :6a )
98:2) in 81% yield. (Z)-5a : bp 125 °C (0.5 Torr, bath tempera-
ture). IR (neat) 3419 (br s, OH), 2956, 2925, 2854, 1601 (CdC),
1464 cm-1
;
1H NMR (CDCl3) δ 0.78-1.03 (m, 18H), 1.12-1.64
2
(m, 27H), 3.85-3.94 (m, 1H), 5.99 (dd, J ) 12.7, 0.8 Hz, J SnH
(coupling constant between 119Sn and H nuclei) ) 65.7 Hz, 1H),
1
In homolytic hydrostannylation of alkynes (eq 1), it has
been reported that the Z-adducts are kinetically favored
over the E-adducts because hydrogen abstraction of the
â-stannylvinyl radical intermediates from hydrostan-
nanes occurs exclusively in the less congested side
opposite to the stannyl group. However, the hydrostan-
nylation usually shows low stereoselectivity because of
isomerization of the initially formed Z-adducts by addi-
tion-elimination of the stannyl radicals.5 The origin of
the present Z-selectivity would be that the isomerization
is inhibited by the Sn-O coordinate bond in the Z-adduct
(Scheme 1). In addition, the conformational fixation of
6.49 (dd, J ) 12.7, 7.5 Hz, J SnH ) 136.4 Hz, 1H); 13C NMR
3
(CDCl3) δ 10.82 (3 × CH2), 13.66 (3 × CH3), 14.08 (CH3), 22.66
(CH2), 25.54 (CH2), 27.32 (3 × CH2), 29.18 (3 × CH2), 29.25
(CH2), 29.55 (CH2), 29.70 (CH2), 31.87 (CH2), 37.35 (CH2), 75.73
(CH), 130.73 (CH), 150.43 (CH). Anal. Calcd for C23H48OSn: C,
60.14; H, 10.53. Found: C, 60.09; H, 10.79.
Ack n ow led gm en t. This work was partly supported
by Grants-in-Aid for Scientific Research from the Min-
istry of Education, Culture, Sports, Science, and Tech-
nology, Government of J apan.
Su p p or tin g In for m a tion Ava ila ble: Analytical and spec-
tral characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
(13) Bassindale, A. R.; Taylor, P. G. In The Chemistry of Organic
Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley, Chichester,
UK, 1989; Part 2, pp 893-963.
J O035012S
8732 J . Org. Chem., Vol. 68, No. 22, 2003