The [2 + 4] Diels-Alder cycloadditon product of a probable dialuminene, Ar′AlAlAr′ (Ar′ = C6H3- 2,6-Dipp 2; Dipp = C6H3-2,6-Pri2), with toluene

Article abstract of DOI:10.1021/ja034478p

Reduction of Ar′AlI₂ (Ar′ = Ar′= C₆H₃-2,6-Dipp₂; Dipp = C₆H₃-2,6-Prⁱ₂) with KC₈ in diethyl ether most probably affords the first dialuminene , Ar′AlAlAr′; it was characterized by its reaction with toluene which yielded a [2 + 4] cycloaddition product incorporating the Ar′AlAlAr′ unit. Copyright

Full text of DOI:10.1021/ja034478p

Published on Web 08/16/2003
The [2 + 4] Diels-Alder Cycloadditon Product of a Probable Dialuminene,
Ar′AlAlAr′ (Ar′ ) C6H3- 2,6-Dipp2; Dipp ) C6H3-2,6-Pri2), with Toluene
Robert J. Wright, Andrew D. Phillips, and Philip P. Power*
Department of Chemistry, UniVersity of California, DaVis, One Shields AVenue, DaVis, California 95616
Received February 3, 2003; E-mail: pppower@ucdavis.edu
Heavier group 14 element alkene analogues (R2MMR2; M )
Si1 and Ge,2 R ) organo or related group) react with dienes to
give cycloaddition products in a manner similar to the Diels-Alder
reaction. In contrast, the unavailability of potentially double-bonded
compounds RMMR (M ) B-Tl) has prevented the analogous
studies for group 13 species. Calculations for the hydrides HMMH
(M ) Ga, In)3 and IR data on matrix-isolated species at low
temperature,4 indicate a trans bent HMMH skeleton and weak
M-M bonding (ca. 3 kcal mol-1). The recently synthesized
compounds Ar′MMAr′ (M ) Ga,5a In,5b and Tl;5c Ar′ ) C6H3-2,6-
Dipp2, Dipp ) C6H3-2,6-Pri2) were also found to have a trans bent
Figure 1. Selected bond lengths (Å) and angles (deg) for 2. H atoms are
structure and M-M distances that were longer than single bonds.
not shown. Al(1)-Al(1A) ) 2.609(2), Al(1)-C(1) ) 1.964(4), Al(1)-
Moreover, they dissociated to Ar′M monomers in hydrocarbons at
I(1) ) 2.5020(12), C(1)-Al(1)-Al(1A) ) 128.16(12), C(1)-Al(1)-I(1)
room temperature which is consistent with weak M-M bonding.
Nonetheless, the “dialuminene” isomer, HAlAlH, was predicted to
have a stronger M-M bond (ca. 10 kcal mol-1) than its heavier
congeners,6 and the Al-Al distance was calculated to be 2.613 Å,
which is shorter than most Al-Al single bonds in the dialanes R2-
AlAlR2.7 These data suggested the possibility of isolating an
aluminum species of formula RAlAlR and the study of addition
reactions to the Al-Al double bond. We now show that reaction
of Ar′AlI2 (1) with KC8 afforded the 1,2-diiodoalane (2) and
probably, the “dialuminene” (3) according to
) 118.25(11), I(1)-Al(1)-Al(1A) ) 113.56(6).
2KC8
2KC8
2Ar′AlI2
-2KI8 I(Ar′)Al-Al(Ar′)I
8
Ar′AldAlAr′
-2KI
3
2
1
Figure 2. Selected bond lengths (Å) and angles (deg) for 4. H atoms are
not shown. Al(1)-Al(2) ) 2.5828(7), Al(1)-C(1) ) 2.0004(16), Al(2)-
C(4) ) 2.0032(15), Al(1)-C(8) ) 1.9946(15), Al(2)-C(38) ) 1.9990-
(15), C(2)-C(3) ) 1.337(2), C(5)-C(6) ) 1.344(2), C(1)-C(2) ) 1.509(2),
C(3)-C(4) ) 1.503(2), C(4)-C(5) ) 1.509(2), C(6)-C(1) ) 1.496(2),
C(1)-Al(1)-Al(2) ) 91.95(5), C(4)-Al(2)-Al(1) ) 93.45(5), C(8)-Al-
(1)-Al(2) ) 142.18(5), C(38)-Al(2)-Al(1) ) 141.08(5).
