Facile synthesis of cyclopropene analogues of aluminum and an aluminum pinacolate, and the reactivity of LA1[η2-C2(SiMe3)2] toward unsaturated molecules (L = HC[(CMe)(NAr)]2, Ar = 2,6-i-Pr2

Article abstract of DOI:10.1021/ja003185i

Reduction of LA1I₂ (1) (L = HC[(CMe)(NAr)]₂, Ar = 2,6-i-Pr₂C₆H₃) with potassium in the presence of alkynes C₂(SiMe₃)₂, C₂Ph₂, and C₂Ph

Full text of DOI:10.1021/ja003185i

J. Am. Chem. Soc. 2001, 123, 9091-9098
9091
Facile Synthesis of Cyclopropene Analogues of Aluminum and an
Aluminum Pinacolate, and the Reactivity of LAl[η2-C2(SiMe3)2]
toward Unsaturated Molecules (L ) HC[(CMe)(NAr)]2, Ar )
2,6-i-Pr2C6H3)
Chunming Cui,§ Sinje Ko1pke,§ Regine Herbst-Irmer,§ Herbert W. Roesky,*,§
Mathias Noltemeyer,§ Hans-Georg Schmidt,§ and Bernd Wrackmeyer‡
Contribution from the Institute of Inorganic Chemistry, UniVersity of Goettingen, Tammannstrasse 4,
D-37077 Goettingen, Germany, and Department of Chemistry, UniVersity of Bayreuth,
D-95447 Bayreuth, Germany
ReceiVed August 28, 2000
Abstract: Reduction of LAlI2 (1) (L ) HC[(CMe)(NAr)]2, Ar ) 2,6-i-Pr2C6H3) with potassium in the presence
of alkynes C2(SiMe3)2, C2Ph2, and C2Ph(SiMe3) yielded the first neutral cyclopropene analogues of aluminum
LAl[η2-C2(SiMe3)2] (3), LAl(η2-C2Ph2) (4), and LAl[η2-C2Ph(SiMe3)] (5), respectively, whereas reduction of
1 in the presence of Ph2CO gave an aluminum pinacolate LAl[O2(CPh2)2] (6), irrespective of the amount of
Ph2CO employed. The unsaturated molecules CO2, Ph2CO, and PhCN inserted into one of the Al-C bonds of
3 leading to ring enlargement to give novel aluminum five-membered heterocyclic systems LAl[OC(O)C2-
(SiMe3)2] (7), LAl[OC(Ph)2C2(SiMe3)2] (8), and LAl[NC(Ph)C2(SiMe3)2] (9) in high yields. In contrast, 3
reacted with t-BuCN, 2,6-Trip2C6H3N3 (Trip ) 2,4,6-i-Pr3C6H2), and Ph3SiN3 resulting in the displacement of
the alkyne moiety to afford LAl[N2(Ct-Bu)2] (10) with an unprecedented aluminum-containing imidazole ring,
and the first monomeric aluminum imides LAlNC6H3-2,6-Trip2 (11) and LAlNSiPh3 (12). All compounds
have been characterized spectroscopically. The variable-temperature 1H NMR studies of 3 and ESR
measurements of 3 and 4 suggest that the Al-C-C three-membered-ring systems can be best described as
metallacyclopropenes. The 27Al NMR resonances of 2 and 3 are reported and compared. Molecular structures
of compounds 3, 4, 6‚OEt2, 8‚OEt2, and 9 were determined by single-crystal X-ray structural analysis.
Introduction
has been centered only on that of boron.2,4 To the best of our
knowledge, stable neutral metallacyclopropenes of the heavier
congeners (Al, Ga, and In) have not yet been described in the
literature.
Heavier main group cyclopropene analogues are of great
interest in synthesis due to their high reactivity.1,2 Previous
studies on such species have centered on Group 14 compounds,
and the chemistry of such compounds, to a large extent, is
related to their carbene analogues R2M: (M ) Si, Ge, Sn). They
have been formed either by the reaction of R2M: with alkynes
or the reductive coupling of R2MX2 in the presence of alkynes.3
However, the chemistry of Group 13 cyclopropene analogues
The recent surge in the number of Group 13 species with a
low valent central atom has prompted us to design appropriate
ligand systems and synthetic methodologies to access novel
Group 13 species. We and others have successfully used the
bulky â-diketiminato ligand L (L ) HC[(CMe)(NAr)]2, Ar )
2,6-i-Pr2C6H3) for the preparation of the monomeric compound
LM (M ) Al (2) Ga), with the central atom being low valent
and two coordinated.5 Ab initio calculations indicate that the
monomeric Al(I) complex 2 possesses a nonbonded electron
pair, and is isoelectronic with a singlet carbene.5a Initial
examination of the two monovalent Al and Ga species showed
that they are highly reactive toward organic azides.6 We now
turn our attention to alkynes in the hope of preparing cyclo-
propene analogues of aluminum since alkynes are well-known
labile and effective trapping reagents for main group carbene
analogues.3 Another interesting approach is the reduction of
§ University of Goettingen.
‡ University of Bayreuth.
(1) Volpin, M. E.; Koreshkov, Yu. D.; Dolova, V. C.; Kursanov, D. N.
Tetrahedron 1962, 18, 107.
(2) (a) Eisch, J. J. In AdVances in Organic Chemistry; West, R. Y.; Stone,
F. G. A., Eds.; Academic Press: New York, 1977; Vol. XVI, p 67. (b) van
der Kerk, S. M.; Budzelaar, P. H. M.; van der Kerk-van Hoof, A.; van der
Kerk, G. J. M.; Schleyer, P. v. R. Angew. Chem. 1983, 95, 61; Angew.
Chem., Int. Ed. Engl. 1983, 22, 48. (c) Habben, C.; Meller, A. Chem. Ber.
1984, 117, 2531. (d) Berndt, A.; Pues, C. Angew. Chem. 1984, 96, 306;
Angew. Chem., Int. Ed. Engl. 1984, 23, 313 (e) Eisch, J. J.; Shafii, B.;
Rheingold, A. L. J. Am. Chem. Soc. 1987, 109, 2526. (f) Eisch, J. J.; Shafii,
B.; Odom, J. D.; Rheingold, A. L. J. Am. Chem. Soc. 1990, 112, 1847.
(3) Examples of Group 14 cyclopropene analogues, see: (a) Egorov,
M. P.; Kolesnikov, S. P.; Struchkov, Yu. T.; Antipin, M. Yu.; Sereda, S.
V.; Nefedov, O. M. J. Organomet. Chem. 1985, 290, C27. (b) Sita, R. L.;
Bicherstaff, R. D. J. Am. Chem. Soc. 1988, 110, 5208. (c) Pea, D. H.; Xiao,
M.; Chiang, M. Y.; Gaspar, P. P. J. Am. Chem. Soc. 1991, 113, 1281. (d)
Tokitoh, N.; Matsumoto, T.; Okazaki, R. J. Am. Chem. Soc. 1997, 119,
2337. (e) Sakamato, K.; Tsutsui, S.; Sakura, H.; Kira, M. Bull. Chem. Soc.
Jpn. 1997, 70, 251.
(4) Kropp, M. A.; Schuster, G. B. J. Am. Chem. Soc. 1989, 111, 2316.
(5) (a) Cui, C.; Roesky, H. W.; Schmidt, H.-G.; Noltemeyer, M.; Hao,
H.; Cimpoesu, F. Angew. Chem. 2000, 112, 4444; Angew. Chem., Int. Ed.
