Reversible and irreversible higher-order cycloaddition reactions of polyolefins with a multiple-bonded heavier group 13 alkene analogue: Contrasting the behavior of systems with π-π, π-π*, and π-n + frontier molecular orbital symmetry

Article abstract of DOI:10.1021/ja301247h

The heavier group 13 element alkene analogue, digallene AriPr ₄GaGaAriPr₄ (1) [AriPr₄ = C₆H ₃-2,6-(C₆H₃-2,6-ⁱPr ₂)₂], has been shown to react rea

Full text of DOI:10.1021/ja301247h

Article
pubs.acs.org/JACS
Reversible and Irreversible Higher-Order Cycloaddition Reactions of
Polyolefins with a Multiple-Bonded Heavier Group 13 Alkene
Analogue: Contrasting the Behavior of Systems with π−π, π−π*, and
π−n+ Frontier Molecular Orbital Symmetry
Christine A. Caputo,† Jing-Dong Guo,‡ Shigeru Nagase,‡ James C. Fettinger,† and Philip P. Power*,†
†Department of Chemistry, University of California, Davis, 1 Shields Avenue, Davis, California 95616, United States
‡Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan
S
* Supporting Information
ABSTRACT: The heavier group 13 element alkene analogue, digallene
iPr4
AriPr GaGaAr (1) [AriPr = C6H3-2,6-(C6H3-2,6-iPr2)2], has been shown to
react readily in [n + 2] (n = 6, 4, 2 + 2) cycloaddition reactions with
norbornadiene and quadricyclane, 1,3,5,7-cyclooctatetraene, 1,3-cyclopenta-
4
4
diene, and 1,3,5-cycloheptatriene to afford the heavier element deltacyclane
iPr4
species AriPr Ga(C7H8)GaAr (2), pseudoinverse sandwiches AriPr Ga(C8H8)-
4
4
GaAriPr (3, 3iso), and polycyclic compounds AriPr Ga(C5H6)GaAr (4) and
iPr4
4
4
AriPr Ga(C7H8)GaAr (5, 5iso), respectively, under ambient conditions. These
iPr4
4
reactions are facile and may be contrasted with other all-carbon versions, which
require transition-metal catalysis or forcing conditions (temperature, pressure),
iPr4
or with the reactions of the corresponding heavier group 14 species AriPr EEAr (E = Ge, Sn), which give very different product
4
structures. We discuss several mechanistic possibilities, including radical- and non-radical-mediated cyclization pathways. These
mechanisms are consistent with the improved energetic accessibility of the LUMO of the heavier group 13 element multiple
bond in comparison with that of a simple alkene or alkyne. We show that the calculated frontier molecular orbitals (FMOs) of
iPr4
AriPr GaGaAr are of π−π symmetry, allowing this molecule to engage in a wider range of reactions than permitted by the usual
4
π−π* FMOs of C−C π bonds or the π−n+ FMOs of heavier group 14 alkyne analogues.
Scheme 1. Reactions of COT with AriPr EEAr (E = Sn,6
iPr4
4
INTRODUCTION
■
Ge7)
The Diels−Alder [4π + 2π],1 homo-Diels−Alder [2π + 2π +
2π],2 and higher-order [6π + 2π] cyclizations3 are of significant
general and synthetic interest as they readily produce complex
ring systems in a generally efficient manner.4 Such polycycles
are found in many natural products, but the synthetic organic
cyclizations used to generate complex ring architectures
generally require forcing conditions or the use of transition-
metal catalysts.5
We showed recently that the reactions of heavier group 14
dimetallynes with cyclic polyolefins did not give the expected
cyclization products. For example, the reactions of
variety of inorganic reagents.9 It also reacts with 2,3-dimethyl-
1,3-butadiene to form 1,6-digallacyclo-3,8-octadiene.9 We
recently showed that it reacts with the simple external olefins
ethylene, propene, 1-hexene, and styrene to afford a series of
compounds with 1,4-digallacyclohexane cores in which there is
no Ga−Ga bonding.10 However, under similar conditions, no
reactions with internal olefins were observed.
We now show that the multiple-bonded group 13 heavy
alkene analogue 1 reacts readily with norbornadiene (NBD)
and quadricyclane, COT, 1,3-cyclopentadiene (CpH), and
1,3,5-cycloheptatriene (CHT) to afford the deltacyclane species
iPr4
iPr4
AriPr SnSnAr and AriPr GeGeAr [AriPr = C6H3-2,6-(C6H3-
4
4
4
2,6-iPr2)2] with 1,3,5,7-cyclooctatetraene (COT) afforded the
unique π-bound inverse-sandwich complexes [(AriPr Sn)2(μ2-
4
6
7
2
2
η2:η3-COT)] and [(AriPr Ge)2(μ2-η :η -COT)], respectively,
the latter of which isomerized to a unprecedented tetracyclic
cage digermane species (Scheme 1). Other cycloadditions with
group 14 dimetallenes have been limited to reactions with 2π
and 4π reaction partners, and no higher-order polyolefin
cyclizations have been reported.
4
Very little is known, however, about the reactions of the
iPr4
neighboring heavier group 13 element analogues AriPr MMAr
4
(M = Al, Ga, In, Tl) with polyolefins.8 The digallene derivative,
Received: February 15, 2012
Published: March 15, 2012
AriPr GaGaAr (1), has been shown to be reactive toward a
iPr4
4
© 2012 American Chemical Society
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iPr4
Scheme 2. Cycloadditions of Various Polyolefins with AriPr GaGaAr (1)
4
CH(CH3)2, 3JHH = 6.6 Hz, 4H), 6.84 (d, Ar, 3JHH = 7.8 Hz, 4H), 6.95
iPr4
AriPr Ga(C7H8)GaAr
(2) and the polycyclic compounds
4
3
iPr4
iPr4
AriPr Ga(C8H8)GaAr (3, 3iso), AriPr Ga(C5H6)GaAr (4),
4
4
(m, Ar, 2H), 7.03−7.06 (m, Ar, 8H), 7.15 (t, JHH = 7.8 Hz, Ar, 4H).
