Highly regio- and stereoselective hydrostannylation of alkynols with a new Lewis acidic hydrostannane

Article abstract of DOI:10.1021/ol0474397

(Chemical Equation Presented) Bu₂Sn(OTf)H (1a), easily prepared from Bu₂SnH₂ and TfOH, was found to be very valuable for highly regio- and stereoselective hydrostannylation of various propargyl alcohols leading to (Z)-γ-st

Full text of DOI:10.1021/ol0474397

ORGANIC
LETTERS
2005
Vol. 7, No. 3
503-505
Highly Regio- and Stereoselective
Hydrostannylation of Alkynols with A
New Lewis Acidic Hydrostannane
Katsukiyo Miura,* Di Wang, Yukihiro Matsumoto, and Akira Hosomi*
Department of Chemistry, 21st Century COE, Graduate School of Pure and Applied
Sciences, UniVersity of Tsukuba, and CREST, Japan Science and Technology
Corporation (JST), Tsukuba, Ibaraki 305-8571, Japan
hosomi@chem.tsukuba.ac.jp
Received December 13, 2004
ABSTRACT
Bu2Sn(OTf)H (1a), easily prepared from Bu2SnH2 and TfOH, was found to be very valuable for highly regio- and stereoselective hydrostannylation
of various propargyl alcohols leading to (Z)- -stannylated allyl alcohols. The stannylation with 1a is applicable to the synthesis of hydroxy-
γ
substituted (Z)-vinylstannanes from terminal alkynes bearing a hydroxy group at the homoallylic or bishomoallylic position. The coordination
of the hydroxy group to the Lewis acidic tin center plays an important role for the observed regio- and stereochemistry.
Vinylstannanes are important reagents working as vinyl anion
equivalents for stereo-controlled alkene synthesis.1 Among
various synthetic routes to vinylstannanes, hydrostannylation
of alkynes with hydrostannanes is a most straightforward
and convenient route. Much attention has been paid to the
development of regio- and stereoselective hydrostannylation
based on the elaboration of the promoter.2-5 Radical-initiated
reactions are valuable for the synthesis of functionalized
vinylstannanes; however, the stereoselectivity is generally
low.2 We have recently described that the Et3B-initiated
hydrostannylation of γ-unsubstituted propargyl alcohols with
Bu2SnClH (1b) shows high levels of regio- and stereocontrol
at low temperatures.6,7 Unfortunately, this method is in-
effective in selective hydrostannylation of other alkynols.
We herein report that Bu2Sn(OTf)H (dibutyl(trifluoromethane-
sulfoxy)stannane, 1a), a more Lewis acidic hydrostannane,
realizes highly regio- and stereoselective hydrostannylation
of various alkynols at room temperature.
(1) (a) Davies, A. G. Organotin Chemistry; Wiley-VCH: Weinheim,
2004. (b) Pereyre, M.; Quintard, J.-P.; Rahm, A. Tin in Organic Synthesis;
Butterworths: London, 1987.
(2) (a) Leusink, A. J.; Budding, H. A. J. Organomet. Chem. 1968, 11,
533-539. (b) Nozaki, K.; Oshima, K.; Utimoto, K. J. Am. Chem. Soc. 1987,
109, 2547-2549. (c) Mitchell, T. N.; Amamria, A. J. Organomet. Chem.
1983, 252, 47-56. (d) Nakamura, E.; Imanishi, Y.; Machii, D. J. Org. Chem.
1994, 59, 8178-8186.
(3) (a) Ichinose, Y.; Oda, H.; Oshima, K.; Utimoto, K. Bull. Chem. Soc.
Jpn. 1987, 60, 3468-3470. (b) Zhang, H. X.; Guibe´, F.; Balavoine, G. J.
Org. Chem. 1990, 55, 1857-1867. (c) Betzer, J.-F.; Delaloge, F.; Muller,
B.; Pancrazi, A.; Prunet, J. J. Org. Chem. 1997, 62, 7768-7780. (d)
Mitchell, T. N.; Moschref, S.-N. Synlett 1999, 1259-1260. (e) Kazmaier,
U.; Pohlman, M.; Schauss, D. Eur. J. Org. Chem. 2000, 2761-2766. (f)
Smith, N. D.; Mancuso, J.; Lautens, M. Chem. ReV. 2000, 100, 3257-
3282 and references therein.
Hydrostannane 1a can easily be prepared by the dehy-
drogenative reaction of Bu2SnH2 with 1 equiv of TfOH
without solvent at 0 °C.8 The hydrostannane thus obtained
(5) Shibata, I.; Suwa, T.; Ryu, K.; Baba, A. J. Am. Chem. Soc. 2001,
123, 4101-4102.
(6) Miura, K.; Wang, D.; Matsumoto, Y.; Fujisawa, N.; Hosomi, A. J.
Org. Chem. 2003, 68, 8730-8732.
(7) Quite recently, Mitchell et al. have reported similar results. Thiele,
C. M.; Mitchell, T. N. Eur. J. Org. Chem. 2004, 337-353.
(8) For the preparation of Bu3SnOTf from Bu3SnH, see: Corey, E. J.;
Eckrich, T. M. Tetrahedron Lett. 1984, 25, 2419-2422.
(4) Asao, N.; Liu, J.-X.; Sudoh, T.; Yamamoto, Y. J. Chem. Soc., Chem.
Commun. 1995, 2405-2406.
10.1021/ol0474397 CCC: $30.25
© 2005 American Chemical Society
Published on Web 01/13/2005

