Ruthenium-catalyzed trans-selective hydrostannation of alkynes

Article abstract of DOI:10.1002/anie.201311080

In contrast to all other transition-metal-catalyzed hydrostannation reactions documented in the literature, the addition of Bu₃SnH across various types of alkynes proceeds with excellent trans selectivity, provided the reaction is catalyzed by [CpRu]-based complexes. This method is distinguished by a broad substrate scope and a remarkable compatibility with functional groups, including various substituents that would neither survive under the conditions of established Lewis acid mediated trans hydrostannations nor withstand free-radical reactions. In case of unsymmetrical alkynes, a cooperative effect between the proper catalyst and protic functionality in the substrate allows outstanding levels of regioselectivity to be secured as well. Unorthodox: Ruthenium catalysts allow stannanes to be added in a trans fashion across the triple bonds of terminal, internal, silylated, and chlorinated alkynes. This pattern violates the basic mechanism of transition-metal catalysis which otherwise secures high cis selectivity in hydrometalations. Cooperative effects between the ruthenium species and protic functionality render reactions of unsymmetrical substrates regioselective. Cp=η⁵-C ₅Me₅.

Full text of DOI:10.1002/anie.201311080

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Angewandte
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DOI: 10.1002/anie.201311080
Reaction Mechanisms
Ruthenium-Catalyzed trans-Selective Hydrostannation of Alkynes**
Stephan M. Rummelt and Alois Fꢀrstner*
Dedicated to Professor Walter Thiel on the occasion of his 65th birthday
Abstract: In contrast to all other transition-metal-catalyzed
hydrostannation reactions documented in the literature, the
addition of Bu3SnH across various types of alkynes proceeds
with excellent trans selectivity, provided the reaction is cata-
lyzed by [Cp*Ru]-based complexes. This method is distin-
guished by a broad substrate scope and a remarkable compat-
ibility with functional groups, including various substituents
that would neither survive under the conditions of established
Lewis acid mediated trans hydrostannations nor withstand
free-radical reactions. In case of unsymmetrical alkynes,
a cooperative effect between the proper catalyst and protic
functionality in the substrate allows outstanding levels of
regioselectivity to be secured as well.
W
e have recently disclosed preliminary results on ruthe-
nium-catalyzed hydrogenation as well as hydroboration
reactions of internal alkynes, reactions which are strictly
trans selective and hence violate the stereochemical princi-
ples which have governed these transformations since their
inception.[1,2] Although our understanding for the origin of
the high trans selectivity is provisional, both transformations
are thought to be different incarnations of a common
mechanism, which supposedly also underlies the trans-hydro-
silylation chemistry pioneered by Trost and co-workers
shortly after the turn of the millennium.[3–5] Details aside,
these processes are believed to involve loaded complexes of
Scheme 1. Mechanistic hypothesis explaining the trans addition of
ꢀ
reagents of the type E H [E=H, B(pin), SiR3] across acetylene
derivatives in the presence of (cationic) [Cp*Ru]-based catalysts. A
priori, one may conceive that the conversion of the loaded complex A
into D is either stepwise or concerted. Cp*=h5-C5Me5, pin=4,4,5,5-
tetramethyl[1,3,2]dioxaborolanyl (pinacolyl).
ꢀ
type A which carry the reagent E H [E = H, B(pin), SiR3] in
s-bound form (Scheme 1).[6–8] This assumption is based on
calculations for the hydrosilylation case[9] and on control
experiments with a pertinent s-H2 complex for the trans
hydrogenation.[1] Hydride delivery to the bound alkyne may
or may not occur directly, without formation of discrete metal
hydride intermediates.[10] The resulting metallacyclopropene
intermediates (h2-vinyl complexes) are fluxional by reversible
hapticity change (BÐCÐD),[11] which allows the larger
substituent, R, to get out of the way of the bulky Cp* ligand
which blocks one face of the coordination sphere about the
central metal. In accord with this interpretation, the extended
umbrella of the Cp* ring is necessary for high trans selectivity,
independent of whether E = H, SiR3, or B(pin).[1–4,12,13]
Alternatively, one may conceive a concerted pathway, in
which hydride delivery is geared to a rotary motion which
leads to intermediates of type D without intervention of open
cationic species.[9] In any case, a final reductive elimination via
E explains the formation of the observed trans adducts.
