The parent borylene: Betwixt and between

Article abstract of DOI:10.1002/anie.201107238

Still elusive: The reduction of dimethylimidazol-2-ylidene dichloroborane by sodium naphthalenide has been suggested to provide a borylene that cycloadds to naphthalene to make a borirane. Evidence has been provided that this borirane instead arises from

Full text of DOI:10.1002/anie.201107238

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DOI: 10.1002/anie.201107238
Borylenes
The Parent Borylene: Betwixt and Between**
Dennis P. Curran,* Anne Boussonniꢀre, Steven J. Geib, and Emmanuel Lacꢁte
ˇ
In memory of Daniel Bellus (1938–2011)
N-Heterocyclic carbenes (NHCs) are effective at stabilizing
unusual bonding patterns on boron.[1,2] Rare boron-based
reactive intermediates including NHC-boryl radicals,[3] boryl
anions,[4] and borenium cations[5] have all been generated and
directly observed by spectroscopy and sometimes even
crystallography. Very recently, evidence for NHC-borylenes
has appeared. Bertrand and co-workers just reported a
trivalent borylene bearing two carbenes.[6] Isoelectronic with
an amine, this fascinating molecule can also be viewed as a
borylene–Lewis base complex.
Prior to that, Braunschweig and co-workers reported
tantalizing evidence for “Trapping the Elusive Parent Bor-
ylene”.[7] Specifically, addition of 2 equiv of sodium naphtha-
lenide (NaN) to diMe-Imd-BHCl2 (1; diMe-Imd = 1,3-dime-
thylimidazol-2-ylidene) provided a 1:1 mixture of stereoiso-
meric boriranes (boracyclopropanes), 2-exo and 2-endo, in
88% yield (Scheme 1). The isolation of these novel, stable
boriranes galvanized NHC-borylene research.
Borylene 5 is isoelectronic with a carbene. Though the boron
atom has only six valence electrons, it still has a formal
negative charge. This is offset by a formal positive charge on
the NHC ring, so 5 is neutral.
The lower part of Scheme 1 shows a plausible route to this
borylene through sequential electron transfer reactions of
NaN to provide a boryl radical 3, then boryl anion 4 (a boron
analogue of a carbenoid). These two steps have precedent in
NHC-borane chemistry.[1c,4b] a-Elimination of chloride from 4
provides 5, whose cycloaddition with naphthalene has a low
calculated activation barrier (< 3 kcalmolÀ1).[7]
Here we suggest a “radical–radical anion coupling” path
to 2 that does not involve a borylene intermediate. This
mechanism is supported by literature data and by isolation of
a coupled product from NaN reduction of a related carbene-
borane. We then change the reduction partners to discourage
radical–radical anion coupling, and we observe instead
À
products of C H insertion. We speculate that these products
The authors concluded that boriranes 2 formed from
cycloaddition of parent NHC-borylene 5 and naphthalene.
indeed arise from trapping of the elusive parent NHC-
borylene.
Braunschweig and co-workers commented in their Sup-
porting Information that naphthalene was the uniquely
successful trap for borylene 5. Various alkenes and alkynes
did not trap it, even when used in excess. This contrasts with
carbene chemistry, where cyclohexene is a better trap than
naphthalene.[8] Extending the B/C analogy, carbenes can be
generated by reductions of dihaloalkenes with NaN, but they
are further reduced by NaN more rapidly than they add to
naphthalene.[8,9] This analysis questions the viability of 5 as an
intermediate.
Since borylene 5 was not observed directly, its existence is
necessarily a mechanistic conclusion. We suggest that the
mechanism shown in Scheme 2 may instead account for the
formation of 2. Here, 1 is again reduced by sodium
Scheme 1. Formation of novel boriranes (boracyclopropanes) 2 (top)
through a suggested borylene mechanism (bottom).
[*] Prof. D. P. Curran, Dr. A. Boussonniꢀre, Dr. S. J. Geib[+]
Department of Chemistry, University of Pittsburgh
Pittsburgh, PA 15260 (USA)
E-mail: curran@pitt.edu
Dr. E. Lacꢁte
ICSN, CNRS, Av. de la Terrasse, 91198 Gif-sur-Yvette (France)
[+] Corresponding author for X-ray analysis.
