Group VI metal aminoborylene complex-catalyzed demercuration reactions of bis(alkynyl)mercurials

Article abstract of DOI:10.1039/b715597b

The first catalytic application of the Group VI metal borylene complexes [(CO)₅M=BN(SiMe₃)₂] involves the demercuration reaction of bis(alkynyl)mercurials, [Hg(C≡CR)₂], with formation of a series of buta-1,3-diynes. The Royal Society of Chemistry.

Full text of DOI:10.1039/b715597b

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Group VI metal aminoborylene complex-catalyzed demercuration
reactions of bis(alkynyl)mercurials{
a
Leonardo Apostolico, Holger Braunschweig,* Andrew G. Crawford, Thomas Herbst and Daniela Rais
a
b
a
a
Received (in Cambridge, UK) 10th October 2007, Accepted 12th November 2007
First published as an Advance Article on the web 15th November 2007
DOI: 10.1039/b715597b
The first catalytic application of the Group VI metal borylene
unexpected outcome of the reaction. Indeed, a first control
complexes [(CO) MLBN(SiMe ) ] involves the demercuration
experiment showed that heating equimolar amounts of 1 and 4a in
5
3 2
C H
6 6
at 90 uC resulted in the formation of PhCMC–CMCPh (7a)
along with elemental mercury and recovery of 1. Since the
2
reaction of bis(alkynyl)mercurials, [Hg(CMCR) ], with forma-
tion of a series of buta-1,3-diynes.
borylene fragment would not transfer in the absence of
3c
irradiation, 1 was photolyzed in a second control experiment in
1
a
Since their first synthesis in 1998, the reactivity of Group VI
metal terminal aminoborylene complexes [(CO)
5
MLBN(SiMe
3
)
2
]
the presence of the diyne 7a, thus leading to the bisborirene
compound 5. Hence, we assumed that the aforementioned
photolysis of 1 and 4a proceeded not only with transfer of the
borylene to the C–C triple bond, but interestingly also with
formation of an intermediate diyne obtained by demercuration
of 4a.
1b
(
M = Cr, 1; M = Mo, 2; M = W, 3) has been investigated in
2
some detail. A particularly useful property of these compounds is
their ability to transfer the aminoborylene moiety from the Group
VI element to other transition metals, under thermal or photolytic
3
conditions, depending on the borylene acceptor. Recently,
successful borylene transfer was achieved employing alkynes as
borylene acceptors, allowing for a new, high yielding synthesis of a
Herein, we describe the aminoborylene complex-catalyzed
demercuration reaction of [Hg(CMCR) ]. This represents the very
2
3c
first catalytic application of borylene complexes, and the first
instance in which a transition metal complex with a direct metal–
boron bond participates in a catalytic cycle in which the boron-
based ligand is not transferred to a substrate.
number of borirenes. In light of the interesting photophysical
4
properties displayed by boron-containing conjugated materials,
and with a view to incorporating a heavy metal into such
frameworks, we wondered about the possibility of transferring the
borylene moiety to transition metal alkynyl complexes. Mercury
The stoichiometric reaction described in Scheme 1 was
1
11
monitored by H and B{ H} NMR spectroscopy.
1
alkynyls [Hg(CMCR)
basis of their linear, and thus sterically unhindered, geometry.
2
] (4) were initially chosen, principally on the
5
This revealed gradual consumption of 4a, simultaneous
formation of 7a and the presence of 1 (or 3) as the only boron-
1
containing compound. The H NMR and GC-MS data for 7a
Upon irradiation of equimolar amounts of chromium complex 1
and [Hg(CMCPh)
formation of a new boron-containing species, 5, was obtained.
2 6 6
] (4a) in C D , spectroscopic evidence for the
6
were perfectly consistent with those obtained from an authentic
sample. To establish the scope of the transformation, the reaction
11
However, the similarity of the B NMR chemical shift of 5 (d =
4.0) to that of the bisborirene 6a (d = 28.0) and the borirene 6b
was performed with a range of complexes [Hg(CMCR)
2
] (R =
2
3c
C H -4-Me, 4b; C H -4-OMe, 4c; C H -4-NMe , 4d), under the
6
4
6
4
6
4
2
(d = 25.0), respectively (Fig. 1), along with the presence of
same conditions. Choosing a convenient substitution pattern on
1
the alkynyl ligand allowed monitoring of the reaction using H
metallic mercury in the reaction vessel, led us to hypothesize an
NMR spectroscopy. Gradual disappearance of the methyl
resonance of the [Hg(CMCR) ] complexes was noted, with the
2
simultaneous appearance of the corresponding signal for the buta-
1,3-diynes. The values obtained, compared with those of authentic
samples, proved formation of the buta-1,3-diynes, which could
7a,b
then be isolated in moderate yields (Table 1, a). As reported in
Table 1{, different reaction times reflect a dependence on the
electronic properties of the alkynyl ligands.
Having ascertained the thermal stability of [Hg(CMCR)
2
] (4)
under the reaction conditions, formation of RCMC–CMCR (7) and
recovery of unreacted aminoborylene complex 1 led us to believe
that 1 could play a catalytic role in the demercuration process. To
verify our hypothesis, the same reactions were performed with a
Fig. 1 Bis(borirenes) 5, 6a and borirene 6b.
a
Institut f u¨ r Anorganische Chemie, Julius-Maximilians-Universit a¨ t,
W u¨ rzburg, Am Hubland, 97074 W u¨ rzburg, Germany.
E-mail: h.braunschweig@mail.uni-wuerzburg.de;
Fax: (+49)931-888-4623; Tel: (+49)931-888-5260
Department of Chemistry, University Science Laboratories, South
Road, Durham, UK DH1 3LE
b
{ Electronic supplementary information (ESI) available: Synthesis and
spectroscopic data of 5; experimental details for catalytic experiments. See
DOI: 10.1039/b715597b
Scheme 1 Stoichiometric demercuration reaction of [Hg(CMCPh)
2
].
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Chem. Commun., 2008, 497–498 | 497

