Distannylations and silastannylations of in situ generated allenes

Article abstract of DOI:10.1002/anie.200705976

(Chemical Equation Presented) Propargylic ethers and acetates can be converted into distannylated alkenes in the presence of Bu₃SnH and a Pd catalyst. The reaction proceeds via a stannylated allyl alcohol derivative, which undergoes elimination and subsequent dimetalation. If a molybdenum catalyst is used in the hydrostannylation step, and the stannylated intermediates can be reacted with other dimetallic compounds.

Full text of DOI:10.1002/anie.200705976

Communications
DOI: 10.1002/anie.200705976
Metalations
Distannylations and Silastannylations of In Situ Generated Allenes**
Alexander O. Wesquet and Uli Kazmaier*
Dedicated to Professor Theophil Eicher on the occasion of his 75th birthday
Owing to their high versatility, organometallic compounds
have become extremely valuable building blocks and have
found a wide range of applications in organic synthesis,
especially in modern cross-coupling chemistry.[1] This devel-
opment has been possible through the tremendous progress
over the past 40 years in the synthesis and applicability of
organometallic substrates. Besides the typically used mono-
metalated compounds, also dimetalated derivatives are of
enormous interest, especially if different organometallic
centers can made to undergo reactions selectively.
In this context, Mitchell et al. reported on palladium-
catalyzed distannylations and silastannylations of alkynes and
Scheme 1. Stannylations of allenes and alkynes.
allenes.[2] The catalytic dimetalation of allenes has the
advantage that products are formed that have a vinylic and
an allylic metal group as well.[3] Both functionalities can be
coupled independently and selectively, which makes the
products of the catalytic dimetalation of allenes highly
valuable building blocks for organic synthesis.[4–6] Besides
diborations,[6] the addition of stannagermanes[7] and silabor-
anes[5,8] to alkenes and alkynes[9] have been reported as well.
Disilylations of allenes are also possible, but in general this
reaction requires higher reaction temperatures.[10] In the last
few years, Cheng et al. have reported on palladium-catalyzed
carbostannylations,[11] carbosilylations,[12] and acyl metala-
tions.[13]
lated product 3a (Scheme 2). At the beginning it was unclear
how this product could be formed, but we speculated that also
the palladium-catalyzed reaction provided 2a as an inter-
mediate, which might undergo elimination to an allene under
the reaction conditions. Therefore, we investigated the
Our group has also been involved for several years in
hydro- and distannylations of alkynes[14,15] and allenes,[16]
especially in the presence of molybdenum and tungsten
catalysts. Depending on the catalyst used, propargyl acetate A
can selectively be converted either into the vinyl stannane
B[14b] or the dimetalated allyl acetate C (Scheme 1).[15] The
molybdenum-catalyzed hydrostannylation of allenyl alcohols
D provided allyl stannanes E in good yields.[16]
Scheme 2. Distannylation of 1a. a) 3 mol% MoBI3, 2 equiv Bu3SnH,
5 bar CO, THF, 608C, 22 h; b) 2 mol% [Pd(PPh3)4], 3.2 equiv Bu3SnH,
THF, 608C, 22 h; c) 2 mol% [Pd(PPh3)4], 1.05 equiv (Bu3Sn)2, THF,
608C, 6 h.
Herein we present a new protocol towards dimetalated
alkenes starting from alkynes or vinyl stannanes. This reaction
came into our focus during our investigations of the influence
of different catalysts on the regioselectivity of hydrostanny-
lations. While Bu3SnH addition towards the alkyne 1a gave
rise to the expected vinyl stannane 2a[17] in the presence of the
molybdenum catalyst [Mo(CO)3(CNtBu)3] (MoBI3), the pal-
ladium-catalyzed reaction surprisingly provided the distanny-
reaction by NMR, which confirmed that tributylstannylphe-
nolate[18] was formed during the reaction, which is a clear
indication for such an elimination process.[19] Because it is well
known that Pd complexes can convert Bu3SnH into (Bu3Sn)2
by elimination of H2, a subsequent distannylation of the
in situ formed allene might explain the outcome of the
reaction. To prove this possibility, we treated the vinyl
stannane 2a with (Bu3Sn)2 under the same reaction con-
ditions. Indeed, the distannane 3a was formed in high yield
after a shorter reaction time. In contrast, the attempt to obtain
3a directly from 1a using (Bu3Sn)2 was unsuccessful. This is
also a good indication that a hydrostannylation is the first
step.