2000. 39, 4274. (b) Hardman, N. J.; Eichler, B. E.; Power, P. P. Chem.
Commun. 2000, 1991.
(6) (a) Cui, C.; Roesky, H. W.; Schmidt, H.-G.; Noltemeyer, M. Angew.
Chem. 2000, 112, 4705; Angew. Chem., Int. Ed. 2000, 39, 4531. (b)
Hardman, N. J.; Cui, C.; Roesky, H. W.; Fink, W. H.; Power, P. P. Angew.
Chem. 2001, 113, 2230; Angew. Chem., Int. Ed. 2001, 40, 2172.
10.1021/ja003185i CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/22/2001

9092 J. Am. Chem. Soc., Vol. 123, No. 37, 2001
Cui et al.
LAlI2 (1)5a in the presence of alkynes for trapping reduction
transients and generating unusual aluminum compounds. The
comparison between two reactions would be useful in under-
standing the reductive coupling mechanism.
X-ray Structure Determinations and Refinements. Data for crystal
structures of 3 and 6‚OEt2 were collected on a STOE-AED2 four-circle
diffractometer and data for the structures of 4, 8‚OEt2, and 9 were
collected on a Stoe-Siemens-Huber four circle diffractometer equipped
with a CCD area detector using Mo KR radiation (λ ) 0.71073 Å).
All structures were solved by direct methods (SHELXS-96)15 and
refined against F2 using SHELXL-97.16 All heavy atoms were refined
anisotropically. Hydrogen atoms were included using the riding model
with Uiso tied to the Uiso of the parent atom. The Et2O molecule in
6‚OEt2 was disordered over two positions. It was refined with distance
restraints and restraints for the anisotropic displacement parameters.
The structure of 4 was refined as a pseudomerohedral twin. The twin
law is [-100 010 001]. The fractional contribution of the second domain
refines to 0.4511(8). Additionally the structure shows pseudosymmetry.
A great number of the atoms fulfill the symmetry of space group P21/
n. But refinement in this space group was much worse even if disorder
was modeled. Crystal data, data collection details, structural solution
and refinement procedures for all compounds are summarized in Table
1.
The reactions of aluminum compounds (alkyls, hydrides, and
halides) with alkynes have been explored extensively.7 The
resulting aluminum vinyl derivatives undergo interesting thermal
and photoinduced rearrangements and have proved to be
valuable precursors for organic chain growing reactions.8
Aluminum cyclopropene analogues have been proposed as
reactive intermediates in the formation of the related 1,4-
(dialumina)cyclohexadienes via dimerization9 and in the reaction
of unstable Al(I) compounds with alkynes.10 We reasoned that
such strained ring systems would show high chemical reactivities
and unique reaction patterns. Herein, we report on the synthesis
of cyclopropene analogues of aluminum and an aluminum
pinacolate by reductive coupling reaction in the presence of
alkynes and benzophenone, and the reaction of a cyclopropene
analogue of aluminum LAl[η2-C2(SiMe3)2] (3) with CO2,
Ph2CO, nitriles PhCN and t-BuCN, and bulky azides Ph3SiN3
and 2,6-Trip2C6H3N3 (Trip ) 2,4,6-i-Pr3C6H2).
Synthesis of LAl[η2-C2(SiMe3)2] (3). A solution of LAlI2 (1)5a (1.40
g, 2 mmol) and bis(trimethylsilyl)alkyne (0.34 g, 2 mmol) in toluene
(20 mL) was added to a suspension of finely divided potassium (0.16
g, 4.1 mmol) at room temperature. The mixture was stirred at room
temperature for 2 d. The solution developed a red-black color and all
the potassium appeared consumed. Subsequently all volatiles were
removed under vacuum, and the residue was extracted with n-hexane
(20 mL). After filtration the red black filtrate was concentrated (ca 6
mL) and stored at -30 °C overnight affording red-black crystals of 3
(0.68 g, 55.4%). Mp: 182-184 °C. 1H NMR (d8-toluene, 298 K, 500.13
MHz): δ 6.97-7.09 (m, 6 H, Ar-H), 4.78 (s, 1 H, γ-CH), 3.33 (sept,
4 H, J ) 6.8 Hz, CHMe2), 1.43 (s, 6 H, Me), 1.42 (d, 12 H, J ) 6.8
Hz, CHMe2), 1.11 (d, 12 H, J ) 6.8 Hz, CHMe2), 0.10 (s, 18 H, SiMe3).
13C NMR (C6D6, 100.60 MHz): δ 228.5 (Al-C), 174.1(CN), 143.5,
140.4, 124.3 (Ph), 97.2 (γ-C), 28.7 (CHMe2), 25.6, 24.1 (CHMe2), 24.0
(Me), 1.8 (SiMe3). 27Al NMR (d8-toluene, 130.32 MHz): δ 90 ( 5
(ν1/2 ≈ 2500 Hz). 29Si NMR (C6D6, 79.46 MHz): δ -24.7.
Experimental Section
General Procedures. All experiments were carried out under an
atmosphere of dry nitrogen or argon using Schlenk techniques or inside
a MBraun MB 150-GI glovebox filled with dry nitrogen where the O2
and H2O level were strictly controlled below 1 ppm. All solvents were
dried using standard methods prior to use.11 The samples for analytical
measurements were prepared inside the glovebox. Commercially
available chemicals were purchased from Fluka or Aldrich and used
as received. The other compounds used in this paper were prepared
according to published procedures: H2C[(CMe)(NAr)]2 (Ar ) 2,6-i-
Pr2C6H3),12 Ph3SiN3,13 and 2,6-Trip2C6H3N3 (Trip ) 2,4,6-i-Pr3C6H2).14
Physical Measurements and Analysis. The melting points of all
compounds described in this paper were measured on a Bu¨hler SPA-1
apparatus in sealed capillaries and uncorrected.1H, 13C, 27Al, and 29Si
NMR spectra were recorded on Avance-500, Avance-200, Bruker MSL-
400, AM-250, and AM-200 instruments. The chemical shifts are
reported in ppm with reference to external standards, namely SiMe4
Synthesis of LAl(η2-C2Ph2) (4). Compound 4 was prepared in a
similar manner as 3. LAlI2 (1) (1.4 g, 2 mmol), diphenylalkyne (0.36
g, 2 mmol), and potassium (0.16 g, 4.1 mmol) were used. After filtration
and partial removal of the solvents, the solution was stored at -30 °C
overnight affording orange crystals of 4 (0.86 g, 70%). Mp: 260 °C
1
(dec). H NMR (C6D6, 200.13 MHz): δ 6.8-7.15 (m, 16 H, Ar-H),
1
for H, 13C, and 29Si nuclei and 1 M aqueous AlCl3 for 27Al NMR.
4.89 (s, 1 H, γ-CH), 3.39 (sept, 4 H, J ) 6.8 Hz, CHMe2), 1.53 (s, 6
H, Me), 1.21 (d, 12 H, J ) 6.8 Hz, CHMe2), 1.11 (d, 12 H, J ) 6.8
Hz, CHMe2). 13C NMR (C6D6, 125.76 MHz): δ 177.2 (Al-C), 173.1
(CN), 144.2, 144.0, 139.2, 131.9, 128.6, 128.4, 124.5, 124.0 (Ph), 97.3
(γ-C), 29.1 (CHMe2), 25.0, 24.4 (CHMe2), 23.5 (Me).
