Polyborane reactions in ionic liquids: New efficient routes to functionalized decaborane and o-carborane clusters

Article abstract of DOI:10.1021/ja048018n

In contrast to reactions that have been observed in traditional organic solvents, decaborane olefin-hydroboration and alkyne-insertion reactions have been found to proceed in ionic liquid solvents without the need of a catalyst. These reactions now provide important new, high-yield synthetic pathways to functionalized decaborane and o-carborane clusters. Copyright

Full text of DOI:10.1021/ja048018n

Published on Web 06/22/2004
Polyborane Reactions in Ionic Liquids: New Efficient Routes to
Functionalized Decaborane and o-Carborane Clusters
Upal Kusari,1a Yuqi Li,1a Mark G. Bradley,1b and Larry G. Sneddon*,1a
Departments of Chemistry, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6323, and
Widener UniVersity, Chester, PennsylVania 19013-5792
Received April 6, 2004; E-mail: lsneddon@sas.upenn.edu
Ionic liquids (IL) are salts that are liquid at low temperatures
(<100 °C) and have relatively low viscosities. The unique properties
of ILs have made them attractive substitutes for conventional
solvents for organic and inorganic syntheses, as well as for metal-
catalyzed reactions.2 The recent report by Vaultier3 that transition
metal-catalyzed hydrosilylation and hydroboration of alkynes and
alkenes can be carried out in various ionic liquids stimulated our
interest in the use of ILs as noncoordinating solvents for the metal-
catalyzed reactions of decaborane. Much to our surprise, we found
that, unlike in conventional organic solvents where either platinum
or titanium catalysts are required,4 when decaborane olefin-hy-
droboration reactions (eq 1) are carried out in ILs, they give high
yields of hydroboration products eVen in the absence of a metal
catalyst!
These differences suggest that efficient polyalkylations may be
achievable in other ILs in which alkylboranes are more soluble.
Equally important as the above results, we have found that
decaborane reactions with alkynes in ILs do not yield hydroboration
products but instead result in alkyne-insertion, thereby providing
an important new route to o-carboranes.
Previously, o-carboranes have been best synthesized by the base-
promoted, two-step process given in eqs 2 and 3.6
B10H14 + 2 L
8 B10H12L2 + H2
(2)
(L ) SEt2 or CH3CN)
B10H12L2 + RCtCH f 1-R-1,2-C2B10H11 + 2 L + H2 (3)
The reactions involve in situ formation of the B10H12L2 adduct
followed by alkyne insertion to yield the carborane product.
Reactions times are normally 4-5 h, and typical yields for terminal
alkynes range from 6 to 75%. This reaction method gives much
lower yields (or does not work at all) for many internal alkynes.
In contrast to reactions in conventional solvents, decaborane
reacts readily with both terminal and internal alkynes in ILs without
a base catalyst to produce o-carborane compounds in high yields
(eq 4).
B10H14 + RCtCR′ ionic liquid8
Best results were found for reactions in either 1-butyl-3-
methylimidiazolium (bmim) or 1-butyl-4-methylpyridium (bmpy)
salts containing the BF4- anion. Because hydrocarbons are insoluble
in both of these ILs, the reactions can be carried out under biphasic
conditions. Thus, when decaborane, olefin, and toluene are added
to (bmim)BF4, a two-layer system is formed. NMR analysis of the
IL layer showed that decaborane partially dissolves in (bmim)BF4.
The reaction mixture is then heated at 125 °C while being stirred
vigorously enough to form an emulsion. As the 6-R-B10H13 product
is formed, it is extracted into the toluene. When the reaction is
complete, the product is removed in the toluene layer. Any residual
IL can be separated by running the toluene solution down a short
silica gel column. The resulting 6-R-B10H13 products are obtained
in excellent yields (∼90%) and purities.5 It is also important to
note that, unlike the metal-catalyzed decaborane olefin-hydrobo-
ration reactions,4 the IL reaction method works well for both in-
ternal and functionalized olefins. Initial studies have shown reac-
tions with a range of olefins, including 1-octene, allylsilane, sty-
rene, allylbenzene, 6-bromo-1-hexene, acetic acid-5-hexen-1-yl
ester, ethyl 2-methylpent-4-enoate, 1,5-cyclooctadiene, 1,7-octadi-
ene, cis-cyclooctene, 2,3-dimethyl-2-butene, and vinylmethylcy-
clotetrasiloxane.