[*] A. O. Wesquet, Prof. Dr. U. Kazmaier
Institut für Organische Chemie, Universität des Saarlandes
Im Stadtwald, Geb. C4.2, 66123 Saarbrücken (Germany)
Fax: (+49)681–302–2409
E-mail: u.kazmaier@mx.uni-saarland.de
To figure out if this reaction is a specialty of phenyl ethers,
or if it can be applied also to other propargylic substrates, we
focused on the stannylated allylic acetate B, which should give
Homepage: http://www.uni-saarland.de/fak8/kazmaier/
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(Ka880/6 and Ka880/8) and the Fonds der Chemischen Industrie.
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Chemie
rise to the same product 3a (Table 1).[20] Indeed, the expected
product could be obtained, although the yield was a littler
lower in this case (Table 1, entry 1). On the basis of our
assumption that the (Bu3Sn)2 required for the distannylation
Table 1: Synthesis of distannylated alkenes 3.
Entry Substrate R1 R2 [Sn]
Product Yield[b] [%]
1
2
3
4
5
B
B
2b
2b
1b
Ac
Ac
H
H
1.05 equiv (Bu3Sn)2 3a
2.1 equiv Bu3SnH 3a
62
74
65
74
41
Ac Ph 1.05 equiv (Bu3Sn)2 3b
Ac Ph 2.1 equiv Bu3SnH
Ac Ph 3.2 equiv Bu3SnH
3b
3b
[a] 2% [Pd(PPh3)4], THF, 608C, 2.5–6 h. [b] Compound 3 was obtained
together with (Bu3Sn)2 after flash chromatography. The yields were
calculated on the basis of 1H und 119Sn NMR spectroscopy.
Scheme 4. Postulated mechanism for the Pd-catalyzed distannylation
of alkynes 1.
is formed in situ from Bu3SnH, we carried out the same
reaction also with the tin hydride directly (Table 1, entry 2).
The yield obtained under these conditions was even higher
than with the distannane. The lower yield obtained with
(Bu3Sn)2 might be caused by the moderate stability of this
compound, and therefore, the in situ generation from Bu3SnH
is a significant advantage. Substituents adjacent to the leaving
group do not seem to have a negative influence, because with
the phenyl-substituted derivative 2b the same yields were
obtained (Table 1, entries 3 and 4). With the phenylated
alkyne 1b we could show that also propargyl acetates can be
converted into the distannanes directly with Bu3SnH (Table 1,
entry 5).
The reaction is not restricted to terminal vinyl stannanes,
but can also be applied to substituted derivatives, as
illustrated with vinyl stannane 4 (Scheme 3). Compound 4
was used as an E/Z mixture, and the E/Z ratio did not change
during the reaction. Interestingly, the formation of the
regioisomeric product 3b was not observed.
tions.[23] After elimination, the palladium center probably
stays coordinated to the allene (A). Otherwise, it would be
difficult to explain the good yields obtained, because uncoor-
dinated allene would immediately evaporate from the reac-
tion mixture. Such a palladium complex also explains why the
dimetalation occurs exclusively at the double bond which is
formed by elimination, and not at the other double bond.[5b]
Therefore, the regioisomeric substrates 2b and 4 provide
selectively the isomeric products 3b and 5, and the E/Z ratio
remains unchanged during the conversion.
The (Bu3Sn)2 required for the distannylation is independ-
ently formed, also by Pd catalysis, from Bu3SnH. Oxidative
addition of the distannane towards the allene-coordinated Pd
center and subsequent insertion/reductive elimination gives
rise to the distannylated alkene 3.
Therefore, in this reaction the Pd catalyst has four
functions:
1) hydrostannylation of the alkyne 1;
2) elimination of tributylstannylphenolate;
3) formation of the distannane from Bu3SnH;
4) distannylation of the in situ formed allene.
With the assumption that the palladium–allene complex A
is the key reaction intermediate, stannylated allyl alcohol
derivatives 2 should also react with other dimetallic com-
pounds. This option was investigated by using the phenyl
ether 2a (Scheme 5).
Scheme 3. Regioselective stannylations.
