Ring Expansion to 1-Bromo-1-alumacyclonona-2,4,6,8-tetraene by Insertion of Two Alkyne Molecules into the AlC Bonds

Article abstract of DOI:10.1002/anie.201504468

Treatment of 1-bromo-2,3,4,5-tetraethylalumole (1) with 3-hexyne afforded the corresponding product 1-bromo-1-alumacyclonona-2,4,6,8-tetraene (2), accompanied by the formation of hexaethylbenzene. In the crystalline state, 2 forms a Br-bridged dimer with a pseudo C₂-symmetric and twisted AlC₈ nine-membered ring. Deuterium-labeling experiments and DFT calculations on the reaction of 1 with 3-hexyne suggested that 1-bromo-1-alumacyclohepta-2,4,6-triene, which is formed by the insertion of one molecule of 1-hexyne into the AlC bond of alumole 1, is the key intermediate for the generation of 2 as well as hexaethylbenzene. Expanding Al's horizons: A stable alumacyclononatetraene 1 was obtained by the reaction of the corresponding alumole with 3-hexyne accompanied by the formation of hexaethylbenzene. Deuterium-labeling experiments and DFT calculations suggested that an alumacycloheptatriene is the key intermediate (see picture).

Full text of DOI:10.1002/anie.201504468

.
Angewandte
Communications
International Edition: DOI: 10.1002/anie.201504468
German Edition: DOI: 10.1002/ange.201504468
Aluminum Compounds
Ring Expansion to 1-Bromo-1-alumacyclonona-2,4,6,8-tetraene by
À
Insertion of Two Alkyne Molecules into the Al C Bonds**
Tomohiro Agou, Tatsuya Wasano, Takahiro Sasamori, Jing-Dong Guo, Shigeru Nagase, and
Norihiro Tokitoh*
Abstract: Treatment of 1-bromo-2,3,4,5-tetraethylalumole (1)
with 3-hexyne afforded the corresponding product 1-bromo-1-
alumacyclonona-2,4,6,8-tetraene (2), accompanied by the for-
mation of hexaethylbenzene. In the crystalline state, 2 forms
a Br-bridged dimer with a pseudo C -symmetric and twisted
2
AlC8 nine-membered ring. Deuterium-labeling experiments
and DFT calculations on the reaction of 1 with 3-hexyne
suggested that 1-bromo-1-alumacyclohepta-2,4,6-triene, which
is formed by the insertion of one molecule of 1-hexyne into the
AlÀC bond of alumole 1, is the key intermediate for the
[4]
Scheme 1. Reaction of benzoalumole I with diphenylacetylene.
In the course of our studies on heavier Group 13 hetero-
les, we have recently reported the synthesis, structure, and
some reactivities of 1-bromo-2,3,4,5-tetraethylalumole (1).
[
5]
[6]
generation of 2 as well as hexaethylbenzene.
Owing to the electron-withdrawing bromo group on the
aluminum center, the electrophilic character of 1 would be
enhanced, and thus 1 is expected to show a unique reactivity
pattern toward alkynes. In this Communication, we describe
the formation of 1-bromo-1-alumacyclonona-2,4,6,8-tetraene
(2) by the reaction of 1 with 3-hexyne. The structure and
formation mechanism of 2 are also discussed.
R
eactions of heteroles with compounds containing carbon–
carbon multiple bonds, such as alkynes, are useful methods to
construct new heterocyclic skeletons with specific structures
and properties. For instance, reactions of pentaarylboroles
with internal alkynes afforded various cyclic boranes includ-
ing 7-borabicyclo[2.2.1]hepta-2,5-diene and borepin deriva-
[1]
tives. In contrast, reactivities of heavier Group 13 heteroles
toward unsaturated compounds have been little unexplored
to date. Eisch and Kaska investigated the reaction of
A mixture of 1-bromo-2,3,4,5-tetraethylalumole (1) and 3-
hexyne (18 equiv) was stirred at 608C for 1 d. Complete
consumption of 1 and generation of 2 and hexaethylbenzene
[2]
[3]
1
benzoalumole I, which was generated in situ from AlPh and
(3) were shown by a H NMR spectrum of the crude reaction
3
[
4]
diphenylacetylene, with diphenylacetylene (Scheme 1). At
008C, benzoalumole I was reported to react with diphenyl-
mixture (Scheme 2). Compound 2 is sparingly soluble in 3-
hexyne and hexane, and thus 2 could be easily isolated as
a colorless, thermally stable solid (m.p. 134–1358C) by
washing the crude material with cold hexane. In contrast,
reaction of 1 with 2 equivalents of 3-hexyne in C D at 608C
2
acetylene to give a mixture of products containing naphtha-
lene derivative II. Formation of II was explained in terms of
the generation of compound III as the intermediate, though
the reaction mechanism has not been elucidated. Further-
more, aluminum-containing heterocyclic compounds were
not obtained in this reaction.