1-R-2-R′-C2B10H10 + 2 H2 (R ) R or H) (4)
As shown in the NMR spectra in Figure 1 for the reaction of 5
mmol of decaborane with 1-octyne in (bmim)Cl,7 reaction times
can be on the order of only a few minutes. The bottom spectrum
shows the initial decaborane starting material in the toluene layer
above the IL. The 1-octyne was then added and the stirred emulsion
heated at 120 °C. The middle spectrum shows the reaction mixture
after 4 min where the peaks of the 1-(C6H13)-1,2-C2B10H11 product
are clearly visible. The top spectrum shows that after only 7 min
the reaction has gone to completion. Workup as described above
then gave a ∼90% isolated yield of product. Initial screening
reactions have shown similar reactions for a variety of both internal
(including, for example, 3-hexyne, diphenylethyne, and 2-butyn-
1-ol) and terminal alkynes. The IL route provides a new one-step,
high-yield route to functionalized o-carboranes, and we are expand-
ing our studies to include reactions of decaborane with a wide range
of substituted acetylenes.
The reactions of alkynes with the 6-substituted decaborane
derivatives that are now readily available through the new IL route
given in eq 1 also provide a new general route to B-substituted
o-carboranes. Previously, B-substituted o-carborane derivatives have
been shown to have potential anticancer (BNCT) applications8 but
could only be produced by reactions of RBCl2 with the dicarbollide
anion or by a multistep process involving initial carborane iodi-
nation, followed by reaction with Grignards in the presence of
In the (bmim)BF4 reactions, only the monoalkylated 6-R-B10H13
product forms. The absence of higher alkylated species undoubtedly
arises because of the low solubility of the 6-R-B10H13 products in
(bmim)BF4. On the other hand, reactions in (bmpy)BF4 showed
that with extended reaction times 6,9-R2-B10H12 species are formed.
9
8662
J. AM. CHEM. SOC. 2004, 126, 8662-8663
10.1021/ja048018n CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
alkyne-insertion reactions occur in different ionic liquids suggests
the ILs may have a more complex function. Regardless of the
detailed mechanism of activation, it is clear that, in ILs, decaborane
exhibits reactivities not found in conventional organic solvents and
that these reactions now provide important new synthetic pathways
to both substituted decaborane and o-carborane derivatives. These
results strongly suggest that advantageous IL-based reactions can
be developed for the syntheses of an even wider range of
polyboranes, as well as for other classes of inorganic compounds.
We are presently exploring these possibilities.
Acknowledgment. We thank the U.S. Department of Energy,
Office of Basic Energy Sciences, and the National Science
Foundation for the support of this project.
References
(1) (a) University of Pennsylvania. (b) Widener University.
(2) For general reviews, see: (a) Ionic Liquids in Synthesis; Wassersheid, P.,
Welton, T., Eds.; Wiley-VCH: Weinheim, 2003. (b) Dyson, P. J. Appl.
Organomet. Chem. 2002, 16, 495-500. (c) Zhao, H.; Malhotra, S. V.
Aldrichimica Acta 2002, 35, 75-83. (d) Sheldon, R. Chem. Commun.
2001, 2399-2407. (e) Wasserscheid, P.; Keim, W. Angew. Chem., Int.
Ed. 2002, 39, 3772-3789. (f) Dupont, J.; de Souza, R. F.; Suarez, P. A.
Z. Chem. ReV. 2002, 102, 3667-3692.
(3) Aubin, S.; Floch, F. L.; Carrie´, D.; Guegan, J. P.; Vaultier, M. In Ionic
Liquids: Industrial Applications for Green Chemistry; American Chemical
Society: Washington, DC, 2002; pp 334-346.
(4) (a) Mazighi, K.; Carroll, P. J.; Sneddon, L. G. Inorg. Chem. 1993, 32,
1963-1969. (b) Pender, M.; Wideman, T.; Carroll, P. J.; Sneddon, L. G.
J. Am. Chem. Soc. 1998, 120, 9108-9109. (c) Pender, M.; Carroll, P. J.;
Sneddon, L. G. J. Am. Chem. Soc. 2001, 123, 12222-12231.
(5) In a typical reaction, 0.30 g (2.5 mmol) of decaborane and 0.70 g (4.9
mmol) of acetic acid-5-hexen-1-yl ester were reacted under Ar in a biphasic
mixture of toluene (3 g) and (bmim)BF4 (1.0 g) at 125 °C for 14 h to
give, following toluene elution from a silica gel column, 0.57 g (2.2 mmol,
87.3% yield) of 6-(acetic acid-hexyl ester)-B10H13 (oily liquid). Exact mass
12C8 H28
B
10
16O2: m/z calcd, 266.3019; measd, 266.3026. 11B NMR
1
11
(160.5 MHz, C6D6) δ 25.6 (s, B6), 10.2 (d, B1, 3), 8.9 (d, B9), 0.33 (d,
B5, 7), -3.2 (d, B8, 10), -33.9 (d, B2), -38.5 (d, B4). 1H NMR (500.1
MHz, C6D6) δ 4.10 (m, OCH2), 1.91 (s, CH3), 1.83 (m, CH2), 1.60 (m,
CH2), 1.38 (m, CH2), 0.37 (m, CH2CH2), -1.7 (br, 2 BHB), -2.1 (br, 2
BHB). IR (NaCl plates, cm-1) 2900(vs), 2860(s), 2560(vs), 1920(b,w),
1720(vs), 1560(m), 1520(s), 1510(s), 1460(s), 1430(s), 1390(m),
1360(s), 1260(vs), 1100(m), 1040(s), 1000(s), 960(m), 930(w), 860(w),
840(w), 810(m), 730(m).
