Syntheses and synthetic applications of stannylated allylic alcohols

Article abstract of DOI:10.1021/jo052611l

Allenyl carbinols undergo regioselective hydrostannation in the presence of MoBl₃, a catalyst originally developed for the hydrostannation of alkynes, giving rise to allyl stannanes. These allyl stannanes can easily be converted into useful synthetic building blocks such as allyl iodides or vinyl epoxides.

Full text of DOI:10.1021/jo052611l

Syntheses and Synthetic Applications of Stannylated Allylic Alcohols
Uli Kazmaier,* Simon Lucas, and Manuela Klein
Institut fu¨r Organische Chemie, UniVersita¨t des Saarlandes, D-66123 Saarbru¨cken, Germany
u.kazmaier@mx.uni-saarland.de
ReceiVed December 20, 2005
Allenyl carbinols undergo regioselective hydrostannation in the presence of MoBl3, a catalyst originally
developed for the hydrostannation of alkynes, giving rise to allyl stannanes. These allyl stannanes can
easily be converted into useful synthetic building blocks such as allyl iodides or vinyl epoxides.
SCHEME 1. Possible Reaction Products of Allene
Hydrostannations
Introduction
During recent years functionalized allenes became rather
important as synthetic intermediates for the synthesis of complex
molecules.1 For example, carbon-carbon and carbon-hetero-
atom bond formations are of major interest, triggering the
development of preparative useful methods, such as additions2
or palladium-catalyzed transformations.3 In addition to cycliza-
tions,4 especially transition-metal-catalyzed hydrometalations
such as hydroborations,5 hydrozirconations,6 or hydrostanna-
tions7 became very popular, giving rise to allylic or vinylic
organometallics depending of the regioselectivity in the addition
step. As illustrated for hydrostannations (Scheme 1), in principle
four different products (A-D) can be expected, and the product
distribution depends on the reaction conditions used.
by a regioselective stannylpalladation transferring the stannyl
groups to the terminal position of the allene. Mitchell et al.
investigated systematically the influence of substituents and
heteroatoms on the regio- and stereoselectivity of these hy-
drostannations.11 They also obtained preferentially the terminal
allyl stannanes (>85% rs), but in general mixtures of (E/Z)-
isomers were obtained. Similar is the situation in the hydrostan-
nation of alkoxy allenes, as reported by Gore´ et al.12 These
authors also used Mo(CO)6 as a catalyst and observed a higher
Both the allyl as well as the vinyl stannanes are important
intermediates that can be used for, e.g., allylation reactions8 or
Stille couplings.9 Oshima et al. were the first to report about
hydrostannations of allenes in the presence of Pd(PPh3)4.10 Allyl
stannanes C were obtained preferentially, which was explained
(8) (a) Methoden der Organischen Chemie, Houben-Weyl, 4th ed.;
Helmchen, G., Hoffmann, R. W., Mulzer, J., Schaumann E., Eds.;
Erga¨nzungsband, Vol. E21b, pp 1516-1540. (a) Thomas, E. J.; Hull, C.;
Mortlock, S. V. Tetrahedron 1989, 45, 1007-1015. (c) Costa, A. L.; Piazza,
M. G.; Tagliavini, E.; Trombini, C.; Umani-Roncho, A. J. Am. Chem. Soc.
1993, 115, 7001-7002. (d) Keck, G. E.; Tarbert, K. H.; Geraci, L. S. J.
Am. Chem. Soc. 1993, 115, 8467-8468. (e) Bach, T. Angew. Chem. 1994,
106, 433-435. (f) Marshall, J. A.; Seletzky, B. M.; Luke, G. P. J. Org.
Chem. 1994, 59, 3413-3420. (g) Marshall, J. A.; Beaudoin, S. J. Org. Chem.
1996, 61, 581-586.
* To whom correspondence should be addressed. Tel: 49 681 3023409.
Fax: 49 681 3022409.