6
6
1
afforded 3 as the major product (65%, estimated by H NMR
spectroscopy), and only a trace amount of 2 was detected. In
both experiments, aluminum-containing products except for 2
could not be characterized.
The molecular structure of 2 was elucidated by X-ray
[
7]
crystallographic analysis (Figure 1). In the crystalline state,
formed a Br-bridged dimer with a crystallographic center of
symmetry. The AlÀBr bond lengths (2.5453(11) and
.4694(12) Š) are comparable to those of the previously
[
*] Dr. T. Agou, Dr. T. Wasano, Prof. Dr. T. Sasamori, Dr. J.-D. Guo,
Prof. Dr. N. Tokitoh
Institute for Chemical Research, Kyoto University
Gokasho, Uji, Kyoto 611-0011 (Japan)
2
2
reported Br-bridged bromoalumane dimer [Tip Al(m-Br)]
E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp
2
2
Dr. J.-D. Guo, Prof. Dr. S. Nagase
Fukui Institute for Fundamental Chemistry, Kyoto University
Kyoto, Kyoto 606-8103 (Japan)
[
**] This work was partially supported by the JSPS KAKENHI (Nos.
4109013, 24550048, 2662028, and 15H05477), the MEXT Project of
2
Integrated Research on Chemical Synthesis from the Ministry of
Education, Culture, Sports, Science, and Technology (Japan), the
“
Molecular Systems Research” project of the RIKEN Advanced
Science Institute, and the Collaborative Research Program of the
Institute for Chemical Research, Kyoto University. T.A. is thankful for
the Ube Industries Award in Synthetic Organic Chemistry (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504468.
Scheme 2. Reaction of alumole 1 with 3-hexyne. [a] Yield estimated
using H NMR spectroscopy.
1
9
568
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Figure 2. Crystal structure of complex 4 (thermal ellipsoids set at 50%
Figure 1. Crystal structure of 2 (thermal ellipsoids set at 50% proba-
bility). Hydrogen atoms are omitted for clarity. Selected bond lengths
probability). Hydrogen atoms are omitted for clarity. Selected bond
lengths [Š], bond angles [8], and torsion angles [8]: Al1ÀBr1 2.3722(5),
Al1ÀO1 1.9381(12), Al1ÀC1 1.9736(16), Al1ÀC8 1.9772(17), C1ÀC2
[
Š], bond angles [8], and torsion angles [8]: Al1ÀBr1 2.5453(11), Al1À
Br1A 2.4694(12), Al1ÀC1 1.964(2), Al1ÀC8 1.959(2), C1ÀC2 1.346(3),
C2ÀC3 1.498(3), C3ÀC4 1.352(3), C4ÀC5 1.516(3), C5ÀC6 1.347(3),
C6ÀC7 1.490(3), C7ÀC8 1.343(3); C1-Al1-C8 139.31(10), C1-Al1-Br1
1
1
.352(2), C2ÀC3 1.497(2), C3ÀC4 1.348(2), C4ÀC5 1.507(2), C5ÀC6
.356(2), C6ÀC7 1.490(3), C7ÀC8 1.347(2); C1-Al1-C8 135.41(7), C1-
Al1-Br1 107.42(5), C1-Al1-O1 97.08(6), C8-Al1-Br1 109.47(5), C8-Al1-
O1 102.16(6), O1-Al1-Br1 97.30(4); C1-C2-C3-C4 53.4(2), C3-C4-C5-C6
À125.37(17), C5-C6-C7-C8 63.3(2).
1
1
03.34(7), C1-Al1-Br1A 106.44(7), C8-Al1-Br1 101.67(7), C8-Al1-Br1A
06.22(8), Br1-Al1-Br1A 87.52(3), Al1-Br1-Al1A 92.49(3); C1-C2-C3-C4
À62.9(3), C3-C4-C5-C6 122.9(2), C5-C6-C7-C8 À54.5(3).
generation of [D ]-2 and hexaethylbenzene [D ]-3
(Scheme 3). Resonance signals in the H NMR spectrum of
20
10
[8]
1
(
Tip = 2,4,6-triisopropylphenyl; 2.500(3) and 2.475(4) Š).