Heteroatomic spectra were recorded 1H-decoupled. If not otherwise
stated, the operation temperature was in the range from 293 to 300 K.
ESR measurements were performed on a Varian (9.6 MHz) instrument.
UV spectra were recorded on a PERKIN ELMER 320 instrument.
Synthesis of LAl[η2-C2Ph(SiMe3)] (5). Compound 5 was prepared
similarly to 3: LAlI2 (1) (1.4 g, 2 mmol), 1-phenyl-2-(trimethylsilyl)-
alkyne (0.35 g, 2 mmol), and potassium (0.16 g, 4.1 mmol) were
employed. Crystallization from toluene at -30 °C gave orange crystals
of 5 (0.74 g, 60%). Mp: 242 °C. 1H NMR (d8-toluene, 200.13 MHz):
δ 7.06-7.12 (m, 11 H, Ar-H), 4.87 (s, γ-CH), 3.36, 3.21 (dsept, 2 H,
J ) 6.8 Hz, CHMe2), 1.53 (s, 6 H, Me), 1.45 (d, 6 H, J ) 6.8 Hz,
CHMe2), 1.13 (d, 6 H, J ) 6.7 Hz, CHMe2), 1.12 (d, 6 H, J ) 6.8 Hz,
CHMe2), 1.08 (d, 6 H, J ) 6.8 Hz, CHMe2), -0.04 (s, 9 H, SiMe3).
13C NMR (d8-toluene, 125.76 MHz): δ 211.9 (Al-C), 187.2 (Al-C),
173.2 (CN), 148.9, 144.2, 143.6, 139.6, 137.5, 128.8, 128.1, 125.3,
125.1, 124.9, 123.6 (Ph), 97.3 (γ-C), 29.1, 28.9 (CHMe2), 25.4, 24.7,
24.5, 24.1 (CHMe2), 23.6 (Me), 2.2 (SiMe3). 29Si NMR (d8-toluene,
99.36 MHz): δ -20.5.
Synthesis of LAl[O2(CPh2)2] (6). A solution of LAlI2 (1) (1.4 g, 2
mmol) and Ph2CO (0.36 g, 2 mmol) in toluene (15 mL) was added to
a suspension of finely divided potassium (0.16 g, 4.1 mmol). Im-
mediately a purple color was observed. The mixture was stirred at room
temperature for 24 h and the solution finally developed a red color.
After filtration the filtrate was concentrated and stored at -30 °C
overnight to afford yellow crystals of 6 (0.56 g, 35%). Mp: 272 °C.
(7) For examples: (a) Scha¨fer, W. Angew. Chem. 1966, 78, 716; Angew.
Chem., Int. Ed. Engl. 1966, 5, 669. (b) Eisch, J. J.; Harrell, R. L., Jr. J.
Organomet. Chem. 1969, 20, 257. (c) Clark, G. M.; Zweifel, G. J. Am.
Chem. Soc. 1971, 93, 527. (d) Albright, M. J.; Butler, W. M.; Anderson, T.
J.; Glick, M. D.; Oliver, J. P. J. Am. Chem. Soc. 1976, 98, 3995. (e) Hoberg,
H.; Gotor, V. J. Organomet. Chem. 1977, 127, C32. (d) Hoberg, H.; Aznar,
F. J. Organomet. Chem. 1980, 193, 161.
(8) (a) Eisch, J. J.; Amtmann, R. J. Org. Chem. 1972, 37, 3410. (b)
Stucky, G. D.; McPherson, A. M.; Rhine, W. E.; Eisch, J. J.; Considine, J.
L. J. Am. Chem. Soc. 1974, 96, 1941. (c) Hoberg, H.; Aznar, F. J.
Organomet. Chem. 1978, 141, 1. (d) Hoberg, H.; Aznar, F. J. Organomet.
Chem. 1980, 193, 155.
(9) (a) Hoberg, H.; Gotor, V.; Milchereit, A.; Kru¨ger, C.; Sekutowski,
J. C. Angew. Chem. 1977, 89, 563; Angew. Chem., Int. Ed. Engl. 1977, 16,
539. (b) Hoberg, H.; Aznar, F. J. Organomet. Chem. 1979, 164, C13.
(10) (a) Schno¨ckel, H.; Leimku¨hler, M.; Lotz, R.; Mattes, R. Angew.
Chem. 1986, 98, 929; Angew. Chem., Int. Ed. Engl. 1986, 25, 921. (b)
U¨ ffing, C.; Ecker, A.; Ko¨ppe, R.; Merzweiler, K.; Schno¨ckel, H. Chem.
Eur. J. 1998, 4, 2142.
(11) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon: London, 1988.
(12) (a) Qian, B.; Ward, D. L.; Smith, M. R., III Organometallics 1998,
17, 3070. (b) Budzelaar, D. H. M.; van Oort, A. B.; Orpen, A. G. Eur. J.
Inorg. Chem. 1998, 1485.
(13) Schulz, S.; Voigt, A.; Roesky, H. W.; Ha¨ming, L.; Herbst-Irmer,
R. Organometallics 1996, 15, 5252.
(14) Twamley, B.; Hwang, C.-S.; Hardman, N. J.; Power, P. P. J.
Organomet. Chem. 2000, 609, 152.
(15) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.
(16) Sheldrick, G. M. SHELXL, Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.

Facile Synthesis of Cyclopropene Analogues of Aluminum
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9093
Table 1. Crystallographic Data for Compounds 3, 4, 6·OEt2, 8·OEt2, and 9
3
4
6‚OEt2
8‚OEt2
9
formula
formula weight
temp (K)
C37H59AlN2Si2
615.02
200(2)
C43H51AlN2
622.84
133(2)
C59H71AlN2O3
883.16
203(2)
C54H79AlN2O2Si2
871.35
133(2)
C44H64AlN3Si2
718.14
133(2)
cryst color
cryst syst
space group
a, Å
b, Å
c, Å
red-black
triclinic
P1h
yellow
monoclinic
P21
18.785(3)
20.832(4)
18.823(3)
90
90.08(3)
90
7366(2)
8
1.123
0.086
2688
yellow
monoclinic
P21/c
22.027(4)
11.464(2)
20.857(4)
90
105.02(2)
90
5086.8(16)
4
1.153
0.086
1904
colorless
orthorhombic
P212121
12.767(3)
17.086(3)
23.583(5)
90
orange
triclinic
P1h
11.160(4)
11.978(5)
16.460(7)
85.68(3)
80.20(2)
64.11(2)
1950.6(14)
2
11.445(2)
12.591(3)
16.521(3)
106.30(3)
90.82(3)
108.90(3)
2147.3(7)
2
R, deg
â, deg
90
90
γ, deg
V, Å3
5144.3(19)
4
1.125
0.126
1896
Z
d(calcd), mg/m3
abs coeff, mm-1
F(000)
1.047
0.138
672
1.111
0.135
780
cryst size, mm
θ range, deg
no. of reflns collected
no. of indep. reflns
Rint
1.0 × 0.6 × 0.3
3.51 to 25.12
7806
6894
0.0708
0.7 × 0.4 × 0.15
2.23 to 27.50
143864
32699
0.0839
0.8 × 0.6 × 0.5
3.52 to 25.03
12230
8973
0.0434
0.4 × 0.3 × 0.2
2.10 to 28.02
109910
12113
0.0710
0.5 × 0.3 × 0.3
2.17 to 27.76
58404
9997
0.0519
GOF/F2
1.033
1.028
1.034
1.053
1.065
R indices [I > 2σ (I)]
R1 ) 0.0698
wR2 ) 0.1853
R1 ) 0.0849
wR2 ) 0.2049
R1 ) 0.0506
wR2 ) 0.1045
R1 ) 0.0780
wR2 ) 0.1177
0.00176(12)
0.249-0.270
R1 ) 0.0476
wR2 ) 0.1015
R1 ) 0.0710
wR2 ) 0.1161
R1 ) 0.0395
wR2 ) 0.0856
R1 ) 0.0486
wR2 ) 0.0897
R1 ) 0.0437
wR2 ) 0.1030
R1 ) 0.0548
wR2 ) 0.1084
(all data)
extinction coeff
largest diff. peak and hole, eÅ-3
0.491-0.490
0.295-0.245
0.241-0.229
0.394-0.308
1H NMR (C6D6, 200.13 MHz): δ 7.01-6.58 (m, 26 H, Ar-H), 5.04
(s, 1 H, γ-CH), 3.55 (sept, 4 H, CHMe2), 1.39 (s, 6 H, Me), 1.11 (d,
12 H, J ) 6.9 Hz, CHMe2), 1.07 (d, 12 H, J ) 6.8 Hz, CHMe2). 13C
NMR (C6D6, 125.76 MHz): δ 174.2 (CN), 149.6, 143.9, 142.5, 130.4,
129.3, 128.5, 126.1, 125.6, 124.9, 124.8 (Ph), 99.3 (γ-C), 92.7 (CO),
28.8 (CHMe2), 25.2, 24.6, 24.0 (CHMe2), 23.1 (Me).