(1) (a) Allenes in Organic Synthesis; Schuster, H. F., Copolla G. M.,
Eds.; Wiley: New York, 1984. (b) Modern Allene Chemistry; Krause, N.,
Hashmi, A. S. K., Eds. Wiley-VCH: Weinheim, 2004.
(2) ) Reviews: (a) Zimmer, R. Synthesis 1993, 165-178. (b) Back T.
G. Tetrahedron 2001, 57, 5263-5301.
(3) ) Review: Zimmer, R.; Dinesh, C. U.; Nandanan, E.; Khan, F. A.
Chem. ReV. 2000, 100, 3067-3125.
(4) ) Reviews: (a) Bates, R. W.; Satchavoen, V. Chem. Soc. ReV. 2002,
31, 12-21. (b) Krause, N.; Hoffman-Ro¨der, A.; Canisius, J. Synthesis 2002,
1759-1774.
(9) (a) Stille, J. K. Angew. Chem. 1986, 98, 504-519. (b) Mitchell, T.
N. Synthesis 1992, 803-816. (c) Pattenden, G.; Sinclair, D. J. J. Organomet.
Chem. 2002, 653, 261-268.
(5) ) Yamamoto, Y.; Fijikawa, R.; Yamada, A.; Miyaura, N. Chem. Lett.
1999, 1069-1070.
(10) Ichinose, Y.; Oshima, K.; Utimoto, K. Bull. Chem. Soc. Jpn. 1988,
61, 2693-2695.
(6) ) (a) Maeta, H.; Hasegawa, T.; Suzuki, K. Synlett 1993, 341-343.
(b) Chino, M.; Matsunoto, T.; Suzuki, K. Synlett 1994, 359-362. (c) Pi, J.
H.; Huang, X. Synlett 2003, 2413-2415.
(11) Mitchell, T. N.; Schneider, U. J. Organomet. Chem. 1991, 405, 195-
199.
(7) ) Review: Smith, N. D.; Mancuso, J.; Lautens, M. Chem. ReV. 2000,
100, 3257-3282.
(12) Koerber, K.; Gore´, J.; Vatele, J. M. Tetrahedron Lett. 1991, 32,
1187-1190.
10.1021/jo052611l CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/07/2006
J. Org. Chem. 2006, 71, 2429-2433
2429

Kazmaier et al.
SCHEME 2. Hydrostannations of Allenes
SCHEME 3. Mechanism of Pd-Catalyzed Hydrostannations
of Allenes
Surprisingly very high (E)-selectivities were obtained for aryl-
substituted allenes such as 5. This was explained by a kinetic
reaction control, especially because the isomeric ratio did not
depend on the reaction temperature.
Lautens et al. were able to show that vinyl stannanes can
also be obtained under palladium catalysis, for example, if Pd-
(OH)2/C is used as catalyst (Scheme 2).15 Under these conditions
several allenes, allenylamines, and allenols such as 8 were
converted into the corresponding vinyl stannanes (9). Again,
Pd(PPh3)4 provided the expected allyl stannane 10 in comparable
yield as a 1:1 (E/Z) mixture.
Besides palladium catalysts, other metal complexes can be
used for hydrostannations, such as rhodium16 or molybdenum.17
Guibe et al. described the application of MoBr(allyl)(CO)2(CH3-
CN)2 for hydrostannations of propargylic alcohols, unfortunately
without significant regioselectivity. The catalytically active
species probably is Mo0, formed in situ via reduction of the
Mo-complex with the tin hydride. On the basis of this first report
on molybdenum catalysis, we developed a new catalyst, Mo-
(CO3(CNtBu)3 (MoBl3),18 which allows highly regioselective
hydrostannations of various types of alkynes.19 As reported
previously in a communication, this catalyst also allows
regioselective hydrostannations of allenols.20 Herein, we report
details and applications of this reaction.
Z-selectivity in the reaction of 1 (Scheme 2), although the
molybdenum catalyst proved to be less reactive.