Thus far, the “dialuminene” 3 has not been isolated as a pure species
but has been found to crystallize only from toluene. It reacts with
this solvent to form the cycloaddition product 4 as shown by
1
The air- and moisture-sensitive 4 was characterized by H and
1
13C NMR, and UV-vis. The H NMR spectrum featured charac-
teristic alkane and alkene hydrogen signals at 2.63-2.74 and 5.91
ppm that were indicative of localized CdC double bonding in a
toluene that had undergone cycloaddition to Ar′AlAlAr′. Attempts
to observe an 27Al NMR signal were unsuccessful. The absence of
a signal is probably a result of the unsymmetric aluminum
environment which broadens the signal into the baseline.11 The 1H
and 13C NMR data were confirmed by X-ray crystallography as
shown in Figure 2. The respective bond lengths for C(2)-C(3) and
C(5)-C(6) are 1.337(2) and 1.344(2) Å and are characteristic of
CdC double bonding.12 The other four ring C-C distances are in
the range 1.496(2)-1.509(3) Å which indicated C-C single
bonding between approximately sp2 hybridized carbons.12 The Al-
(1)-Al(2) distance, 2.5828(7) Å, is shorter than that in 2 and is
outside the known range (2.647(3)-2.751(2) Å) for Al-Al single
bonds in three-coordinate R2AlAlR2 compounds.7 The Al(1)-C(1)
and Al(2)-C(4) bond lengths are 2.0004(16) and 2.0032(15) Å
which are typical for Al-C bond distances in trivalent organoalu-
Compound 3 was generated by the reduction of Ar′AlI2 with 2
equiv of KC8 in Et2O. Removal of the solvent and subsequent
extraction into toluene, followed by separation of 2, which is less
soluble, and cooling in a ca. -30 °C freezer, afforded 4 as red
crystals in 17% yield.8a Compound 2, an isolable intermediate en
route to 3 can also be synthesized as a pale yellow solid in 40%
yield from the reduction of 1 with 1 equiv of KC8.8b Yellow crystals
of 2 were grown from C6D6 solution, and its structure (Figure 1)9
shows that 2 is a unique example of an unsolvated halogen-
substituted dialane.10 It has a short Al-Al single bond 2.609(2)
due to the contraction of the effective radius of Al by the iodine
substituent. The large Ar′ groups are orientated trans to each other,
and the C-Al-Al angle is nearly 15° wider than the I-Al-Al
angle.
9
10784
J. AM. CHEM. SOC. 2003, 125, 10784-10785
10.1021/ja034478p CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
minum species.13 The internal angles of the complexed toluene ring
are narrowest at the Al-bound carbons C(1) and C(4) (109.4(1) and
109.4(1°)) and are in the range (120.8(1)-121.7(1°)) at C(2), C(3),
C(5), and C(6). The C(1)-Al(1)-Al(2) and C(4)-Al(2)-Al(1)
angles are close to 90°, and the external angles C(8)-Al(1)-Al(2)
and C(38)-Al(2)-Al(1) involving the terphenyl ligands are wide
at 142.18(5) and 141.08(5)°sprobably as a result of steric repulsion.
The aluminums are almost planar coordinated with angular sums
of 359.25(6)° at Al(1) and 358.55(6)° at Al(2). The C(1)-Al(1)-
Al(2)-C(4) torsion angle is only 24.5° which indicates the bulky
substituents (Ar′) are nearly cis with respect to each other.
There are a handful of reactions between unstable boron(I) or
aluminum(I) monomers and unsaturated molecules. The first
example was the reaction between BX (X ) F,14a Cl14b) and
acetylene to give 1,4-diboracyclohexadiene. Similarly, the reaction
of AlCl with the alkynes, RCtCR (R ) Me, Et), gave the cage
species, (AlCl‚RCtCR)4, which are dimers of substituted 1,4-
dialuminacyclohexadiene.15 In addition, the reaction of AlCl with
2,3-dimethylbutadiene (DMB) produced the cyclic hexamer (AlCl‚
DMB)6.16 More recently, the formal [1 + 2] cycloaddition product
formed from the potassium reduction of I2Al[{DippN(Me)C}2CH]
in the presence of RCtCR; (R ) Ph, or TMS) gave the aluminum
(4) Himmel, H.-J.; Manceron, L.; Downs, A. J.; Pullumbi, P. Angew. Chem.,
Int. Ed. 2002, 41, 796. Himmel, H.-J.; Manceron, L.; Downs, A. J.;
Pullumbi, P. J. Am. Chem. Soc. 2002, 124, 4448. Downs, A. J.; Himmel,
H.-J.; Manceron, L. Polyhedron 2002, 21, 473.
(5) Hardman, N. J.; Wright, R. J.; Phillips, A. D.; Power, P. P. Angew. Chem.,
Int. Ed. 2002, 41, 2842. (b) Wright, R. J.; Phillips, A. D.; Hardman, N.