13C{1H} NMR (C6D6, 100.6 MHz, 298 K): δ 11.3, 13.3, 23.7, 24.4,
25.2, 25.4, 30.8, 35.7, 36.3, 43.9, 122.9, 123.6, 126.2, 127.0, 128.4,
128.9, 129.3, 144.8, 156.5. IR (nujol mull) cm−1 (intensity): 2725 (m),
2670 (m), 2360 (m), 2340 (m). UV−vis λmax (ε): 226 nm (3.3 × 104
M−1 cm−1), 303 nm (8.6 × 103 M−1 cm−1), 488 nm (2.4 × 103 M−1
and AriPr Ga(C7H8)GaAr
(5, 5iso), respectively, under
iPr4
4
ambient conditions (Scheme 2). Unlike the heavier group 14
dimetallynes, the group 13 dimetallene reacts to afford
cycloadducts that are in several instances analogous to the all-
carbon versions. We show that they react much more readily
than alkenes, without the need for substrate activation or the
use of transition-metal catalysts. We propose that this effect is
due to differences in the energy and symmetry of their frontier
molecular orbitals (FMOs). In contrast to the all-carbon
systems, the reactions are readily reversible. We show one
example of onward isomerization of the cycloadduct that
involves breaking of the Ga−Ga bond.
cm−1).
iPr4
AriPr Ga(C8H8)GaAr (3). To a solution of 1 (0.30 g, 0.32 mmol)
4
in dry degassed hexane (20 mL) was added freshly distilled COT (0.08
mL, 0.67 mmol) via syringe at ca. 25 °C. An immediate color change
from dark-green to bright-yellow was observed. The solution was
stirred for 1 h, concentrated to ca. 10 mL under reduced pressure, and
then stored at ca. −18 °C to afford yellow crystals of 2. Yield: 0.14 g,
1
42%. Mp: 170 °C (dec). Data for 3: H NMR (600 MHz, C7D8, 253
3
K): δ 1.04 (d, o-CH(CH3)2, JHH = 6.8 Hz, 24H), 1.13 (m, 4H), 1.24
We have previously shown that in hydrocarbon solution, the
3
(d, o-CH(CH3)2, JHH = 6.8 Hz, 24H), 2.42 (br, Ga[CH(CHCH)-
dimetallene AriPr GaGaAr
exists in equilibrium with the
iPr4
4
3
CH]2Ga, 2H), 2.88 (sept, CH(CH3)2, JHH = 6.8 Hz, 8H), 4.00 (br,
9,11
corresponding AriPr Ga: monomers,
consistent with the
4
Ga[CH(CHCH)CH]2Ga, 4H), 7.07 (m, Ar, 12H), 7.19 (m, Ar, 2H),
7.26 (m, Ar, 4H). 13C{1H} NMR (C7D8, 100.6 MHz, 273 K): δ 22.9,
23.2, 25.6, 25.9, 30.2, 30.7, 121.8, 123.3, 123.6, 127.6, 141.9, 146.6,
150.7. IR (KBr) cm−1 (intensity): 2964 (m), 2969 (m), 1942 (w),
1603 (w), 1464 (m), 1385 (m), 1363 (m), 1263 (s), 1099 (s), 1022
(s) 803 (s). UV−vis λmax (ε): 416 nm (4.1 × 103 M−1 cm−1), 330 nm
(6.7 × 103 M−1 cm−1), 242 nm (8.3 × 103 M−1 cm−1). Data for 3iso:
1H NMR (800 MHz, C7D8, 313 K): δ 1.14−1.19 (m, CH(CH3)2,
dimetallene binding energy of ca. 9 kcal/mol calculated using
density functional theory (DFT).12 However, the reaction
products described here all involve species that feature a Ga−
Ga bond. The preservation of the Ga−Ga bond in the isolated
products 2, 3iso, 4, and 5iso suggests that the initial reaction
involves the AriPr GaGaAr dimer and the cyclic polyolefin. In
iPr4
4
no case was a product with just a single gallium isolated. These
3
18H), 1.25 (d, CH(CH3)2, JHH = 7.2 Hz, 6H), 2.97−3.01 (m,
observations tend to support the formulation of these heavier
CH(CH3)2, 8H), 3.71 (q, 3JHH = 4.8 Hz, 1H), 4.27 (dd, 3JHH = 4.8 Hz,
1H), 4.58 (d, 3JHH = 12.8 Hz, 1H), 6.40 (dd, 3JHH = 12.8, 4.8 Hz, 1H),
7.16−7.32 (m, Ar, 18H). (For 2D HSQC and COSY spectra of a
mixture of 3 and 3iso, see Figures S5 and S6 in the Supporting
group 13 dimetallenes as multiply bonded AriPr GaGaAr
iPr4
4
dimers rather than as weakly associated AriPr Ga: monomers.
4
EXPERIMENTAL SECTION
Information.)
■
iPr4
AriPr Ga(C5H6)GaAr (4). To a solution of 1 (0.21 g, 0.23 mmol)
4
General Procedures. All manipulations were carried out under
anaerobic and anhydrous conditions. All reagents were trap-to-trap
vacuum-distilled and dried over 4 Å molecular sieves prior to use. 1
was prepared according to literature procedures.9 1H, 13C{1H}
correlation spectroscopy (COSY), and heteronuclear single-quantum
correlation (HSQC) NMR spectra were recorded on Varian
spectrometers and referenced to known standards.
in dry degassed pentane (20 mL) was added freshly distilled CpH
(0.15 mL, 1.4 mmol) via syringe at ca. 25 °C. An immediate color
change from dark-green to dark-orange was observed. The solution
was stirred for 1 h, concentrated to 5 mL under reduced pressure, and
then stored at ca. −18 °C to afford orange crystals of 4. Yield: 0.09 g,
40%. 1H NMR (400 MHz, C7D8, 298 K): δ 0.95−1.08 (m, o-
CH(CH3)2, 12H), 1.11−1.67 (m, o-CH(CH3)2, 12H), 1.20−1.31 (m,
(CHGa)2, CHH′(CHGa)2, 2H), 2.71−3.12 (m, o-CH(CH3)2,
CHH′(CHGa)2, 9H), 5.89 (s, HCCH, 2H), 7.08−7.31 (m, Ar,
18H). 13C{1H} NMR (C6D6, 150.6 MHz, 298 K): δ 13.9, 22.6, 24.0,
24.1, 25.5, 30.4, 30.5, 122.5, 123.1 (br), 126.7, 128.6 (br), 131.1, 140.7,
146.5, 146.7, 147.1. IR (neat) cm−1 (intensity): 3055 (w), 2958 (s),
2927 (m), 2866 (m), 1943 (w), 1651 (w), 1507 (m). UV−vis λmax (ε):
264 nm (3.1 × 104 M−1 cm−1), 351 nm (9.6 × 103 M−1 cm−1), 435 nm
(4.1 × 103 M−1 cm−1).
iPr4
AriPr Ga(C7H8)GaAr
(2). To a solution of 1 (0.69 g, 0.073
4
mmol) in dry degassed hexane (50 mL) was added freshly distilled
NBD (0.74 mL, 0.73 mmol) via syringe at ca. 25 °C. An immediate
color change from dark-green to bright-orange was observed. The
solution was stirred for 1 h, concentrated to ca. 10 mL under reduced
pressure, and then stored at ca. −18 °C to afford orange crystals of 2.
Yield: 0.41 g, 56%. Mp: 207−210 °C. 1H NMR (600 MHz, C7D8, 348
3
K): δ −0.26 (br d, GaCH, JHH = 4.8 Hz, 2H), 0.76 (s,
Ga(CHCH)2CHCH2, 2H), 0.81 (s, GaCHCH and Ga-
(CHCH)2CHCH2, 2H), 0.85−0.89 (m, Ga(CHCH)2CHCH2, 2H),
iPr4
AriPr Ga(C7H8)GaAr
(5 and 5iso). To a solution of
4
iPr4
0.98 (d, CH(CH3)2, 3JHH = 6.6 Hz, 12H), 1.02 (d, CH(CH3)2, 3JHH
=
AriPr GaGaAr (0.28 g, 0.30 mmol) in dry degassed pentane (20
4
3
6.6 Hz, 12H), 2.90 (sept, CH(CH3)2, JHH = 6.6 Hz, 4H), 2.99 (sept,
mL) was added freshly distilled CHT (0.04 mL, 0.34 mmol) via
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syringe at ca. 25 °C. An immediate color change from dark-green to
dark-reddish-orange was observed. The solution was stirred for 1 h and
concentrated to dryness under reduced pressure.
