Carbon-carbon bond cleavage by strongly electrophilic boranes

Article abstract of DOI:10.1002/chem.201002047

Make and break: Strongly electrophilic boranes RB(C₆F ₅)₂ (R=Me, CH₂CH₂Ph) react with an aminodihydropentalene substrate by C=C bond cleavage with concomitant borylene insertion to yield the ring-enla

Full text of DOI:10.1002/chem.201002047

DOI: 10.1002/chem.201002047
Carbon–Carbon Bond Cleavage by Strongly Electrophilic Boranes
Bao-Hua Xu,[a] Gerald Kehr,[a] Roland Frçhlich,[a, b] and Gerhard Erker*[a]
Attack and cleavage of strong unactivated bonds for a se-
lective functionalization of the resulting compounds has
been a continuing major challenge and important task in
chemistry. While the activation of dihydrogen[1] and subse-
quently carbon–hydrogen bond activation have been ach-
ieved with a large selection of powerful methods,[2–5] the ac-
When the two reagents were mixed in pentane at room tem-
perature, a precipitate began to appear after 1 h and we iso-
lated the product 3a in greater than 60% yield after two
days. It was characterized spectroscopically, by elemental
analysis, high-resolution mass spectrometry, and by X-ray
diffraction. The X-ray crystal structure analysis of 3a
(Figure 1) shows that the boron atom has cleaved a C=C
ꢀ
tivation and cleavage of carbon–carbon (C C) bonds has at-
tracted much less attention.[4–7] Methodologically it is to be
regarded as problematic. There are classical methods known
ꢀ
for the selective cleavage of strong unactivated C C bonds,
such as the ozonolysis cycloaddition/cycloreversion se-
quence[8,9] or the metal-catalyzed olefin metathesis reac-
tion.[10,11] However, the stage of the development of C C
ꢀ
bond activation does not even come close to that of, for ex-
ꢀ
ꢀ
ample, C H activation chemistry. We have now found a C
C bond cleavage reaction that takes place straightforwardly
upon treatment of a suitable substrate with strongly electro-
philic boranes. The reaction sequence forms two new
ꢀ
ꢀ
carbon–boron (C B) bonds at the expense of the C C link-
age.
We have found that MeBACHTUNRGTNE(NUG C6F5)2 (2a) reacts cleanly with
the aminodihydropentalene derivative 1 at room tempera-
ꢀ
ture to yield the C C cleavage product 3a (Scheme 1).
Figure 1. A view of the molecular structure of 3a.
Scheme 1. Reaction of dimethylaminodihydropentalene derivative 1 with
the borane reagent MeBACHTUNGTRENNUNG(C6F5)2.
bond of the substrate and formed two new boron–carbon
ꢀ
ꢀ
linkages (B6 C5 1.658(3), B6 C7 1.623(4) ꢀ). The boron in-
sertion was coupled with a selective migration of the methyl
group from the boron atom to the carbon atom to yield the
observed methyl-substituted borata-cyclohexadiene subunit
found in 3a. This is part of a doubly conjugated iminium
[a] Dr. B.-H. Xu, Dr. G. Kehr, Dr. R. Frçhlich, Prof. Dr. G. Erker
Organisch-Chemisches Institut der Universitꢁt Mꢂnster
Corrensstrasse 40, 48149 Mꢂnster (Germany)
E-mail: erker@uni-muenster.de
ꢀ
ꢀ
ꢀ
salt (N1 C3 1.313(3), C4 C9 1.366(3), C8 C7 1.349(3) ꢀ)
inside the zwitterionic heterobicyclic framework of 3a.
In solution, compound 3a features a 11B NMR signal at
d=ꢀ14.1 ppm and a set of typical “borate-C6F5” 19F NMR
resonances [Dd (p,m-F)=2.8 ppm]. It shows the 7-CH3
1H NMR signal at d=2.41 ppm and 13C NMR signals of the
[b] Dr. R. Frçhlich
X-ray structure analyses.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002047.
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Chem. Eur. J. 2010, 16, 12538 – 12540