J.; Power, P. P. J. Am. Chem. Soc. 2002, 124, 8538. (c) Wright, R. J.;
Phillips, A. D.; Power, P. P. Unpublished results.
(6) Schwerdtfeger, P.; Heath, G. A.; Dolg, M.; Bennett, M. A. J. Am. Chem.
Soc. 1992, 114, 7518. Pala´gyi, Z.; Schaefer, H. F., III. Chem. Phys. Lett.
1993, 203, 195.
(7) Pluta, K.-R.; Po¨rschke, C.; Kruger, K.; Hildenbrand, K. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 388. Uhl, W.; Vester, A.; Kaim, W.; Poppe, J. J.
Organomet. Chem. 1993, 454, 9. Uhl, W.; Schutz, U.; Kaim, E.; Waldho¨r,
E. J. Organomet. Chem. 1995, 501, 79.
(8) (a) Under anaerobic and anhydrous conditions, Ar′AlI2 (1.36 g, 2.0 mmol)
was dissolved in Et2O (85 mL). The solution was added dropwise over
30 min to freshly synthesized KC8 (0.54 g, 4.0 mmol) and stirred for 18
h. The deep red solution was allowed to settle (4 h) and was decanted
from the graphite. The ether was removed under reduced pressure, and
the dark red residue was extracted with toluene (15 mL). Overnight storage
at ca. -30 °C afforded red crystals of 4 (0.2 g, 0.18 mmol). The
supernatant liquid was removed from the crystals and further concentrated
to 10 mL. Storage at -30 °C for 9 d gave 4 as red X-ray quality crystals.
Yield: 0.19 g, 17%; mp ) 103-107 °C (upon melting turned from red
to pale yellow). Calcd. for C67H82Al2: C ) 85.49, H ) 8.78. Found: C
) 85.88, H ) 8.03. 1H NMR (300 MHz, C6D6, 25 °C): δ 0.964 (d, 12H,
o-CH(CH3)2) 3JHH ) 6.6 Hz, 0.985 (d, 12H, o-CH(CH3)2) 3JHH ) 6.6 Hz,
3
1.511 (s, 3H, Ph-CH3) 2.645 (m, CdC-CH), 2.745 (t, CdC-CH) JHH
)
)
3
6.9 Hz, 2.909 (broad mult, 8H, CH(CH3)2), 5.907 (t, CHdCH) JHH
3
7.2 Hz, 6.771 (d, 4H, m-C6H3) JHH ) 7.5 Hz, 7.081 (m, 10H, m-Dipp
and p-C6H3), 7.197 (t, 4H, p-Dipp) 3JHH ) 7.5 Hz. 13C{1H} NMR (C6D6,
74.46 MHz, 25 °C): δ 23.66 (CH(CH3)2), 25.66 (CH(CH3)2), 30.92
(CH(CH3)2), 123.75 (m-Dipp), 127.48 (p-C6H3), 128.97 (m-C6H3), 129.21
(p-Dipp), 141.93 (i-Dipp), 147.04 (o-Dipp), 147.14 (o-C6H3), 150.91 (i-
C6H3). UV/vis (hexanes) λmax nm (ꢀ mol‚L-1‚cm-1): 320 (3400); (b)
Ar′AlI2 (2.5 g, 3.69 mmol) was dissolved in Et2O (65 mL). The solution
was added dropwise over 30 min to KC8 (0.50 g, 3.69 mmol) at ca. -78
°C and stirred for 12 h. The red solution was decanted from the graphite.
The ether was removed under reduced pressure, and the residue was
washed with hexane (15 mL). The supernatant was removed from the
pale yellow product. Yield: 0.81 g, 40%; mp ) 226-229 °C. Calcd. for
cyclopropene analogue, HC{C(Me)DippN}2Al(Ph)CdC(Ph).17
However, none of these reactions afforded products with group 13‚‚
13 element bonds, and it is notable that concentrated toluene
solutions of Ar′MMAr′ (M ) Ga, In, and Tl) display no trace of
cycloaddition products. The relative inertness of the heavier
“dimetallenes” toward [2 + 4] cycloadditions may be due to their
ready dissociation in to monomeric Ar′M species in solution.5
Among heavier main group element compounds, the reactions
which most resemble the addition of PhMe to 3 is the addition of
unstable Me2SiSiMe2 to aromatic molecules such as benzene,
naphthalene, or anthracene to give disilabicyclo[2.2.2]octadiene
derivatives with structures similar to that of 4.18 However, irradiation
with UV (ca. 250 nm) regenerated the disilene by a photolytic [2
+ 4] cycloreversion.19 The analogous generation of Ar′AlAlAr′ from
4 via photochemical methods is under investigation. In fact, future
work may show that the chemistry of the lighter group 13
“dimetallenes”, RMMR (M ) B and Al) will parallel that of the
group 14 alkene analogues. Current work is focused on the isolation
and characterization of an uncomplexed “dialuminene” and the
examination of its reaction chemistry.