Synthesis of LAl[OC(O)C2(SiMe3)2] (7). When a solution of 3 (0.31
g, 0.5 mmol) in n-hexane (15 mL) was exposed to dry CO2 at room
temperature, the red-black color changed to yellow within 10 min. The
solvent was removed under vacuum to give a slightly yellow powder,
which was subsequently washed with cold pentane (5 mL) to give
analytically pure 7 (0.30 g, 91%). Mp: 258-260 °C. 1H NMR (C6D6,
200.13 MHz): δ 7.03 (m, 6 H, Ar-H), 5.11 (s, 1 H, γ-CH), 3.10,
3.35 (sept, 4 H, J ) 6.8 Hz, CHMe2), 1.51 (s, 6 H, Me), 1.23 (dd, 12
H, J ) 6.8 Hz, CHMe2), 1.02 (dd, 12 H, J ) 6.8 Hz, CHMe2), 0.49,
-0.09 (s, 9 H, SiMe3). 13C NMR (C6D6, 125.76 MHz): δ 178.1 (â-
CSiMe3), 172.9 (CN), 161.5 (CO), 145.4, 143.1, 142.8, 139.8, 125.8,
125.2, 124.2, 123.6 (Ph), 100.7 (γ-C), 29.0, 28.6, 28.1, 25.6, 24.9, 24.7,
24.5, 24.4, 23.8, 23.4 (CHMe2 and Me), 2.37, 2.29 (SiMe3). 29Si NMR
(C6D6, 99.36 MHz): δ -7.9, -12.0 (SiMe3).
Ar-H), 7.15-7.05 (m, 7 H, Ar-H), 5.27 (s, 1 H, γ-CH), 3.25 (sept,
4 H, J ) 6.8 Hz, CHMe2), 1.60 (s, 6 H, Me), 1.34 (d, 6 H, J ) 6.7 Hz,
CHMe2), 1.31 (d, 6 H, J ) 6.8 Hz, CHMe2), 1.13 (d, 6 H, J ) 6.7 Hz,
CHMe2), 1.06 (d, 6 H, J ) 6.7 Hz, CHMe2), 0.09 (s, 9 H, SiMe3), 0.03
(s, 9 H, SiMe3). 13C NMR (C6D6, 125.76 MHz): δ 183.8 (Al-C), 181.5
(â-C), 177.3 (PhCN), 172.4 (CN), 149.7, 145.2, 143.5, 141.3, 127.4,
127.3, 127.2, 124.8, 124.4 (Ph), 102.0 (γ-C), 28.7, 28.2 (CHMe2), 25.9,
25.1, 25.0 (CHMe2), 24.0 (Me), 3.4, 3.2 (SiMe3). 29Si NMR (C6D6,
99.36 MHz): δ -10.3, -11.8.
Synthesis of LAl[N2(Ct-Bu)2] (10). To a solution of 3 (0.31 g, 0.5
mmol) in diethyl ether (20 mL) was added t-BuCN (0.042 g, 0.5 mmol)
at room temperature. The black-red color disappeared within 2 h. The
solution was stirred for an additional 4 h. The solvents were partially
removed (ca. 10 mL) and the solution was stored at -5 °C for 2 days
1
to give colorless crystals (0.10 g, 42%). Mp: 249-250 °C. H NMR
(C6D6, 200.13 MHz): δ 7.01 (m, 6 H, Ar-H), 5.08 (s, 1 H, γ-CH),
3.42 (sept, 4 H, J ) 6.8 Hz, CHMe2), 1.58 (s, 6 H, Me), 1.47 (d, 12 H,
J ) 6.7 Hz, CHMe2), 1.25 (br s, 18 H, t-Bu), 1.06 (12 H, J ) 6.8 Hz,
CHMe2). 13C NMR (C6D6, 125.76 MHz): δ 193.6 (NCt-Bu), 170.6,
170.5 (CN), 144.9, 143.6, 143.3, 142.1, 141.9, 127.3, 127.2, 124.9,
124.5 (Ph), 97.8 (γ-C), 41.4, 40.8 (CMe3), 29.9, 29.5 (CHMe2), 29.3,
29.1, 29.0, 28.9, 27.8 (CHMe2), 25.5 (Me), 24.9, 24.8, 24.5, 24.0, 23.8
(CMe3).
Synthesis of LAl[OC(Ph)2C2(SiMe3)2] (8). To a mixture of 3 (0.61
g, 1 mmol) and Ph2CO (0.18 g, 1 mmol) was added diethyl ether (20
mL). The mixture was stirred at room temperature for 30 min. The
solution was concentrated and stored at -30 °C overnight to give
Synthesis of LAlNC6H3-2,6-Trip2 (11)6b and LAlNSiPh3 (12).
Method A: To a mixture of {HC[(CMe)(NAr)]2}Al (2) (0.22 g, 0.5
mmol) and 2,6-Trip2C6H3N3 or Ph3SiN3 (0.15 g, 0.5 mmol) was added
toluene (15 mL) at -60 °C. The mixture was allowed to warm to room
temperature and a slow color change from orange red to yellow was
observed during 1 h. After being stirred at room temperature for an
additional 3 h, the solution was concentrated (ca. 5 mL) and stored at
room temperature for 2 days to afford the pale yellow solid of 11 (0.26
g, 52%) or the white solid of 12 (0.28 g, 80%), respectively.
Method B: To a mixture of 3 (0.31 g, 0.5 mmol) and equal amounts
of the azides used in method A was added toluene (15 mL) at 0 °C.
The mixture was allowed to warm to room temperature within 1 h.
During this period, the solution developed a slightly yellow color. After
the solution was stirred for additional 1 h, all volatiles were removed.
