Synthesis of allylsilanes and dienylsilanes by a one-pot catalytic C-H borylation-Suzuki-Miyaura coupling sequence

Article abstract of DOI:10.1021/ol801203u

(Chemical Equation Presented) Allylsilane and various vinyl substrates undergo selective C-H bond functionalization based boronation reactions using iridium catalysis. The products were further reacted with aryl and vinyl halogenides in a one-pot sequence. This method is suitable for regio- and stereoselective synthesis of various allylsilanes and dienylsilanes.

Full text of DOI:10.1021/ol801203u

ORGANIC
LETTERS
2008
Vol. 10, No. 14
3129-3131
Synthesis of Allylsilanes and
Dienylsilanes by a One-Pot Catalytic
C-H Borylation-Suzuki-Miyaura
Coupling Sequence
Vilhelm J. Olsson and Ka´lma´n J. Szabo´*
Stockholm UniVersity, Arrhenius Laboratory, Department of Organic Chemistry,
SE-106 91 Stockhom, Sweden
kalman@organ.su.se
Received May 27, 2008
ABSTRACT
Allylsilane and various vinyl substrates undergo selective C-H bond functionalization based boronation reactions using iridium catalysis.
The products were further reacted with aryl and vinyl halogenides in a one-pot sequence. This method is suitable for regio- and stereoselective
synthesis of various allylsilanes and dienylsilanes.
Allyl and vinylsilanes have been widely applied as useful
building blocks in complex organic transformations and
natural product synthesis.1 Therefore, development of new
methods for selective synthesis of these species has attracted
much interest in modern organic chemistry.1h–p In this
respect, direct functionalization of allylsilanes offers a
powerful approach for the synthesis of regio- and stereode-
fined derivatives.1b,c
Recent works by Marder, Hartwig, Miyaura, Smith, and
their co-workers2a–g as well as our laboratory2h have
demonstrated that organic substrates can be efficiently
functionalized by C-H bond activation based borylation,3a
which is often followed by a C-C bond formation reaction
in a one-pot sequence.
We have envisaged that a similar strategy can also be
employed for synthesis of functionalized allylsilanes starting
from the parent compound 1a (eq 1, Table 1).
(1) (a) Fleming, I.; Barbero, A.; Walter, D Chem. ReV 1997, 97, 2063.
(b) Brook, M. A. Silicon in Organic, Organometallic, and Polymer
Chemistry; Wiley: Chichester, 2002. (c) Sarkar, T. K. Science of Synthesis,
Houben-Weyl Methods of Molecular Transformations; Fleming, I., Ed.;
Thieme: Stuttgart, 2002; Vol. 4, p 837. (d) Langkopf, E.; Schinzer, D. Chem.
ReV. 1995, 95, 1375. (e) Chabaud, L.; James, P.; Landais, Y. Eur. J. Org.
Chem. 2004, 3173. (f) Chan, T. H. Chem. ReV. 1995, 95, 1279. (g)
Yamamoto, Y.; Asao, N. Chem. ReV. 1993, 93, 2207. (h) Kacprzynski,
M. A.; May, T. L.; Kazane, S. A.; Hoveyda, A. H. Angew. Chem., Int. Ed.
2007, 46, 4554. (i) Nakamura, S.; Uchiyama, M.; Ohwada, T. J. Am. Chem.
Soc. 2005, 127, 13116. (j) Huber, J. D.; Perl, N. R.; Leighton, J. L. Angew.
Chem., Int. Ed. 2008, 47, 3037. (k) Auer, G.; Oestreich, M. AdV. Synth.
Catal. 2005, 347, 637. (l) Karabelas, K.; Westerlund, C.; Hallberg, A. J.
Org. Chem. 1985, 50, 3896. (m) Macsa´ri, I.; Hupe, E.; Szabo´, K. J. J. Org.
Chem. 1999, 64, 9547. (n) Macsa´ri, I.; Szabo´, K. J. Chem. Eur. J. 2001, 7,
4097. (o) Barbero, A.; Cuadrado, P.; Fleming, I.; Gonza´lez, A. M.; Pulido,
F. J. J. Chem. Soc., Perkin Trans. 1 1992, 327. (p) Hayashi, T.; Konishi,
M.; Ito, H.; Kumada, M. J. Am. Chem. Soc. 1982, 104, 4962.
Indeed, we have found that allylsilanes 8a-i can be
obtained from 1a by employing a one-pot CH-borylation/
CC-coupling sequence (entries 1-8). The iridium-
catalyzed (2) regio- and stereoselective borylation of 1a
was performed at 80 °C (4 h) using 3 as boronate source.
Subsequently, the in situ generated boronate 4a was
reacted with aryl (5a-c) and vinyl (6a-e) halogenides
10.1021/ol801203u CCC: $40.75
Published on Web 06/19/2008
2008 American Chemical Society