Analogously to the synthesis of 2a (Table 1), 2,3-bis(tri-
methylstannyl)propene (6a) could also be synthesized with
(Me3Sn)2.[24] The application of Bu3SnSiMe3 led to formation
of the silylated products 7a and 8a,[25] whereby the formation
of 8a was quite surprising. Interestingly, the outcome depends
on the amount of silastannane used. If stoichiometric or
substoichiometric amount of this reagent is used, only the
expected product 7a is formed. With two equivalents or more,
only the disilylated product 8a is obtained. With one to two
equivalents silastannane and after shorter reaction times, a
On the basis of these results, we can postulate the
mechanism illustrated for alkyne 1 in Scheme 4. The palla-
dium-catalyzed hydrostannylation of propargylic ether 1 gives
rise to a mixture of the two regioisomers 2 and 2’.[21] Only 2 is
able to undergo elimination of tributylstannylphenolate,
probably promoted by the Pd catalyst.[22] We assume that
the elimination proceeds via a p-allyl-Pd intermediate, at
least in the case of the corresponding acetates, because with
good nucleophiles these compounds undergo allylic alkyla-
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Communications
mixture turned brown from pale yellow, but was yellow again after
3 h. After 22 h the reaction mixture was cooled by liquid N2 and
during slow warm-up, the solvent was removed in high vacuum. The
crude product was dissolved in hexanes, and the catalyst and all polar
side products were removed on a short chromatography column
(SiO2, hexanes). The impurities caused by the excess of the (Bu3Sn)2
could not be removed under these conditions. Therefore, the yield
was calculated from the NMR spectra. Yield: 428 mg (689 mmol,
69%) 3a as a colorless liquid.
Synthesis of 3a from 2a: The synthesis and the workup were
carried out as described before. Vinyl stannane 2a (212 mg, 500 mmol)
was treated with [Pd(PPh3)4] (11.6 mg, 10 mmol) and freshly purified
(Bu3Sn)2[28] (609 mg, 1.05 mmol) in THF (2 mL). No color change was
observed in this case. The reaction was monitored by TLC and was
finished after 5.5 h. Yield: 287 mg (462 mmol, 92%) 3a. 1H NMR:
(400 MHz, CDCl3): d = 0.69–1.05 (m, 30H), 1.23–1.35 (m, 12H), 1.37–
1.52 (m, 12H), 2.02 (d, J = 1.3 Hz, JSn = 59 Hz, 2H), 4.81 Hz (d, J =
2.5 Hz, JSn = 63, 22 Hz, 1H), 5.44 ppm (dt, J = 2.5, 1.3 Hz, JSn = 144,
20 Hz, 1H); 13C NMR (100 MHz, CDCl3): d = 9.5 (JSn = 307 Hz), 9.6
(JSn = 307 Hz), 13.7, 22.4, 27.4 (JSn = 55 Hz), 27.5 (JSn = 55 Hz), 29.2
(JSn = 20 Hz), 120.7, 154.3 ppm; 119Sn NMR (150 MHz, CDCl3): d =
À48.4, À19.8 ppm.
Received: December 28, 2007
Published online: March 10, 2008
Scheme 5. Reactions of vinyl stannanes 2 with dimetallic compounds:
a) 2 mol% [Pd(PPh3)4], THF, 608C, 18 h; b) 2 mol% [Pd(PPh3)4], THF,
608C, 2 h; c) 2 mol% [Pd(PPh3)4], THF, 608C, 11 h.
Keywords: allenes · dimetalation · hydrostannylation ·
molybdenum · palladium
.
mixture of both products is observed. These observations, in
combination with detailed 1H and 119Sn NMR studies, indicate
that in both cases the silastannane 7a is formed first. After
longer reaction times and in the presence of an excess of the
silyl reagent, 7a is converted into 8a. A similar behavior was
also observed in reactions of allenyl stannanes, which run via
allyl stannane intermediates.[26] This direct pathway to the
disilylated compound 8a offers an interesting alternative to
the disilylation of allenes.[27]
This protocol can also be used for substituted substrates,
whereby the nature of the substituent apparently does not
have a significant influence on the reaction (Scheme 5).
Similarly good yields were obtained with both the phenyl-
substituted 2b and isopropyl-substituted 2c.
In conclusion, we could show that propargylic ethers and
esters could be converted directly into distannylated alkenes.
Only Bu3SnH is necessary in this Pd-catalyzed reaction. The
reaction probably proceeds via a stannylated allyl alcohol
derivative, which undergoes elimination and subsequent
dimetalation. The two processes can be decoupled, for
example, by using a molybdenum catalyst in the hydro-
stannylation step. This procedure allows the selective reaction
of the stannylated intermediates with other dimetallic com-
pounds. Further mechanistic studies as well as synthetic
applications are currently under investigation.