The residue was crystallized from toluene to give pure 11 and 12 in
ca. 80% yield, respectively. Data for 12: Mp: 203 °C. 1H NMR (CDCl3,
200.13 MHz): δ 7.32-6.88 (m, 21 H, Ar-H), 4.18 (s, 1 H, γ-CH),
1
colorless crystals of 8 (0.59 g, 74%). Mp: 215 °C. H NMR (C6D6,
200.13 MHz): δ 7.70 (m, 2 H, Ar-H), 7.07-6.84 (m, 14 H, Ar-H),
4.91 (s, 1 H, γ-CH), 3.20 (sept, 4 H, J ) 6.8 Hz, CHMe2), 1.38 (s, 6
H, Me), 1.33 (d, 6 H, J ) 6.8 Hz, CHMe2), 1.20 (d, 6 H, J ) 6.8 Hz,
CHMe2), 1.12 (d, 6 H, J ) 6.7 Hz, CHMe2), 1.02 (d, 6 H, J ) 6.8 Hz,
CHMe2), 0.64 (s, 9 H, SiMe3), 0.01 (s, 9 H, SiMe3). 13C NMR (C6D6,
125.8 MHz): δ 195.5 (Al-C), 190.8 (â-C), 171.5 (CN), 148.5, 144.7,
143.5, 141.8, 132.0, 130.2, 130.0, 127.3, 126.6, 125.6, 125.2, 124.3
(Ph), 99.9 (γ-C), 90.1 (CO), 28.7, 28.6 (CHMe2), 25.3, 25.2, 25.1, 24.7
(CHMe2), 24.4 (Me), 5.7, 4.1 (SiMe3).
Synthesis of LAl [NC(Ph)C2(SiMe3)2] (9). To a solution of 3 (0.31
g, 0.5 mmol) in diethyl ether (20 mL) was added neat PhCN (0.052 g,
0.5 mmol) at room temperature, and the mixture was stirred at room
temperature overnight. The solution was concentrated (ca. 5 mL) and
stored at -30 °C overnight to give orange crystals of 9 (0.22 g, 61%).
1
Mp: 235-236 °C. H NMR (C6D6, 200.13 MHz): δ 7.65 (m, 2 H,

9094 J. Am. Chem. Soc., Vol. 123, No. 37, 2001
Cui et al.
3.40 (sept, 2 H, CHMe2), 2.35 (sept, 2 H, CHMe2), 1.47 (s, 6 H, Me),
0.85 (d, 6 H, J ) 6.8 Hz, CHMe2), 0.68 (d, 6 H, J ) 6.8 Hz, CHMe2),
0.56 (d, 6 H, J ) 6.8 Hz, CHMe2), 0.49 (d, 6 H, J ) 6.8 Hz, CHMe2).
13C NMR (CDCl3, 125.76 MHz): δ 172.0 (CN), 145.3, 143.6, 140, 4,
137.1, 136.5, 128.4, 127.3, 125.4, 124.8 (Ph), 102.3 (γ-C), 28.4, 28.1
(CHMe2), 25.4 (Me), 24.8, 24.7, 24.6, 24.5 (CHMe2).
be involved in the partial reduction of 1 to give the transient
radical A, which couples with alkynes. Another possible
mechanism is that the radical anion C, formed by electron
transfer from K to the alkyne, substitutes the iodide in 1. Both
mechanisms would generate the same intermediate LAlI(RCCR)•-
(B), which undergoes a further electron-transfer reaction to give
the desired products. However, we were unable to monitor the
reaction under vigorous stirring due to its heterogeneous nature.
Results and Discussion
Reduction of LAlI2 (1) in the Presence of Alkynes and
Ph2CO. The diiodide LAlI2 (1) (L ) HC[(CMe)(NAr)]2, Ar )
2,6-i-Pr2C6H3) is easily accessible in high yield through the
To make a comparison, we performed the reduction of 1 in
the presence of Ph2CO since Ph2CO can be rapidly reduced by
Na and K at room temperature to give the ketyl M+(OCPh2)•-(M
) Na, K) with a typical deep blue color. When a solution of
benzophenone was added to a suspension of 1 and potassium
in toluene, immediately a deep blue color was formed, indicating
the formation of the ketyl K+(OCPh2)•-.17 The deep blue color
gradually changed to purple, then red over a period of 6 h. This
reaction in the presence of 1 or 2 equiv of benzophenone yielded
the aluminum pinacolate {HC[(CMe)(NAr)]2}Al[O2(CPh2)2] (6)
(Scheme 1). Therefore it is assumed that this reaction mainly
proceeds through the initial replacement of the iodide by the
ketyl to form the radical LAl(OCPh2)•, which could easily couple
with another ketyl, followed by elimination of KI, to yield 6.
The facile formation of 6 instead of an aluminum η2-ketone
complex LAl(η2-OCPh2) also supports this mechanism. More-
over the reaction implies that in the former reaction RCCR
should not be as easily and rapidly reduced as Ph2CO in toluene,
otherwise alkyne coupling would be observed. Therefore it
seems reasonable to assume that the former reaction mainly
proceeds through transient A. This mechanism was further
supported by the following experiment: in a NMR tube,
compounds 1, 2, and alkyne PhCCPh were dissolved in
12a
reaction of LAlMe2 with 2 equiv of I2 in toluene. 1 can be
reduced at room temperature to afford the first two coordinated
aluminum compound LAl (2),5a which is not paramagnetic as
disclosed by an ESR study. Ab initio calculations show that
the electronic structure of 2 is carbene-like. Unlike reactive
carbenes, 2 does not undergo a [1 + 2] cycloaddition with bis-
(trimethysilyl)alkyne at room temperature. However, as 1 can
be easily reduced, we reasoned that the reductive coupling
reactions of 1 in the presence of selected alkynes may be an
alternative route to access unique species which otherwise are
not easily available. As expected, reduction of 1 with potassium
in the presence of bis(trimethylsilyl)alkyne, diphenylalkyne, and
1-phenyl-2-trimethylsilylalkyne at room temperature afforded
LAl[η2-C2(SiMe3)2] (3), LAl(η2-C2Ph2) (4), and LAl[η2-C2Ph-
(SiMe3)] (5) in modest yields, respectively (Scheme 1). Com-
pound 3 was isolated as red-black crystals, soluble in hydro-
carbon solvents, and extremely air- and moisture-sensitive as
indicated by the immediate color change from red-black to
yellow when exposed to air. Yellow crystals of 4 and orange-
red crystals of 5 were obtained from toluene and are sparingly
soluble in n-hexane.
1
d8-toluene at room temperature; the H NMR spectra indicate
the formation of compound 4 in this system within 2 h. We
reasoned that the radical A could be easily generated in this
system by the reaction of 1 and 2. It was trapped by the alkyne
to generate B, which could be further oxidized by low-valent
species existing in the system.
Scheme 1
Compounds 3-6 have been characterized by EI-mass,1H, 13
C
NMR, and IR spectra. The EI-MS spectrum of 3 shows the (M+
- SiMe3) ion as the fragment with the highest mass, while those
of 4, 5, and 6 give the parent ions. The 13C NMR spectrum of
3 shows a weak resonance at δ 228.5 ppm, corresponding to
the coordinated CdC group. In the 13C NMR spectrum of 4
the corresponding resonance appears at δ 177.2 ppm, and in
that of 5 two resonances (δ 211.9, 187.2 ppm) for the carbon
atoms of the coordinated CdC group were observed. The IR
spectra for the three compounds all show a characteristic
absorption band centering at 1590 cm-1, which could be
assigned to the stretching frequencies of the coordinated CdC
groups.