conditions employed in the one-pot sequence gave a high
level of functional group tolerance as silyl, hydroxy, acetal
vinyl ether, and carbonyl groups remained unchanged
under the process.
Table 1. Synthesis of Allyl- and Vinylsilanes via CH
Borylationa
The high regio- and stereoselectivity is a very attractive
feature of the presented one-pot sequence. Although aryla-
llylsilanes (such as 8a-c) are relatively simple compounds,
their synthesis is usually encumbered by rearrangement and
desilylation processes.1a–c,l However, using the presented
method, these side reactions can be completely avoided,
and the corresponding arylallylsilanes (8a-c) can be pre-
pared efficiently and with high selectivity (entries 1-3).
Stereodefined allyl dienylsilanes are particularly useful
synthons1a–g,5 but also challenging synthetic targets. Con-
sidering the high selectivity of the presented borylation and
coupling reactions (entries 4-9), these types of compounds
can easily be obtained by the above methodology (eq 1).
The employed one-pot approach is suitable for synthesis of
both isolated (8i) and conjugated (8d-g) allyl dienylsilanes.
Even dienes with different double-bond geometries (8f) could
be prepared without any isomerization reactions (entry 6).
The coupling reaction could also be smoothly performed in
the presence of a bulky phenyl group affording 8g in high
selectivity (entry 7).
Silyl-substituted butadienes are useful substrates in
Diels-Alder and related reactions.1a–d,6 Due to the high
selectivity of the borylation of 1b-e and the subsequent
coupling with vinyl bromide 6e, these types of reagents are
also easily accessible by the above-described C-H activa-
tion/C-C coupling sequence. Dienylsilanes 9c,d are structur-
ally interesting species, as in these compounds the linear
conjugation is extended through three π-bonds.
Probably the most intriguing mechanistic feature of the
presented one-pot reactions is the highly selective formation
of vinyl boronate 4a from the parent allylsilane 1a. The
regioselectivity of the C-H activation based borylation
(2) (a) Mkhalid, I. A. I.; Coventry, D. N.; Albesa-Jove, D.; Batsanov,
A. S.; Howard, J. A. K.; Perutz, R. N.; Marder, T. B. Angew. Chem., Int.
Ed. 2006, 45, 489. (b) Coapes, R. B.; Souza, F. E. Z.; Thomas, R. L.; Hall,
J. J.; Marder, T. B. Chem. Commun. 2003, 614. (c) Mkhalid, I. A. I; Coapes,
B.; Edes, N.; Coventry, D. N.; Souza, F. E. S.; Thomas, R. L.; Hall, J. J.;
Bi, S.-W.; Lin, Z.; Marder, T. B. Dalton Trans. 2008, 1055. (d) Burgess,
K.; van der Donk, W. A.; Westcott, S. A.; Marder, T. B.; Baker, R. T.;
Calabrese, J. C. J. Am. Chem. Soc. 1992, 114, 9350. (e) Tzschucke, C. C.;
Murphy, J. M.; Hartwig, J. F. Org. Lett. 2007, 9, 761. (f) Boller, T. M.;
Murphy, J. M.; Hapke, M.; Ishiyama, T.; Miyaura, N.; Hartwig, J. F. J. Am.
Chem. Soc. 2005, 127, 14263. (g) Cho, J. Y.; Tse, M. K.; Holmes, D.;
Maleczka, R. E.; Smith, M. R. Science 2002, 295, 305. (h) Olsson, V. J.;
Szabo, K. J. Angew. Chem., Int. Ed. 2007, 46, 6891. (i) Brown, J. M.; Lloyd-
a The borylation reactions were carried out in neat 1a-e (4-8 equiv)
with diboronate 3 in the presence of catalyst 2 (2 mol %) at 80 °C for 4 h.
b This step was followed by addition of 5 and 6, Cs2CO3, and Pd(PPh3)4 (5
mol %) in a mixture of THF/water and stirring the mixture for the given
times and temperatures. c Isolated yield. d Palladium pincer-complex catalyst
(2.5 mol %) was used as catalyst in the second coupling step (see ref4 and
the Supporting Information). e This product was obtained in a 10:1 trans/
cis ratio. f Pd(OAc)2 (5 mol %) and dppf (10 mol %) were used instead of
Pd(PPh3)4.
Jones, G. C. J. Am. Chem. Soc. 1994, 116, 866
.
as well as vinyl epoxide derivative 7 under Suzuki-Miyaura
conditions3 affording allyl and dienyl silanes 8a-i. To
our delight, under similar reaction conditions, selective
functionalization of several vinyl substrates 1b-e could
also be achieved. By employment of vinylsilane 6e as
coupling reagent, this reaction could be applied for
synthesis of vinyl dienylsilanes 9a-d. The regio- and
stereoselectivity of the overall transformations is excellent,
since only the formation of the trans-substituted linear
products (8 and 9) (Table 1) was detected. The only
exception was the reaction giving 8i (entry 9), which
resulted in about 10% of the cis isomer as well. The mild
(3) (a) Hall, D. G. Boronic Acids; Wiley: Weinheim, 2005. (b) Miyaura,
N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (c) Miyaura, N. Top. Curr.
Chem. 2002, 219, 11
.
(4) Kjellgren, J.; Aydin, J.; Saltanova, I. V.; Wallner, O. A.; Szabo, K.
Chem. Eur. J. 2005, 11, 5260
.
(5) (a) Lee, T. W.; Corey, E. J J. Am. Chem. Soc. 2001, 123, 1872. (b)
Meyers, C. Y.; Hua, D. H.; Peacock, N. J. J. Org. Chem. 1980, 45, 1721.
(c) Yasuda, H.; Nishi, T.; Miyanaga, S.; Nakamura, A. Organometallics
1985, 4, 359
.
(6) (a) Li, D.; Liu, G.; Hu, Q.; Wang, C.; Xi, Z. Org. Lett. 2007, 9,
5433. (b) Urabe, H.; Mitsui, K.; Ohata, S.; Sato, F. J. Am. Chem. Soc. 2003,
125, 6074. (c) Koreeda, M.; Ciufolini, M. A. J. Am. Chem. Soc. 1982, 104,
2308. (d) Luh, T.-Y.; Wong, K.-T. Synthesis 1993, 349
.
(7) (a) Pereira, S.; Srebnik, M. Organometallics 1995, 14, 3127. (b)
Morrill, C.; Grubbs, R. H. J. Org. Chem. 2003, 68, 6031. (c) Jankowska,
M.; Pietraszuk, C.; Marciniec, B.; Zaidlewicz, M. Synlett 2006, 1695
.
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Org. Lett., Vol. 10, No. 14, 2008