[**] We thank the National Science Foundation and CNRS for support of
this work and Prof. Holger Braunschweig for helpful discussions.
Scheme 2. A mechanism based on radical–radical anion coupling also
accounts for the formation of 2 and explains why the reagent NaN
(not naphthalene) is a unique trap that cannot be outcompeted by
added molecules.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107238.
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naphthalenide and the resulting boryl radical 3 again reacts
with a second equivalent of sodium naphthalenide. However,
we suggest that radical–radical anion coupling occurs in this
second encounter rather than electron transfer. This provides
anion 6, which undergoes intramolecular substitution to give
the products 2. Recent studies on diverse substitution
reactions of NHC-boryl halides and triflates provide prece-
dent for this last step.[10]
The reaction between 3 and sodium naphthalenide is
effectively a radical–radical combination whose rate could
approach the diffusion-controlled limit. This explains why
external traps cannot “outcompete” the reagent for trapping.
Radical–molecule reactions cannot compete with radical–
radical reactions when one of the radicals (here NaN) is
present in high concentration. The negligible barrier of this
reaction also accounts for the lack of stereoselectivity in
formation of the two diastereomers of 6 and hence 2.
This mechanism has precedent in the reactions of sodium
naphthalenide with alkyl halides summarized in Scheme 3.
These reactions have low preparative value because they give
complex product mixtures.[11] For example, treatment of alkyl
À37.4 ppm that corresponds to the doubly reduced, proton-
ated product 7 (diMe-Imd-BH3, < 5%) and an intriguing
sharp triplet at À24.6 ppm (15%).
The formation of 7 suggested that our reaction medium
had traces of a proton source (water?) that was not present in
Braunschweigꢀs experiments. This in turn suggested that some
diMe-Imd-BH2Cl (8) was formed during our reaction. Could
the triplet at À24.6 ppm be due to a coupling product derived
from 8 and NaN?
To answer this question, we prepared 8 (see Supporting
Information) and reduced it with sodium naphthalenide. The
results of two key experiments are shown in Scheme 4. The
Scheme 3. Reductions of alkyl halides with NaN provide complex
À
mixtures of aliphatic and naphthalene-derived products. (R( H) is an
alkene formed from disproportionation.)
Scheme 4. Reduction of mono-chloroborane 8 gives stable coupled
product 10.
halides with sodium naphthalenide provides aliphatic prod-
ucts of reduction (RH), coupling (RR) and disproportiona-
tion (R(-H)) along with assorted naphthalene coupling
products.[12] All the aliphatic products are said to arise directly
or indirectly from alkylsodium intermediates. The ratio of
11B NMR spectrum of the reaction product resulting from the
addition of 2.5 equiv NaN in THF to 8 showed no resonances
for 2. There were three resonances: the sharp triplet at
À24.6 ppm (about 25%), the quartet for 7 (about 25%), and
in between a new, very broad triplet at À33.0 ppm (about
50%). This last product might be diborane 9,[15] but it did not
survive flash chromatography.
Inverse addition of 8 to excess NaN (7.5 equiv) provided a
cleaner crude product; the broad triplet resonance was absent
and the sharp triplet resonance predominated over the
quartet of diMe-Imd-BH3 (7) (about 65% to 35%). Flash
chromatography provided the new product 10 in 24%
isolated yield. The initial structure assignment by an HH
COSY experiment was confirmed by X-ray crystallography
(Figure 1).
naphthalene-derived products to aliphatic products increases
[12a,13]
À
in order of R X primary < secondary < tertiary.
This trend originates from a competition in the reactions
of alkyl radicals with sodium naphthalenide between electron
transfer (reduction) and radical–radical anion coupling. As
the radical RC becomes more difficult to reduce to an anion
(primary < secondary < tertiary), the radical–radical anion
coupling takes over and the amounts of naphthalene-derived
products increase. Tellingly, chemical and electrochemical
reduction studies have shown that NHC-boryl radicals are
easy to oxidize[5a,14] but difficult to reduce.[4b] So boryl radicals
are prime candidates for coupling.
