Synthesis and properties of stable alumoles

Article abstract of DOI:10.1080/10426507.2015.1128911

Stable alumole and gallole derivatives have been synthesized, and their reactivity was investigated. Mes? -substituted alumole and gallole have tri-coordinated heavier group 13 element moieties even in the presence of Lewis bases, because of the steric hindrance of the bulky Mes? group. Treatment of the Mes? -substituted heavier group 13 heteroles with Li metal afforded the Li salts of the corresponding heterole dianions. Furthermore, 1-bromo- and 1-chloro-alumole derivatives were synthesized by applying the same synthetic method as used for the synthesis of the Mes? -substituted alumole. In the solid state, the 1-haloalumoles exhibit dimeric structures due to the halogen-aluminum coordination interactions. Utilization of the 1-bromoalumole for the synthesis of new cyclic organoaluminum compounds through the reactions with nucleophiles and alkynes are also described.

Full text of DOI:10.1080/10426507.2015.1128911

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Synthesis and Properties of Stable Alumoles
Norihiro Tokitoh, Tomohiro Agou, Tatsuya Wasano & Takahiro Sasamori
To cite this article: Norihiro Tokitoh, Tomohiro Agou, Tatsuya Wasano & Takahiro Sasamori
(2016): Synthesis and Properties of Stable Alumoles, Phosphorus, Sulfur, and Silicon and the
Related Elements, DOI: 10.1080/10426507.2015.1128911
To link to this article: http://dx.doi.org/10.1080/10426507.2015.1128911
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Date: 03 February 2016, At: 21:44