was used without purification. Propargyl alcohol 2a reacted
spontaneously with 1a at room temperature. Treatment of
the resulting mixture with BuLi gave (Z)-vinylstannane 3a
in a high isolated yield (entry 1 in Table 1). The regio- and
γ-stannylation showed complete Z-selectivity. Under the
same conditions, the stannylation with 1b took place mainly
at the â-position as the stannylation with 1c does, but with
low Z-stereoselectivity (entry 7). Internal alkyne 2f was
stannylated with better γ-regioselectivity than 2e (entry 8).
In the reactions of 2e,f with 1a, heating the reaction mixture
before butylation led to exclusive formation of (Z)-3e,f,
although the total yields of stannylated products decreased.
The seeming improvement in regioselectivity is probably due
to thermal decomposition of the precursors leading to 4e,f
by deoxystannylation.11b Actually, the formation of 1,2-
tridecadiene was observed in the reaction of 2f by the
improved method. Propargyl alcohols 2g,h underwent highly
regio- and stereoselective hydrostannylation to give (Z)-3g,h
in moderate yields (entries 9 and 10). The use of Et3B as
initiator effected higher yields of (Z)-3g,h.2b
Table 1. Hydrostannylation of Propargyl Alcohols 2 with 1aa
propargyl alcohol
entry
R1
R2
R3
yield (%)b
(2a) 89
(2a) 83
(2b) 93
(2c) 85
(2d) 90
(2e) 88 (50)f
(2e) 77
(Z)-3:4
>99:1c
93:7
>99:1c
>99:1
96:4
71:26 (>99:1)f
7:93 g
91:9 (>99:1)f
>98:2 (>99:1)h
97:3 (97:3)h
1
n-C8H17
n-C8H17
c-C6H11
H
H
H
H
Me
H
H
H
H
H
H
H
H
H
H
Me
Me
2d
3
We next examined the hydrostannylation of other alkynols
with 1a (Scheme 1). Terminal alkynes 5a,b, bearing a
4e
5
H
Me
H
6
7d
8
9
10
H
H
H
n-C10H21 (2f) 91 (77)f
Scheme 1. Hydrostannylation of Alkynols 5, 8, and 10 with
1aa
SiMe3
(2g) 64 (85)h
Me
n-C6H13 (2h) 72 (89)h
a Unless otherwise noted, the initial step was carried out with 2 (1.00
mmol) and 1a (1.10 mmol) in hexane (2 mL) at room temperature for 3 h.
The resultant mixture was diluted with Et2O (2 mL) and treated with BuLi
(1.6 M in hexane, 2.2-2.5 mmol) at 0 °C for 20 min. b Isolated yield of a
mixture of (Z)-3 and 4. c A trace amount of (E)-3 (<1%) was formed. d 1b
was used instead of 1a. e BuMgBr (in Et2O) was used instead of BuLi.
f After the reaction of 2 with 1a, the reaction mixture was concentrated
under reduced pressure and heated at 80 °C for 3 h. The resultant mixture
was diluted with Et2O and treated with BuLi. The results are shown in
parentheses. g (Z)-3e:(Z)-4e:(E)-4e ) 7:64:29. h Et3B (0.10 mmol) and dry
air were used as initiator. The results are shown in parentheses. (Z)-3h could
be isolated in 86% yield.
stereoisomers, 4a and (E)-3a, were hardly formed. Since the
hydrostannylation of 2a was effectively inhibited by adding
5 mol % of galvinoxyl, a radical scavenger, it would involve
a radical chain mechanism. Hydrostannane 1b is known to
have a similar reactivity causing spontaneous, homolytic
hydrostannylation.9 The use of 1b instead of 1a resulted in
lower regioselectivity under the same conditions (entry 2).10
The present method using 1a is valuable also for highly
selective, efficient hydrostannylation of other γ-unsubstituted
propargyl alcohols 2b-d (entries 3-5).
It has been reported that the radical-initiated reaction of
γ-substituted propargyl alcohols (e.g., 2e) with Bu3SnH (1c)
yields (Z)-â-stannylated products (e.g., (Z)-4e) exclusively.11
In contrast, 1a added spontaneously to 2e to form (Z)-γ-
stannylated product 3e as the major product (entry 6).
Although the regiochemistry was not well controlled, the
a A small excess of an alkynol was used (alkynol:1a ) 1.1:1) in
the reactions marked with an asterisk (*).
hydroxy group at the homopropargylic position, were stan-
nylated with complete regio- and stereoselectivity.12 The
reaction of internal alkyne 5c formed both regioisomers with
low regioselectivity, although the stannylation at the sp-
carbon more remote from the hydroxy group gave only the
Z-isomer of 6c. Similar to the case of 5a,b, the stannylation
of terminal alkynes 8b,c proceeded with high Z-selectivity.
(9) (a) Neumann, W. P.; Pedain, J. Tetrahedron Lett. 1964, 2461-2465.
(b) Davies, A. G.; Kinart, W. J.; Osei-Kissi, D. K. J. Organomet. Chem.
1994, 474, C11-C13. (c) Mitchell, T. N.; Moschref, S.-N. Chem. Commun.
1998, 1201-1202.
(10) The hydrostannylattion of 2a with Bu3SnH was much slower even
in the presence of Et3B (rt, 6 h, 49% conversion with 10 mol % Et3B) and
showed low Z-selectivity ((Z)-3a:(E)-3a:4a ) 66:26:8).
(11) (a) Ensley, H. E.; Buescher, R. R.; Lee, K. J. Org. Chem. 1982, 47,
404-408. (b) Konoike, T.; Araki, Y. Tetrahedron Lett. 1992, 33, 5093-
5096.
(12) In general, homolytic hydrostannylation of terminal alkynes proceeds
with high regioselectivity. See ref 2. As an exception, the reaction of
γ-unsubstituted propargyl alcohols shows slightly lower regioselectivity.
504
Org. Lett., Vol. 7, No. 3, 2005

