1,1-carboboration of 1-alkynes: A conceptual alternative to the hydroboration reaction

Article abstract of DOI:10.1021/ol102544x

Strongly electrophilic boranes R-B(C₆F₅)₂ react readily with a variety of 1-alkynes by means of a 1,1-carboboration reaction to yield alkenylborane products, which can subsequently be used as reagents in metal catalyzed cr

Full text of DOI:10.1021/ol102544x

ORGANIC
LETTERS
2011
Vol. 13, No. 1
62-65
1,1-Carboboration of 1-Alkynes: A
Conceptual Alternative to the
Hydroboration Reaction
Chao Chen, Tanja Voss, Roland Fro¨hlich, Gerald Kehr, and Gerhard Erker*
Organisch-Chemisches Institut, Westfa¨lische Wilhelms-UniVersita¨t, Corrensstrasse 40,
48149 Mu¨nster, Germany
erker@uni-muenster.de
Received October 20, 2010
ABSTRACT
Strongly electrophilic boranes R-B(C6F5)2 react readily with a variety of 1-alkynes by means of a 1,1-carboboration reaction to yield alkenylborane
products, which can subsequently be used as reagents in metal catalyzed cross-coupling reactions.
The 1,1-carboboration of alkynes previously had been a
rather specific reaction leading to boryl-functionalized alk-
enes.1 Formally it might be regarded as an insertion reaction
of the vinylidene isomer of the acetylene into a boron-carbon
bond of the borane reagent.2 However, mechanistically it
proceeds by means of an abstraction/rearrangement se-
quence.3 So far it required the presence of particular
substituents at the alkyne, mostly main group or transition
metal derived, that served as good 1,2-migrating groups in
this scheme.1,3 We had quite recently found that 1,1-
carboboration reactions of simple 1-alkynes can easily be
performed with the strongly electrophilic R-B(C6F5)2 re-
agents,4 which are readily available.5 In this account we will
report on some significant progress including utilization of
the resulting 1,1-carboboration products in cross-coupling
reactions.
In a typical example, we mixed the terminal alkyne
5-phenyl-1-pentyne (1a) with B(C6F5)3 (2a)6 in CD2Cl2.
Inspection by NMR spectroscopy (1H, 13C, 19F, 11B) revealed
that the 1,1-carboboration reaction was complete within
minutes to give a close to quantitative conversion to a ca.
1:1.2 mixture of the isomeric products E-3a and Z-3a (for
details see the Supporting Information). Subsequent pho-
tolysis (HPK 125, Pyrex filter, 15 min, rt) resulted in a >95%
conversion of the E-3a to the Z-3a isomer (see Scheme 1).
We then performed this reaction on a preparative scale. In
an one-pot procedure the thermally induced 1,1-carboboration
reaction of the 1-alkyne 1a with the borane 2a was carried
out in pentane at ambient conditions. Subsequent photolysis
(1) Wrackmeyer, B. Coord. Chem. ReV. 1995, 145, 125–156.
(2) In a formal sense this reaction is remotely related to the reverse of
the Fritsch-Buttenberg-Wiechell reaction. Review: Tykwinski, R. R.
Chem. Commun. 2010, 46, 3235–3249.
(3) Review: (a) Wrackmeyer, B. Heteroat. Chem. 2006, 17, 188–208,
and references cited therein. See also selected examples: (b) Wrackmeyer,
B.; Horchler, K.; Boese, R. Angew. Chem. 1989, 101, 1563–1565. (c)
Wrackmeyer, B.; Horchler, K.; Boese, R. Angew. Chem., Int. Ed. Engl.
1989, 28, 1500–1502. (d) Wrackmeyer, B.; Kehr, G.; Boese, R. Angew.
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(g) Wrackmeyer, B.; Kundler, S.; Milius, W.; Boese, R. Chem. Ber. 1994,
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Ohki, Y.; Tatsumi, K.; Grimme, S.; Kehr, G.; Erker, G. Chem. Asian J.
2010, 5, 2199–2208. (b) Chen, C.; Kehr, G.; Fro¨hlich, R.; Erker, G. J. Am.
Chem. Soc. 2010, 132, 13594–13595.
(5) For their preparation, see the Supporting Information. See also:
Spence, R. E. v. H.; Piers, W. E.; Sun, Y.; Parvez, M.; MacGillivray, L. R.;
Zaworotko, M. J. Organometallics 1998, 17, 2459–2469.
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250. (b) Wang, C.; Erker, G.; Kehr, G.; Wedeking, K.; Fro¨hlich, R.
Organometallics 2005, 24, 4760–4773.
10.1021/ol102544x  2011 American Chemical Society
Published on Web 12/03/2010

Scheme 1
Figure 1. Molecular structure of Z-3c.
a Isolated yield. b Not determined.
(3 h) followed by workup eventually gave the pure Z-3a
product in 63% yield. Similarly, the products Z-3b and Z-3c
were obtained in good yield starting from 1b and 1c,
respectively. The X-ray crystal structure analysis of Z-3c
clearly shows the substituent pattern at the double bond
originating from the 1,1-carboboration reaction after subse-
quent irradiation.
Scheme 2
Figure 2. Molecular structures of the 1,1-carboboration products
E-5a (top) and E-5b (bottom).
a Isolated yield. b 75 °C for 5 h.
Org. Lett., Vol. 13, No. 1, 2011
63