Figure 1. 11B{1H} NMR spectra showing the progress of the reaction of
decaborane with 1-octyne at 120 °C when employing (bmim)Cl/toluene
biphasic media.
palladium reagents.8,9 In contrast to the previous routes, the new
ester derivative shown in eq 5 is efficiently synthesized in high
yield by the reaction of the previously unknown ester-functionalized
decaborane with 3-octyne under (bmim)Cl/toluene biphasic reaction
conditions.10
(6) (a) Heying, T. L.; Ager, J. W., Jr.; Clark, S. L.; Mangold, D. J.; Goldstein,
H. L.; Hillman, M.; Polak, R. J.; Szymanski, J. W. Inorg. Chem. 1963, 2,
1089-1092. (b) Fein, M. M.; Bobinski, J.; Mayes, N.; Schwartz, N.;
Cohen, M. S. Inorg. Chem. 1963, 2, 1111-1115. (c) Fein, M. M.;
Grafstein, D.; Paustian, J. E.; Bobinski, J.; Lichstein, B. M.; Mayes, N.;
Schwartz, N.; Cohen, M. S. Inorg. Chem. 1963, 2, 1115-1119.
(7) Reaction of 0.25 g (2.1 mmol) of decaborane and 0.72 (6.5 mmol) of
1-octyne under Ar in a biphasic mixture of toluene (∼5 mL) and (bmim)-
Cl (0.204 g) with vigorous stirring at 120 °C for 7 min gave, following
hexanes elution from a silica gel column, 0.43 g (1.86 mmol, 90.8% yield)
6c
of the known compound 1-hexyl-1,2-C2B10H11
.
(8) Chen, W.; Diaz, M.; Rockwell, J. J.; Knobler, C. B.; Hawthorne, M. F.
Chemistry 2000, 3, 223-229.
(9) (a) Chen, W.; Rockwell, J. J.; Knobler, C. B.; Harwell, D. E.; Hawthorne,
M. F. Polyhedron 1999, 18, 1725-1734. (b) Zheng, Z.; Jiang, W.; Zinn,
A. A.; Knobler, C. B.: Hawthorne, M. F. Inorg. Chem. 1995, 34, 2095-
2100 and references therein.
(10) Reaction of 0.50 g (1.9 mmol) of 6-(acetic acid hexyl ester)-B10H13 and
0.61 g (7.6 mmol) of 3-hexyne under Ar in a biphasic mixture of toluene
(2.0 g) and (bmim)Cl (0.5 g) with vigorous stirring at 95 °C for 4 h gave,
following CH2Cl2 elution from a silica gel column, 0.60 g (1.75 mmol,
92.6% yield) of 3-(acetic acid-hexyl ester)-1,2-diethyl-C2B10H9 (oily
12
11
liquid). Exact mass
C B
1H34 16O2: m/z calcd, 344.3489; measd,
14 10
344.3478. 11B NMR (160.5 MHz, C6D6) δ -0.7 (s, 1, B3), -4.6 (d, 2),
-10.1 (s, 2), -11.4 (d, 3), -12.5 (d, 1), -14.1 (d, 1). 1H NMR (500.1
MHz, C6D6) δ 3.99 (m, OCH2), 1.75 (m, CH2), 1.69 (s, CH3), 1.67 (m,
CH2), 1.52 (m, CH2), 1.4 (m, CH2), 1.25 (m, CH2), 1.0-0.65 (m, CH3,
CH2). IR (NaCl plates, cm-1) 2940(vs), 2860(sh,vs), 2590(vs), 1740(vs),
1460(s), 1390(s), 1370(s), 1250(vs), 1180(m), 1120(m), 1040(s),
940(vw), 910(w), 890(w), 850(vw), 740(m).
The exact role that the ILs play in inducing the reactions in eqs
1, 4, and 5 is still to be determined. The ILs may be simply
providing an inert reaction medium that allows thermal activation
of decaborane, but the fact that the olefin-hydroboration and
JA048018N
9
J. AM. CHEM. SOC. VOL. 126, NO. 28, 2004 8663