Grigg et al. combined elegantly the regioselective hydrostan-
nation with a cyclization, both reactions catalyzed by Pd0.13 This
straightforward protocol allows the synthesis of five- to seven-
membered heterocycles such as 4, starting from allene 3. In
1997 Yamamoto et al. compared the Pd0-catalyzed hydrostan-
nation of allenes such as 5 with the Lewis acid catalyzed
versions.14 As reported previously, the Pd variant provided
preferentially the allyl stannanes 7,7 whereas in the presence of
B(C6F5)3 the corresponding vinyl stannanes 6 were obtained.
This was explained by different reaction mechanisms. It is
postulated that in the Lewis acid catalyzed mechanism the
B(C6F5)3 adds to the internal double bond, followed by a hydride
transfer onto the zwitterionic intermediate and transmetalation
B versus Sn. On the other hand, the palladium-catalyzed version
starts with an oxidative addition of the tin hydride to Pd0
(Scheme 3), giving rise to the highly reactive palladium hydride
species E. This intermediate can undergo either hydropalladation
on the allene, providing allylpalladium species F, or palla-
dostannylation yielding G. Both intermediates can form the allyl
stannane C via reductive eliminations. Alkyl-substituted allenes
again gave mixtures of the isomeric (E/Z)-allyl stannanes.
Results and Discussion
A well-established method for the synthesis of terminal
allenes is found in the Crabbe´ homologization of terminal
alkynols,21 which were easily obtained via a Grignard reaction
(Scheme 4). The propargylic alcohols 11 were reacted with
(15) Lautens, M.; Ostrovsky, D.; Tao, B. Tetrahedron Lett. 1997, 38,
6343-6346.
(16) Kikukawa, K.; Umekawa, H.; Wada, F.; Matsuda, T. Chem. Lett.
1988, 881-884.
(17) Zhang, H. X.; Guibe, F.; Balavoine, G. J. Org. Chem., 1990, 55,
1857-1867.
(18) Similar molybdenum isonitril complexes were used for allylic
alkylations: Trost, B. M.; Merlic, C. A. J. Am. Chem. Soc. 1990, 112, 9590-
9600.
(19) (a) Kazmaier, U.; Schauss, D.; Pohlmann, M. Org. Lett. 1999, 1,
1017-1019. (b) Kazmaier, U.; Pohlmann, M.; Schauss, D. Eur. J. Org.
Chem. 2000, 2761-2766. (c) Kazmaier, U.; Schauss, D.; Pohlmann, M.;
Raddatz, S. Synthesis 2000, 914-916. (d) Kazmaier, U.; Schauss, D.;
Raddatz, S.; Pohlmann, M. Chem. Eur. J. 2001, 7, 456-464. (e) Braune,
S.; Kazmaier, U. J. Organomet. Chem. 2002, 641, 26-29. (f) Braune, S.;
Pohlman, M.; Kazmaier, U. J. Org. Chem. 2004, 69, 468-474. (g)
Kazmaier, U.; Wesquet, A. Synlett, 2005, 1271-1274.
(20) Kazmaier, U.; Klein, M. Chem. Commun. 2005, 501-503.
(13) Grigg, R.; Sansano, J. M. Tetrahedron 1996, 52, 13441-13445.
(14) Gevorgyan, V.; Liu, J.-X.; Yamamoto, Y. J. Org. Chem. 1997, 62,
2963-2967.