ꢀ
This rationale insinuates that other reagents, E H (or
ꢀ
even E E), might also be amenable to similar trans-addition
processes. Stannanes are obvious candidates,[14] not least for
their known ability to form s-complexes with different
electron-deficient metal fragments.[15] The available data
ꢀ
indicate that the Sn H bond of a s-bound stannane is
[*] M. Sc. S. M. Rummelt, Prof. A. Fꢀrstner
Max-Planck-Institut fꢀr Kohlenforschung
45470 Mꢀlheim/Ruhr (Germany)
significantly more elongated than that of an analogous s-
bound silane. This higher degree of activation might either
translate into particularly good trans-donor qualities or prime
the complexes to decomposition with release of H2 upon
contact with excess R3SnH.[15a] Anyway, it was not clear
whether the conceived trans hydrostannation could prevail
over this potentially facile but unproductive side track.
Building upon on our previous experiences,[1,2,4] a few test
reactions sufficed to answer this question. Using the cyclo-
E-mail: fuerstner@kofo.mpg.de
[**] Generous financial support from the MPG and the Fonds der
Chemischen Industrie is gratefully acknowledged. We thank Dr. B.
Sundararaju for preliminary studies and Dr. C. Farꢁs for NMR
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201311080.
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Table 1: Optimization of the trans-hydrostannation reaction.[a]
(Table 2). In many cases, the isomer ratio reached the
detection limit (ꢁ 99:1, 1H NMR). Somewhat ironically
though, 1, serving as the model compound in the initial
screening exercise, gave the lowest Z/E ratio of all substrates
investigated (see Table 2, entry 13). Using simple 5-decyne,
we probed the robustness of the method, which basically
furnished identical results on both a 80 mg and 1.4 g scale
(Table 2, entry 1).
In close analogy to the trans hydroboration,[2] the reaction
tolerates a variety of functional groups and is applicable to
substrates containing electron-deficient arene rings.[21] Most
Table 2: trans-Selective hydrostannation of symmetrical internal alky-
nes.[a]
Entry Major Product
Z/E[b]
Yield [%][c]
96 (94)[d]
1
99:1
Entry
Catalyst
Solvent
Yield [%][b]
Z/E[c]
2
3
99:1
99:1
80
98
1
2
3
4
5
6
7
8
9
10
3
4
5
6
3
3
3
3
3
3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
97 (94)[d]
83
34
85:15
63:37
84:16
89:11
84:16
83:17
82:18
83:17
85:15[g]
86:14[h]
85
97
ClCH2CH2Cl
acetone
MeCN[f]
diglyme
4
5
97:3
98:2
80 (X=Br)
56 (X=N3)
41[e]
66[f]
81
6
99:1
88
CH2Cl2
94[g]
78[h]
[g]
[h]
CH2Cl2
7
8
99:1
99:1
76 (R=H)
89 (R=TBS)
[a] Bu3SnH was added dropwise over 5 min to a 0.1m solution of the
substrate and the catalyst (5 mol%) in the indicated solvent under Ar
atmosphere; the reaction was worked up after 15 min. [b] Yield of
isolated product. [c] Determined by 1H NMR spectroscopy. [d] In 0.5m
solution. [e] Conversion (1H NMR). [f] The stannane is hardly soluble in
this medium. [g] The reaction was performed in the dark. [h] In the
presence of TEMPO (1 equiv). TEMPO=2,2,6,6-tetramethyl-1-piperi-
dinyloxy, free radical.