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SYNTHESIS AND REACTIVITY OF ALUMOLES
Synthesis and Properties of Stable Alumoles
Norihiro Tokitoh*, Tomohiro Agou, Tatsuya Wasano, and Takahiro Sasamori
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
Abstract
Stable alumole and gallole derivatives have been synthesized, and their reactivity was
investigated. Mes*-substituted alumole and gallole have tri-coordinated heavier group 13
element moieties even in the presence of Lewis bases, because of the steric hindrance of the
bulky Mes* group. Treatment of the Mes*-substituted heavier group 13 heteroles with Li metal
afforded the Li salts of the corresponding heterole dianions. Furthermore, 1-bromo- and 1-
chloro-alumole derivatives were synthesized by applying the same synthetic method as used for
the synthesis of the Mes*-substituted alumole. In the solid state, the 1-haloalumoles exhibit
dimeric structures due to the halogen-aluminum coordination interactions. Utilization of the 1-
bromoalumole for the synthesis of new cyclic organoaluminum compounds through the reactions
with nucleophiles and alkynes are also described.
Keywords
Alumole; gallole; alumepin; 1-haloalumole
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INTRODUCTION
In contrast to the recent development of the chemistry of boroles,1 reports on stable heavier
group 13 heteroles have been limited to gallole and indacyclopentadiene derivatives.2,3 Their
aluminum analogues, i.e., alumoles, were postulated as reactive intermediates in the reactions of
some organometallic compounds with electrophiles or unsaturated compounds,4 though the
precise structures and reactivity of alumoles have not been thoroughly revealed because of the
lack of isolable alumole derivatives.5 Recently, we have reported a stable Lewis base-free
alumole bearing Mes* group (Mes* = 2,4,6-(t-Bu)3C6H2)6 and an isolable 1-bromoalumole.7
Herein, we describe the synthesis and properties of the Mes*-substituted heavier group 13
heteroles and 1-haloalumole derivatives.
RESULTS AND DISCUSSION
Synthesis of Mes*-substituted alumole and gallole
Mes*-substituted alumole 1 and gallole 2 were obtained by the treatment of 1,4-dilithiobutadiene
9
10
derivative 38 with Mes*AlCl2 and Mes*GaCl2 , respectively (Scheme 1).6,11 Heteroles 1 and 2
are colorless and thermally stable under inert atmosphere. X-Ray crystallographic analysis
revealed that 1 and 2 have planar EC4 (E = Al, Ga) five-membered rings with C-C bond lengths
alternations, showing their apparent butadiene-type structures. The bulky Mes* groups prevent
from the coordination of Lewis bases towards the aluminum and gallium moieties in 1 and 2,
respectively.
Synthesis and structure of the dianions of the Mes*-substituted heteroles
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Treatment of alumole 1 and gallole 2 with excess amount of Li in thf afforded the lithium salts of
heterole dianions 4 and 5, respectively (Scheme 2). Dianions 4 and 5 were isolated as orange-
colored crystals and fully characterized by means of NMR spectroscopy and X-ray
crystallographic analysis (Figures 1 and 2). In the solid state, 4 and 5 posses contact-ion-pair
structures, and the C-C bond alternations in the EC4 (E = Al, Ga) five-membered rings
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decreased, indicating the delocalization of the -electrons in the butadiene moieties. The Li
NMR signals of 4 and 5 were observed in the high field region (4: --6.0 ppm, 5: --6.8 ppm) as
expected, suggesting the shielding effects of the dianionic EC4 five-membered rings.
Synthesis and structure of 1-haloalumoles
1-Chloroalumole 6 and 1-bromoalumole 7 were obtained by the treatment of 3 with AlCl3 and
AlBr3, respectively (Scheme 3).7 1-Haloalumoles 6 and 7 are both colorless and thermally stable
under inert atmosphere. In the solid state, 6 and 7 dimerize by means of aluminum-carbon
coordination interactions (Figures 3 and 4),12 while in solution these dimers likely dissociate to
generate 6 and 7 as indicated by the VT-NMR measurements.
Reactivity of the 1-bromoalumole
Reactivity of 1-bromoalumole 7 toward nucleophilic reagents were investigated, and the results
are shown in Scheme 4. Treatment of 7 with thf and IPr [1,3-bis(diisopropylphenyl)imidazol-2-
ylidene] afforded the corresponding Lewis base complexes 8 and 9, respectively. The structure
1
of the IPr complex 9 was elucidated by X-ray crystallographic analysis (Figure 5).13 In the H
NMR spectrum of 9, the methylene protons of the ethyl substituents attached to the C1 and C4
atoms showed two sets of multiplets, one of which was observed in characteristic, higher field
region (0.12-0.19 ppm), probably because of the shielding effect of the proximate 2,6-
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diisopropylphenyl substituents of the IPr ligand. Nucleophilic substitution of the bromo
substituent of 7 with Mes*Li yielded the Mes*-substituted alumole 1. Treatment of 7 with
LiN(SiMe3)2 in thf or toluene gave the 1-aminoalumole derivative 10 or 11, respectively.14
Compounds 10 and 11 are interconverted to each other by the ligand exchange reactions. These
reactions demonstrate the potential of 1-haloalumoles as modular synthetic precursors for new
alumole derivatives bearing functional groups on the aluminum moiety.
1-Bromoalumole 7 reacted with excess amount of 3-hexyne to afford a mixture of
hexaethylbenzene (12) and 1-bromoalumonin 13 (Scheme 5).15 Formation of 12 and 13 can be
explained in terms of the intermediate generation of 1-bromoalumepin 14. Reactions of 7 with
other unsaturated compounds are currently being investigated.
CONCLUSION
In conclusion, we have shown the synthesis and reactivity of stable alumole derivatives. Lewis