2430 J. Org. Chem., Vol. 71, No. 6, 2006

Stannylated Allylic Alcohols
TABLE 2. Optimization of the Reaction Conditions
SCHEME 4. Syntheses of Allenols
ratio
temp equiv of
yield
entry (°C) Bu3SnH time (%) (E)-13a
:
(Z)-13a
:
14a
1
2
3
4
5
6
7
55
rt
0
rt
rt
3
3
3
4
10 h
18 h
18 h
18 h
3 d
68
71
23
60
61
34
58
3
2
2
2
3
3
2
:
:
:
:
:
:
:
4
3
3
3
2
1
1
:
:
:
:
:
:
:
1
-
-
TABLE 1. Synthesis of Allenols
b
-
-
-
-
3a
1.1a
1.1a
b
entry
R
yield, %
b
rt
55
3 d
12 h
1
Ph
11a
11b
11c
11d
11e
11f
11g
11h
11i
90
12a
12b
12c
12d
12e
12f
60
69
58
63
66
26
34
56
20
59
40a
68
b
2
4-MePh
84
91
56
74
90
77
92
86
67
67
71
a 1 M in THF; slow addition of the tin hydride (0.1 equiv/h). b Traces
of diene 14a detected
3
4
5
2-MeOPh
2,4,6-(MeO)3Ph
2-BrPh
6
4-ClPh
SCHEME 6. Hydrostannations of Various Allenols 12
7
8
2,6-Cl2Ph
2-NO2Ph
12g
12h
12i
9
4-NO2Ph
10
11
12
PhCH2
(CH3)2CHCH2
3-Cyclohexenyl
11k
11l
11m
12k
12l
12m
a THF was used as solvent.
TABLE 3. Hydrostannations of Various Allenols 12
ratio
SCHEME 5. Hydrostannation of Allenols
time temp yield (E)-
(h) (°C) (%) 13
(Z)-
13
entry allene
R
:
:
14
1
2
3
4
5
6
7
8
12a Ph
12b 4-MePh
12c 2-MeOPh
12d 2,4,6-(MeO)3Ph 18
12d 2,4,6-(MeO)3Ph
18
18
18
rt
rt
rt
rt
55
rt
55
55
55
rt
rt
55
rt
71
78
62
56a
60b
86
68
74
76
84
68
79
49
63
57
75
2
10
2
6
-
4
1
1
1
4
1
2
3
3
2
2
:
:
:
:
:
:
:
:
:
:
:
:
:
:
:
:
3
10
5
4
1
3
1
2
1
3
2
3
5
5
3
3
:
:
:
:
:
:
:
:
:
:
:
:
:
:
:
:
-
1
1
1
2
-
-
-
-
-
-
-
-
-
-
-
4
18
4
4
4
18
18
18
18
18
paraformaldehyde in the presence of DIPA and a Cu(I) catalyst,
giving rise to the required allenols. We applied this protocol to
various substituted aromatic (Table 1, entries 1-9) and a few
aliphatic aldehydes (entries 10-12). We used strong electron-
donating and -withdrawing groups on the aromatic ring to find
out if there is an electronic influence of these substituents on
the selectivity and the product distribution of the hydrostanna-
tion. The substrates prepared and the yields are summarized in
Table 1.
12e
12f
2-BrPh
4-ClPh
12g 2,6-Cl2Ph
12h 2-NO2Ph
c
9
c
10
11
12
13
14
15
16
12i
4-NO2Ph
12k PhCH2
12k PhCH2
12l
12l
(CH3)2CHCH2
(CH3)2CHCH2
55
rt
55
12m 3-Cyclohexenyl 18
12m 3-Cyclohexenyl 18
In the hydrostannations of alkynes we obtained best results
with 3 equiv od Bu3SnH in refluxing THF.19 To determine if
this is also true with allenes, we investigated the influence of
the reaction conditions, especially on the yield and selectivity,
with allenol 12a (Scheme 5). The ratio of the (E/Z)-stereoiso-
mers was determined by NMR. In general, 2 mol % MoBl3
was used and hydroquinone was added to suppress radical
hydrostannation. The results obtained are summarized in Table
2. If the reaction conditions for alkyne hydrostannations were
used (3 equiv of Bu3SnH, refluxing THF), the isomeric allyl
stannanes 13a were obtained in good yield (entry 1). Interest-
ingly, a third product 14a was formed, obviously by elimination.