The above reactions differ greatly from the reactions of NBD
iPr4
with heavier group 14 alkyne analogues AriPr EEAr (E = Ge,
4
Sn), which react with 2 equiv of NBD to afford two reversible
[2π + 2π] reactions wherein the E−E σ bond is maintained
(Scheme 3, bottom).19 This behavior is in sharp contrast to the
all-carbon alkyne reactivity, which exclusively undergoes [2π +
2π + 2π] cycloaddition (via thermal and transition-metal-
catalyzed routes).20
Our studies of the reaction of digallene with simple
unactivated alkenes (ethylene, propene, 1-hexene, and styrene)
revealed a double [2π + 2π] cyclization in which the Ga−Ga
bond is broken and 2 equiv of the alkene are added.10 We
attributed this reactivity to the in-phase interaction of the
FMOs, namely, the highest occupied molecular orbital
(HOMO) (in-plane π orbital) of the olefin and the lowest
occupied molecular orbital (LUMO) (out-of-plane π orbital) of
the digallene, which is symmetry-allowed.12
Addition of NBD to a solution of 1 in hexane results in an
immediate color change from dark-green to bright-orange.
Structural data revealed the formation of a polycyclic structure
arising from a formal [2π + 2π + 2π] cycloaddition reaction to
furnish the homo-endo digalladeltacyclane product 2 (Figure
1). The Ga−Ga distance [2.4704(2) Å] lies at the longer end of
1
Data for 5: Yield: 0.16 g, 54%. Mp: 122−124 °C (dec). H NMR
(600 MHz, C7D8, 298 K): δ 0.55 (br, o-CH(CH3)2, 12H), 0.87 (br, o-
CH(CH3)2, 12H), 1.17 (br, o-CH(CH3)2, 12H), 1.25 (br, o-
2
2
CH(CH3)2, 12H), 1.57 (d, JHH = 13.8 Hz, 1H), 2.31 (d, JHH
=
16.8 Hz, 1H), 2.88 (br, o-CH(CH3)2, 4H), 2.94 (br, o-CH(CH3)2,
4H), 3.31 (br, 2H), 4.16 (br, 1H), 4.97 (br, 1H), 7.01 (br s, Ar, 4H),
2
7.02−7.12 (m, Ar, 10H), 7.22 (t, JHH = 7.8 Hz, Ar, 4H). 13C{1H}
NMR (C7D8, 150.6 MHz, 298 K): δ 23.2, 25.3, 26.0, 30.1, 30.7, 31.4,
78.3, 98.9, 123.5, 126.8, 130.0, 141.8, 145.3, 146.7, 148.8, 155.2. UV−
vis λmax (ε): 440 nm (8.0 × 104 M−1 cm−1), 350 nm (2.9 × 104 M−1
cm−1), 262 nm (1.2 × 105 M−1 cm−1).
After the NMR sample was heated to 373 K under nitrogen for 10
min, the solution changed from red to yellow. The yellow residue was
taken up in a minimal amount of dry diethyl ether (ca. 5 mL) and then
stored at ca. −18 °C to afford yellow crystals of 5iso.
Data for 5iso: Yield: 100% by 1H NMR. Mp: 220−221 °C. 1H NMR
3
2
(600 MHz, C7D8, 373 K): −0.39 (dd, JHH = 9.0 Hz, JHH = 15.6 Hz,
2
1H), −0.25 (d, JHH = 15.6 Hz, 1H), 1.00−1.26 (m, 24H), 2.52 (t,
3JHH = 6.6 Hz, 1H), 2.74 (sept, o-CH(CH3)2, 4H), 2.83−2.95 (m, o-
CH(CH3)2 and CH, 5H), 4.14 (pt, 1H), 4.78 (d, 2JHH = 13.2 Hz, 1H),
3
2
5.67 (dd, JHH = 6.6, Hz, JHH = 13.2 Hz, 1H), 5.77 (pt, 1H), 7.03−
7.23 (m, Ar, 18H). 13C{1H} NMR (C7D8, 150.6 MHz, 298 K): δ 8.7,
23.3, 23.4, 30.6, 30.7, 30.7, 37.8, 52.1, 122.8, 123.1, 123.3, 123.4, 127.1,
131.6, 134.5, 138.9, 141.9, 142.2, 146.7, 146.8, 147.0, 147.1, 147.3,
150.4, 151.2. (See Figure S11 in the Supporting Information for the
2D COSY spectrum of a mixture of 5 and 5iso.)
RESULTS AND DISCUSSION
■
Reaction of 1 with Norbornadiene and Quadricy-
clane. The homo-Diels−Alder cycloaddition reactions of
organic dienophiles with NBD and quadricyclane to furnish
deltacyclanes has been well-documented experimentally and
studied extensively by DFT analysis.13 In general, the addition
of an activated alkene to NBD proceeds via a [2π + 2π + 2π]
cyclization, which can be carried out either thermally14 or in the
presence of a transition-metal catalyst (Ni,2,14 Cu,15 Ru16) to
give a homo-endo cyclization product. However, the addition of
the same dienophiles to quadricyclane, a structural isomer of
NBD, results in a [2π + 2σ + 2σ] cyclization (Scheme 3) to give
an exo-type product. The reactivity of NBD has previously been
attributed to the dependence on a through-space interaction17
and spatial proximity of the π bonds.18 Both reactions can
proceed either via a biradical stepwise mechanism or a
concerted two-step mechanism.
Figure 1. Thermal ellipsoid (30% probability) plot of 2. Selected H
atoms, a crystallographically independent deltacyclane molecule, and a
cocrystallized hexane molecule are not shown. Selected bond lengths
(Å) and angles (deg): Ga(1)−Ga(2), 2.470(4); Ga(1)−C(1),
1.990(2); Ga(2)−C(31), 1.983(3); Ga(1)−C(63), 2.081(4);
Ga(2)−C(61), 2.037(4); C(61)−C(62), 1.570(6); C(61)−C(65),
1.514(5); C(62)−C(63), 1.573(6); C(62)−C(67), 1.514(7); C(65)−
C(66), 1.501(8); C(66)−C(67), 1.483(6); C(1)−Ga(1)−Ga(2),
141.66(4); C(1)−Ga(1)−C(63), 129.21(1); Ga(2)−Ga(1)−C(63),
87.80(12); C(31)−Ga(2)−Ga(1), 141.78(4); C(31)−Ga(2)−C(61),
121.8(12); C(61)−Ga(2)−Ga(1), 90.47(11).
