Synthesis and Ring Strain of a Benzoborirene-N-Heterocyclic Carbene Adduct

Article abstract of DOI:10.1002/chem.201804629

The reduction of an N-heterocyclic carbene (1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene, IiPr_Me2) adduct of dichloro(ortho-bromophenyl)borane by tert-butyl lithium at low temperature yields the IiPr_Me2 adduct A of parent benzoborirene, a highly strained boron-containing bicyclic compound. A is unstable at room temperature and dimerizes at low temperature to the bis-IiPr_Me2 adduct of 9,10-dihydro-9,10-diboraanthracene, characterized by single-crystal X-ray crystallography.

Full text of DOI:10.1002/chem.201804629

DOI: 10.1002/chem.201804629
Communication
&
Strained Molecules
Synthesis and Ring Strain of a Benzoborirene-N-Heterocyclic
Carbene Adduct
Jennifer Hahn,[a] Constanze Keck,[a] Cꢀcilia Maichle-Mçssmer,[b] Esther von Grotthuss,[c]
Paul Niklas Ruth,[d] Alexander Paesch,[d] Dietmar Stalke,[d] and Holger F. Bettinger*[a]
Dedicated to Professor W. E. Billups in appreciation of his seminal contributions to strained molecules
benzoborirene were detected to result from photoreactions of
diiodophenylborane and phenylborylene, respectively, in
matrix isolation experiments.[10] Benzoborirene (1) is isoelec-
tronic to the benzocyclopropenium ion (2). Halton et al. ob-
tained the first NMR spectral evidence for the existence of a
derivative of 2 in 1974, but noted its limited stability that
made full characterization difficult.[11] The neutral compounds,
cycloproparenes, have received considerable attention[12] since
Anet and Anet[13] reported the first derivative, and Vogel and
Billups described syntheses of the parent benzocyclopropene
3.[14]
Abstract: The reduction of an N-heterocyclic carbene (1,3-
diisopropyl-4,5-dimethylimidazolin-2-ylidene, IiPrMe
)
2
adduct of dichloro(ortho-bromophenyl)borane by tert-
butyl lithium at low temperature yields the IiPrMe adduct
2
A of parent benzoborirene, a highly strained boron-con-
taining bicyclic compound. A is unstable at room temper-
ature and dimerizes at low temperature to the bis-IiPrMe
2
adduct of 9,10-dihydro-9,10-diboraanthracene, character-
ized by single-crystal X-ray crystallography.
Although the strain of 1 is much smaller than that of known
all-carbon scaffolds 2 and 3 (Scheme 1), efforts aimed at the
targeted synthesis of benzoborirenes under conventional con-
ditions were unsuccessful. Schacht and Kaufmann reduced (2-
Aromaticity,[1] conjugation,[2] hyperconjugation,[3] and ring
strain[4] are classical concepts of organic chemistry. The inter-
play between strain and aromaticity has attracted quite some
attention in efforts to understand the influence of bond-length
alternation on the properties of aromatic compounds, and re-
sulted in the synthesis of an impressive array of highly strained
annulated aromatic systems.[5] This research was motivated by
the Mills–Nixon effect,[6] and culminated in the development of
the concept of strain-induced bond localization.[7]
Besides experiment work, theory was also invoked to study
the influence of small heterocyclic rings on the aromaticity and
bond length alternation in systems that are not accessible ex-
perimentally, a case in point being benzoborirene (1).[7a,8] This
heteroaromatic compound was first observed as a product of
the reaction of boron atoms and benzene under single-colli-
sion conditions.[9] Later on, the B-iodo derivative and parent
Scheme 1. Benzoborirene (1), benzocyclopropenium ion (2), benzocyclopro-
pene (3), and their strain energies (in kcalmolÀ1) according to homodesmot-
ic equations computed at the CCSD(T)/cc-pVTZ//B3LYP/6–311+G** level of
theory.
bromophenyl)phenylbromoborane 4 with Li and postulated
formation of a benzoboratirene 5 based on degradation ex-
periments (Scheme 2, reaction a).[15] Attempted benzoborirene
formation by thermal elimination of trimethylchlorosilane from
2-(dichloroboryl)phenyl trimethylsilane 6 resulted in 7 by HCl
elimination (Scheme 2, reaction b) instead. These experiments
indicate the resistance towards benzoborirene formation, pos-
sibly due to a combination of strain and the inherent reactivity
of a planar, sterically unprotected boron center.