This is not surprising, because R-metalated alcohols and
derivatives thereof undergo elimination toward alkenes, and
herein we have an analogous situation. To suppress this side
reaction, we lowered the reaction temperature to room temper-
ature, and indeed no elimination was observed under these
milder conditions (entry 2). The yield was slightly higher and
the (E/Z)-selectivity was not affected significantly; however, it
is worth to mention that the terminal double bond was attacked
exclusively. Further decreasing of the temperature to 0 °C
resulted in a dramatic loss of yield, but the reaction still worked
a 35% (rel.) vinyl stannane 15d formed. b 25% (rel.) vinyl stannane 15d
formed. c Traces of diene 14 formed.
(entry 3). It seems that allenols are significantly more reactive
than alkynols, which show nearly no reaction at room temper-
ature. Increasing the amount of Bu3SnH does not result in a
better yield (entry 4); the same is true if the tin hydride is added
slowly via a syringe pump (entry 5). Here, also traces of
elimination product were observed. Reducing the amount of tin
hydride to 1.1 equiv in the slow addition mode provided a low
yield even after days (entry 6). Interestingly, for the first time
a significant selectivity for the (E)-isomer was observed.
Increasing the reaction temperature to 55 °C provided a higher
yield in a less selective reaction (entry 7).
The best results were obtained if 3 equiv Bu3SnH was added
at once and if the reactions were carried out between room
temperature and refluxing THF. Therefore, we used these
reaction conditions for the hydrostannation of the other allenols
(Scheme 6, Table 3). The yields obtained with the aromatic
allenols were generally good, especially for those substrates with
electron-withdrawing substituents (12e-i, entries 6-10). In-
terestingly, electron-donating groups are obviously less suitable.
Not only are the obtained yields lower, but the selectivity is
(21) (a) Crabbe´, P.; Andre´, D.; Fillion, H. Tetrahedron Lett. 1979, 20,
893-896. (b) Crabbe´, P.; Fillion, H.; Andre´, D.; Luche, J.-L. J. Chem.
Soc., Chem. Commun. 1979, 859-860. (c) Searles, S.; Li, Y.; Nassim, B.;
Robert Lopes, M.-T.; Tran, P. T.; Crabbe´, P. J. Chem. Soc., Perkin Trans.
1 1984, 747-751.
J. Org. Chem, Vol. 71, No. 6, 2006 2431

Kazmaier et al.
SCHEME 8. Allyl Iodides 16 via Metal-Halogen Exchange
FIGURE 1. Elimination of stannylated allyl alcohols 13.
SCHEME 7. Proposed Mechanism for the
Molybdenum-Catalyzed Hydrostannation of Allenes
SCHEME 9. Conversion of Allyl Stannanes 13 into Vinyl
Epoxides 17
result from coordination of the molybdenum to the two
diastereotopic faces of the terminal double bond. The lower
regioselectivity obtained in the hydrostannation of 11d might
have its explanation by an increased electron density of the
internal double bond. Therefore this double bond might be a
better ligand for the molybdenum favoring the formation of
complex H, which then undergoes hydrometalation to H′ and
reductive elimination to 15d. In this case obviously electronic
and steric effects keep the balance, providing a mixture of both
regioisomers.
Probably this mechanism is different from the one proposed
for the hydrostannation of alkynes, which seems to start with a
transfer of the stannyl group and a subsequent hydrogen transfer
in the reductive elimination step, but both possible mechanisms
were also discussed for palladium-catalyzed hydrostannations.7
Therefore, it is reasonable that the situation with the molybde-
num complexes is similar.
We focused our investigations on the allenols because we
were interested to convert the allyl stannanes obtained into other
synthetically useful intermediates such as vinyl epoxides.