Scheme 3. Reactions of NBD and Quadricyclane with
iPr4
AriPr EEAr (E = Ga, Ge, Sn) and Other Dienophiles (R =
4
H or Electron-Withdrawing Group, e.g., CN or COOR)
the Ga−Ga single bond range.21 The Ga−C distances to the
NBD unit [2.037(4) and 2.081(4) Å] are slightly longer than
those to the aryl ligand [1.990(2) and 1.983(2) Å]. All of the
C−C bonds within the deltacyclane are within the range of
single C−C bonds [avg 1.525(6) Å].22 Each Ga atom is almost
trigonal-planar-coordinated, although there is a significant twist
about the Ga−Ga bond [C(31)−Ga(1)−Ga(2)−C(1) torsion
angle = 55.80(1)°] to accommodate the size of the large
ligands, which makes the sum of the angles slightly less than
360° [Ga(1), 358.67(9)°; Ga(2), 354.06(9)°].
We hypothesized that the multiple-bonded digallene would
react readily with quadricyclane since the digallene can be
considered as a highly π-electrophilic molecule because of the
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relatively low energy of the LUMO (out-of-plane π orbital) in
comparison to the LUMO of ethylene. Quadricyclane was
independently synthesized by literature methods23 and added
to 1 in hexane at room temperature. An immediate color
change from dark-green to bright-orange was observed, and an
can act as an electrophile in the reaction with the strained σ
bonds of quadricyclane.
iPr4
The group 14 analogue, AriPr SnSnAr , reacts with 2 equiv
4
of NBD in an exo-type cyclization, though the known
diradicaloid nature of the distannyne may result in a preference
for the radical mechanism and preferential double [2π + 2π]
cycloaddition (Scheme 3 bottom).19
In the absence of steric restraints, we would expect to see
some of the analogous [2π + 2π] cycloaddition with the
digallene, as seen with the group 14 analogues. Since we
observed no evidence of this type of reactivity, we propose that
the digallene reaction proceeds via a nonradical cyclization
mechanism. The reaction with NBD likely proceeds via a
concerted 6π cyclization, on the basis of the exclusive formation
of the digalladeltacyclane product 2.
1
orange solid was isolated that was identified as 2 by H NMR
spectroscopy (Scheme 3). The same product is formed through
a formal [2π + 2σ + 2σ] cycloaddition in which the digallene is
the 2π component. In the presence of excess NBD, 2 is formed
as the sole product (as determined by 1H NMR spectroscopy),
unlike the group 14 systems (Scheme 3 bottom), and there was
no evidence for double [2π + 2π] cycloaddition of NBD.
UV−vis spectroscopy of 2 revealed three strong absorbances
at 226, 303, and 488 nm (ε = 3.3 × 104, 8.6 × 103, and 3.3 ×
102 M−1 cm−1, respectively). DFT calculations [M06-HF//6-
311+G(2df)<Ga>/6-31G(d)<others>] of the molecular orbi-
tals of 2 predicted strong absorbances with values that are in
good agreement with the experimental values (Table 1 and
Reaction of 1 with Cyclooctatetraene. Addition of COT
to a solution of 1 in hexane at room temperature resulted in an
immediate color change from dark-green to bright-yellow. X-ray
structural data for the crystals obtained from the solution
iPr4
revealed an addition of a AriPr GaGaAr unit across the COT
4
Table 1. Experimental and Calculated UV−Vis Absorbances
a
and Assigned Transitions for 2−5
ring to give 3, which possesses a central twisted 3,7-
cyclooctadiene core with each side bridged by a gallium
above and below the carbon ring (see Figure 3). This bonding
mode is quite rare and has not previously been seen with
addition of neutral COT to a main-group species.29 Nickel and
silver complexes that possess similar twisted ring structures
have been reported, but in these the four double bonds of the
COT moiety are maintained, and there are two η2-bound
double bonds at each metal atom.29,30 This formal [2π + 2π +
2π] addition of COT to the digallene is in sharp contrast to the
λ/nm (ε/M−1 cm−1
)
b
exptl
calcd
assigned transition
2
3
4
5
226 (3.3 × 104) 258 (0.5478) σGa−Ga/σGa−C to pGa
303 (8.6 × 103) 322 (0.0053) σGa−C/σC−C to pGa
488 (3.3 × 102) 510 (0.0108) σGa−Ga to π*Ar
242 (8.3 × 103) 244 (0.1877) σGa−C to pGa(1)/π*Ar
330 (6.7 × 103) 317 (0.0055) σGa−C to pGa(1)/π*Ar
416 (4.1× 103)
384 (0.1058) σGa−C to pGa(1)
264 (3.1 × 104) 274 (0.2710) σGa−C/πCC to πAr*
addition of COT to the heavier group 14 analogues
351 (9.6 × 103) 364 (0.0801) pGa(1)/pC(3)
AriPr EEAr (E = Ge, Sn), both of which gave a reduction
iPr4
4
435 (4.1 × 103) 402 (0.0202) σGa−C/πCC to pGa(2)/pC(2)
262 (1.2 × 105) 250 (0.1889) σGa−C/πCC to π*Ar
of the polyolefin and resulted in the splitting of the E−E bond
and formation of a π-bound inverse-sandwich compound with a
planar aromatic C8H8 ring (see Scheme 1).6,7 It also differs
from the result reported by Roesky in which the base-stabilized
bis(silylene) [PhC(NtBu)2Si]2 undergoes a [4 + 1] cyclo-
addition with COT to give a Si−N (ligand) opened structure
that maintains a Si−Si bond.31
350 (2.9 × 104) 349 (0.0207) σGa−C/πCC to pGa(1)/Ga(2)/π*CC
/π*Ar
440 (8.0 × 103) 412 (0.0167) σGa−C/πCC to pGa(1)/Ga(2)
a
M06-HF//6-311+G(2df) for Ga and 6-31G(d) for all other
b
elements. Calculated oscillator strengths.
1
Analysis of the room-temperature H and 13C{1H} NMR
spectra revealed the fluxional nature of the central ring, since
Figures S1 and S2 in the Supporting Information). They are a
result of transitions between the σGa−Ga (HOMO) and σGa−C
(HOMO−1) and the empty π orbitals centered on the two
gallium atoms (LUMO) and π* orbitals of the ligand (LUMO
+1 and LUMO+2), as specified in Table 1.
only signals arising from the AriPr ligands were observed.