[a] Dr. J. Hahn, C. Keck, Prof. Dr. H. F. Bettinger
Institut fꢀr Organische Chemie, Universitꢁt Tꢀbingen, Auf der Morgenstelle
18, 72076 Tꢀbingen (Germany)
E-mail: holger.bettinger@uni-tuebingen.de
[b] Dr. C. Maichle-Mçssmer
Institut fꢀr Anorganische Chemie, Universitꢁt Tꢀbingen, Auf der Morgen-
stelle 18, 72076 Tꢀbingen (Germany)
[c] E. von Grotthuss
Institut fꢀr Anorganische und Analytische Chemie
Universitꢁt Frankfurt
On the other hand, monocyclic borirenes are well-known.[16]
These are less strained than benzoborirenes (32.0 vs. 48.4 kcal
molÀ1 according to homodesmotic equations, Scheme 1), and
were found to undergo coordination to Lewis bases such as N-
heterocyclic carbenes (NHC).[16i] As interaction with an NHC
could protect the boron center by reducing its Lewis acidity
Max-von-Laue-Str. 7, 60438 Frankfurt (Germany)
[d] P. N. Ruth, A. Paesch, Prof. Dr. D. Stalke
Institut fꢀr Anorganische Chemie, Universitꢁt Gçttingen
Tammannstrasse 4, 37077 Gçttingen (Germany)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/chem.201804629.
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spectrum, the signal at À23.8 ppm due to compound B splits
1
into a triplet with a JBH coupling constant of 87.0 Hz, indicat-
ing that two hydrogen atoms are bound to boron. Comparison
of the spectral data allows assignment to the known NHC
adduct of phenylborane.[20]
Compound A is the NHC adduct of benzoborirene based on
the following arguments. The proton coupled 11B NMR spec-
trum results in splitting of the signal at À26.4 ppm into a dou-
1
blet with a JBH coupling constant of 120.5 Hz, showing that a
single hydrogen atom is bonded to the boron center. This hy-
1
drogen atom can be observed at 3.70 ppm in a H{11B} spec-
Scheme 2. Attempted benzoborirene syntheses by reductive elimination (re-
action a) or flash vacuum pyrolysis (FVP, reaction b) according to Kaufmann
et al.[15]
trum, and an HETCOR NMR experiment proves that a coupling
exists with the boron signal at À26.4 ppm. Both the 11B chemi-
1
cal shift and the JBH value are very similar to those reported
1
and providing steric shielding, we employed NHC coordination
in the synthesis of a benzoborirene. We here report the first
synthesis of a benzoborirene-NHC adduct under conventional
solution phase conditions and its characterization by multinu-
clear NMR spectroscopy.
by Curran et al. for a NHC-borirane.[21] In the H NMR spectrum
an AA’BB’ spin system with multiplets at 7.97 and 7.51 ppm
and the signals of an intact coordinated NHC molecule are ob-
served that integrate in the required ratio and also disappear
quickly at room temperature. The aromatic protons of benzo-
cyclopropene (3) resonate at 7.12 ppm.[14a] The AA’BB’ spin
system in A is associated with two 13C NMR signals at 126.6
and 122.4 ppm that each carry a single hydrogen atom accord-
ing to a DEPT-135 NMR experiment. The boron-bound fusion
centers were observed in a low temperature 13C NMR experi-
ment at 149.0 ppm, but the carbene center could not unam-
biguously be detected. The assignment to A is supported by
density functional theory computations (Table 1) of the chemi-
cal shifts.
We reasoned that the bromo-lithium exchange of a NHC
adduct (9) of the known dichloro-2-bromophenylborane 8[17]
might provide access to the benzoborirene scaffold
(Scheme 3). The NHC adducts 9 were obtained as colorless
Table 1. Comparison of experimental and computed (B3LYP/def2-TZVP//
B3LYP/6–311+G**) chemical shifts of A.
Molecule A
Nucleus
dexp [ppm]
dcalc [ppm][a]
C2/C3
C4/C7
C6/C5
C12
149.0
122.4
126.6
not observed
À26.4
147.1
120.1
125.4
173.3
À26.9
B
Scheme 3. Synthesis of the benzoborirene-NHC adduct A and its decomposi-
tion.