Therefore, we subjected the isomeric mixture (E/Z)-13h to a
halogen metal exchange (Scheme 8). Although the isomeric
mixture was used, the corresponding allyl iodide (E)-16h was
obtained as a single isomer in high yield. Under these conditions
only the thermodynamically most stable product is formed. The
metal halogen exchange occurs nearly immediately, but the
subsequent isomerization to the thermodynamic (E)-iodide takes
about 20-30 min. This isomerization can easily be monitored
by TLC. By this protocol we could also obtain the iodides 16a
and 16i in reasonably good yield.
Unfortunately, these highly reactive iodides are rather un-
stable. The results shown are the best obtained; other iodides
decompose during workup. Removal of the solvent even at room
temperature resulted in a deiodination, and the compounds could
not be stored for a longer time. Therefore, we tried to convert
the iodides 16 directly without separation into vinyl epoxides.
Deprotonation of the hydroxy functionality should provide
epoxides 17 via intramolecular SN2′ reaction.
With allyl stannane 13k we investigated the influence of the
base on this two-step vinyl epoxide formation (Scheme 9, Table
4). The best results were obtained with NaH. The trans epoxide
17k was obtained in 69% over both steps. To probe the influence
of the counterion, we also performed the reaction in the presence
worse and side reactions such as eliminations become more
relevant. This is clearly indicated by going from the initial
substrate 12a to the more and more electron-rich derivatives
12d (entries 1-5). At higher temperatures the elimination
product 14d becomes the major product. Surprisingly, at 55 °C
only the (Z)-configured allyl stannane 13d could be isolated,
whereas at room temperature this isomer is the minor one. The
(E)-isomer seems to be the one undergoing elimination pref-
erentially. This assumption is in good agreement with the
product ratio (Z)-13d/14d obtained, and the elimination seems
to be limited to substrates that can stabilize a carbenium ion in
the allylic position (Figure 1).
Compound 12d is an interesting substrate, because it is the
only one that reacted relatively unselectively. Here, significant
amounts (up to 35%) of vinyl stannane 15d were isolated, a
regioisomer, which was not formed with all of the other
substrates. Aliphatic substrates (11k-m) are less reactive
compared to the aromatic ones, but the yields could be increased
at higher temperatures without the formation of side products
(entries 11-16).
The high selectivity toward the allyl stannanes 13 can be
explained by the following mechanistic rational (Scheme 7):
Probably in the first step some isonitrile ligands dissociate from
the molybdenum, opening free coordination sites for the
oxidative addition of the tin hydride and coordination of the
allene. With respect to the sterically high demanding molyb-
denum complex, coordination to the less hindered terminal
double bond is reasonable, giving rise to intermediate G.
Subsequent hydrometalation should provide intermediate G′,
with the molybdenum fragment added to the less sterically
hindered position. The allyl stannanes 13 were formed from
this intermediate via reductive elimination. The (E/Z)-isomers
2432 J. Org. Chem., Vol. 71, No. 6, 2006

Stannylated Allylic Alcohols
TABLE 4. Conversion of Allyl Stannanes 13 into Vinyl Epoxides
17
from allenol 12a (198 mg, 1.50 mmol) as a colorless liquid (463
mg, 1.06 mmol, 71%) in a 2:3 (E/Z) mixture. (Z)-13a: 1H NMR
(500 MHz, CDCl3) δ 0.82-0.89 (m, 15H), 1.22-1.30 (m, 6H),
1.41-1.47 (m, 6H), 1.74 (m, 2H), 1.77 (d, J ) 3.5 Hz, 1H), 5.37
(m, 1H), 5.50 (dd, J ) 8.5, 3.5 Hz, 1H), 5.75 (m, 1H), 7.22-7.40
(m, 5H); 13C NMR (125 MHz, CDCl3) δ 9.4, 11.3, 13.7, 27.3, 29.1,
69.5, 126.1, 126.4, 127.8, 128.4, 132.1, 144.2. (E)-13a (selected
signals): 1H NMR (500 MHz, CDCl3) δ 5.12 (dd, J ) 7.4, 3.5 Hz,
1H), 5.46 (ddd, J ) 37.5, 15.0, 7.4 Hz, 1H), 5.85 (ddt, J ) 49.8,
15.0, 8.8 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ 9.3, 13.6, 14.5,
29.0, 75.6, 126.0, 128.3, 133.0, 144.0; HRMS (CI) calcd for
C18H29O120Sn [M - Bu]+ 381.1186, found 381.1213.