Scheme 2
The variable-temperature 1H NMR investigations of 3 (193-
373 K, see Supporting Information) show that 3 does not
dissociate up to 373 K, indicating that the CdC moiety is
strongly bonded to the aluminum atom of 3 and the Al-C-C
ring is intact in toluene solution even at high temperature. This
result suggests that the compound can be described as rather a
metallocyclopropene (Al(III)) than an aluminum alkyne complex
(π complexation). The assumption is also supported by ESR
studies of compounds 3 and 4, in whose spectra no resonances
were observed for both compounds. Furthermore, the 27Al NMR
spectrum of 3 shows a resonance (90 ppm) in the range of those
The inertness of 2 to alkynes at room temperature suggests
that the reductive coupling reactions should not proceed through
a [1 + 2] cycloaddition. Two possible mechanisms may be
assumed for the reductive coupling reaction (Scheme 2) because
both (LAlI)• (A) and K+(RCCR)•- (C) could be generated under
the reducing conditions considering the long period of the
reductive coupling reaction (2-3 d). One of the species may
(17) Carey, F. A.; Sundberg, R. J. AdVanced Organic Chemistry, Part
A: Structure and Mechanisms; Plenum Press: New York and London, 1991;
p 668.

Facile Synthesis of Cyclopropene Analogues of Aluminum
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9095
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Compounds 3, 4, and 6·OEt2
3
Al(1)-N(1)
C(6)-C(7)
Al(1)-C(7)
1.889(2)
1.382(4)
1.908(3)
Al(1)-C(6)
Al(1)-N(2)
1.899(3)
1.892(2)
C(6)-Al(1)-C(7)
Al(1)-C(7)-C(6)
N(1)-Al(1)-C(6)
N(2)-Al(1)-C(6)
C(7)-C(6)-Si(2)
Si(1)-C(7)-Al(1)
42.57(11) Al(1)-C(6)-C(7)
68.39(15) N(1)-Al(1)-N(2)
126.85(11) N(1)-Al(1)-C(7)
128.25(11) N(2)-Al(1)-C(7)
69.04(15)
97.33(10)
124.63(11)
132.17(11)
133.7(2)
134.0(2)
C(6)-C(7)-Si(1)
156.35(16) Si(2)-C(6)-Al(1)
156.73(16)
4
Al(1)-N(1A)
Al(1)-C(6A)
C(6A)-C(7A)
1.885(3)
1.889(4)
1.356(5)
Al(1)-N(2A)
Al(1)-C(7A)
1.875(4)
1.894(3)
C(6A)-Al(1)-C(7A) 42.02(14) C(7A)-C(6A)-Al(1)
C(6A)-C(7A)-Al(1) 68.80(19) N(1A)-Al(1)-C(6A)
69.2(2)
126.98(15)
Figure 1. ORTEP drawing of 3 (50% thermal ellipsoids). Hydrogen
atoms have been omitted for clarity.
N(1A)-Al(1)-C(7A) 124.09(15) N(2A)-Al(1)-N(1A) 97.03(13)
N(2A)-Al(1)-C(6A) 130.52(15) Al(1)-C(7A)-C(70A) 160.9(3)
N(2A)-Al(1)-C(7A) 131.08(15) C(7A)-C(6A)-C(60A) 130.4(3)
Al(1)-C(6A)-C(60A) 160.0(2)
C(6A)-C(7A)-C(70A) 129.8(3)
6‚OEt2
1.7324(14) Al(1)-O(2)
Al(1)-O(1)
Al(1)-N(1)
O(1)-C(6)
C(6)-C(7)
1.7355(14)
1.9038(16)
1.427(2)
1.8978(17) Al(1)-N(2)
1.430(2)
1.670(3)
O(2)-C(7)
N(1)-Al(1)-N(2)
O(1)-Al(1)-N(1)
O(1)-C(6)-C(7)
O(2)-Al(1)-N(2)
C(6)-O(1)-Al(1)
96.91(7)
O(1)-Al(1)-O(2)
92.70(6)
124.68(7) O(1)-Al(1)-N(2)
106.75(15) O(2)-Al(1)-N(1)
123.09(7) O(2)-C(7)-C(6)
110.56(7)
111.39(7)
106.03(14)
116.08(11)
115.73
C(7)-O(2)-Al(1)
(av 1.96 Å).12a The C(6)-C(7) bond length (1.382(4) Å) in 3
is consistent with double bond character. The C(6)-Al-C(7)
angle (42.57(11)°) is very acute. The Al(1) and C(3) atoms lie
out of the N(1)-C(1)-C(4)-N(2) plane (mean deviation of the
plane ) 0.006 Å) by 0.37 and 0.11 Å, respectively. The two
fused planes N(1)-Al(1)-N(2) and C(6)-Al(1)-C(7) (angle
94.3°) are arranged nearly perpendicular to each other. Com-
pound 4 crystallizes in the monoclinic space group P21. The
asymmetric unit contains four molecules only marginally
different in bond lengths and angles. Figure 2 shows one of the
four independent molecules. The structure of 4 is comparable
to that of 3.
The molecular structure of compound 6‚OEt2 also has been
determined by single-crystal X-ray structural analysis. The
structure is shown in Figure 3, and selected bond distances and
angles are tabulated in Table 2. The aluminum atom is four
coordinated and acts as the center atom for the two fused rings.
The Al-N distances (av 1.90 Å) are similar to those of
LAl(SeH)2 (av 1.90 Å)19 and LAl[(NSiMe3)2N2] (av 1.89 Å).6a
It is noteworthy that the C(6)-C(7) bond length (1.670(3) Å)
is longer by 0.13 Å compared to normal C-C single bond
lengths (ca. 1.54 Å)20 due to the electronic and steric effects
afforded by two phenyl groups on each carbon atom. The O(1)-
Al(1)-O(2) angle (92.70(7)°) is the smallest among the internal
angles of the five-membered AlOCCO cycle.
Figure 2. ORTEP drawing of one molecule of 4 (50% thermal
ellipsoids). Hydrogen atoms have been omitted for clarity.
for four-coordinated aluminum compounds. In contrast, the Al(I)
compound 2 gives the most highly shielded resonance (590 (
40 ppm, ν1/2 ≈ 30 000 Hz) experimentally observed so far. The
detailed explanation for these data is currently unavailable
although the 27Al NMR resonances for monovalent aluminum
compounds have been computed ranging from -170 to 850
ppm.18
The UV spectra of the colored solution of compounds 3 and
4 in n-hexane show several maxima in the range of 240 to 400
nm. A maximum at 320 nm (ꢀ ≈ 4000 M-1 cm-1) for 3 as well
as a maximum at 340 (ꢀ ≈ 3000 M-1 cm-1) for 4 disappears
when the solutions are exposed to air. The absorptions might
be due to electronic transitions within the Al-C-C three-
membered-ring system.