[1] Reviews: a) K. C. Nicolaou, P. G. Bulger, D. Sarlah, Angew.
Chem. 2005, 117, 4516 – 4563; Angew. Chem. Int. Ed. 2005, 44,
4442 – 4489; b) J.-P. Corbet, G. Mignani, Chem. Rev. 2006, 106,
2651 – 2710.
[2] a) T. N. Mitchell, U. Schneider, J. Organomet. Chem. 1991, 407,
319 – 327; b) T. N. Mitchell, Synthesis 1992, 803 – 815; c) M.
Niestroj, W. P. Neumann, T. N. Mitchell, J. Organomet. Chem.
1996, 519, 45 – 68.
[3] Reviews: a) R. Zimmer, C. U. Dinesh, E. Nandanan, F. A. Khan,
Chem. Rev. 2000, 100, 3067 – 3125; b) A. S. K. Hashmi, N.
Krause, Modern Allene Chemistry, Wiley-VCH, Weinheim,
2004; c) S. Ma, Chem. Rev. 2005, 105, 2829 – 2871.
[4] a) C.-M. Yu, J. Youn, J. Jung, Angew. Chem. 2006, 118, 1583 –
1586; Angew. Chem. Int. Ed. 2006, 45, 1553 – 1556; b) S.-K. Kang,
Y.-H. Ha, B.-S. Ko, Y. Lim, J. Jung, Angew. Chem. 2002, 114,
353 – 355; Angew. Chem. Int. Ed. 2002, 41, 343 – 345; c) T. N.
Mitchell, U. Schneider, K. Heesche-Wagner, J. Organomet.
Chem. 1991, 411, 107 – 120; d) T. N. Mitchell, K. Kwetkat, D.
Rutschow, U. Schneider, Tetrahedron 1989, 45, 969 – 978.
[5] a) K.-J. Chang, D. K. Rayabarapu, F.-Y. Yang, C.-H. Cheng, J.
Am. Chem. Soc. 2005, 127, 126 – 131; b) M. Suginome, Y. Ito, J.
Organomet. Chem. 2003, 680, 43 – 50; c) M. Suginome, Y.
Ohmori, Y. Ito, Synlett 1999, 1567 – 1568.
[6] a) N. F. Pelz, A. R. Woodward, H. E. Burks, J. D. Sieber, J. P.
Morken, J. Am. Chem. Soc. 2004, 126, 16328 – 16329; b) N. F.
Pelz, J. P. Morken, Org. Lett. 2006, 8, 4557 – 4559.
[7] T. N. Mitchell, U. Schneider, B. Fröhling, J. Organomet. Chem.
1990, 384, C53 – C56.
[8] S. Onozawa, Y. Hatanaka, M. Tanaka, Chem. Commun. 1999,
1863 – 1864.
À
[9] Comparable reactions on alkynes: a) (Sn Si): T. N. Mitchell, R.
Experimental Section
Wickenkamp, A. Amamria, R. Dicke, U. Schneider, J. Org.
À
Synthesis of 3a from 1a: Phenyl propargyl ether 1a (132 mg,
1.00 mmol) was dissolved in THF (2 mL) in a pyrex flask with a
teflon stopper. A solution of [Pd(PPh3)4][28] (23.1 mg, 20 mmol) in
THF (2 mL) was added, followed by Bu3SnH (917 mg, 3.15 mmol).
The mixture was heated to 608C in a heating block. The reaction
Chem. 1987, 52, 4868 – 4874; b) (Sn Al): S. Sharma, A. C.
Oehlschlager, J. Org. Chem. 1989, 54, 5064– 5073; c) (Sn B):
S. Sharma, A. C. Oehlschlager, Tetrahedron Lett. 1988, 29, 261 –
264; d) (Sn Ge): E. Piers, R. T. Skerlj, J. Chem. Soc. Chem.
À
À
À
Commun 1987, 1025 – 1026; e) (Si B): K. Nozaki, K. Waka-
3052
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Chemie
matsu, T. Nonaka, W. Tückmantel, K. Oshima, K. Utimoto,
Tetrahedron Lett. 1986, 27, 2007 – 2010.
[10] a) H. Watanabe, M. Saito, N. Sutou, Y. Nagai, J. Chem. Soc.
Chem. Commun. 1981, 617 – 718; b) H. Watanabe, M. Saito, N.
Sutou, K. Kishimoto, J. Inose, Y. Nagai, J. Organomet. Chem.
