Cycloaddition of Dialkylalumanyl Anion toward Unsaturated Hydrocarbons in (1+2) and (1+4) Modes

Article abstract of DOI:10.1002/chem.201905830

The reactivity of dialkylalumanyl anion (1) towards naphthalene, anthracene, diphenylacetylene, and (E)/(Z)-stilbenes was investigated. The compound 1 reacts with naphthalene and anthracene through (1+4) cyclization, giving Al-containing norbornadiene derivatives. In the reaction of 1 with diphenylacetylene and (E)/(Z)-stilbenes, (1+2) cyclization proceeded to form Al-C-C three-membered rings. Cyclization toward (E)- or (Z)-stilbenes solely gave a trans-cycloadduct. DFT calculations revealed that the cycloaddition of 1 with (Z)-stilbene proceeds via a single transition state with a carbanion character, which results in the selectivity towards the trans-cycloadduct.

Full text of DOI:10.1002/chem.201905830

A Journal of
Accepted Article
Title: Cycloaddition of Dialkylalumanyl Anion toward Unsaturated
Hydrocarbons in (1+2) and (1+4) Modes
Authors: Makoto Yamashita, Kengo Sugita, and Ryo Nakano
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Cycloaddition of Dialkylalumanyl Anion toward Unsaturated
Hydrocarbons in (1+2) and (1+4) Modes
[a]
Kengo Sugita, Ryo Nakano, Makoto Yamashita*
[a]
K. Sugita, Dr. R. Nakano, Prof. Dr. M. Yamashita
Department of Molecular and Macromolecular Chemistry
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603 Aichi (Japan)
E-mail: makoto@oec.chembio.nagoya-u.ac.jp
Abstract: The reactivity of dialkylalumanyl anion (1) toward
naphthalene, anthracene, diphenylacetylene, and (E)/(Z)-stilbenes is
calculations, a possible reaction mechanism for the cycloaddition
reaction was proposed.
investigated. The compound
1 reacts with naphthalene and
anthracene through (1+4) cyclization, affording Al-containing
norbornadiene derivatives. In the reaction of 1 with diphenylacetylene
and (E)/(Z)-stilbenes, (1+2) cyclization proceeded to form Al–C–C
three-membered rings. Cyclization toward (E)- or (Z)-stilbenes solely
Base-stabilized and neutral Al(I)
R5
H
Dip =
Al
Al
N
N
Dip
Al
Dip
L
L
A
tBu
C
Dip
N
B
C
R
5
= H
2
3
L =
gave
a trans-cycloadduct. DFT calculations revealed that the
cycloaddition of 1 with (Z)-stilbene proceeds via a single transition
state with a carbanion character, which results in the selectivity toward
the trans-cycloadduct.
Anionic Al(I)
Dip
Dip
N
Al
N
Dip
Me
3
Si
Si
SiMe
3
Si
N
K
Si
O
Si
O
Al
Al K(tol)
SiMe
1
2
Si
N
K
Dip
Me
3
3
D
Main group element compounds, especially in low oxidation state,
have been known to undergo oxidative addition at the main group
element center to give oxidized compounds.[1] Among them,
aluminum-containing molecules in the low oxidation state have
been extensively studied in the last three decades,[2] especially
for dialumane derivatives having an Al–Al bond,[3] oligomeric form
of Al(I) species,[4] and Al=Al doubly bonded compounds.[3o-s, 5]
While the parent monomeric halogenated Al(I) species "AlX" (X =
Cl, Br) are unstable and only can be handled in matrices,[6]
isolation of monomeric and nucleophilic Al(I) species are still
limited to a handful of examples (Figure 1), including CpAl(I)
species A,[7] nacnac-substituted species B,[8] Al(I) hydride C
stabilized by two CAAC ligands,[9] diamino-Al anion D,[10] and
oxygen-stabilized diamino-Al anions E[11] and F.[12] We recently
reported an alkyl-substitued alumanyl anion 1,[13] featuring strong
basicity to deprotonate benzene at room temperature as an
anionic aluminum nucleophile.
Base-stabilized and anionic Al(I)
tBu
tBu
tBu
Dip
Dip
Dip
Dip
Dip
Al
N
K
N
N
N
O
Al
Al
O
O
N
K
N
F
Dip
tBu
E
tBu
tBu [K(cryptand)]
Figure 1. Isolated examples of monomeric Al(I) compounds.