4
Variable-temperature NMR spectroscopy permitted the reso-
1
lution of two broad singlets in the H NMR spectrum, one
signal for the four olefinic CH groups of the ring and one for
the four aliphatic CH groups (Figure 2). Below 273 K, these
signals appeared at 2.40 and 4.15 ppm. When the sample was
heated to 353 K, the two signals coalesced to one broad singlet
at 3.20 ppm that was well-resolved. The 13C{1H} NMR
spectrum also showed similar behavior, with the appearance of
one olefinic signal at 121.9 ppm and one aliphatic signal at 30.3
We propose that the mechanism of the formal [2π + 2π +
2π] cycloaddition to furnish 2 is similar to the all-carbon
version. The reaction may proceed either via a concerted or
stepwise nonradical cyclization pathway or by a radical
pathway.24 NBD and quadricyclane differ in that they have
FMOs of inverse symmetry,25 which explains their different
reactivities with alkenes. The all-carbon [2π + 2π + 2π] homo-
endo-type cyclization reaction13a,26 between an electron-
deficient dienophile and NBD is symmetry-allowed and gives
a five-membered ring, whereas the [2π + 2π] exo-type
cyclization reaction resulting in a four-membered ring is
symmetry-forbidden (see Scheme 3). These same dienophiles
react preferentially to form exo-type products when added to
quadricyclane.27 In contrast, 1 reacts in a homo-endo fashion
with both substrates (Scheme 3). Several examples of
electrophilic ring-opening reactions of a strained cyclopropane,
and more specifically with quadricyclane, are known for all-
carbon systems.28 The relatively low-lying π-type LUMO of 1
1
ppm upon cooling to 243 K. The H−13C correlation for the
new peaks was confirmed by low-temperature two-dimensional
HSQC NMR spectroscopy (see Figure S5 in the Supporting
Information).
The molecular orbitals of 3 were calculated by DFT analysis
[M06-HF//6-311+G(2df)<Ga>/6-31G(d)<others>] and the
UV−vis absorptions were also predicted (see Figures S3 and S4
in the Supporting Information). The main absorbance,
predicted to be at 416 nm, is attributed to the transition
from the HOMO, which is centered on the Ga−C σ bonds
between gallium and the central carbon ring, to the LUMO,
involving the empty π orbital at Ga(1) and the π* system of the
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1
Figure 2. Stacked plot of the aliphatic region of the H NMR spectra
of 3 in C7D8. A small amount of 3iso was present, as indicated by peaks
labeled with *.
terphenyl ligand (Figure S3 in the Supporting Information).
The other transitions, predicted to be at 244 and 317 nm, are
from the HOMO to the LUMO+1 (aryl ligand π* orbital) and
the HOMO to the LUMO+2 [empty π orbital on Ga(2)]. The
predicted and experimental absorption bands of 3 were in good
agreement (see Table 1).
The solid-state structure of 3 showed two crystallographically
independent molecules in the unit cell (Figure 3). This is
consistent with the fluxional behavior of the central ring seen in
the 1H NMR spectrum. There are two slightly different
orientations of the central ring, which result in different
interactions with gallium. There is an increased twist in the
ring, with short Ga(1)−C(34), Ga(1A)−C(33), Ga(2)−C(74),
and Ga(2A)−C(77) contacts [2.454(5), 2.538(5), 2.508(4),
and 2.405(5) Å, respectively] as a result. The gallium p orbital
can interact with the remaining π bond of the resultant 1,5-
cyclooctadiene ring. The bond distances between gallium and
the ligand [avg 1.955(14) Å] as well as gallium and the central
1,5-cyclooctadiene (COD) ring [avg 1.988(2) Å] are within the
expected range for Ga−C single bonds,21 though the distances
from the central ring to Ga are slightly longer than the aryl−Ga
distances in the starting compound 1.12 The two C−C bonds at
either end of the central ring remain unbroken double bonds
[avg 1.345(8) Å], while the bridging C−C bonds are slightly
longer [avg 1.563(5) Å] than the rest of the C−C single bonds
in the ring [1.485(6)−1.534(6) Å]. The cross-ring Ga···Ga
distance of 3.880(4) Å is too long for significant bonding. The
geometry at each gallium is nearly perfectly trigonal-planar,
since the sum of the angles at gallium are 359.7(1)° and
360.0(1)°.
Figure 3. Thermal ellipsoid (30%) plot of the two crystallographically
independent molecules of 3. Selected H atoms, a symmetry-related
second central COD ring for each independent molecule, and
isopropyl group disorder are not shown. Selected bond lengths (Å):
Ga(1)···Ga(1A), 3.880(4); Ga(1)−C(1), 1.959(9); Ga(1)−C(31),
2.005(3); Ga(1)···C(34), 2.454(5); Ga(1)−C(35), 1.999(3); Ga-
(1A)···C(32), 1.954(3); Ga(1A)−C(36), 2.005(3); Ga(1A)···C(33),
2.528(5); Ga(2)−C(41), 1.9505(14); Ga(2)−C(71), 2.022(3);
Ga(2)···C(74), 2.508(4); Ga(2)−C(75), 1.974(3); Ga(2A)−C(72),
2.034(3); Ga(2A)−C(76), 1.916(3); Ga(2A)···C(77), 2.405(5);
C(31)−C(32), 1.566(5); C(32)−C(33), 1.509(6); C(33)−C(34),
1.354(9); C(34)−C(35), 1.500(7); C(35)−C(36), 1.564(4); C(36)−
C(37), 1.493(7); C(37)−C(38), 1.346(9); C(38)−C(31), 1.488(8);
C(71)−C(72), 1.561(5); C(72)−C(73), 1.508(7); C(73)−C(74),
1.291(8); C(74)−C(75), 1.534(6); C(75)−C(76), 1.560(4); C(76)−
C(77), 1.485(7); C(77)−C(78), 1.390(7); C(71)−C(78), 1.485(6).
Scheme 4. Possible Mechanism for the Formation of 3 and
Isomerization Back to the Cyclization Adduct 3iso
The tub shape of free COT is known to prevent a planar
alignment of at least one diene unit, making COT an ineffective
4π component in [4π + 2π] cycloaddition reactions with
alkenes. The formation of a highly reactive valence tautomer of
COT, bicyclo[4.2.0]octa-2,4,7-triene, which does possess a
planar 4π component, has been shown to be present in
equilibrium in solution.32 It is possible that 1 can react in a [2π
+ 2π] radical fashion with the cyclobutene fragment of this
isomer to give a cycloadduct intermediate (3iso) or equally in a
[4π + 2π] fashion (Scheme 4). A [6π + 2π] cyclization with
COT would furnish intermediate 3iso via a nonradical pathway
(which would proceed in an antarafacial fashion under thermal
conditions for all-carbon systems) and homolytic cleavage of
the Ga−Ga bond followed by reaction across the central olefin
could give 3. Crystals of 3 are stable under a N2 atmosphere,
1
but we observed gradual isomerization of 3 in solution by H
NMR spectroscopy. The isomerization gives an equilibrium
1
mixture of 3 and a product with four magnetically distinct H
signals for the ring protons. We were led to the conclusion that
this isomer could be the intermediate 3iso in the mechanistic
pathway shown (Scheme 4).
and could also lead to 3. A [1,3]-shift of the AriPr Ga unit
4
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Two-dimensional NMR spectroscopy (COSY and HSQC)
supported the assignment for the structure of the isomeric
product, although additional definitive structural character-
ization of this isomer could not be obtained (see the
Supporting Information) since the structure of 3iso could not
be confirmed by X-ray crystallographic analysis. Other
supporting evidence for the structure of 3iso is based on a
comparison with the related [6π + 2π] metal-promoted
cyclization observed with alkenes and alkynes.3,33 We also
1
observed the formation of free COT (5.77 ppm) in the H
NMR spectrum of the sample after the solution was stored for
several days at room temperature under a nitrogen atmosphere.