X-ray Structures of Compounds 3, 4, and 6‚OEt2. The red-
black crystals of 3 and yellow crystals of 4 suitable for X-ray
structure analysis were obtained from n-hexane and toluene at
-30 °C, respectively. The structures of 3 and 4 are shown in
Figures 1 and 2 and important bond lengths and angles are listed
in Table 2. The Al-C-C ring system of 3 forms an isosceles
triangle with the Al-C bond lengths of 1.899(3) and 1.908(3)
Å, which are shorter than the Al-C bond lengths of the
corresponding dimethyl derivative {HC[(CMe)(NAr)]2}AlMe2
Reaction of LAl[η2-C2(SiMe3)2] (3) with CO2, Ph2CO, and
PhCN. Reduction of the carbonyl groups by activated metals
or metal complexes constitutes a powerful strategy for C-C
bond formation reactions.21 The success in the synthesis of
compounds 3-5 with the highly strained three-membered
(19) Cui, C.; Roesky, H. W.; Hao, H.; Schmidt, H.-G.; Noltemeyer, M.
Angew Chem. 2000, 112, 1885; Angew. Chem., Int. Ed. 2000, 39, 1815.
(20) See ref 17; p 11.
(18) Gauss, J.; Schneider, U.; Ahlrichs, R.; Dohmeier, C.; Schno¨ckel,
H. J. Am. Chem. Soc. 1993, 115, 2402.
(21) Kahn, B. E.; Rieke, R. D. Chem. ReV. 1988, 88, 733.

9096 J. Am. Chem. Soc., Vol. 123, No. 37, 2001
Cui et al.
Figure 4. ORTEP drawing of 8 (50% thermal ellipsoids). Hydrogen
atoms and solvent Et2O have been omitted for clarity.
Figure 3. ORTEP drawing of 6 (50% thermal ellipsoids). Hydrogen
atoms and solvent Et2O have been omitted for clarity.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
Compounds 8‚OEt2 and 9
Scheme 3
8‚OEt2
Al(1)-O(1)
Al(1)-N(2)
O(1)-C(3)
C(2)-C(3)
1.7273(12) Al(1)-N(1)
1.9108(14) Al(1)-C(1)
1.4268(18) C(1)-C(2)
1.574(2)
1.9124(15)
1.9852(16)
1.359(2)
N(1)-Al(1)-N(2) 97.90(6)
O(1)-Al(1)-C(1) 93.56(6)
O(1)-Al(1)-N(2) 112.03(6)
N(1)-Al(1)-C(1) 118.54(7)
O(1)-Al(1)-N(1) 111.13(6)
N(2)-Al(1)-C(1) 124.08(7)
9
Al(1)-N(1)
Al(1)-N(3)
N(3)-C(6)
C(7)-C(8)
1.9109(13) Al(1)-N(2)
1.8678(15) Al(1)-C(8)
1.2741(19) C(6)-C(7)
1.366(2)
1.8887(13)
1.9917
1.552(2)
C-C-Al ring systems prompted us to examine the reactivities
of this unique species toward unsaturated molecules. It is well-
known that reactivities of base-stabilized neutral aluminum
complexes are dramatically retarded compared to those of base-
free aluminum compounds. However, ring strain within the Al-
C-C cycles would increase the reactivities of the Al-C bonds
toward unsaturated molecules, leading to organic chain growing
reactions, although the Al atom in these systems is four
coordinated. In addition, this type of reaction should be an
effective route for the synthesis of novel aluminum-containing
ring systems.
N(1)-Al(1)-N(2) 96.39(6)
N(1)-Al(1)-N(3) 108.35(6)
N(2)-Al(1)-N(3) 106.48(6)
N(1)-Al(1)-C(8) 125.92(6)
N(2)-Al(1)-C(8) 124.56(6)
N(3)-Al(1)-C(8) 93.33(7)
compound with a metalladihydrofuran ring structure. The two
low-field resonances (δ 195.5 and 190.8 ppm) and a resonance
at δ 90.1 ppm for the carbon atoms in the Al-C-C-C-O
ring in the 13C NMR spectrum are consistent with the aluminum
dihydrofuran structure.
Compound 3 also reacts with benzonitrile PhCN at room
temperature to give a new aluminum-containing unsaturated ring
system LAl[NC(Ph)C2(SiMe3)2] (9) in a moderate yield (Scheme
3). Compound 9 crystallized from diethyl ether as orange
crystals, very soluble in aromatic solvents but only sparingly
soluble in n-hexane. The EI-MS spectrum shows a peak of
highest mass at (M+ + H). The 1H and 29Si NMR spectra give
two distinct resonances for the Me3Si nuclei, respectively,
indicating that PhCN inserts only into one of the Al-C bonds
in 3. The 13C NMR spectrum of 9 shows three resonances (δ
183.8, 181.5, and 173.3 ppm) corresponding to the carbon atoms
of the Al-C-C-C-N five-membered ring.
Molecular Structures of 8‚OEt2 and 9. Single crystals of
8‚OEt2 were obtained from diethyl ether at 0 °C. The structure
is shown in Figure 4, and some important bond distances and
angles are summarized in Table 3. The aluminum atom is four
coordinate and has a distorted tetrahedral geometry. The angles
N(1)-Al(1)-N(2) (97.90(6)°) and O(1)-Al(1)-C(1) (93.56(6)°)
are smaller than 109.28° for an ideal tetrahedral array. The
Al(1)-O(1) distance (1.7273(12) Å) is comparable to those of
6 (av 1.73 Å) and shorter than the sum of covalent radii of Al
and O, indicating increased ionic interaction between the two
atoms. The bond length C(1)-C(2) (1.359(2) Å) is consistent
Reaction of 3 with CO2 proceeds smoothly at room temper-
ature leading to C-C coupling with the formation of a new
class of aluminum heterocycles, LAl[OC(O)C2(SiMe3)2] (7), in
high yield (Scheme 3). The EI-MS spectrum shows the
molecular ion with high intensity consistent with the formulation
1
as a monomer. The H, 13C, and 29Si NMR spectra all gave
two distinct resonances for the Me3Si nuclei, indicating that CO2
only inserts into one of the Al-C bonds. The IR spectrum shows
a very strong band at 1666 cm-1, suggesting the formation of
an R,â-unsaturated ketone unit.
Reaction of 3 with benzophenone (Ph2CO) in diethyl ether
at room temperature also leads to C-C coupling with the
formation of an aluminadihydrofuran LAl[OC(Ph)2C2(SiMe3)2]
(8) despite the bulkiness of the ketone (Scheme 3). It has been
observed previously that trialkyl aluminum compounds can
reduce carbonyl groups with competitive alkyl insertion and
â-hydrogen insertion to form aluminum oxides.22 The neat
insertion of carbonyls into an Al-C (vinyl) bond was observed
in this case. Compound 8 is the first example of an aluminum
(22) Eisch, J. J. In ComprehensiVe Organometallic Chemistry II; Wilkin-
son, G., Stone, F. G. A., Abel, E. W., Eds.; Elsevier: Oxford, UK, 1982;
p 644.

Facile Synthesis of Cyclopropene Analogues of Aluminum
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9097
with PhCN and t-BuCN is currently not clear. It is possible
that initial coordination of a nitrile molecule to 3 takes place.
As a consequence of steric bulk and electron donation capacity
of the t-Bu group, the carbon of the CN group is not easily
attacked by the alkyne carbon like in the case of PhCN, instead,
t-BuCN replaces Me3SiCCSiMe3 to give a metal η2 nitrile
intermediate,24 which is highly reactive and immediately couples
with another molecule of t-BuCN to give 10. The isolation of
this interesting intermediate is under investigation.