We suggest that 10 arises from the coupling of the boryl
radical derived from 8 with NaN followed by protonation.[16]
Although this radical (diMe-Imd-BH2C) is not the same as the
previous radical 3 (diMe-Imd-BHClC), it is similar. So this
result provides additional evidence that the radical–radical
anion coupling mechanism for formation of 2 is viable.
To start, we repeated the experiment to prepare 2.[7]
Looking for minor side products, we recorded an 11B NMR
spectrum of the crude product mixture. As expected, the
major peaks at À33.2 and À36.9 ppm are from the two
diastereomers of 2.[7] We also observed a small quartet at
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other broad resonances (see Supporting Information). A
cleaner crude product (40% 12b and 60% 13b) was formed
À
by adding NaN to 11b. The new C H insertion product 13b
was isolated from this reaction in 36% yield by flash
chromatography. The crystal structures of both 13a
(Figure 1) and 13b (Supporting Information) were solved.
Robinson and Braunschweig have just described the
À
formation of related C H insertion products from potassium
graphite reductions,[17] so such insertions may be general. If
À
so, then is the formation of C H insertion products a reliable
signature of the borylene? Perhaps, but other mechanisms for
insertion must also be considered.[18]
Where does this leave the elusive parent NHC-borylene?
Betwixt and between the various product types, apparently.
Currently, we hypothesize that products 2 are formed from
radical–radical coupling, not cycloaddition of borylene and
naphthalene. Likewise, 10 is also formed from coupling. On
the other hand, we hypothesize that products 13 are formed
Figure 1. Crystal structures of 10 (left) and 13a (right).
Preparatively, the problems of arene alkylation shown in
Scheme 3 were solved by the introduction of lithium di-tert-
butylbiphenylide (LDBB).[13] This reagent was designed to
block radical–radical anion coupling (every ring carbon has a
large substituent either ipso or ortho). Indeed, we showed that
LDBB reduced a boryl radical to a boryllithium reagent by
both 11B NMR studies and product analysis.[4b] No radical–
radical anion coupling products were observed.
À
by C H insertion of either a free borylene or another reactive
intermediate (borylenoid) with borylene-like reactivity.
Methods to test these hypotheses, especially by direct
observation of borylenes, are important objectives.
Received: October 13, 2011
Published online: January 4, 2012
To discourage radical–radical anion coupling, we reduced
two more hindered boryl dichlorides 11a,b with LDBB. The
results of two experiments are shown in Scheme 5. Addition
of 11a to LDBB (2.5 equiv, THF, À788C) provided a
relatively clean crude product containing 18% of doubly
Keywords: borylenes · N-heterocyclic carbenes · radical–
radical coupling · reaction mechanisms
.
À
reduced product 12a and 82% of a new C H insertion
product 13a. This insertion product was isolated in 60% yield
by flash chromatography.
[1] a) Y. Wang, G. H. Robinson, Inorg. Chem. 2011, 50, 12326 –
12337; b) Y. Wang, B. Quillian, P. Wei, Y. M. Xie, C. S. Wannere,
R. B. King, H. F. Schaefer, P. v. R. Schleyer, G. H. Robinson, J.
Am. Chem. Soc. 2008, 130, 3298 – 3299; c) Y. Wang, B. Quillian,
P. Wei, C. S. Wannere, Y. Xie, R. B. King, H. F. Schaefer, P. v. R.
Schleyer, G. H. Robinson, J. Am. Chem. Soc. 2007, 129, 12412 –
12413.
Similar addition of 11b to LDBB provided a product
mixture containing resonances for 12b and 13b along with
[2] D. P. Curran, A. Solovyev, M. Makhlouf Brahmi, L. Fensterbank,
M. Malacria, E. Lacꢁte, Angew. Chem. 2011, 123, 10476 – 10500;
Angew. Chem. Int. Ed. 2011, 50, 10294 – 10317.
[3] a) J. C. Walton, M. Makhlouf Brahmi, L. Fensterbank, E.
Lacꢁte, M. Malacria, Q. Chu, S.-H. Ueng, A. Solovyev, D. P.