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Table 1 Synthesis of buta-1,3-diynes via stoichiometric (a) and
catalytic (b) demercuration reactions of [Hg(CMCR)
(0.066 mmol) in benzene (1.5 mL). The mixture was heated at 90 uC until
the reaction had reached completion (as judged from GC-MS spectra, see
Table 1 for reaction times). The solvent was then removed. Complex 1 and
2
]
R
4
7
Time/h
Yield (%)
6
small amounts of [Cr(CO) ] were removed by high vacuum sublimation.
Crystals of 7 were obtained upon extraction of the diynes from the residue
with hexane, filtration and subsequent crystallization (see Table 1 for
isolated yields).
a
a
C
C
C
C
6
H
H
H
H
5
4
4
4
4a
4b
4c
4d
7a
7b
7c
7d
32
9
62
65
b
b
1
a
b
a
b
a
6
6
6
-4-Me
21
0
11
4
64
66
b
7
a
1 (a) H. Braunschweig, C. Kollann and U. Englert, Angew. Chem., 1998,
110, 3355–3357, (Angew. Chem., Int. Ed., 1998, 37, 3179–3180); (b)
B. Blank, H. Braunschweig, M. Colling-Hendelkens, C. Kollann,
K. Radacki, D. Rais, K. Uttinger and G. R. Whittell, Chem.–Eur. J.,
-4-OMe
-4-NMe
47
51
b
3
a
a
2
1.5
1
52
55
b
1
b
2
007, 13, 4770–4781.
For reviews see: (a) H. Braunschweig, Angew. Chem., 1998, 110,
882–1898, (Angew. Chem., Int. Ed., 1998, 37, 1786–1801); (b)
H. Braunschweig and M. Colling, Eur. J. Inorg. Chem., 2003,
93–403; (c) H. Braunschweig, Adv. Organomet. Chem., 2004, 51,
63–192; (d) H. Braunschweig and D. Rais, Heteroat. Chem., 2005, 16,
66–571; (e) H. Braunschweig, C. Kollann and D. Rais, Angew. Chem.,
006, 118, 5380–5400, (Angew. Chem., Int. Ed., 2006, 45, 5254–5274).
a
Stoichiometric reaction conditions: 4 (0.066 mmol), 1 (0.066 mmol),
4
2
b
C
(
6
H
6
(1.5 mL), 90 uC.
0.066 mmol), 1 (0.0066 mmol), C
Catalytic reaction conditions:
(1.5 mL), 90 uC.
1
6
H
6
3
1
5
2
catalytic amount (10 mol%) of 1. In all cases, the reactions reached
1
full conversion as judged from H NMR spectroscopy of the
3
(a) H. Braunschweig, M. Colling, C. Kollann, H. G. Stammler and
B. Neumann, Angew. Chem., 2001, 113, 2359–2361, (Angew. Chem., Int.
Ed., 2001, 40, 2298–2300); (b) H. Braunschweig, M. Colling, C. Hu and
K. Radacki, Angew. Chem., 2003, 115, 215–218, (Angew. Chem., Int.
Ed., 2003, 42, 205–208); (c) H. Braunschweig, T. Herbst, D. Rais and
F. Seeler, Angew. Chem., 2005, 117, 7627–7629, (Angew. Chem., Int. Ed.,
2005, 44, 7461–7463); (d) H. Braunschweig, M. Forster and K. Radacki,
Angew. Chem., 2006, 118, 2187–2189, (Angew. Chem., Int. Ed., 2006, 45,
reaction mixture and the absence of soluble degradation products,
demonstrating that 1 catalyzes the demercuration of bis(alkynyl)-
mercurials (Table 1, b).
All reactions were also performed with tungsten complex 3,
yielding comparable results to those obtained with 1. Furthermore,
control experiments with [Cr(CO) ] proved that the hexacarbonyl
6
2132–2134); (e) H. Braunschweig, M. Forster, K. Radacki, F. Seeler and
complex showed no catalytic activity. Bis(alkynyl)mercurials find
application primarily as air and moisture stable transalkynylating
9
reagents for transition metal, lanthanide and main group element
G. R. Whittell, Angew. Chem., 2007, 119, 5304–5306, (Angew. Chem.,
Int. Ed., 2007, 46, 5212–5214).
4 (a) N. Matsumi and Y. Chujo, in Contemporary Boron Chemistry, Spec.