yield (%)
entry
stannane
R
base
trans-17
cis-17
1
2
3
4
5
6
7
8
9
13k
13k
13k
13k
13k
13b
13c
13e
13i
Bn
Bn
Bn
Bn
Bn
4-MePh
2-MeOPh
2-BrPh
NaH
69
72
29
33
5
31
45
52
52
NaH, 15-C-5
KOtBu
KOtBu, 18-C-6
DBU
NaH
NaH
17
21
19
NaH
NaH
4-NO2Ph
(E)-4-Iodo-1-phenylbut-2-en-1-ol (16a). A solution of iodine
(381 mg, 1.50 mmol) in ether (40 mL) was added to a solution of
the allyl stannane (E/Z)-13a (650 mg, 1.45 mmol) in THF (10 mL).
The reaction was monitored by TLC, and after 1 h the isomerization
was complete. The organic solution was washed with saturated
Na2S2O3 and stirred overnight with TBAF (600 mg) as an ether/
THF/water mixture at room temperature. The layers were separated,
and the aqueous layer was extracted twice with ether. The combined
organic layers were dried (Na2SO4), and the solvent was evaporated
in vacuo in the cold. Flash chromatography (Hex/EtOAc (1) 100/
0; (2) 80/20) of the crude product provided 16a (290 mg, 1.06
mmol, 72%) as a rather unstable yellow oil and as a single
stereoisomer: 1H NMR (500 MHz, CDCl3) δ 2.00 (d, J ) 3.8 Hz,
1H), 3.88 (d, J ) 8.2 Hz, 2H), 5.20 (m, 1H), 5.88 (dd, J ) 15.3,
6.0 Hz, 1H), 6.04 (ddt, J ) 15.3, 8.2, 0.9 Hz, 1H), 7.22-7.40 (m,
5H); 13C NMR (125 MHz, CDCl3) δ 4.4, 73.8, 126.3, 127.9,128.7,
135.5, 142.3.
General Procedure for the Conversion of Stannylated Allyl
Alcohols into Vinyl Epoxides. A solution of iodine (381 mg, 1.50
mmol, 1.05 equiv) in ether (40 mL) was added to a solution of the
allyl stannane (E/Z)-13 (1.45 mmol) in THF (10 mL). The reaction
was monitored by TLC, and after the isomerization was complete,
this solution was added to a suspension of NaH (72 mg, 3.0 mmol)
in THF (5 mL). After stirring overnight, the organic layer was
washed with saturated Na2S2O3 and brine and dried (Na2SO4), and
the solvent was evaporated in vacuo. The crude product was purified
by flash chromatography.
cis/trans-2-(4-Methylphenyl)-3-vinyloxiran (17b). According
to the general procedure for vinyl epoxide formation, 17b was
obtained from stannane 13b (990 mg, 2.19 mmol), iodine (583 mg,
2.30 mmol), and NaH (106 mg, 4.40 mmol) as a colorless liquid
(168 mg, 1.05 mmol, 48%) in a 1:3 cis/trans-mixture. trans-17b:
1H NMR (500 MHz, CDCl3) δ 3.24 (s, 3H), 3.34 (dd, J ) 7.3, 1.8
Hz, 1H), 3.73 (d, J ) 1.8 Hz, 1H), 5.32 (dd, J ) 10.4, 0.9 Hz,
1H), 5.50 (dd, J ) 17.4, 0.9 Hz, 1H), 5.72 (ddd, J ) 17.4, 10.4,
7.3 Hz, 1H), 7.14-7.24 (m, 4H); 13C NMR (125 MHz, CDCl3) δ
21.2, 60.2, 62.8, 119.4, 125.4, 129.2, 132.2, 135.2, 138.0. cis-17b:
1H NMR (500 MHz, CDCl3) δ 3.24 (s, 3H), 3.63 (dd, J ) 8.2, 4.3
Hz, 1H), 4.20 (d, J ) 4.3 Hz, 1H), 5.26 (dd, J ) 10.4, 1.8 Hz,
1H), 5.40 (ddd, J ) 17.1, 10.4, 8.2 Hz, 1H), 5.53 (dd, J ) 17.1,
1.8 Hz, 1H), 7.14-7.24 (m, 4H); 13C NMR (125 MHz, CDCl3) δ
21.2, 58.8, 59.8, 121.7, 126.3, 128.8, 132.0, 135.9, 137.4; HRMS
(CI) calcd for C11H13O [M + H]+ 161.0968, found 161.0967.