[a] Carbon with reference to benzene and its experimental chemical shift
d
mental shift d11B=À23.8 ppm.
13C=128.06 ppm, boron with reference to compound B and its experi-
solids in good to moderate yields by treating 8 with the free
NHC in toluene and could be fully characterized by multinu-
clear NMR spectroscopy and single-crystal X-ray crystallogra-
phy (see Supporting Information, Figure S16, S17).[18] The
11B NMR shifts (1.8 ppm for 9a and 1.1 ppm for 9b) are as ex-
pected.[19]
Electron impact mass spectrometry shows that compound A
ionizes with loss of one hydrogen atom. The high resolution
measurement of the resulting [MÀH]+ mass is in agreement
with the assignment to A. The formation of borane-NHC ad-
ducts from dihaloborane-NHC adducts under reductive condi-
tions has been observed before and was explained to result
from reaction with solvent or with an unspecified hydrogen
atom source.[22]
Treatment of compound 9a with two equivalents of tert-
BuLi in [D8]toluene at low temperature was monitored by NMR
spectroscopy. Two new signals were observed by 11B{1H} NMR
at À26.4 ppm (compound A) and À23.8 ppm (compound B),
with the former one showing the highest intensity. After keep-
ing the solution at room temperature for a day, the signal at
À26.4 ppm had disappeared completely, whereas the signal
for compound B was still present. In a proton-coupled 11B NMR
Attempted crystallization of A at À358C in acetonitrile solu-
tion resulted in the formation of colorless crystals. Single-crys-
tal X-ray crystallography identified these as 10a, the formal
dimer of A (Figure 1). The two NHC ligands are in syn orienta-
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tions.[16f,i] Zwitterion 12a is a minimum on
the potential energy surface, which lies
30 kcalmolÀ1 higher in energy than ben-
zoborirene. This energy difference is re-
duced to 25 kcalmolÀ1 after taking into
account acetonitrile solvent effects using
a polarizable continuum model (see the
Supporting Information). The NHC stabiliz-
es the borenium ion center and, without
the NHC, the zwitterion would collapse to
benzoborirene according to computa-
Scheme 4. Zwitter-
ionic isomer 12a of
A.
tions. Hence, the NHC, though providing steric protection, can
open a decomposition pathway that is not available to uncoor-
dinated benzoborirenes with bulky boron substitution. Due to
the high exothermicity of the dimerization reaction of A, the
existence of other hitherto unidentified lower energy pathways
that avoid the zwitterion intermediate is possible.
Figure 1. Molecular structure of 10a in the solid state. Anisotropic displace-
ment parameters are depicted at the 50% probability level. Hydrogen atoms
bonded to carbon atoms were omitted for clarity. For bond lengths and
angles, see Figure S18 in the Supporting Information.
In summary, we could provide for the first time NMR spec-
troscopic evidence for the existence of the benzoborirene mo-
lecular framework in the form of an NHC adduct under con-
ventional solution-phase conditions. The dimerization of the
compound is very strongly exothermic despite the coordinat-
ing NHC, and this reaction may be facilitated by the NHC
through stabilization of a zwitterionic isomer. Future research
is aimed at introducing bulky groups on the boron center in
benzoborirene as an alternative means of steric protection that
avoids Lewis base coordination.
tion, similar to the bis-dimethylsulfide adduct described previ-
ously by Wagner et al.[23] We synthesized 10a independently
by treating 9,10-dihydro-9,10-diboraanthracene[24] 11 in
[D8]THF with two equivalents of the NHC IiPrMe . The spectral
2
data of the poorly soluble dimer 10a (13C, 11B=À17.5 ppm,
1JBH ꢀ62 Hz) differ so strongly from that assigned to A (e.g.,
boron-bound carbon atoms of the diboraanthracene backbone
resonate at 156.9 ppm, see the Supporting Information for de-
tails) that solvent effects cannot be responsible. Most impor-
tantly, dimer 10a ionizes with the loss of one hydrogen atom,
and the high-resolution mass spectrum is in agreement with
the [MÀH]+ ion of 10a. These analyses of 10a confirm that
the dimer is not observed initially, but forms slowly during at-
tempted crystallization of monomeric A. Formation of dimer
10a can be considered an indirect proof of the assignment to
A.