(
a) (1+2)
(b) (1+4)
R
N
N
Dip
Al
N
Dip
R
N
N
or R
B
+
Dip
Al
Dip
R
B + Dip
Ca Dip
J
N
N
G
R
R
Dip
Ca Dip
N
O
[B(C F ) ]
6 5
4
Si
N
Si
N
Some of these Al(I) compounds in Chart 1 reacted with
unsaturated hydrocarbons to give the corresponding
cycloadducts (Scheme 1). The nacnac-stabilized Al(I) species B
underwent (1+2) cycloaddition with alkynes and alkenes to form
Al-containing three-membered ring species G.[14] The diamino-Al
6 5 4
[B(C F ) ]
Dip
Al
Dip
D +
K
(c) (1+2) to (1+4)
n
H
O
Si
N
Si
N
t-Bu
Dip
H + 18-c-6
Dip
Al
Dip
anion D cyclized with cyclooctatetraene to give H via (1+2)
cycloaddition.[10a] Similarly, the monomeric form of oxygen-
N
K
F
+
O
Al
stabilized Al anion F reacted with ethylene to form (1+2)
cycloadduct I. On the other hand, B also reacted with a benzene
K(18-c-6)
N
Dip
I
complex of [nacnac-Ca]+ to furnish an alumanorbornadiene
t-Bu
(dinuclear complex via K )
+
product J.[15] The anionic bicyclic compound H rearranged to the
(
1+4) product K
upon addition of 18-crown-6.[10a] Thus,
Scheme 1. Reactions of isolated monomeric Al(I) compounds with unsaturated
compounds B and F are examples of Al(I) compound which
exhibit reactivity for two different modes of cycloaddition reaction.
It should also be noted that there is no stereochemical information
in these Al-containing cycloaddition reactions. Herein, we report
dialkyl-substituted Al anion 1 undergoes oxidative cycloaddition in
both (1+4) and (1+2) modes to give the corresponding Al-
containing norbornadiene, cyclopropene, and cyclopropane
compounds. From the structure of the products and DFT
hydrocarbons.
Reaction of 1 with unsaturated hydrocarbons furnished the
corresponding (1+4) or (1+2) cycloaddition products (Scheme 2).
The reaction of 1 with naphthalene at room temperature gave an
aluminum-containing benzonorbornadiene derivative
2
as
1
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COMMUNICATION
colorless crystals in 96% isolated yield, through (1+4) cyclization
The single crystal X-ray diffraction analysis revealed
characteristic structural parameters of 2, 3, 4, and trans-5 in
comparison with those of reference compounds (Table 1).
reaction. This is in contrast to that F reacted with naphthalene at
8
0 °C to form an isomeric mixture of (naphthyl)(hydrido)-
aluminate,[12]
and the isoelectronic dialkylsilylene
:Si[C(SiMe , 6) bearing the same “helmet” substituents
Compounds
2 and 3 are the first examples of anionic
(
3
)
2
CH
2
]
2
aluminanorbornadiene derivatives. Two newly formed Al–
C(bridgehead) bonds of 2 and 3 [2: 2.1118(15), 2.1087(15) Å; 3:
2.1125(12), 2.1154(15) Å] are longer than those of the previously
reacted with naphthalene to furnish mono- or bis-(1+2) adducts
under the irradiation of visible light.[16] Similarly, 1 was treated with
anthracene to afford (1+4) cyclization product 3 as a colorless
solid in 66% yield. The bicyclic structures of 2 and 3 was
unambiguously confirmed by a single crystal X-ray diffraction
[20]
J,[15] K[10a]),
reported Al-incorporated bicyclic compounds (L,
reflecting the steric repulsion between four trimethylsilyl
substituents and benzoannulated moiety in 2 and 3. As a result,
the C–Al–C angles in 2 and 3 [2: 75.10(6)°; 3: 75.21(6)°] are
slightly smaller than those of reference compounds. Similar
tendencies were found in the Al–C bonds and C–Al–C angles of
4 and trans-5 in comparison with those of reference compounds
(G1-G6,[14] H[10a]). One exception from these structural tendencies
could be found in the structure of reference compound M probably
analysis (Figure 2a,b). The 1H NMR spectrum of 2 in THF-d
8
exhibited a C symmetrical pattern which is matched with the
s
anionic part of the crystal structure of 2 with a characteristic
multiplet signal of two bridgehead protons at d 2.89 ppm which is
correlated with a signal of alkenyl protons at d 5.99 ppm. In the
1
case of 3, the H NMR exhibited one singlet signal of magnetically
equivalent four TMS groups and one signal of the bridgehead
due to the steric hindrance among bulky CH(SiMe
substituents.[21]
3 2
)
protons at
aluminadibenzonorbornadiene structure. The reaction of 1 with
diphenylacetylene furnished (1+2) cyclized product,
aluminacyclopropene 4, which was recrystallized from Et O. X-
d
3.34 ppm, indicating the
C2v-symmetrical
a
2
ray crystallographic analysis revealed a three-membered ring
structure of 4 (Figure 2c). The resulting 4 exhibited carbanionic
character, as confirmed by a hydrolysis to give (Z)-stilbene (see
SI). The 1H NMR spectrum of 4 suggested a C2v-symmetrical
structure of 4, similar to the case of 3. Cyclization of 1 also took
place with (Z)- or (E)-stilbenes to give the same single product
(
Fig S9), trans-diphenylaluminacyclopropane (trans-5), which
was crystallized after complexation with [2.2.2]-cryptand. X-ray
diffraction study unambiguously confirmed the trans
stereochemistry of two phenyl substituents in trans-5 (Figure 2d).