1
The reformation of 1 was difficult to confirm by H NMR
spectroscopy because the isopropyl methine and diastereotopic
methyl resonances overlapped and were obscured by the
significantly more intense resonances of 3, which was also
present. However, the reappearance of COT supports the
proposed mechanism and the overall reversibility of these
cyclization reactions.
The product 3 could potentially form via a radical
mechanism with two double bonds of COT without invoking
an initial cyclization reaction. However, treatment of 1 with 1,5-
cyclooctadiene gave no apparent color change, and no reaction
could be detected by 1H NMR spectroscopy, even under more
forcing conditions (65 °C, 3 h). Treatment of 1 with 1,4-
cyclohexadiene also did not afford a reaction under similar
conditions (85 °C, 3 h). These results indicate that the
conjugation of the cyclic polyolefins is crucial for reaction with
the digallene, ruling out the possibility for a [2π + 2π + 2π]
reaction with cyclic nonconjugated polyolefins.
Figure 4. Thermal ellipsoid (30%) plot of 4. A pentane solvent
molecule and ligand H atoms are not shown. Selected bond lengths
(Å) and angles (deg): Ga(1)−Ga(2), 2.4534(7); Ga(1)−C(1),
2.042(2); Ga(2)···C(3), 2.444(3); Ga(2)−C(4), 2.076(2); Ga(2)−
C(6), 1.995(2); Ga(1)−C(36), 1.983(2); C(1)−C(2), 1.479(4);
C(2)−C(3), 1.338(4); C(3)−C(4), 1.426(4); C(4)−C(5), 1.514(4);
C(1)−C(5), 1.546(4); C(1)−Ga(1)−C(36), 121.95(8); Ga(2)−
Ga(1)−C(36), 149.12(5); C(1)−Ga(1)−Ga(2), 88.31(6); C(6)−
Ga(2)−C(4), 121.01(9); C(6)−Ga(2)−C(3), 119.47(8); C(4)−
Ga(2)−C(3), 35.61(10); C(6)−Ga(2)−Ga(1), 148.82(5); C(4)−
Ga(2)−Ga(1), 88.84(7); C(3)−Ga(2)−Ga(1), 81.47(6).
iPr4
pentenyl ring is bound. In the starting AriPr GaGaAr , the
4
diastereotopic methyl groups appear as a pair of overlapping
doublets at 1.08 ppm and the ligand isopropyl methine as a
septet at 2.96 ppm. In 4, however, the signals for the methyl
and methine groups appear as complex multiplets at ca. 1.1 and
2.9 ppm, respectively. They also overlap with the aliphatic CH
and CH2 groups of the cyclopentene ring. A distinct signal
attributable to the olefinic CH groups of the cyclopentene
appear as a singlet at 5.89 ppm. The 13C{1H} NMR spectrum
displays seven aliphatic signals that are also indicative of the
lower symmetry of the ligand environment but also possesses a
new olefinic signal at 120.5 ppm attributable to the cyclo-
pentene ring. The UV−vis spectrum of 4 revealed strong
absorptions at 264 nm (ε = 3.1 × 104 M−1 cm−1), 351 nm (ε =
9.6 × 103 M−1 cm−1), and 435 nm (ε = 4.1 × 103 M−1 cm−1).
DFT calculations [(M06-HF//6-311+G(2df)<Ga>/6-31G(d)
<others>] on 4 revealed that the HOMO is centered on the
Ga−Cring σ bonds and the CC π bonds in the ring (Figures
S7 and S8 in the Supporting Information). The absorbances
predicted by the DFT calculations involve transitions from the
HOMO to the LUMO and LUMO+1 [involving the empty p
orbitals at Ga(1) and Ga(2)] and the LUMO+2 (ligand π*Ar)
and agree well with the experimental results (Table 1).
Reaction of 1 with Cyclopentadiene. Weidenbruch and
co-workers previously showed that a tetra-tert-butyldisilene
(generated in situ from the photolysis of hexa-tert-butylcyclo-
trisilane) reacted with CpH to provide the [4π + 2π]
cycloadduct along with other unidentified products.34 We
treated 1 with CpH and observed an immediate color change
from green to orange. Concentration and cooling of the
solution to ca. −18 °C for 3 weeks afforded X-ray-quality
crystals of 4, and the structure of 4 is shown in Figure 4.
The Ga−Ga bond is maintained [2.4534(7) Å] and is within
the range expected for a Ga−Ga single bond.21 The
iPr4
cyclopentene fragment is bound to the AriPr GaGaAr moiety
4
by three short contacts, Ga(1)−C(1) [2.042(2) Å], Ga(2)−
C(3) [2.444(3) Å], and Ga(2)−C(4) [2.076(2) Å]. The
average aryl−Ga distance [1.989(2) Å] is shorter than the Ga−
ring carbon distance seen in all of the previous structures.
Within the ring, the shortest distance involves C(2)−C(3)
[1.338(4) Å], as would be expected if a [4π + 2π] reaction had
taken place. The distances for C(1)−C(2) and C(3)−C(4) are
1.479(4) and 1.426(4) Å, respectively, which are significantly
shorter than normal C−C single bonds (ca. 1.54 Å) but longer
than CC double bonds (ca. 1.33 Å).22 These single bonds
are likely shortened as a result of an additional interaction and
short contact of C(3) with Ga(2). The two remaining C−C
bonds in the ring, C(4)−C(5) [1.514(4)] Å and C(1)−C(5)
[1.546(4) Å], are within the range expected for C−C single
bonds.22 The large terphenyl rings are twisted [C(6)−Ga(1)−
Ga(2)−C(36) torsion angle = 26.58(14)°] to accommodate
the cyclopentene ring within the isopropyl groups. The sum of
the angles at each Ga is consistent with trigonal-planar
coordination [Ga(1), 359.37(6)°; Ga(2), 359.69(7)°].
Finally, we note that the reaction of 1 with CpH stands in
sharp contrast to that of its germanium counterpart
iPr4
5
AriPr GeGeAr , which releases H2 and forms AriPr Ge(η -
C5H5).35 In effect, the germanium species behaves as a reducing
agent analogous to sodium, whereas 1 behaves as a simple 2π
electron moiety in its interaction with the 4π electrons of CpH.
Reaction of 1 with 1,3,5-Cycloheptatriene. We then
tested a larger conjugated ring to determine whether it would
undergo the same [4π + 2π] reaction or an [nπ + 2π]
cyclization, where n is the number of conjugated π electrons in
the ring. In comparison, the [6π + 2π] cycloaddition of CHT
with alkenes has been shown to proceed under transition-metal
catalysis conditions with iron(0),33a chromium(0), and
cobalt(0) complexes of CHT3,33b,36 as well as with
4
4
1
The H NMR spectrum of 4 is complicated as a result of its
lower symmetry arising from the way in which the cyclo-
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1
Figure 5. Stacked plot of the H NMR spectra recorded in C7D8 illustrating the isomerization of 5 to 5iso with the appearance of free CHT (*).