Acknowledgements
We thank Professors Lars Wesemann and Alexey Timoshkin for
helpful discussions, Dr. Fabian Uhlemann for providing the N-
heterocyclic carbenes, and Deutsche Forschungsgemeinschaft
for financial support of this research. C. K. thanks the Fonds
der chemischen Industrie for a Kekulꢂ fellowship. The compu-
tations were performed on the BwForCluster JUSTUS. The au-
thors acknowledge support by the state of Baden-Wꢃrttem-
berg through bwHPC and the German Research Foundation
(DFG) through grant no INST 40/467-1 FUGG.
The dimerization of sterically unprotected borirenes is
known to be a low-barrier process.[25] Likewise, the dimeriza-
tion of benzoborirene is associated with a zero-point energy
corrected barrier of only E0 =7 kcalmolÀ1 (DG° =17 kcalmolÀ1
)
and with considerable energy gain (DG=À84 kcalmolÀ1) ac-
cording to quantum chemical computations (DLPNO-CCSD(T)/
cc-pVTZ//B3LYP/6–311+G**).[26] The transition state for dimeri-
zation involves interaction of the three-coordinate boron cen-
ters with the ring fusion atoms (see the Supporting Informa-
tion, Figure S20). A similar transition state is difficult to imagine
for A as the boron center is electronically saturated and the
bulky NHC ligands provide some steric protection. But despite
the bulkiness of the NHC ligands, dimerization of A to 10a is a
very strongly exergonic process (DG=À83 kcalmolÀ1), similar
to that of parent benzoborirene. The interaction of the NHC
with the boron center is so strong (E0 =42 kcalmolÀ1, DG=
31 kcalmolÀ1) that dissociation of the NHC is not expected to
be important at room temperature or below. One conceivable
dimerization mechanism may involve the zwitterionic inter-
mediate 12a (Scheme 4). Similar zwitterions were invoked pre-
viously to explain the increased tendency of pyridine-coordi-
nated borirene molecules to undergo ring-opening reac-
Conflict of interest
The authors declare no conflict of interest.
Keywords: boron · fused ring systems · heterocycles · strained
molecules · ylides
[1] a) M. Faraday, Philos. Trans. R. Soc. London 1825, 115, 440–466; b) A.
Kekulꢂ, Lehrbuch der organischen Chemie oder der Chemie der Kohlen-
stoffverbindungen, F. Enke, Erlangen, 1866.
[2] J. Thiele, Liebigs Ann. Chem. 1899, 306, 87–142.
[3] a) R. S. Mulliken, J. Chem. Phys. 1939, 7, 339–352; b) R. S. Mulliken, C. A.
Rieke, W. G. Brown, J. Am. Chem. Soc. 1941, 63, 41–56.
[4] A. Baeyer, Ber. Dtsch. Chem. Ges. 1885, 18, 2269–2281.
[5] a) R. Diercks, K. P. C. Vollhardt, J. Am. Chem. Soc. 1986, 108, 3150–3152;
b) R. Boese, D. Blꢀser, W. E. Billups, M. M. Haley, A. H. Maulitz, D. L.
Mohler, K. P. C. Vollhardt, Angew. Chem. Int. Ed. Engl. 1994, 33, 313–317;
Chem. Eur. J. 2018, 24, 1 – 5
www.chemeurj.org
3
ꢁ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Communication
Angew. Chem. 1994, 106, 321–325; c) N. L. Frank, K. K. Baldridge, J. S.
Siegel, J. Am. Chem. Soc. 1995, 117, 2102–2103; d) H.-B. Bꢃrgi, K. K. Bal-
dridge, K. Hardcastle, N. L. Frank, P. Gantzel, J. S. Siegel, J. Ziller, Angew.
Chem. Int. Ed. Engl. 1995, 34, 1454–1456; Angew. Chem. 1995, 107,
1575–1577; e) N. L. Frank, K. K. Baldridge, P. Gantzel, J. S. Siegel, Tetrahe-
dron Lett. 1995, 36, 4389–4392; f) F. Cardullo, D. Giuffrida, F. H. Kohnke,
F. M. Raymo, J. F. Stoddart, D. J. Williams, Angew. Chem. Int. Ed. Engl.
1996, 35, 339–341; Angew. Chem. 1996, 108, 347–349.