Scheme 3
a A: starting from the isolated alkenylboranes Z-3 or E-5, respectively. B: one-pot procedure. b For the respective alkenylborane see ref 4a. c Trimethylsilyl
groups were removed during workup.
The 1,1-carboboration reaction of 1-alkynes is chemo-
selective with regard to the substituents employed at the
borane. We treated 1-pentyne with the reagent H3C-B(C6F5)2
(2b) under similar conditions and found that exclusively the
methyl substituent was transferred from boron to carbon in
the course of the 1,1-carboboration reaction to give a mixture
of the Z-3d and E-3d isomers. Subsequent photolysis again
gave the pure Z-3d product (69% isolated, see Scheme 1).
Similarly, we obtained the product Z-3e selectively from the
1,1-carboboration/photochemical isomerization sequence, in
which only the 2-phenylethyl substituent had migrated.
Since we could selectively prepare substituted alkenyl
boranes (Z-3) with a Z-configuration by this facile 1,1-
carboboration/photolysis sequence, we searched for an E-
selective alternative. This was found by employing 1-alk-
yne substrates bearing alkoxyalkyl or trimethylsiloxyalkyl
substituents. A typical example is the reaction of bis-
propargyl ether (4a) with B(C6F5)3 (2a). The 1,1-carbobo-
ration reaction took place readily at room temperature
selectively with one alkynyl moiety to generate a ca. 1:1.8
mixture of the products Z-5a and E-5a. Subsequent photolysis
again resulted in a fast and efficient isomerization of the
carbon-carbon double bond of the product, but in this case
the photostationary equilibrium lay on the side of the E-5a
isomer (>95%, see Scheme 2). We isolated this crystalline
product in 62% yield and characterized it by X-ray diffraction
(see Figure 2). The X-ray crystal structure analysis confirmed
the formation of 5a by the 1,1-carboboration sequence and
revealed stabilization of the borane product by internal
oxygen to boron coordination (B1-O5 1.617(2) Å). This
structural feature of compound E-5a is retained in solution
(e.g., 11B NMR, δ 9.1; 19F NMR, ∆δm,p[B(C6F5)2] ) 8.5).7
The 1,1-carboboration/photolysis sequence of the -OCH3
or -OTMS functionalized 1-alkynes 4b-d proceeds analo-
gously. In each case we eventually isolated the respective
E-alkenylborane product in good yield (see Scheme 2).
Compound 5b was also characterized by X-ray diffraction
(7) (a) Piers, W. E. AdV. Organomet. Chem. 2005, 52, 1–76. (b)
Beringhelli, T.; Donghi, D.; Maggioni, D.; D’Alfonso, G. Coord. Chem.
ReV. 2008, 252, 2292–2313, and references cited therein.
64
Org. Lett., Vol. 13, No. 1, 2011

(see Figure 2). The crystal structure analysis confirmed the
E-configuration of the CdC double bond in 5b and shows
the internal stabilization of the borane by internal oxygen to
boron coordination. The oxygen-boron bond in 5b (B2-O1
1.595(3) Å) is shorter than in 5a.
shows a different pattern of forming the essential new bonds
and, thus, provides a useful alternative to the hydroboration
reaction,13 especially in cases (e.g., 3d, 3e) where the latter
might encounter regioselectivity problems. This new se-
quence makes the 1,1-carboboration reaction of 1-alkynes
an interesting and useful methodological addition to the
synthetic repertoire of organo-boron reagent chemistry.
The alkenylboranes 3 and 5 are interesting products in
themselves as functionalized strong boron Lewis acids. They
have now also been employed as reactive reagents in Pd-
catalyzed cross-coupling reactions. For this purpose the
isolated alkenylborane products were used (method A,
Scheme 3), but it was also possible to alternatively employ
these reagents generated in situ without additional workup
in an one-pot procedure (method B). The reaction starting
from 5-phenyl-1-pentyne (1a) is a typical example. Selective
1,1-carboboration with CH3B(C6F5)2 took place at room
temperature; this was followed by photolysis and then
Pd(PPh3)4 (10 mol %) catalyzed coupling with phenyl iodide
(see Scheme 3)8 to selectively give the product E-6b, which
was isolated in a yield of 76%. The -OTMS functionalized
alkynes reacted similarly, only in these cases the correspond-
ing alcohols (E-6d, Z-6e, see Scheme 3) were isolated in 90
or 73% yield, respectively. The p-cyanophenyl substituted
alkene Z-6f was obtained in good yield from the Pd-catalyzed
cross-coupling reaction of Z-3c with p-iodobenzonitrile.9
The new 1,1-carboboration reaction of simple 1-alkynes
makes Z-substituted reactive alkenylboranes readily available.
In principle, such products could also be prepared by a
hydroboration route of the corresponding alkyne backbones
with Pier’s borane HB(C6F5)2.10-12 Of course, our procedure
Acknowledgment. Financial support from the Deutsche
Forschungsgemeinschaft is gratefully acknowledged. C.C.
thanks the Alexander von Humboldt-Stiftung for a stipend.
Supporting Information Available: Experimental pro-
cedures and spectroscopic data for all new compounds 3, 5
and 6. CIF files for compounds Z-3c, E-5a and E-5b. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL102544X
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