C
30H37AlI: C ) 65.33, H ) 6.76. Found: C ) 65.01, H ) 7.01. 1H
3
NMR (400 MHz, C6D6, 25 °C): δ 0.987 (d, 24H, o-CH(CH3)2) JHH
)
3
6.8 Hz, 1.151 (d, 24H, o-CH(CH3)2) JHH ) 6.8 Hz, 3.012 (sept, 8H,
3
3
CH(CH )2) JHH ) 6.8 Hz, 7.09-7.06 (m, 6H, m-C6H3, p-C6H3), 7.146
(d, 8H, m-Dipp), 7.328 (t, 4H, p-Dipp) JHH ) 7.6 Hz. 13C{1H} NMR
3
(C6D6, 100.531 MHz, 25 °C): δ 25.25 (CH(CH3)2), 26.52 (CH(CH3)2),
30.51 (CH(CH3)2), 124.16 (m-Dipp), p-C6H3 (unobserved, probably
obscured by C6D6), 129.87 (m-C6H3), 130.10 (p-Dipp), 139.59 (i-Dipp),
145.85 (i-C6H3), 146.93 (o-C6H3), 147.809 (o-Dipp).
(9) Crystal data for 2 at 90 K with Mo KR (λ ) 0.71073 Å) radiation: a )
13.5724(12) Å, b ) 19.0587(17) Å, c ) 15.5757(14) Å, R ) 90° ) γ, â
) 114.603(3)°, monoclinic, space group P21/n, Z ) 4, R1 ) 0.0426 for
5199 (I > 2σ(I)) data, wR2 ) 0.1245 for all (5731) data. Crystal data for
4 at 90 K with Mo KR (λ ) 0.71073 Å) radiation: a ) 13.9134(15) Å,
b ) 13.9628(18) Å, c ) 19.057(2) Å, R ) 106.6.45(3)°, b ) 107.171-
(4)°, c ) 91.862(5)°, triclinic, space group P1h, Z ) 2, R1 ) 0.0396 for
10326 (I > 2σ(I)) data, wR2 ) 0.1114 for all (13096) data.
(10) The structure of a base-stabilized dialane, [(Me3Si)Si(Cl)(THF)Al]2, has
been determined in Klemp, C.; Uffing, C.; Baum, E.; Schno¨ckel, H. Z.
Anorg. Allg. Chem. 2000, 626, 1787.
(11) Akitt, J. W. Prog. Nucl. Magn. Reson. Spectrosc. 1989, 21, 1.
(12) Smith, M. B.; March, J. March’s AdVanced Organic Chemistry, 5th ed.;
Wiley: New York, 2001; p 20.
(13) Chemistry of Aluminum, Gallium, Indium, and Thallium; Downs, A. J.,
Ed.; Blackie-Chapman & Hall: London, 1993.
Acknowledgment. We are grateful to the National Science
Foundation (CHE-0096913) for financial support.
Supporting Information Available: X-ray data (CIF) for 2 and 4.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(14) (a) Timms, P. L. J. Am. Chem. Soc. 1968, 90, 4585. (b) Timms, P. L.
Acc. Chem. Res. 1973, 6, 118.
(15) Schno¨ckel, H.; Leimku¨hler, M.; Lotz, R.; Mattes, R. Angew. Chem., Int.
Ed. Engl. 1986, 25, 921. U¨ ffing, C.; Ecker, A.; Ko¨ppe, R.; Merzweiler,
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(16) Dohmeier, C.; Mattes, R.; Schno¨ckel, H. J. Chem. Soc., Chem. Commun.
1990, 358.
(17) Cui, C.; Ko¨pke, S.; Herbst-Irmer, R.; Roesky, H. W.; Noltemeyer, M.;
Schmidt, H. G.; Wrackmeyer, B. J. Am. Chem. Soc. 2001, 123, 9091.
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Soc., Chem. Commun. 1991, 341.
(19) Masamune, S.; Murakami, S.; Tobita, H. Organometallics 1983, 2, 1464.
Yokelson, H. B.; Maxka, J.; Siegal, D. A.; West, R. J. Am. Chem. Soc.
1986, 108, 4239.
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