Shieh, P. G. Werness, J. L. Wood, J. Am. Chem. Soc. 1973, 95, 7878–7880;
e) W. E. Billups, A. J. Blakeney, W. Y. Chow, Org. Synth. 1976, 55, 12–15.
[15] a) D. E. Kaufmann, W. Schacht, Pure Appl. Chem. 1991, 63, 383–386;
b) W. Schacht, D. Kaufmann, J. Organomet. Chem. 1987, 331, 139–152.
[16] a) J. J. Eisch, F. Shen, K. Tamao, Heterocycles 1982, 18, 245–250; b) C.
Habben, A. Meller, Chem. Ber. 1984, 117, 2531–2537; c) C. Pues, A.
Berndt, Angew. Chem. Int. Ed. Engl. 1984, 23, 313–314; Angew. Chem.
1984, 96, 306–307; d) B. Pachaly, R. West, Angew. Chem. Int. Ed. Engl.
1984, 23, 454; Angew. Chem. 1984, 96, 444; e) J. J. Eisch, B. Shafii, A. L.
Rheingold, J. Am. Chem. Soc. 1987, 109, 2526–2528; f) J. J. Eisch, B.
Shafii, J. D. Odom, A. L. Rheingold, J. Am. Chem. Soc. 1990, 112, 1847–
1853; g) H. Braunschweig, T. Herbst, D. Rais, F. Seeler, Angew. Chem. Int.
Ed. 2005, 44, 7461–7463; Angew. Chem. 2005, 117, 7627–7629; h) C. E.
Anderson, H. Braunschweig, R. D. Dewhurst, Organometallics 2008, 27,
6381–6389; i) H. Braunschweig, A. Damme, R. D. Dewhurst, S. Ghosh, T.
Kramer, B. Pfaffinger, K. Radacki, A. Vargas, J. Am. Chem. Soc. 2013, 135,
1903–1911.
[6] a) W. H. Mills, I. G. Nixon, J. Chem. Soc. 1930, 2510–2524; b) A. Stanger,
J. Am. Chem. Soc. 1991, 113, 8277–8280; c) J. S. Siegel, Angew. Chem.
Int. Ed. Engl. 1994, 33, 1721–1723; Angew. Chem. 1994, 106, 1808–
1810.
[7] a) A. Stanger, J. Am. Chem. Soc. 1998, 120, 12034–12040; b) A. Stanger,
E. Tkachenko, J. Comput. Chem. 2001, 22, 1377–1386; c) S. M. Bachrach,
J. Organomet. Chem. 2002, 643–644, 39–46; d) A. J. Boydston, M. Las-
koski, U. H. F. Bunz, M. M. Haley, Synlett 2002, 981–983; e) C. Cohrs, H.
Reuchlein, P. W. Musch, C. Selinka, B. Walfort, D. Stalke, M. Christl, Eur. J.
Org. Chem. 2003, 901–906; f) T. Rosenau, G. Ebner, A. Stanger, S. Perl, L.
Nuri, Chem. Eur. J. 2005, 11, 280–287; g) A. Patel, F. Liebner, T. Netscher,
K. Mereiter, T. Rosenau, J. Org. Chem. 2007, 72, 6504–6512; h) K. J.
Donald, M. Bober, Chem. Eur. J. 2011, 17, 1936–1945; i) A. D. Payne,
B. W. Skelton, A. H. White, D. Wege, ChemistrySelect 2016, 1, 5339–
5346.
[17] D. Kaufmann, Chem. Ber. 1987, 120, 901–905.
[18] CCDC 1862384, 1862385, and 1862386 contain the supplementary crys-
tallographic data for this paper. These data are provided free of charge
by The Cambridge Crystallographic Data Centre.
[19] J. J. Dunsford, I. A. Cade, K. L. Fillman, M. L. Neidig, M. J. Ingleson, Orga-
nometallics 2014, 33, 370–377.
´
´
[8] a) M. Eckert-Maksic, Z. Glasovac, Z. B. Maksic, I. Zrinski, Theochem 1996,
[20] J. C. Walton, M. M. Brahmi, J. Monot, L. Fensterbank, M. Malacria, D. P.
Curran, E. Lacꢄte, J. Am. Chem. Soc. 2011, 133, 10312–10321.
[21] T. R. McFadden, C. Fang, S. J. Geib, E. Merling, P. Liu, D. P. Curran, J. Am.
Chem. Soc. 2017, 139, 1726–1729.