COMMUNICATION
conjugated p system at d=191.5 (C7), 188.6 (C9), 181.3 (C=
N), 128.9 (C4), 123.4 ppm (C8).
The related strongly Lewis acidic borane PhCH2CH2B-
shift inside the substituted cyclopentadiene leads to the zwit-
terion 5. In this intermediate the ring position adjacent to
the boron atom becomes activated for nucleophilic attack
by conjugation with the iminium ion functionality. Conse-
quently, nucleophilic R-migration from boron to carbon can
now take place to give 6. The topological reversal of the last
step opens a pathway for the migration of the C5 alkyl
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
gously with the aminodihydropentalene substrate 1 by C=C
cleavage/boron insertion to yield the product 3b (isolated
yield 58%), which was also characterized by X-ray diffrac-
tion (see Figure 2, further details see the Supporting Infor-
mation).
ꢀ
group to boron with concurrent cleavage of the C7 C5
carbon–carbon bond to yield the observed product 3.
This pathway leading to C=C cleavage of 1 is supported
by the direct observation of the intermediate 5b (R=
CH2CH2Ph) by performing the 1 plus 2b reaction under
similar conditions with direct NMR control and by isolating
the related product 5c from the reaction of the aminodihy-
dropentalene derivative 1 with BACHTNUTRGNE(UNG C6F5)3 (2c) (Scheme 3;
Scheme 3. Reaction of dimethylaminodihydropentalene derivative 1 with
borane reagent B(C6F5)3.
AHCTUNGTRENNUNG
compound 5b gave ca. 90% pure product 3b upon keeping
the NMR sample for 4 h at 558C; compound 5c was isolated
in 85% yield, for details of its characterization see the Sup-
porting Information). Compound 5c was characterized by
X-ray diffraction (see Figure 3). It shows a typical conjugat-
Figure 2. Molecular structure of 3b. Selected bond lengths [ꢀ] and angle
ꢀ
ꢀ
ꢀ
ꢀ
[8]: B6 C5 1.659(3), B6 C7 1.621(3), C7 C8 1.348(3), C8 C9 1.439(3),
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ed zwitterionic bicyclic framework (B1 C6 1.623(5), C5 C6
C9 C4 1.369(3), C4 C3 1.427(3), C3 N1 1.321(3); C7-B6-C5 112.6(2).
ꢀ
1.520(4), C6 C7 1.364(4) ꢀ) with a pair of anellated five-
membered rings. We assume that the lower nucleophilicity
ꢀ
The formation of the C=C cleavage product 3 can be ra-
tionalized by the reaction sequence described in Scheme 2.
The sequence is initiated by boron Lewis acid addition to
the substrate 1 to generate 4. A subsequent 1,5-hydrogen
of the B C6F5 unit in this case has prevented the rapid sub-
Scheme 2. A possible mechanism for C=C bond cleavage by RBACHTUNRGTNEUNG(C6F5)2
boranes.
Figure 3. Molecular structure of 5c.
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www.chemeurj.org
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G. Erker et al.
sequent reaction of 5c to its ring-enlarged isomer (3c) and
hence has allowed its isolation.
Acknowledgements
Financial support from the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie is gratefully acknowledged.
Our new reaction sequence formally involves an unprece-
dented 1,1-carboboration reaction[13–18] of an olefin (1 ! 6)
followed by 1,2-alkyl migration from carbon to boron (6 !
ꢀ
3) that eventually results in the cleavage of the C C bond.
Our example shows that the utilization of such unusual reac-
ꢀ
Keywords: boranes · carboboration · C C activation · ring
tion patterns with selected substrate families can lead to
expansion
ꢀ
new reaction types such as the efficient C C bond activation
described in this report.
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Experimental Section
Synthesis of compound 3a: MeBACTHUGNTERN(UGNN C6F5)2 (100 mg, 0.28 mmol) and 1
(53 mg, 0.30 mmol) were dissolved in pentane (10 mL) at room tempera-
ture and a yellow-red solid began to precipitate after 1 h. After the reac-
tion mixture had been stirred for 48 h, the solid was isolated by cannula
filtration and washed twice with cold pentane (2.5 mL). A solution of the
obtained solid in benzene (5 mL) was heated up to 558C for 4 h. Remov-
al of all volatiles in vacuo yielded 3a (93 mg, 62%) as a red powder.
Crystals suitable for X-ray crystal structure analysis were grown from the
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reaction phase between 1 and MeBACHTUNTRGNEUNG(C6F5)2 in pentane solution at room
temperature. Elemental analysis calcd (%) for C25H20BF10N: C 56.10, H
3.77, N 2.62; found: C 55.43, H 3.60, N 2.39. HRMS (ESI) exact mass for
[M+Na]+ (C25H20BF10NNa): calcd: m/z 558.1426, found: m/z 558.1423.
For more experimental details and complete characterization, see the
Supporting Information.
Synthesis of compound 3b: Styrene (40 mL, 0.34 mmol) and HBACHTNUTRGNEUNG(C6F5)2
(117 mg, 0.34 mmol) were dissolved in pentane (6 mL) and stirred for
15 min. A solution of 1 (65 mg, 0.37 mmol) in pentane (3 mL) was added
at room temperature, and after 1 h a yellow-red solid began to precipi-
tate. After the reaction mixture had been stirred for 24 h, the solid was
isolated by cannula filtration and washed twice with cold pentane
(2.5 mL). A solution of the obtained solid in benzene (5 mL) was heated
up to 558C for 4 h. Removal of all volatiles in vacuo yielded 3b (123 mg,
58%) as a red powder. Crystals suitable for X-ray crystal structure analy-
sis were grown by slow diffusion of pentane into a solution of 3b in ben-
zene at room temperature. Elemental analysis calcd (%) for
C32H26BF10N·0.5C6H6: C 64.55, H 4.25, N 2.03; found: C 64.57, H 3.93, N
1.74. For more experimental details and complete characterization of 3b
and involving intermediate 5b, see the Supporting Information.
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Synthesis of compound 5c: B
in pentane (6 mL) and stirred for 5 min.
G
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ja106365j.
A
1
0.22 mmol) in pentane (3 mL) was added at room temperature and a
white precipitate was immediately formed. After the reaction mixture
had been stirred for 24 h, the solid was isolated by cannula filtration and
washed twice with cold pentane (2.5 mL). Removal of all volatiles in
vacuo yielded 5c (117 mg, 85%) as a white powder. Elemental analysis
calcd (%) for C30H17BF15N: C 52.43, H 2.49, N 2.04; found: C 52.48, H
2.40, N 2.08. For more experimental details and complete characteriza-
tion, see the Supporting Information.
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Received: July 19, 2010
Published online: September 28, 2010
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Chem. Eur. J. 2010, 16, 12538 – 12540