base-free alumole and gallole were obtained by taking advantage of the steric protection with
bulky Mes* group. These stable heavier group 13 heteroles reacted with lithium to afford the
lithium salt of the corresponding heterole dianions. By applying the same synthetic method as
used for the Mes*-substituted alumole, 1-chloroalumole and 1-bromoalumole were synthesized
as stable compounds. These 1-haloalumole derivatives dimerize in the solid state via halogen-
aluminum coordination interactions. The 1-bromoalumole reacted with various nucleophilic
reagents and 3-hexyne to afford the substituted alumole derivatives and the first alumonin
derivative, respectively. Such a variety of reaction modes of the 1-bromoalumole suggest the
usefulness of 1-haloalumoles for the development of new aluminum-containing cyclic
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compounds with unique structure and properties. Further studies on the reactivity of the 1-
haloalumoles are now in progress.
EXPERIMENTAL
1-Chloroalumole 6: A toluene (50 mL) solution of 3 (0.132 g, 0.741 mmol) was mixed with a
toluene (50 mL) solution of AlCl3 (0.199 g, 0.746 mmol) at room temperature, and the mixture
was stirred for 12 h. The solvent was removed under reduced pressure, and the residue was
dissolved in hexane and filtered. The filtrate was concentrated and stored at --35 ºC to afford 6 as
1
a colorless solid (0.128 g, 0.445 mmol, 60%). mp 158 ºC (dec.). H NMR (600 MHz, C6D6) 
[ppm] = 0.99 (t, J = 7.5 Hz, 6H, -CH2CH3), 1,20 (t, J = 7.5 Hz, 6H, -CH2CH3), 2.2-2.3 (broad,
8H, CH2CH3).
IPr-complex 9: To a toluene (10 mL) solution of 7 (44.2 mg, 0.163 mmol) IPr (64.2 mg, 0.165
mmol) was added at room temperature, and the mixture was stirred for 6 h. The solvent was
removed under reduced pressure, and the residue was recrystallized from hexane at --35 ºC to
afford 9 as a colorless solid (96.8 mg, 0.147 mmol, 90%). 1H NMR (300 MHz, C6D6)  [ppm] =
0.12-0.19 (m, 2H, -CH2CH3), 0.95 (d, J = 6.9 Hz, 12H, CH(CH3)2), 1.07-1.11 (m, 12H,
CH2CH3), 1.39 (d, J = 6.6 Hz, 12H, CH(CH3)2), 1.95-2.02 (m, 2H, -CH2CH3), 2.34-2.42 (m,
4H, -CH2CH3), 2.76-2.89 (m, 4H, CH(CH3)2), 6.33 (s, 2H, 4,5-CH of IPr), 7.09-7.12 (m, 4H),
7.19-7.26 (m, 2H).
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11. Crystallographic data for 1-chloroalumole 6: Empirical formula C24H40Al2Cl2, FW 453.42,
monoclinic, space group P21/n, a = 9.037(1) Å, b = 15.8043(1) Å, c = 18.6295(1) Å,  =
100.3693(3)º, V = 2617.05(2) Å3,  = 1.151 g cm-3, Z = 4,  = 0.323 mm--1, 1.70≤≤25.50,
reflections collected 70459, independent reflections 4608 Rint = 0.0524), completeness to
(25.24º) = 99.7%, GOF on F2 = 1.028, R1 [I>2(I)] = 0.0301, wR2 (all data) = 0.0836,
largest difference peak and hole 0.301 and --0.340 e Å--3. CCDC-1417355.
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monoclinic, space group P21/n, a = 10.6985(2) Å, b = 16.9795(3) Å, c = 20.8014(4) Å,  =
90.4499(10)º, V = 3778.57(12) Å3,  = 1.160 g cm-3, Z = 4,  = 1.138 mm--1, 1.55≤≤25.50,
reflections collected 40426, independent reflections 6928 Rint = 0.0545), completeness to
(25.24º) = 99.0%, GOF on F2 = 1.025, R1 [I>2(I)] = 0.0367, wR2 (all data) = 0.0965,
largest difference peak and hole 0.546 and --0.474 e Å--3. CCDC-1417291.
13. Agou, T.; Wasano, T.; Sasamori, T.; Tokitoh, N. Organometallics 2014, 33, 6963--6966.
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14. Agou, T.; Wasano, T.; Sasamori, T.; Guo, J.-D.; Nagase, S.; Tokitoh, N. Angew. Chem. Int.
Ed. 2015, 54, 9568--9571.
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Figure 1 Structure of alumole dianion 4 (50% probability level). Hydrogen atoms are omitted for
clarity
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Figure 2 Structure of gallole dianion 5 (50% probability level). Hydrogen atoms are omitted for
clarity
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Figure 3 Dimeric structure of 1-chloroalumole 6 (50% probability level). Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å): Al1-C1 1.952(1), Al1-C4 2.207(1), Al1-C8
2.042(1), Al1-Cl1 2.1549(5), C1-C2 1.350(2), C2-C3 1.498(2), C3-C4 1.370(2), Al2-C5
1.953(1), Al2-C8 2.209(1), Al2-C4 2.045(1), Al2-Cl2 2.1475(5), C5-C6 1.351(2), C6-C7
1.499(2), C7-C8 1.374(2)
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Figure 4 Dimeric structure of 1-bromoalumole 7 (50% probability level). Hydrogen atoms are
omitted for clarity. Two independent dimers were found in the crystal, and each dimer contains a
C2-axis at the center of the molecule. Selected bond lengths (Å): Al1-C1 1.944(3), Al1-C4
2.192(3), Al1-C4* 2.052(3), Al1-Br1 2.306(1), C1-C2 1.352(5), C2-C3 1.490(3), C3-C4
1.388(5), Al2-C5 1.948(3), Al2-C8 2.193(3), Al2-C8* 2.052(3), Al2-Br2 2.310(1) C5-C6
1.348(5), C6-C7 1.494(5), C7-C8 1.375(5)
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Figure 5 Structure of IPr complex 9 (50% probability level). Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å): Al1-Br1 2.3552(7), Al1-C1 1.967(2), Al1-C4 1.973(2), Al1-
C5 2.081(2), C1-C2 1.354(3), C2-C3 1.522(3), C3-C4 1.353(3)
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Scheme 1 Synthesis of Mes*-substituted alumole 1 and gallole 2
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Scheme 2 Synthesis of lithium salts of heterole dianions 4 and 5
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Scheme 3 Synthesis of 1-haloalumoles 6 and 7
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Scheme 4 Reactions of 1-bromoalumole 7 with nucleophilic reagents
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Scheme 5 Reaction of 1-bromoalumole 7 with 3-hexyne
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