Interestingly, the selectivity with 8a was rather low under
the standard conditions; however, a small excess of 8a over
1a achieved high Z-selectivity.13 Furthermore, 5-hexyn-1-ol
(10) was used as a substrate. As shown in the reaction of
8a, the molar ratio of 10 to 1a strongly affected the
stereoselectivity: a small excess of 1a led to E-selectivity,
whereas Z-selectivity was observed with a small excess of
10.
that with a four-membered chelate ring, 12b (n ) 1), by
thermodynamic control.
Vinylstannanes prepared by the present method can be
directly used for the Pd-catalyzed cross-coupling reaction
with aryl halides (Scheme 3).16 The hydrostannylation of 2
Scheme 3. Pd-Catalyzed Cross-Coupling of Vinylstannanes
The stereochemistry of homolytic hydrostannylation of
alkynes is determined by H-abstraction of â-stannylated vinyl
radical intermediates and the subsequent stannyl radical-
induced isomerization of the vinylstannanes formed.6 The
high Z-selectivity with 1a can be rationalized by conforma-
tional fixation of the intermediate 12a to the Z-form and
suppression of the isomerization of the product (Z)-13a, both
of which are directed by a strong interaction between Sn
and O atoms (Scheme 2).14,15 The origin of the high
with 1a followed by treatment with TBAF, iodoarenes, and
catalytic amounts of Pd2(dba)3 and PPh3 (one-pot procedure)
gave allyl alcohols (Z)-14 (Z:E ) >99:1) in good yields.
In conclusion, we have developed a novel hydrostan-
nylating agent with a strong Lewis acidity, which is very
valuable for highly selective synthesis of functionalized
vinylstannanes from various alkynols. It is noteworthy that
the use of 1a enables γ-selective stannylation of γ-substituted
propargyl alcohols in sharp contrast to â-selective stan-
nylation with 1b and 1c. The present study has demonstrated
that the high Lewis acidity of 1a is quite effective in both
regio- and stereocontrol of homolytic hydrostannylation of
alkynols.
Scheme 2. Origin of Regio- and Stereocontrol
Acknowledgment. This work was partly supported by
Grants-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science, and Technology, Gov-
ernment of Japan.
γ-selectivity with 2 would be that the reversible addition of
•Sn(OTf)Bu2 forms the radical intermediate with a five-
membered chelate ring, (Z)-12a (n ) 1), in preference to
Supporting Information Available: Experimental de-
tails, characterization data (1H NMR, 13C NMR, IR, elemental
analysis), and discussions on the Lewis acidity of 1a and
the reaction mechanism. This material is available free of
charge via the Internet at http://pubs.acs.org.
(13) Under the standard conditions, the excess 1a may interfere with
the O-Sn coordination in (Z)-12a or (Z)-13a when the chelate ring is not
tight.
(14) The NMR analysis of the (trifluoromethanesulfoxy)vinylstannane
prepared from 1a and 2a revealed the presence of a highly coordinated tin
center. See Supporting Information.
OL0474397
(15) The reaction mechanism possibly involves pre-coordination of 1a
with a substrate and intramolecular radical addition leading to 12. See
Supporting Information for further discussion on the reaction mechanism.
(16) For the cross-coupling reaction using organohalostannanes, see:
Fugami, K.; Ohnuma, S.; Kameyama, M.; Saotome, T.; Kosugi, M. Synlett
1999, 63-64 and references therein.
Org. Lett., Vol. 7, No. 3, 2005
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