´
´
366, 173–183; b) Z. B. Maksic, M. Eckert-Maksic, K.-H. Pfeifer, J. Mol.
Struct. 1993, 300, 445–453; c) M. R. Molavian, A. Abdolmaleki, K. Eskan-
dari, Comput. Theor. Chem. 2017, 1099, 102–108.
[9] a) R. I. Kaiser, H. F. Bettinger, Angew. Chem. Int. Ed. 2002, 41, 2350–2352;
Angew. Chem. 2002, 114, 2456–2458; b) H. F. Bettinger, R. I. Kaiser, J.
Phys. Chem. A 2004, 108, 4576–4586; c) F. Zhang, Y. Guo, X. Gu, R. I.
Kaiser, Chem. Phys. Lett. 2007, 440, 56–63.
[22] a) 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; b) R. Kinjo, B. Donnadieu, M. A. Celik, G. Frenking, G. Ber-
trand, Science 2011, 333, 610–613.
[10] a) H. F. Bettinger, Chem. Commun. 2005, 2756–2757; b) H. F. Bettinger, J.
Am. Chem. Soc. 2006, 128, 2534–2535.
[23] A. Lorbach, M. Bolte, H.-W. Lerner, M. Wagner, Chem. Commun. 2010,
46, 3592–3594.
[11] B. Halton, A. D. Woolhouse, H. M. Hugel, D. P. Kelly, J. Chem. Soc. Chem.
Commun. 1974, 247–248.
[12] a) B. Halton, Chem. Rev. 1973, 73, 113–126; b) B. Halton, Chem. Rev.
1989, 89, 1161–1185; c) B. Halton, Chem. Rev. 2003, 103, 1327–1369;
d) W. E. Billups, Acc. Chem. Res. 1978, 11, 245–251; e) W. E. Billups, W. A.
Rodin, M. M. Haley, Tetrahedron 1988, 44, 1305–1338.
[24] A. Lorbach, M. Bolte, H. Li, H.-W. Lerner, M. C. Holthausen, F. Jꢀkle, M.
Wagner, Angew. Chem. Int. Ed. 2009, 48, 4584–4588; Angew. Chem.
2009, 121, 4654–4658.
[25] a) P. H. M. Budzelaar, S. M. Van der Kerk, K. Krogh-Jespersen, P. v. R.
Schleyer, J. Am. Chem. Soc. 1986, 108, 3960–3967; b) M. Krasowska, H. F.
Bettinger, Chem. Eur. J. 2016, 22, 10661–10670.
[13] R. Anet, F. A. L. Anet, J. Am. Chem. Soc. 1964, 86, 525–526.
[14] a) E. Vogel, W. Grimme, S. Korte, Tetrahedron Lett. 1965, 6, 3625–3631;
b) R. Neidlein, D. Christen, V. Poignꢂe, R. Boese, D. Blꢀser, A. Gieren, C.
Ruiz-Pꢂrez, T. Hꢃbner, Angew. Chem. Int. Ed. Engl. 1988, 27, 294–295;
Angew. Chem. 1988, 100, 292–293; c) W. E. Billups, A. J. Blakeney, W. Y.
Chow, J. Chem. Soc. D, Chem. Commun. 1971, 1461–1462; d) W. E. Bill-
ups, W. Y. Chow, K. H. Leavell, E. S. Lewis, J. L. Margrave, R. L. Sass, J. J.
[26] a) C. Riplinger, F. Neese, J. Chem. Phys. 2013, 138, 034106; b) C. Riplin-
ger, B. Sandhoefer, A. Hansen, F. Neese, J. Chem. Phys. 2013, 139,
134101.
Manuscript received: September 10, 2018
Version of record online: && &&, 0000
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COMMUNICATION
Boron under strain: A benzannulated
borirene molecule with coordinated N-
&
Strained Molecules
J. Hahn, C. Keck, C. Maichle-Mçssmer,
E. von Grotthuss, P. N. Ruth, A. Paesch,
D. Stalke, H. F. Bettinger*
heterocyclic carbene IiPrMe is syntheti-
2
cally accessible. The ring strain of the
compound is high and facilitates facile
dimerization.
&& – &&
Synthesis and Ring Strain of a
Benzoborirene-N-Heterocyclic Carbene
Adduct
Chem. Eur. J. 2018, 24, 1 – 5
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