The AlC nine-membered ring of 2 exhibits a highly twisted,
[D ]-2 indicated that two compounds, A and B, were present
8
20
pseudo C -symmetric conformation. The torsion angles
in a 1:1 ratio. These experimental data can be explained by
2
between the adjacent C=C double bonds are À62.9(3),
generation of the alumacycloheptatriene [D ]-5 as the key
10
1
22.9(2), and À54.5(3)8. The twisted structure of 2 is
intermediate by the insertion of [D ]-3-hexyne into an AlÀC
1
0
[4,12]
corroborated by the DFT calculations on the parent com-
bond of alumole 1.
Further reaction of [D ]-5 with the
10
pound HAlC H , which revealed that the twisted C -sym-
second molecule of [D ]-3-hexyne affords [D ]-2 as a one-to-
8
8
2
10
20
metic conformation corresponds to the energy minimum of
one mixture of A and B. Meanwhile, reductive elimination of
[D ]-5 would afford [D ]-3 accompanied by the extrusion of
[
9]
the AlC nine-membered ring.
8
10
10
1
[13,14]
In the H NMR spectrum of 2, the methylene protons of
the ethyl substituents adjacent to the aluminum center (that
is, the ethyl groups attached to the C1 and C8 atoms in
the aluminum(I) species BrAlD (6).
Although attempts to
trap transient species 6 have been unsuccessful so far, it might
react with the alkyne to afford unidentified organoaluminum
[15]
Figure 1) exhibited an ABX pattern signal, showing that
compounds.
3
these methylene protons are diastereotopic. Signals for the
other methylene protons also showed complicated multiplet
signals, probably because of the overlapping of ABX3
1
patterns. The H NMR spectrum did not show notable
changes between 25 and 608C in C D . These spectral data
6
6
suggest that the pseudo C -symmetric structure of the AlC8
2
nine-membered ring detected in the crystalline state is
retained in solution and that ring inversion is slow on the
NMR timescale.
Treatment of 2 with THF afforded THF complex 4 as
a colorless solid, and its structure was elucidated by single-
[10]
crystal X-ray diffraction (Figure 2). The Al1ÀO1 distance
in 4 (1.9381(12) Š) is comparable to those detected for the
THF complexes of sterically hindered alumanes Mes RAl
2
(
1
2
R = Mes, 1.969(5) Š; R = Et, 1.934(4) Š; R = Cl, 1.890(5),
.898(5) Š), demonstrating the moderate Lewis acidity of
[
11]
. Similarly to the case of 2, the AlC nine-membered ring of 4
8
exhibited a highly twisted conformation with a pseudo C -axis
2
passing through the Al1 atom and the midpoint of the C4ÀC5
bond.
To obtain information on the mechanism of the formation
of 2 and 3, the reaction of 1 with [D ]-3-hexyne was carried
out. After heating the mixture at 608C for 2 days, the mass
10
Scheme 3. Reaction of 1 with [D ]-3-hexyne (D C -CꢀC-C D ). The
1
0
5
2
2
5
scheme in square brackets shows the proposed reaction mechanism.
1
spectrum of the hydrolyzed reaction mixture suggested the
[a] Yield estimated using H NMR spectroscopy.
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The mechanism for the reaction of alumole 1 with 3-
C D afforded 3 as the major product, and 2 was generated as
6 6
hexyne was further investigated by DFT calculations at the
M06-2X/6-31G(d) level of theory. The computed reaction
profile is shown in Scheme 4. At the first stage, coordination
a trace product.
[16]
In conclusion, we have demonstrated that the reaction of
alumole 1 with 3-hexyne affords 2 accompanied by the
formation of benzene derivative 3. Compound 2 is, to our
knowledge, the first example of structurally characterized,
stable alumacyclononatetraene derivative. The AlC8 nine-
membered ring of 2 exhibited a highly twisted, pseudo C2-
symmetric structure both in the crystalline state and in
solution. The crystallographic analysis of THF complex 4
suggested that 2 had moderate Lewis acidity. Products 2 and 3
were likely formed from the common intermediate alumacy-
cloheptatriene 5, as suggested by deuterium-labeling experi-
ments and DFT calculations.
Experimental Section
Synthesis of 1-bromo-1-alumacyclonona-2,4,6,8-tetraene (2): A mix-
ture of 1-bromoalumole (1; 0.521 g, 1.92 mmol) and 3-hexyne (2.85 g,
3
4.7 mmol) was stirred at 608C for 1 d. After cooling the reaction
mixture to room temperature, crude precipitated product 2 was
collected by filtration and washed with cold hexane to afford pure 2 as
[17]
a colorless, crystalline solid (0.281 g, 0.647 mmol, 34%).