Bottom to top: 313, 333, 353, 373, and 373 K after 10 min.
TiCl4·Et2AlCl as a catalyst.37 The [6π + 2π] cyclization has
been shown to proceed in a stepwise manner to avoid
Scheme 5. Proposed Mechanism for the Formation of 5 and
the Isomerization to 5iso
symmetry restrictions. The use of a catalyst allows the
symmetry restrictions to be lifted by altering the symmetry of
the HOMO, allowing the complex to undergo pericyclic
reactions and/or by enhancing the rate of a multistep reaction.
Reaction of 1 with CHT in hexane resulted in an immediate
color change of the solution from dark-green to dark-red. The
1H NMR spectrum displayed five resonances that could be
assigned to the ring protons (Figure 5 bottom), which allowed
us to make a tentative assignment of the cyclized adduct as
isomerization from 5 to 5iso, although the steric bulk of the
product 5. However, some of the signals in the spectrum were
broad, so variable-temperature 1H NMR spectroscopy was
terphenyl ligand makes this possibility unlikely. As seen with
undertaken. At 313 K, two broad resonances at 4.2 and 5.1 ppm
were assigned to the four olefinic CH groups, and the signal at
3.3 ppm was assigned to the CH groups adjacent to the Ga
atoms in 5. A distinctive doublet at 2.3 ppm was assigned to the
bridgehead proton oriented toward the diene moiety, with a
the COT cyclization, CHT is known to form a highly strained
bicyclic species, norcaradiene,38 which could also potentially
undergo a radical-mediated cyclization to give 5.
Concentration and cooling of a diethyl ether solution of 1
and excess CHT to −18 °C afforded X-ray-quality crystals. The
solid-state structure revealed that the [4π + 2π] cyclization
product 5iso had preferentially crystallized, as evidenced by the
formation of the Ga(1)−C(61) and Ga(2)−C(65) single
bonds (Figure 6).
2
large JHH coupling constant of 16.8 ppm. The bridgehead
proton oriented toward the Ga−Ga moiety at 1.57 ppm also
had a large coupling, but it was not well resolved. When a
sealed C7D8 solution of 5 was cooled to 253 K, however, the
olefinic signals remained broad (data not shown). The solution
was then gradually heated to 373 K and held at that
temperature for 10 min (Figure 5 top). The disappearance
and temperature-dependent drift in the chemical shift for the
broad resonances of 5 was observed. Free CHT signals were
also identified in the spectra between 353 and 373 K. The
appearance of seven well-resolved signals was observed (the
eighth signal was determined to be buried beneath the
isopropyl methine multiplet by integration and 2D COSY
NMR spectroscopy (Figure S11 in the Supporting Informa-
tion). A color change from red to yellow accompanied the
gradual appearance of seven well-resolved resonances. Cooling
of the sample to room temperature did not restore the red
The ring bond distances revealed that double bonds are
present between C(63)−C(64) [1.367(3) Å] and C(66)−
C(67) [1.337(4) Å] after the cyclization, indicative of a 1,4-
cycloheptadienyl fragment that is bound to the two galliums of
iPr4
AriPr GaGaAr at the C(61) and C(66)/C(65) atoms. The
4
C(66)−C(67) distance [1.337(4) Å] is slightly shorter than
that in the other C−C double bond in the ring [C(63)−C(64),
1.367(3) Å] as a result of the Ga(2)−C(66) interaction, which
is relatively short [2.352(3) Å]. The Ga(2)−C(65) and
Ga(1)−C(61) distances [2.219(4) and 2.020(7) Å, respec-
tively] are reasonable for alkyl Ga−C single bonds21 but longer
than the Ga−aryl distances [1.985(3) and 1.983(6) Å] seen in
the other structures 2, 3, and 4. All of the other C−C bonds in
the ring are single bonds. The Ga(1)−Ga(2) bond distance is
within the expected range for a Ga−Ga single bond.21 The
terphenyl ligands are in an approximately cis orientation as a
result of bonding to the cycloheptadiene ring. The cyclo-
heptadiene ring is positioned at nearly right angles with respect
to the Ga−Ga bond [C(65)−Ga(2)−Ga(1) = 97.18(6)° and
C(61)−Ga(1)−Ga(2) = 95.63(5)°]. The sums of the angles at
the two gallium atoms [359.9(5) and 359.3(6)°] indicate near
trigonal-planar geometry, discounting the weak interaction to
C(66).
1
color or the original H NMR spectrum. Instead, the spectrum
indicated the formation of the isomer 5iso, which is likely the
thermodynamically favored product. The isomerized product
5
iso showed an unsymmetrical and unique environment for each
1
proton in the ring by H NMR spectroscopy.
On the basis of the NMR data, we propose that an initial [6π
+ 2π] cycloaddition occurs to form 5, which is the kinetic
product. At elevated temperatures, the cyclization reaction is
reversible, producing free CHT in solution, which can then
recyclize via a [4π + 2π] cyclization to give the thermodynami-
cally preferred structural isomer, 5iso (Scheme 5). It is also
The UV−vis spectrum of 5iso revealed strong absorptions at
440 nm (ε = 8.0 × 104 M−1 cm−1), 350 nm (ε = 2.0 × 104 M−1
possible that a [1,3]-sigmatropic AriPr Ga shift may cause the
4
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and π*-type LUMO, the calculated FMOs for the heavier group
13 multiple-bonded analogues are both of π symmetry,
although their overall shapes are very different (Figure 7).
Figure 6. Thermal ellipsoid (30%) plot of 5iso. Ligand H atoms and a
cocrystallized diethyl ether solvent molecule are not shown. Selected
bond lengths (Å) and angles (deg): Ga(1)−Ga(2), 2.469(3); Ga(1)−
C(1), 1.985(2); Ga(2)−C(31), 1.983(6); Ga(1)−C(61), 2.020(7);
Ga(2)−C(65), 2.219(4); Ga(2)−C(66), 2.352(3); C(61)−C(62),
1.507(3); C(62)−C(63), 1.447(3); C(63)−C(64), 1.367(3); C(64)−
C(65), 1.479(4); C(65)−C(66), 1.398(4); C(66)−C(67), 1.337(4);
C(67)−C(61), 1.494(3); C(1)−Ga(1)−Ga(2), 136.48(4), C(1)−
Ga(1)−C(61), 128.89(6); C(31)−Ga(2)−Ga(1), 138.13(5); C(31)−
Ga(2)−C(65), 123.98(7); Ga(1)−Ga(2)−C(65), 97.18(6); Ga(2)−
Ga(1)−C(61), 118.09(14).