1982, 225, 343 – 356.
[11] F.-Y. Yang, M.-Y. Wu, C.-H. Cheng, Tetrahedron Lett. 1999, 40,
6055 – 6058.
[12] M.-Y. Wu, F.-Y. Yang, C.-H. Cheng, J. Org. Chem. 1999, 64,
2471 – 2474.
[13] F.-Y. Yang, M. Shanmugasundaran, S.-Y. Chuang, P.-J. Ku, M.-Y.
Wu, C.-H. Cheng, J. Am. Chem. Soc. 2003, 125, 12576 – 12583.
[14] a) U. Kazmaier, D. Schauß, M. Pohlman, Org. Lett. 1999, 1,
1017 – 1019; b) U. Kazmaier, M. Pohlman, D. Schauß, Eur. J.
Org. Chem. 2000, 2761 – 2766; c) S. Braune, U. Kazmaier, J.
Organomet. Chem. 2002, 641, 26 – 29; d) S. Braune, M. Pohlman,
U. Kazmaier, J. Org. Chem. 2004, 69, 468 – 474; e) U. Kazmaier,
A. Wesquet, Synlett 2005, 1271 – 1274; f) A. O. Wesquet, S.
Dörrenbꢀcher, U. Kazmaier, Synlett 2006, 1105 – 1109; g) H. Lin,
U. Kazmaier, Eur. J. Org. Chem. 2007, 839 – 2843.
[20] Unfortunately, propargyl acetate A could not be hydrostanny-
lated in the presence of Pd, because the competing formation of
allenyl-Pd complexes was faster.
[21] The regioselectivity towards the desired a-stannylated product
in general is lower with Pd catalysts in comparison to the
molybdenum catalyst MoBI3 (see reference [14], and references
therein).
[22] No elimination was observed in the absence of palladium.
[23] a) U. Kazmaier, D. Schauß, M. Pohlman, S. Raddatz, Synthesis
2000, 914– 916; b) U. Kazmaier, D. Schauß, S. Raddatz, M.
Pohlman, Chem. Eur. J. 2001, 7, 456 – 464; c) J. Deska, U.
Kazmaier, Angew. Chem. 2007, 119, 4654 – 4657; Angew. Chem.
Int. Ed. 2007, 46, 4570 – 4573.
[24] This reaction was not optimized, owing to the high toxicity of the
methylstannanes.
À
À
À
[25] Dimetalations with Sn Si, Sn B, and Si B show a high degree of
regioselectivity. The following preference for the vinyl position is
observed: B > Si > Sn. (see references [2a,b,5b,c]). Tanaka
et al.[8] und Cheng et al.[5a] reported an inverse regioselectivity
À
in the case of B Si.
[15] S. Braune, U. Kazmaier, Angew. Chem. 2003, 115, 318 – 320;
Angew. Chem. Int. Ed. 2003, 42, 306 – 308.
[26] Y. Yamamoto, M. Al-Masum, N. Fujiwara, Chem. Commun.
1996, 381 – 382.
[16] a) U. Kazmaier, M. Klein, Chem. Commun. 2005, 501 – 503;
b) U. Kazmaier, S. Lucas, M. Klein, J. Org. Chem. 2006, 71,
2429 – 2433; c) U. Kazmaier, S. Dörrenbꢀcher, A. Wesquet, S.
Lucas, M. Kummeter, Synthesis 2007, 320 – 326.
[17] The yields obtained in the hydrostannylation can be increased by
running the reaction in an atmosphere of CO, probably because
of a prolonged lifetime of the catalyst.
[27] The direct disilylation requires reaction temperatures of around
1208C (see reference [10]). The attempt to react 2a with
hexamethyldisilane was unsuccessful.
[28] The reaction is rather sensitive towards impurities in the
distannane and the Pd catalyst. Therefore, the distannane was
purified directly before use by flash chromatography (hexanes).
[Pd(PPh3)4] was obtained from Chempur and should be stored
under Ar. Even small color changes (from yellow to orange) in
[Pd(PPh3)4] result in a dramatic drop in yield.
[18] D. Ballivet-Tkatchenko, O. Douteau, S. Stutzmann, Organo-
metallics 2000, 19, 4563 – 4567.
[19] For Pd-free b eliminations of a-hydroxyvinyl stannanes to
allenes, see: C. Nativi, A. Ricci, M. Taddei, Tetrahedron Lett.
1987, 28, 2751 – 2752.
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