Publ. No. 253, ed. M. G. Davidson, A. K. Hughes, T. B. Marder and
K. Wade, The Royal Society of Chemistry, Cambridge, 2000, pp. 51–58;
8
10
compounds. Demercuration reactions of these species have been
described as a means to access conjugated diynes, as well as more
(b) C. D. Entwistle and T. B. Marder, Angew. Chem., 2002, 114,
11
complex systems. Hence, Takagi et al. studied the Rh-catalyzed
demercuration of range of organomercurials, including
Hg(CMCPh) ], to give the corresponding diyne PhCMC–CMCPh
3051–3056, (Angew. Chem., Int. Ed., 2002, 41, 2927–2931); (c) F. Jaekle,
J. Inorg. Organomet. Polym. Mater., 2005, 15, 293–307; (d) D. Gabel, in
Science of Synthesis: Houben-Weyl Methods of Molecular
Transformation, ed. D. Kaufmann and D. S. Matteson, G. T. Verlag,
Stuttgart, 2005, vol. 6, p. 1277.
a
[
2
1
1b
(
7a). Later Hill and co-workers extended the range of suitable
mercury alkynyl reagents, noting extrusion of elementary mercury
5 (a) J. R. Johnson and W. L. McEwen, J. Am. Chem. Soc., 1926, 48,
469–476; (b) E. Rothstein and R. W. Saville, J. Chem. Soc., 1952,
from [Hg(CMCR) ] and consequent formation of buta-1,3-diynes
2
2987–2991.
H. Braunschweig and T. Herbst, unpublished work. For experimental
as the result of a catalytic process involving complexes of Rh, Ru
11c,d
and Os,
6
and elegantly applied the protocol to the synthesis of
and spectroscopic details see ESI{.
11e–g
dimetallaoctatetraynes.
The reactions reported herein demon-
7 (a) J. G. Rodriguez, A. Lafuente, R. Martin-Villamil and
M. P. Martinez-Alcazar, J. Phys. Org. Chem., 2001, 14, 859–868; (b)
S. V. Damle, D. Seomoon and P. H. Lee, J. Org. Chem., 2003, 68,
strate that earlier transition metal complexes are also capable of
effecting demercuration reactions, when possessing an aminobor-
ylene ligand. However, while for later transition metals a catalytic
cycle involving conventional steps, such as oxidative addition,
extrusion of mercury with organyl group transfer and reductive
elimination, was established, favoured by the electronic and
coordinative unsaturation of the metal complexes involved, it is
unlikely that such a cycle applies to the 18-electron borylene
species. Efforts devoted to a clarification of the operative
mechanism are under way.
7
085–7087.
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9
G. Lin, R. McDonald and J. Takats, Organometallics, 2000, 19,
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Financial support from the DFG and the Fonds der
Chemischen Industrie is gratefully acknowledged.
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998, 1171–1174.
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1
1
(
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Notes and references
1a
1a
{
Aminoborylene complexes 1 and 3 and bis(alkynyl)mercurials
5
[Hg(CMCR)
2
], were prepared according to literature procedures. The
spectroscopic data for compounds 7 are in full agreement with those
7a,b
reported in the literature.
General procedure for the catalytic formation of buta-1,3-diynes (7). Solid
(0.0024 g, 0.0066 mmol) was added to a solution of [Hg(CMCR)
1
2
]
4
98 | Chem. Commun., 2008, 497–498
This journal is ß The Royal Society of Chemistry 2008