Note that corresponding cis product was not observed by NMR
measurements, indicating that the (1+2) cyclization of 1 with
stilbenes took place in
a stepwise manner to give a
thermodynamically stable trans isomer (vide infra). It should be
noted that the reaction of 1 with (E)-stilbene under the dark also
proceeded to afford trans-5. In contrast, compound 1 did not react
with cyclohexene, probably due to electron richness on the C=C
Figure 2. ORTEP drawings of (a) 2, (b) 3, (c) 4, and (d) trans-5 with 50%
probability (hydrogen atoms, co-crystallized solvent molecules, and
intermolecular interactions were omitted for clarity).
[17]
double bond (vide infra). It has been reported that 6
did not
react with (Z)- and (E)-2-butenes at <10 °C under the dark but
reacted with them at a slightly higher temperature or upon
irradiation of visible light (>420 nm) to afford the corresponding
stereoretained silacyclopropane derivatives.[18],[19]
Table 1. Structural parameters (Å, °) of 2, 3, 4, and trans-5 with those of
reference compounds.
cpd
2
3
L
J
K
Al–C
2.1118(15) 2.1125(12) 2.059(4)
2.1087(15) 2.1154(15) 2.055(4)
2.060(3) 2.025(3)
2.073(3) 2.044(3)
Ph
Ph
(
5.0 equiv.)
(3.0 equiv.)
Al
K(tol)
K(tol)
Al
K(OEt2)2
C–Al–C 75.10(6)
75.21(6)
81.5(1)
77.2(1)
88.1(1)
1)
Ph
2
(96%)
4
(68%)
1
Ph
(Z) or (E)
(3.0 equiv.)
cpd
4
G1
G2
G3
M
anthracene
Ph
Ph
(1.0 equiv.)
trans-5 (85%)
sole product)
Al
Al–C
1.945(3)
1.940(3)
1.885(2)
1.878(2)
1.899(3)
1.908(3)
1.889(3) 2.052(3)
1.894(3) 1.951(3)
1.887(3)
Al
2
(
2)
[2.2.2]-
cryptand
K(cryptand]+
[
3
(66%)
3
Me Si
SiMe
Al
SiMe
3
1.886(3)
Al =
C–Al–C 41.10(14) 42.30(7)
42.57(11) 42.02(14) 37.6(1)
42.56(14)
3
Me Si
3
cpd
trans-5
G4
G5
G6
H
Scheme 2. Reaction of 1 with unsaturated hydrocarbons.
Al–C
2.0178(18) 1.940(4)
.9953(19) 1.948(3)
1.9209(18) 1.920(3) 2.061(3)
1.9163(19) 1.924(3) 2.071(3)
1
2
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COMMUNICATION
C–Al–C 45.43(14) 49.35(17) 48.54(8)
48.78(12) 42.0(1)
Acknowledgements
This research was supported by a Grant-in-Aid for Scientific
Research (A) (JSPS KAKENHI grant 17H01191) and by JST
CREST (14529307). Theoretical calculations were carried out
using resources at the Research Center for Computational
Science (Okazaki, Japan).
Dip
Dip
Dip
SiMe
SiMe
3
3
Ph
Ph
Ph
Me
3
Si
N
N
N
R
Al
R
O
Al
Al
Al
Al
Et
N
N
N
AlR
2
Dip
G1
Dip
Dip
G3
Me
3
Si
M
L
G2
Dip
Dip
Dip
PMB
N
Al
N
N
Keywords: aluminium • cycloaddition • low oxidation state • DFT
Al
Al
N
N
Dip
G5
N
Dip
G6
calculations • alumanyl anion
Dip
G4
[
1]
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Dip = 2,6-Pr C H , R = CH(SiMe ) , PMB = CH C H -4-OMe
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To gain insights on the electronic character of transition structures
for of the present cycloadditions, we performed DFT
computations for the reactions of naphthalene, diphenylacetylene,
and (E)-, and (Z)-stilbenes (See SI for details). Initial structures of
these reactions were generated by replacing one of two toluene
molecules in 1 with the reactant. In the reaction of naphthalene, a
transition state was found to give a (1+2) cycloadduct, followed
by a ring-expanding isomerization to (1+4) cycloadduct with a
smaller energy barrier (Figures S12 and S13). This ring expansion
seems to be similar to that in Scheme 1(c). Reactions of
diphenylacetylene and (E)-stilbene take place in a concerted
fashion to form the three-membered ring directly (Figures S14 and
S15). In the reaction of (Z)-stilbene, a similar transition structure
Z_TS was found to connect Z_start and Z_end (trans-5) (Scheme
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Chemistry - A European Journal
10.1002/chem.201905830
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Entry for the Table of Contents
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(1+4) cycloaddition
(1+2) cycloaddition
Me Si
3
SiMe3
Al
Ph
Ph
Ph
Ph
Al =
Me Si
Al
Al
SiMe3
3
(sole product)
The reactivity of dialkylalumanyl anion (1) toward unsaturated hydrocarbons is investigated. Reaction with naphthalene and
anthracene took place through (1+4) cyclization. while the reaction with diphenylacetylene and (E)/(Z)-stilbenes gave (1+2)
cycloadducts. Cyclization toward (E)- or (Z)-stilbenes solely gave a trans-cycloadduct. DFT calculations revealed the selectivity
toward the trans-cycloadduct.
Twitter username of the researcher: @makoto_B
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