9
99:1
94
98
10
99:1
94:6
11
12
69[e]
98
alkyne 1[16,17] as our model substrate and commercial [Cp*Ru-
(MeCN)3]PF6 (3) as the catalyst, the product (Z)-2 was
obtained with appreciable selectivity in fair to excellent yields
(Table 1).[18] Like in our previous studies,[1,2] 3 outperformed
its less bulky sibling [CpRu(MeCN)3]PF6 (4), thus confirming
a large steric component in the stereodetermining step. As
expected for a cationic catalyst, noncoordinating solvents
gave the best results. Typical experiments were carried out in
CH2Cl2 (0.1–0.5m) by slowly adding commercial Bu3SnH[19]
over the course of 5–10 minutes to avoid excessive distannane
formation, which wins out if the stannane is added at once.
For less reactive substrates it is advisable to adjust the
addition time.[20] Most reactions were exceptionally fast and
definitely more rapid than the analogous trans hydroboration
of the same substrate with pin-BH.[2] Control experiments
showed that the trans hydrostannation proceeds with uncom-
promised rate and selectivity in the dark (entry 9) as well as in
the presence of one equivalent of TEMPO (entry 10), which
strongly advocates a nonradical pathway.
(99:1)[e]
98:2
13
14
85:15
97
97
95:5
[a] Unless state otherwise, all reactions were performed on 0.1–0.2 mmol
scale by adding the commercial Bu3SnH (1.1 equiv) over the course of
ca. 5 min to a solution of the substrate and complex 3 (5 mol%) in
CH2Cl2 (0.2m) at RT under Ar. [b] Ratio in the crude reaction mixture, as
determined by 1H NMR spectroscopy. [c] Isolated material. [d] 3.5 mmol
Scale using only 1.05 equiv of the stannane. [e] After flash chromatog-
raphy. TBS=tert-butyldimethylsilyl, Ts=p-toluenesulfonyl.
Next, a selection of symmetrical alkynes was subjected to
trans-hydrostannation under the standard reaction conditions
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Table 3: trans-Hydrostannation of unsymmetrical alkynes (for the full Table, see the
noticeable is the compatibility with primary bromides
(Table 2, entry 4) as well as with azides (entry 5),
which would not survive if free tin radicals were
involved at any stage. Moreover, several other polar
or reducible sites remain intact (ester, ketone,
phthalimide, Weinreb amide, primary tosylate, silyl
ether, unprotected alcohols, and acids). This remark-
able chemoselectivity profile distinguishes the cur-
rent method from an otherwise also highly trans-
selective hydrostannation using strong Lewis acids
such as ZrCl4 in substoichiometric or stoichiometric
amounts.[22] Such harsh promoters, however, do not
tolerate most functionality, and even benzyl ethers
are incompatible.[22]
In an attempt to further extend the scope, we
initially encountered the usual regioselectivity issues
which tend to trouble hydrometalations of unsym-
metrical alkynes. The reaction of pent-3-yne-2-ol
under the standard reaction conditions gave a disap-
pointing 74:26 mixture of the proximally and distally
stannylated products (Table 3, entry 1), both of which
derive from a trans-addition process.[23] Gratifyingly
though, replacement of the cationic complex 3 by
other commercial Cp*-containing precatalysts led to
much more rewarding outcomes (entries 2–4). Spe-
cifically, the use of either [Cp*Ru(cod)Cl] (5), the
oligomeric RuIII species [{Cp*RuCl2}n] (6), or the
tetrameric cluster [{Cp*RuCl}4] (7)[24] resulted in an
almost exclusive formation of a single isomer.