Figure 7. Illustration of the FMOs of 1 calculated using DFT
[B3PW91/6-311+G(2df) for Ga, 6-31G(d) other atoms; contour
value = 0.04] compared with the FMOs of ethylene.12
This π−π FMO arrangement differs not only from that of the
olefins but also from that of the neighboring heavier group 14
analogues, whose FMOs are of π−n+ symmetry. In essence, the
heavier group 13 dimetallene FMOs can interact with both the
HOMO and LUMO FMOs of the polyolefins, regardless of
their symmetry. This difference has broad implications, as these
systems are less restricted by the Woodward−Hoffmann
symmetry rules, specifically as applied to predict cycloaddition
reactivity. This is illustrated in the reactions of NBD and
quadricyclane with 1. These two substrates present a similar
steric profile, but their FMOs have been shown to be of inverse
symmetry.25 Different products are obtained in the reactions of
NBD and quadricyclane with olefins, since the olefins obey the
orbital symmetry restrictions of the Woodward−Hoffmann
rules. Olefins react with NBD to give the homo-endo
cyclization product but react with quadricyclane to give the
exo-type product. The π−π symmetry of the FMOs of 1 allow
it to react with both substrates to give the same homo-endo
digalladeltacyclane product, regardless of the Woodward−
Hoffmann restrictions.
This effect is also demonstrated by the [6π + 2π] cyclization
reactions studied. The thermal cyclization reaction of alkenes
with 6π polyolefins is symmetry-forbidden, but it has been
shown to proceed in the presence of several group 10 metal
catalysts.3,33b,36,37 The presence of electron-withdrawing groups
in the olefin lowers the energy of the π* LUMO, enhancing the
d−π* back-bonding interaction from the zero-valent metal to
the olefin and increasing the reactivity. As we have shown, 1
reacts readily with CHT and COT without the need for a
transition-metal catalyst.
cm−1) and 265 nm (ε = 1.2 × 105 M−1 cm−1). DFT calculations
[M06-HF//6-311+G(2df)<Ga>/6-31G(d)<others>] on 5 af-
forded molecular orbitals similar to those of 4, with the HOMO
centered on the Ga−Cring bonds with a significant contribution
from the π bonds of the ring (see Figures S9 and S10 in the
Supporting Information). In this structure, the DFT-predicted
absorbances are due to transitions from the HOMO to the
LUMO [empty p orbitals on Ga(1) and Ga(2)], LUMO+1
(ring π*CC), and LUMO+2 (ligand π*Ar) and agree
moderately well with the experimental results (Table 1).
GENERAL REMARKS AND CONCLUSIONS
■
As has been shown above, the heavier group 13 dimetallene
iPr4
AriPr GaGaAr (1) participates in cyclization reactions with a
4
range of cyclic polyolefins, including CpH, CHT, COT, NBD,
and the strained polycycle quadricyclane, that are akin to the
corresponding cyclization reactions with alkenes but proceed
under much milder conditions. Furthermore, the dimetallene
either reacts with polyolefins that are unreactive toward the
corresponding heavier group 14 analogues (in the case of
CHT) or reacts to give products that are significantly different.
iPr4
For example, AriPr EEAr (E = Ge, Sn) give inverse-sandwich
4
products when reacted with COT,6,7 π complexes along with
H2 release when reacted with CpH,35 and double [2π + 2π]
cycloaddition products with NBD.19 On the basis of the
products formed, we postulate that the group 13 analogues are
more likely to react in a nonradical fashion, whereas the heavier
group 14 analogues are more diradicaloid character39 making
them more prone to undergo reactive pathways not seen for
carbon. 1 reacts much more readily than olefins and without
the use of transition-metal catalysts or forcing conditions.
The Woodward−Hoffmann rules for all-carbon cyclo-
additions predict that both [2π + 2π] and [6π + 2π]
cycloadditions are symmetry-forbidden, while the [4π + 2π]
Diels−Alder and [2π + 2π + 2π] homo-Diels−Alder reactions
are symmetry-allowed under thermal conditions.40 Unlike the
FMOs for simple alkenes, which consist of a π-type HOMO
DFT calculations showed that, in addition to significant
differences in the symmetry of the FMOs, there are substantial
energy differences between the FMOs of 2π alkenes and the
heavier group 13 analogues. The heavier group 13 analogues
are in effect more electron-deficient and more electron-rich
reactive partners than simple alkenes (cf. Figure 7). This effect
has previously been implicated in the ready reactivity of
disilenes.41 The increased reactivity of the group 13 dimetallene
is also due to the relatively weak Ga−Ga bonding and the
calculated HOMO−LUMO gap of 2.7 eV (ca. 62 kcal mol−1),
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which is significantly smaller than the value of 7.8 eV for
ethylene (Figure 7). This effect is evident experimentally, as an
increase in reactivity is seen for alkenes that are activated
toward cycloaddition reactions by the presence of strongly
electron-withdrawing groups. Tetracyanoethylene is the only
alkene that undergoes [2π + 2π + 2π] cyclization with NBD
and [2π + 2σ + 2σ] cyclization with quadricyclane without the
need for transition-metal catalysis.27a,42
Periselectivity is a concern in carbon chemistry, since higher-
order cyclization reactions are known to afford complex
mixtures of products due to multiple irreversible competitive
pathways (i.e., [2π + 2π] vs [4π + 2π] vs [6π + 2π]).3 While we
observed different cyclization products for the reactions of 1
with the larger cyclic polyolefins, we also observed temperature-
dependent interconversion between them. In the reaction of 1
with CHT, both [6π + 2π] and [4π + 2π] cyclization products
(5 and 5iso, respectively) were observed. The proposed
mechanism for this reaction relies on the reversibility of the
cyclizations, where complete isomerization of the initially
formed [6π + 2π] product 5 occurred to form the [4π + 2π]
adduct 5iso at elevated temperatures. CpH also undergoes a [4π
+ 2π] cyclization with 1 to afford 4. These [4π + 2π] and [6π +
2π] cyclization products are structurally analogous to those of
the all-carbon systems.
alkenes, alkynes, and their heavier group 14 element analogues.
The increased electrophilicity and nucleophilicity of these
species and their propensity to undergo nonradical cyclization
pathways make them closer analogues to the all-carbon system
than the heavier group 14 analogues, which behave in a manner
more consistent with significant diradicaloid character.39
ASSOCIATED CONTENT
■
S
* Supporting Information
Calculated structures, molecular orbital figures for 2−5, 2D
COSY and HSQC experiments, and X-ray crystallographic data
(CIF) for 2−5. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
pppower@ucdavis.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the U.S. Department of Energy (DE-FG02-
07ER46475) for funding and the Natural Sciences and
Engineering Research Council of Canada (NSERC) for a
postdoctoral fellowship. J.-D.G. and S.N. are grateful for a
Grant-in-Aid for Specially Promoted Research from the MEXT
of Japan.
In the reaction of 1 with COT, the [4π + 2π] and [6π + 2π]
cyclization pathways are indistinguishable, as they both afford
the same product, 3. Isomerization of 3 was observed, however,
yielding the structural isomer 3iso in solution over time. This
onward isomerization is not seen in the all-carbon cyclization
product, indicating that even after cyclization, the Ga−Ga bond
remains highly reactive, likely as a result of the remaining low-
lying LUMO, which consists of empty π-type orbitals at each
gallium.
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