Aluminum iminato complexes prepared by reduction of
nitriles with trialkylalane or alkylaluminum hydrides have been
employed as intermediates in the preparation of aldehydes,
primary amines, Schiff bases, and azacyclic compounds.25
However, the utilization of unsaturated aluminum ring systems
for the reduction of nitriles has not been reported to date. Here
formal coupling of an unsaturated molecule with a nitrile
induced by an aluminum complex was observed for the first
time. Obviously, this is a new approach to synthesize novel
cyclic aluminum iminato complexes.
Figure 5. ORTEP drawing of 9 (50% thermal ellipsoids). Hydrogen
atoms have been omitted for clarity.
The 1H NMR spectrum of 10 does not show low-field
resonances for Me3Si protons, but a broad single resonance (δ
1.25 ppm) can be assigned to the t-Bu protons. The integration
of all signals is consistent with the formulation of 10. The
compound shows a typical CdN stretch (1704 cm-1) in the IR
Scheme 4
spectrum, which is comparable to that found in 9 (1697 cm-1
)
but at higher wavenumbers (ca. 60 cm-1) than those of bridged
iminato groups of aluminum compounds.23,26 A low-field
resonance (δ 193.6 ppm) in the 13C NMR spectrum can be
assigned to the carbon atoms in the Al-N-C-C-N five-
membered ring. The EI-mass spectrum gives a peak of highest
mass at [M+ - t-Bu]. The molecular structure of this unique
compound was determined by single-crystal X-ray structural
analysis. Single crystals of 10 were obtained from diethyl ether.
Unfortunately the quality of the crystal measured was not
satisfactory to enable determination of precise bond parameters;
however, the molecular array of 10 has been elucidated as
proposed. The structure is consistent with the spectroscopic data
of 10.
with a CdC double bond character though shorter than that of
3 (C(6)-C(7) (1.382(2) Å) due to the strained three-membered
Al(1)-C(6)-C(7) ring in 3. The Al(1)-C(1) bond length
(1.9852(16) Å) is longer by ca. 0.09 Å than those of the parent
compound 3 (av 1.90 Å).
The reaction of 3 with bulky azides 2,6-Trip2C6H3N3 and
Ph3SiN3 also proceeds through replacement of the alkyne moiety
with the formation of the first monomeric aluminum imides
LAlNC6H2-2,6-Trip2 (11) and LAlNSiPh3 (12) in high yield
(Scheme 4). The two compounds have also been generated by
the direct reaction of the Al(I) species LAl (2) with the
corresponding azides (Scheme 4). The EI-mass spectra of both
compounds show a peak of highest mass corresponding to the
monomeric formula of 11 and 12, respectively. The two
compounds have also been characterized by NMR spectra and
elemental analysis, which are consistent with the proposed
structure and composition. The corresponding compound
LGaNC6H3-2,6-Trip2 has been characterized by X-ray structural
analysis, which shows a monomeric structure with a short Ga-N
distance.6b Unfortunately, attempts to grow X-ray quality crystals
Orange crystals of 9 were obtained from toluene at -5 °C.
The structure is shown in Figure 5, and selected bond distances
and angles are listed in Table 3. The aluminum atom is
coordinated to a terminal iminato group and a vinyl group
forming a nearly planar five-membered Al-N-C-C-C ring
(mean deviation of the plane corresponds to 0.044 Å). The Al-
N(3) distance (1.8678(15) Å) is significantly shorter than those
reported for the aluminum iminato complexes {(i-Bu)2Al[µ2-
NdC(H)(C6H3-2,6-Me2)]}2 (1.946 Å),23 in which the iminato
groups are bridging two aluminum atoms, due to the low
coordination number (2) of the iminato moiety in 9. The N(3)-
C(6) (1.2741(19) Å) and C(7)-C(8) (1.366(2) Å) bond lengths
are consistent with NdC and CdC double bonds. The Al-
C(8) (1.9917 Å) bond length is comparable to that found in 8,
but longer by ca. 0.1 Å than those in 3.
(24) For a few well-characterized η2-nitrile metal complexes see: (a)
Wright, T. C.; Wilkinson, G.; Motevalli, M.; Hursthouse, M. B. J. Chem.
Soc., Dalton Trans. 1986, 2017. (b) Chetcuti, P. A.; Knobler, C. B.;
Hawthorne, M. F. Organometallics 1988, 7, 650.
(25) (a) Cainelli, G.; Panunzio, M.; Andreoli, P.; Martelli, G.; Spunta,
G.; Giacomini, D.; Bandini, E. Pure Appl. Chem. 1990, 62, 605. (b)
Buchwald, S. L.; Watson, B. T.; Wannamaker, M. W.; Dewan, J. C. J. Am.
Chem. Soc. 1989, 111, 4486. (c) Goering, H. L.; Tseng, C. C. J. Org. Chem.
1981, 46, 5252. (d) Andreoli, P.; Billi, L.; Cainelli, G.; Panunzio, M.;
Martelli, G.; Spunta, G. J. Org. Chem. 1990, 55, 4199. (e) Bogdanovic, B.;
Konstantinovic, S. Liebigs Ann. Chem. 1970, 738, 202. (f) Overman, L.
E.; Burk, R. M. Tetrahedron Lett. 1984, 25, 5737.
Reaction of LAl[η2-C2(SiMe3)2] (3) with t-BuCN and Bulky
Azides. Unexpectedly, the reaction of 3 with an equivalent
amount of t-BuCN at room temperature leads to the displace-
ment of the alkyne moiety with the formation of the first
aluminum bis(iminato) complex LAl[N2(Ct-Bu)2] (10) (Scheme
4), which features an unprecedented aluminum-containing
imidazole ring. The detailed mechanism for the reaction of 3
(23) Jensen, J. A. J. Organomet. Chem. 1993, 456, 161.
(26) Wehmschulte, R. J.; Power, P. P. Inorg. Chem. 1998, 37, 6906.

9098 J. Am. Chem. Soc., Vol. 123, No. 37, 2001
Cui et al.
of 11 and 12 in various solvents and using different temperatures
were unsuccessful to date.
the alkyne moiety by bulky nitrile (t-BuCN) and azides to
generate the same products as those of the reaction of LAl (2)
with the corresponding azides, indicating that in some specific
cases 3 could be used as the LAl (2) source. Further investiga-
tions of the reactions of 2 and 3 are currently in progress.
Summary
We have prepared the first neutral cyclopropene analogues
of aluminum incorporating a bulky â-diketiminato ligand L (L
) HC[(CMe)(NAr)]2, Ar ) 2,6-i-Pr2C6H3) by facile alkali metal
reduction LAlI2 (1) in the presence of alkynes. The route also
might be applicable to other Group 13 metals. The utilization
of the very bulky ligand may be crucial to preventing associa-
tion, thus highly strained Al-C-C three-membered rings were
obtained. The bulky chelating ligand might render the mono-
meric Al(I) species LAl (2) unreactive to alkynes. However,
two types of reactions were observed for LAl[η2-C2(SiMe3)2]
(3). One involves the insertion of small unsaturated molecules
into one of the Al-C bonds, and another the substitution of
Acknowledgment. Dedicated to Professor Robert W. Parry.
This work was supported by the Deutsche Forschungsgemein-
schaft and the Go¨ttinger Akademie der Wissenschaften.
Supporting Information Available: VT NMR data for 3,
MS, IR, and elemental analysis data for compounds 3-12, and
details of the X-ray structure determinations of compounds 3,
4, 6‚OEt2, 8‚OEt2, and 9 (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
JA003185I