As manifested in Table 3 (entries 2–11), this
pattern is independent of whether the propargylic
alcohol site is primary, secondary, or tertiary. Increas-
ing the steric demand does not override this pro-
nounced bias, as is often the case in hydrostannations
catalyzed by other transition metals.[25] Comparison
of entries 7 and 8 confirms that the largely improved
regioselectivity is intimately related with the pres-
ence of an unprotected hydroxy group and not
merely caused by dipolar interactions in the transi-
tion state. Even if the OH group was shifted to the
(bis)homopropargylic position, appreciable regiose-
lectivity is retained (entries 12 and 13). Likewise,
a propargylic sulfonamide also showed high prefer-
ence for proximate stannylation when reacted in the
presence of the complex 7 (entry 14). In contrast, the
hydrostannation of an a-methyl branched alkyne led
to product mixtures, irrespective of the chosen
precatalyst.[26] Therefore we conclude that a massive
cooperative effect between the protic functionality
and the catalyst must be operational. The fate of 7 in
the presence of Bu3SnH and a protic substrate and
hence the nature of the active species responsible for
this striking regioselectivity are presently under
investigation.[27–29]
Supporting Information).[a]
Entry
Products
Cat.[b] a/b[c] Z/E[c]
Yield
[%]
1
2
3
4
3
5
6
7
74:26 99:1 (a) 91
97:3 99:1 (a) 73
97:3 99:1 (a) 88[d]
98:2 99:1 (a) 81
5
6
3
7
60:40 99:1 (a) quant.[e]
95:5 99:1 (a) 83[f]
7
8
7
7
98:2 99:1 (a) 84 (R=H)[g]
75:25 94:6 (a) 86 (R=Ac)
9
10
7
7
97:3 99:1 (a) 77 (R=H)
98:2 99:1 (a) 72 (R=Bu)
11
12
13
14
7
7
7
7
99:1 99:1 (a) 97
81:19 95:5 (a) 81
83:17 99:1 (a) 86
99:1 99:1 (a) 90
15
16
3
7
50:50 91:9 (b) 77
90:10 96:4 (a) 87[d,h]
17
18
3
40:60 99:1 (b) 90
6:94 95:5 (b) 71[i,j]
7[i]
19
7
93:7 99:1 (a) 87[h,k]
[a] Unless stated otherwise, all reactions were performed on 0.1–0.2 mmol scale by
adding Bu3SnH (1.1 equiv) over ca. 5 min to a solution of the substrate and the
respective catalyst in CH2Cl2 (0.2m) under Ar. [b] Using either 5 mol% of 3 or 5, or
1.25 mol% of 7. [c] Ratio is that of the crude reaction mixture, as determined by
1H NMR spectroscopy. [d]ꢁ1 mmol Scale. [e] Conversion (1H NMR). [f] 2.1 mmol
Scale. [g] Small amounts of the corresponding ketone were also found. [h] Using
1.0 equiv of Bu3SnH. [i] The stannane was added over 1.5 h. [j] The yield refers to the
pure Z-configured b-stannylated isomer obtained by flash chromatography.
[k] 0.6 mmol Scale.
An equally pronounced effect was recorded for acetylene
carboxylate derivatives. Hydrostannations in the presence of
3, albeit highly trans selective, were regio-indiscriminative
(Table 3, entries 15 and 17). In contrast, the use of 7 forced
the acid to react with high preference at the proximal a-
position (entry 16), most likely by a steering mechanism
which echoes the results of the propargylic alcohol series. If
this cooperativity with the protic functionality is lacking, the
outcome is different. Thus, acetylenic esters exhibit the
opposite preference for stannylation at the distal b-site
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(entry 18). This dichotomy is obviously useful in preparative
terms and nicely distinguishes the current method from other
transition-metal-catalyzed hydrostannations, which tend to be
a-selective even in the acetylenic ester series.[14,30]
The new trans hydrostannation was also successfully
applied to terminal acetylenes,[31,32] to alkynes capped by
a Me3Si-group, as well as to a terminally chlorinated substrate,
none of which is suited for trans hydrogenation or trans
hydroboration at the present stage of development
(Table 4).[1,2] The evidently broader substrate scope suggests
significantly better than other nucleophiles.[34] From a heuris-
tic perspective, the present study adds another important
entry to a growing list of trans-addition reactions which
seemingly violate very basic concepts of metal catalysis. As
such, it invigorates our efforts to further generalize and better
understand this intriguing and enabling reactivity pattern.[35]
Received: December 20, 2013
Published online: February 26, 2014
Keywords: alkynes · reaction mechanisms · ruthenium ·
.
synthetic methods · tin
Table 4: trans-Hydrostannation of terminal, silylated, or chlorinated
alkynes or diynes.[a]
Entry Major Product
Isomer Z/E[b] Yield [%]
[1] K. Radkowski, B. Sundararaju, A. Fꢀrstner, Angew. Chem. 2013,
125, 373 – 378; Angew. Chem. Int. Ed. 2013, 52, 355 – 360.