reactions is particularly difficult to control2c,h for linear
alkenes comprising both sp2 and sp3 C-H bonds. This
observation2c,h suggests that the selectivity of the C-H bond
functionalization in 1a is controlled by the steric or electronic
effects of the silyl functionality.
that the observed product 4a is considerably (by 3.4 kcal/
mol) more stable than its isoelectronic isomer 15. This
relatively large energy difference may explain the selective
formation of 4a in the C-H bond activation process. The
surprisingly high stability of 4a is probably due to the
presence of favorable hyperconjugative interactions between
the B(pπ*) and CC(π) as well as between the C-Si(σ) and
CC(π*) molecular orbitals. As there is no possibility for such
type of electronic interactions in 15, the relative thermody-
namical stability of 4a is higher.
We have found that the catalytic borylation of 1a (step 1,
eq 1) leads to formation of 4a and its saturated counterpart,
without formation of any other boronated products. Accord-
ingly, one boronate group from 3 is used to form 4a, whereas
the other one is sacrificed for hydroboration of 1a. Consider-
ing this, and our previous results2h on C-H bond activation-
borylation reactions of cyclohexene using a similar catalytic
system, we conclude that the borylation reaction probably
proceeds via a dehydrogenative borylation mechanism (eq
2).2b,c,i This mechanism, reported by Marder,2b,c Lloyd-
Jones,2i and Brown2i for rhodium-catalyzed C-H activation
reactions, involves two steps: addition of the metal boronate
complex to the double bond followed by ꢀ-hydride elimina-
tion. Accordingly, the corresponding mechanism involving
1a is supposed to be started by addition of iridium tris-boryl
complex2d,f 11 to the double bond of 1a affording complex
12. After a conformational change, ꢀ-hydride elimination
occurs from the boronated carbon resulting in complex 13.
Subsequently, 4a is released and the formed iridium-hydride
complex 14 undergoes hydroboration with 1a by regenerating
the catalyst.
It is very fortunate that the applied iridium catalyst does
not induce cis-trans isomerization of 4a-e, and thus,
formation of these species is both stereo- and regioselective.
Alternative methods for preparation of these types of
vinylboronates involve hydroboration7a of alkynes or cross-
metathesis,7b,c which often have selectivity issues.
In summary, we have devised a versatile one-pot protocol
for regio- and stereoselective preparation of allylsilanes and
vinyl dienylsilanes from easily accessible starting materials
via a highly selective catalytic CH-borylation/CC-coupling
sequence. The presented method opens operationally simple,
new routes for preparation of densely functionalized allyl-
silanes and dienylsilanes, which are important building blocks
in modern organic chemistry and natural product synthe-
sis.1,5,6
Acknowledgment. This work was supported by the
Swedish Research Council (VR).
Supporting Information Available: Experimental pro-
1
cedures, NMR data, as well as H and 13C NMR spectra of
products 8a-i and 9a-d. This material is available free of
charge via the Internet at http://pubs.acs.org.
An interesting aspect is the selective formation of allyl-
silane 4a instead of vinylsilane 15 in the ꢀ-hydride elimina-
tion step (eq 3). DFT calculations (B3PW91/6-31G**) show
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