of crown ether (15-C-5), although without significant changes.
Other bases such as KOtBu and DBU were definitely less
suitable, but independent of the base used, the trans epoxide
was formed exclusively.
Therefore, we used NaH for our further investigations and
the synthesis of four more vinyl epoxides (Table 4).22 The yields
obtained were in the range of 48-71%. The nitro derivate 13i
also gave the trans epoxide 17i exclusively (entry 4), whereas
the other derivatives gave mixtures of the trans/cis isomers
(entries 1-3), as determined by NMR spectroscopy.
In conclusion, we have shown that the molybdenum-catalyzed
hydrostannation of allenols is an interesting synthetic tool for
the regioselective synthesis of hydroxylated allyl stannanes,
which can be converted into useful synthetic intermediates such
as allyl iodides or vinyl epoxides. Further investigations
concerning synthetic applications are currently in progress.
Experimental Section
General Remarks. All reactions were carried out in oven-dried
glassware (100 °C) under argon. All solvents were dried before
use. THF was distilled from LiAlH4 and stored over molecular
sieves. The products were purified by flash chromatography on
silica gel. Mixtures of EtOAc and hexanes were generally used as
eluents, and 1% NEt3 was added for the purification of the
stannanes. TLC was performed with commercially precoated
Polygram SIL-G/UV 254 plates. Visualization was accomplished
with UV light and KMnO4 solution. Melting points are uncorrected.
Selected signals in the NMR spectra for the minor isomers are
extracted from the spectra of the isomeric mixture. Elemental
analyses were carried out at the Department of Chemistry,
University of Saarbru¨cken.
General Procedure for Hydrostannations of Allenols. In a
Schlenk tube, Mo(CO)3(CNtBu)3 (4.3 mg, 0.01 mmol, 2 mol %)
and hydroquinone (3 mg) were dissolved in THF (1 mL) under
argon. The corresponding allenol (0.5 mmol) in THF (1 mL) was
added at room temperature, and the reaction mixture was either
kept at this temperature or refluxed for 15 min. Bu3SnH (0.46 mL,
1.5 mmol, 3 equiv) was added either at once or slowly as a 0.1 M
solution in THF via syringe pump (see Tables 2 and 3). The reaction
mixture was stirred at the given temperature until all allene was
consumed, as determined by TLC. The solvent was evaporated in
vacuo, and the crude product was purified by flash chromatography.
In general, a mixture of hexane/EtOAc/NEt3 (98:1:1) was used as
eluent.
Acknowledgment. Financial support by the Deutsche For-
schungsgemeinschaft and the Fonds der Chemischen Industrie
is gratefully acknowledged.
Supporting Information Available: Analytical and detailed
NMR spectroscopic data of compounds 13-17. This material is
available free of charge via the Internet at http://pubs.acs.org.
(E/Z)-1-Phenyl-4-tributylstannylbut-2-en-1-ol (13a). According
to the general procedure for hydrostannations 13a was obtained
(22) These are all examples investigated so far.
JO052611L
J. Org. Chem, Vol. 71, No. 6, 2006 2433