[2] B. Sundararaju, A. Fꢀrstner, Angew. Chem. 2013, 125, 14300 –
14304; Angew. Chem. Int. Ed. 2013, 52, 14050 – 14054.
[3] a) B. M. Trost, Z. T. Ball, T. Jçge, J. Am. Chem. Soc. 2002, 124,
7922 – 7923; b) B. M. Trost, Z. T. Ball, J. Am. Chem. Soc. 2005,
127, 17644 – 17655; c) B. M. Trost, M. R. Machacek, Z. T. Ball,
Org. Lett. 2003, 5, 1895 – 1898; d) B. M. Trost, Z. T. Ball, J. Am.
Chem. Soc. 2003, 125, 30 – 31.
ratio[b]
1
97:3
–
73
2
3
76:24[c]
93:7[c]
–
–
89 (R=Me)
96 (R=SiMe3)
[4] a) A. Fꢀrstner, K. Radkowski, Chem. Commun. 2002, 2182 –
2183; b) F. Lacombe, K. Radkowski, G. Seidel, A. Fꢀrstner,
Tetrahedron 2004, 60, 7315 – 7324.
4
5
n.d.
99:1 55[d]
99:1 98
[5] For
a related trans hydrogermylation, see: T. Matsuda, S.
Kadowaki, Y. Yamaguchi, M. Murakami, Org. Lett. 2010, 12,
1056 – 1058.
98:2
[6] G. J. Kubas, Metal Dihydrogen and s-Bond Complexes, Kluwer/
Plenum, Dordrecht, 2001.
[7] a) G. J. Kubas, Catal. Lett. 2005, 104, 79 – 101; b) S. Lachaize, S.
Szabo-Etienne, Eur. J. Inorg. Chem. 2006, 2115 – 2127; c) R. H.
Crabtree, Angew. Chem. 1993, 105, 828 – 845; Angew. Chem. Int.
Ed. Engl. 1993, 32, 789 – 805.
6
7
96:4
99:1
99:1 82
99:1 94
[8] For another recent study into a s-silane complex relevant to
catalysis, see: D. V. Gutsulyak, S. F. Vyboishchikov, G. I. Niko-
nov, J. Am. Chem. Soc. 2010, 132, 5950 – 5951.
[9] L. W. Chung, Y.-D. Wu, B. M. Trost, Z. T. Ball, J. Am. Chem. Soc.
2003, 125, 11578 – 11582.
[10] For the hydrosilylation, computations suggest that hydride
delivery precedes silyl delivery (see Ref. [9]); we assume that
the same phasing applies to the trans hydroboration, although
a reverse order of events cannot be excluded at this point.
Whether differences in the regioselective outcomes of trans
hydrostannations, hydrosilylations, and hydroborations are
caused by changes in the timing of these elementary steps is
speculative at this point.
[a] In case of terminal alkynes, the reactions (0.1–0.2 mmol scale) were
performed by adding a solution of the substrate and Bu3SnH (1.1 equiv)
in CH2Cl2 to a solution of the catalyst (5 mol%) in the same solvent over
ca. 12 min under Ar; all other reactions were performed by adding
Bu3SnH (1.0–1.1 equiv) over ca. 5 min to a solution of the substrate and
complex 3 (5 mol%) in CH2Cl2 (0.2m). [b] Ratio in the crude reaction
mixture as determined by 1H NMR spectroscopy. [c] Refers to the ratio of
stannylation of the two different alkynes present in the substrate.