An
aqueous solution of NaHCO3 was added to the filtrate and the
organic layer was separated. The aqueous layer was extracted with
Scheme 4. Possible mechanism for the reaction of 1 with 3-hexyne.
Relative Gibbs free energies (kcalmol ) were calculated at the M06-
CH Cl . The combined organic layer was dried over MgSO and
À1
2
2
4
evaporated under reduced pressure. The residue was purified by silica
gel chromatography (using hexane as eluent) to afford hexaethyl-
2
X/6-31G(d) level of theory.
[18]
benzene (3) as a colorless solid (0.101 g, 0.410 mmol, 21%).
of 3-hexyne to the aluminum center of 1 affords pre-reactive
complex INT1 through an exothermic reaction (DG =
Keywords: alkynes · aluminum · DFT calculations ·
ring expansion · structure elucidation
À1
À8.3 kcalmol ). After the formation of INT1, insertion of
the coordinated 3-hexyne into the AlÀC bond results in the
exothermic formation of key intermediate 5, which proceeds
How to cite: Angew. Chem. Int. Ed. 2015, 54, 9568–9571
Angew. Chem. 2015, 127, 9704–9707
À1
through TS1 with a reaction barrier of 15.9 kcalmol .
Nucleophilic attack and reaction of the second molecule of
[
1] a) J. J. Eisch, J. E. Galle, J. Am. Chem. Soc. 1975, 97, 4436; b) J. J.
Eisch, J. E. Galle, B. Shafii, A. L. Rheingold, Organometallics
3
-hexyne with 5 affords 2 by means of a strongly exothermic
À1
reaction (DG = À29.1 kcalmol ) through a reaction barrier
1
990, 9, 2342; c) C. Fan, W. E. Piers, M. Parvez, R. McDonald,
À1
of 25.0 kcalmol (TS2). Meanwhile, 5 can isomerize to 7-
Organometallics 2010, 29, 5132.
bromo-7-alumabicyclo[2.2.1]hepta-2,5-diene (7), which is
slightly more stable compared to 5, with an isomerization
barrier of 28.4 kcalmol (TS3). Retro [1+4] cycloaddition of
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aldehydes, see: Z. Xi, P. Li, Angew. Chem. Int. Ed. 2000, 39, 2950;
Angew. Chem. 2000, 112, 3057.
À1
[
[
[
3] J. J. Eisch, W. C. Kaska, J. Am. Chem. Soc. 1966, 88, 2976.
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Chem. 2015, 28, 104.
bicyclic intermediate 7 results in the exothermic formation of
À1
aluminum–arene complex 8 (DG = À14.4 kcalmol ) through
À1
a low reaction barrier of 4.3 kcalmol (TS4). The structure of
the C Et moiety of complex 8 is nearly the same as that of
6
6
free hexaethylbenzene (3), indicating that there is only a weak
[
[
6] a) T. Wasano, T. Agou, T. Sasamori, N. Tokitoh, Chem.
Commun. 2014, 50, 8148; b) T. Agou, T. Wasano, T. Sasamori,
N. Tokitoh, Organometallics 2014, 33, 6963.
interaction between the C Et6 and BrAlD moieties in 8.
6
Dissociation of complex 8 affords 3 as an isolable product,
though the fate of the BrAlD moiety remained unclear. These
calculation results can explain the concentration effect of 3-
hexyne on the product ratio of 2/3 as shown in Scheme 2.
7] Crystallographic data for 2: Empirical formula C H AlBr, M =
2
4
40
r
4
35.45, À1708C, triclinic, P-1 (no. 2), a = 8.512(3), b = 9.956(4),
c = 14.471(5) Š, a = 96.188(3), b = 96.656(4), g = 107.022(5)8,
3
À3
À1
Because the reaction barrier for the insertion of 3-hexyne into
V= 1151.7(8) Š , 1calcd = 1.256 gcm , m = 1.829 mm , Z = 2,
À1
5
(25.0 kcalmol ) is slightly lower than that for the isomer-
2.41 ꢁ q ꢁ 25.50, reflections collected 10177, independent
À1
reflections 4264 (R = 0.0375), completeness to q(25.248) =
ization from 5 to 7 (28.4 kcalmol ), 2 was preferentially
obtained under the neat reaction conditions. When the
amount and concentration of 3-hexyne are decreased, the
reaction of intermediate 5 and 3-hexyne is less feasible.