Curran, J. Am. Chem. Soc. 2010, 132, 2350 – 2358; b) S.-H. Ueng,
A. Solovyev, X. Yuan, S. J. Geib, L. Fensterbank, E. Lacꢁte, M.
Malacria, M. Newcomb, J. C. Walton, D. P. Curran, J. Am. Chem.
Soc. 2009, 131, 11256 – 11262.
[4] a) H. Braunschweig, C.-W. Chiu, K. Radacki, T. Kupfer, Angew.
Chem. 2010, 122, 2085 – 2088; Angew. Chem. Int. Ed. 2010, 49,
2041 – 2044; b) J. Monot, A. Solovyev, H. Bonin-Dubarle, ꢂ.
Derat, D. P. Curran, M. Robert, L. Fensterbank, M. Malacria, E.
Lacꢁte, Angew. Chem. 2010, 122, 9352 – 9355; Angew. Chem. Int.
Ed. 2010, 49, 9166 – 9169.
[5] a) T. Matsumoto, F. P. Gabbaꢃ, Organometallics 2009, 28, 4252 –
4253; b) D. McArthur, C. P. Butts, D. M. Lindsay, Chem.
Commun. 2011, 47, 6650 – 6652; c) B. Inꢄs, M. Patil, J. Carreras,
R. Goddard, W. Thiel, M. Alcarazo, Angew. Chem. 2011, 123,
8550 – 8553; Angew. Chem. Int. Ed. 2011, 36, 8400 – 8403.
[6] R. Kinjo, B. Donnadieu, M. A. Celik, G. Frenking, G. Bertrand,
Science 2011, 333, 610 – 613.
[7] P. Bissinger, H. Braunschweig, K. Kraft, T. Kupfer, Angew.
Chem. 2011, 123, 4801 – 4804; Angew. Chem. Int. Ed. 2011, 50,
4704 – 4707.
À
Scheme 5. Reductions of hindered boryl dichlorides provides stable C
H insertion products 13a,b after flash chromatography.
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[8] G. D. Sargent, C. M. Tatum, R. P. Scott, J. Am. Chem. Soc. 1974,
96, 1602 – 1604.
[9] G. D. Sargent, C. M. Tatum, S. M. Kastner, J. Am. Chem. Soc.
1972, 94, 7174 – 7176.
[10] A. Solovyev, Q. Chu, S. J. Geib, L. Fensterbank, M. Malacria, E.
Lacꢁte, D. P. Curran, J. Am. Chem. Soc. 2010, 132, 15072 – 15080.
[11] J. F. Garst, Acc. Chem. Res. 1971, 4, 400 – 406.
[12] a) G. D. Sargent, G. A. Lux, J. Am. Chem. Soc. 1968, 90, 7160 –
7162; b) J. F. Garst, J. T. Barbas, F. E. Barton,II, J. Am. Chem.
Soc. 1968, 90, 7159 – 7160.
[14] M.-A. Tehfe, M. Makhlouf Brahmi, J.-P. Fouassier, D. P. Curran,
M. Malacria, L. Fensterbank, E. Lacꢁte, J. Lalevꢄe, Macro-
molecules 2010, 43, 2261 – 2267.
[15] Compare this value to (dipp-Imd-BH2)2, d = À31.6 ppm, and
(diMes-Imd-BH2)2, d = À31.2 ppm. See reference [1b,c].
[16] Substantial (40%) deuteration was observed when a ring-
deuterated analog of 8 was used; see Supporting Information.
[17] a) See Figure 5 in reference [1a]; b) P. Bissinger, H. Braunsch-
weig, A. Damme, R. D. Dewhurst, T. Kupfer, K. Radacki, K.
Wagner, J. Am. Chem. Soc. 2011, 133, 19044 – 19047.
[18] For other possible mechanisms, see: a) the discussion in the
Supporting Information; b) reference [17b]; c) A. Prokofjevs,
E. Vedejs, J. Am. Chem. Soc. 2011, 133, 20056 – 20059.
[13] P. K. Freeman, L. L. Hutchinson, Tetrahedron Lett. 1976, 17,
1849 – 1852.
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