[d] Using complex 7 (1.25 mol%). n.d.=not determined.
ꢀ
that Sn H activation by a cationic ruthenium species is
[11] D. S. Frohnapfel, J. L. Templeton, Coord. Chem. Rev. 2000, 206 –
207, 199 – 235.
[12] A. Fꢀrstner, M. Bonnekessel, J. T. Blank, K. Radkowski, G.
Seidel, F. Lacombe, B. Gabor, R. Mynott, Chem. Eur. J. 2007, 13,
8762 – 8783.
particularly favorable. Preliminary data even indicate a useful
differential in reactivity, since terminal acetylenes could be
addressed with decent selectivity in the presence of internal or
silylated triple bonds (entries 2 and 3). Likewise, it is possible
to discriminate between two internal acetylenes, provided one
of them is propargylic (entry 4).
[13] For a striking case in which replacement of Cp* by Cp affected
the stereo- as well as the regioselectivity of alkyne hydro-
silylations, see: S. Ding, L.-J. Song, L. W. Chung, X. Zhang, J.
Sun, Y.-D. Wu, J. Am. Chem. Soc. 2013, 135, 13835 – 13842.
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[15] a) U. Schubert, E. Kunz, B. Harkers, J. Willnecker, J. Meyer, J.
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In summary, we present an exceptionally productive
trans hydrostannation which nicely complements or exceeds
the established methodologies, be they metal-catalyzed or
free-radical-based. It is distinguished by a unique and
rewarding chemo-, regio-, and stereoselectivity profile.
Despite some toxicity concerns, alkenyltin reagents remain
indispensable for synthesis because of their reliability and
versatility.[33] Actually, a number of advanced applications are
documented in the literature where tin reagents fared
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e) A. Khaleel, K. J. Klabunde, Inorg. Chem. 1996, 35, 3223 –
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[28] For an interesting example of a neutral 16-electron cyclopenta-
dienyl half sandwich complex which features an internal hydro-
gen bond between a coordinated propargyl alcohol and the
chloride ligand on ruthenium, see: B. Dutta, B. F. E. Curchod, P.
Campomanes, E. Solari, R. Scopelliti, U. Rothlisberger, K.
Severin, Chem. Eur. J. 2010, 16, 8400 – 8409.
[29] In radical additions of R3SnH, the cis/trans ratio is strongly
influenced by secondary processes caused by the tin radicals.
Propargylic alcohols usually favor a-stannylation via O-com-
plexed tin radicals, although a recent investigation speaks for
a more involved mechanism. For representative studies, see:
a) A. J. Leusink, H. A. Budding, J. Organomet. Chem. 1968, 11,
533 – 539; b) C. Nativi, M. Taddei, J. Org. Chem. 1988, 53, 820 –
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[30] For representative studies, see Ref. [25a] and the following:
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[31] For terminal alkynes, a slightly modified procedure was used, in
which a solution of the substrate and Bu3SnH in CH2Cl2 was
slowly added to a solution of the catalyst in the same solvent; see
the Supporting Information.
[32] With other transition-metal catalysts, the regioselectivity can
vary from modest to excellent. For leading references, see
Ref. [25a,30b] and the following: a) K. Kikukawa, H. Umekawa,
F. Wada, T. Matsuda, Chem. Lett. 1988, 881 – 884; b) Y. Ichinose,
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[18] The trans-addition mode is evident from the spectral data; see
the Supporting Information. Particularly diagnostic is the JSn,H
coupling constant, which is about twice as large if tin and
hydrogen are trans to each other than if they are in a cis
arrangement: B. Wrackmeyer, Annu. Rep. NMR Spectrosc. 1986,
24, 73 – 186.
[19] Commercial Bu3SnH is stabilized with 0.05% of 3,5-di-tert-
butyl-4-hydroxytoluene, which was not removed in any of the
reactions described herein.
[20] Distannane formation plagues many transition-metal-catalyzed
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[26] Specific examples are contained in the Supporting Information.
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