Therefore, the reaction of 1 with 2 equivalents of 3-hexyne in
int
9
0
9.5%, GOF 1.087, R1 [I > 2s(I)] = 0.0320, wR2 (all data) =
.0765, largest difference peak and hole 0.805 and
À3
À0.271 eŠ . CCDC 1059006.
[
8] M. A. Petrie, P. P. Power, H. V. Rasika Dias, K. Ruhlandt-Senge,
K. M. Waggoner, R. Wehmschulte, Organometallics 1993, 12,
1
086.
9
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Angew. Chem. Int. Ed. 2015, 54, 9568 –9571

Angewandte
Chemie
[
9] a) P. von RaguØ Schleyer, L. Nyulµszi, T. Kµrpµti, Eur. J. Org.
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ometallic Chemistry 41 (Eds.: S. Woodward, S. Dagorne),
Springer, Heidelberg, 2013, pp. 91 – 124.
[
10] Crystallographic data for 4: Empirical formula C H AlBrO,
[14] A similar mechanism was proposed for the formation of
hexamethylbenzene from the reaction of ClAlD and 2-butyne.
See: C. Üffing, A. Ecker, R. Kçppe, K. Merzweiler, H.
Schnçckel, Chem. Eur. J. 1998, 4, 2142.
[15] 3-Hexene was detected by GC – MS analysis of the hydrolyzed
reaction mixture, indicating the generation of 1,4-dibromo-1,4-
dialuma-2,3,5,6-tetraethylcyclohexa-2,5-diene as an aluminum-
containing byproduct. For formation of 1,4-dialumacyclohexa-
2,5-diene derivatives from XAlD (X = Cl, Br) and internal
alkynes, see: H. Schnçckel, M. Leimkühler, R. Lotz, R.
Mattes, Angew. Chem. Int. Ed. Engl. 1986, 25, 921; Angew.
Chem. 1986, 98, 929. See also ref. [12].
[16] Y. Zhao, D. G. Truhlar, Theor. Chem. Acc. 2008, 120, 215.
[17] For the characterization data of 2, see the Supporting Informa-
tion.
[18] Hexaethylbenzene (3) was identified by the comparison of its
spectroscopic data with those reported previously. See: T.
Takahashi, F.-Y. Tsai, Y. Li, K. Nakajima, M. Kotora, J. Am.
Chem. Soc. 1999, 121, 11093.
2
8
48
M = 507.55, À1708C, triclinic, P-1 (no. 2), a = 8.4083(1), b =
r
1
0.6258(2), c = 16.4221(3) Š, a = 84.6838(9), b = 79.7948(12),
3
À3
g = 72.6920(16)8, V= 1377.39(4) Š , 1calcd = 1.224 gcm
1
3
, m =
À1
.541 mm , Z = 2, 3.04 ꢁ q ꢁ 25.50, reflections collected
1062, independent reflections 5075 (R = 0.0428), complete-
int
2
ness to q(25.248) = 99.0%, GOF on F 1.109, R1 [I > 2s(I)] =
0
0
.0261, wR (all data) = 0.0678, largest difference peak and hole
.446 and À0.235 eŠ . CCDC 1401074.
2
À3
[
[
[
11] a) M. S. Lalama, J. Kampf, D. G. Dick, J. P. Oliver, Organo-
metallics 1995, 14, 495; b) V. S. J. De Mel, J. P. Oliver, Organo-
metallics 1989, 8, 827.
12] For isolable derivatives related to 5, see: K. Yoshida, T.
Furuyama, C. Wang, A. Muranaka, D. Hashizume, S. Yasuike,
M. Uchiyama, J. Org. Chem. 2012, 77, 729.
13] For reviews on the properties of aluminum(I) halides, see: a) C.
Dohmeier, D. Loos, H. Schnçckel, Angew. Chem. Int. Ed. Engl.
1
996, 35, 129; Angew. Chem. 1996, 108, 141; b) G. Linti, H.
Schnçckel, W. Uhl, N. Wiberg, in Molecular Clusters of the Main
Group Elements (Eds.: M. Driess, H. Nçth), Wiley-VCH,
Weinheim, 2004, pp. 126 – 168; c) H. Schnçckel, A. Schnepf, in
The Group 13 Metals Aluminium, Gallium, Indium and Thal-
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Downs), Wiley, Chichester, 2011, pp. 402 – 488; d) R. J. Wehm-
Received: May 18, 2015
Published online: June 26, 2015
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www.angewandte.org
9571