Activation and Functionalization of C-C σ Bonds of Alkylidene Cyclopropanes at Main Group Centers

Article abstract of DOI:10.1021/jacs.0c03383

Aluminum(I) and magnesium(I) compounds are reported for the C-C σ-bond activation of strained alkylidene cyclopropanes. These reactions result in the formal addition of the C-C σ bond to the main group center either at a single site (Al) or across a metal-metal bond (Mg-Mg). Mechanistic studies suggest that rather than occurring by a concerted oxidative addition, these reactions involve stepwise processes in which substrate binding to the main group metal acts as a precursor to α- or β-alkyl migration steps that break the C-C σ bond. This mechanistic understanding is used to develop the magnesium-catalyzed hydrosilylation of the C-C σ bonds of alkylidene cyclopropanes.

Full text of DOI:10.1021/jacs.0c03383

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Communication
Activation and Functionalization of C–C #-Bonds of
Alkylidene Cyclopropanes at Main Group Centers
Richard Y. Kong, and Mark R. Crimmin
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c03383 • Publication Date (Web): 26 Jun 2020
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Activation and Functionalization of C–C σ-Bonds of Alkylidene
Cyclopropanes at Main Group Centers
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Richard Y. Kong and Mark R. Crimmin*
Department of Chemistry, Molecular Sciences Research Hub, Imperial College London, White City, London, W12 0BZ, UK.
Supporting Information Placeholder
group compounds including silylenes,8 a phosphirene,9 and an
ABSTRACT: Aluminum(I) and magnesium(I) compounds are
reported for the C–C s-bond activation of strained alkylidene cy-
clopropanes. These reactions result in the formal addition of the
C–C s-bond to main group center either at a single site (Al) or
across a metal–metal bond (Mg–Mg). Mechanistic studies sug-
gest that rather than occurring by a concerted oxidative addition,
these reactions involve stepwise processes in which substrate
binding to the main group metal acts as a precursor to a- or b-
alkyl migration steps that break the C–C s-bond. This mechanis-
tic understanding is used to develop the magnesium-catalyzed
hydrosilylation of the C–C s-bonds of alkylidene cyclopropanes.
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alumanyl anion10 are known to insert into a C–C bond of ben-
zene rings. While this reactivity could be described by a formal
oxidative addition process, more precisely it involves a Büch-
ner ring-expansion. The oxidative addition of the cyclobutene
C–C σ-bond in biphenylene has also recently been observed
at an anionic cyclic-alkylamino alumanyl complex.11 Although
these examples are yet to translate into new catalytic methods,
Lewis acid catalysis has been applied to the functionalization
of cyclopropanes through ring-opening reactions that break a
C–C s-bond.12–14
Herein we report C(sp2)–C(sp3) σ-bond activation within
the coordination sphere of well-defined aluminium and mag-
nesium compounds. A combination of DFT and experimental
data show that while a redox reaction is involved in the for-
mation of intermediates, the key C–C bond breaking step
starts formally from an aluminum(III) or magnesium(II) spe-
cies. α-Alkyl and β-alkyl migration mechanisms are in opera-
tion. The redox-neutral nature of the C–C σ-bond activation
is leveraged to develop the first example of catalytic C–C σ-
bond functionalization using magnesium-based catalysis.
Reaction of the aluminium(I) complex 115,16 with the un-
saturated cyclopropane 2a at 25°C in C6D6 initially resulted in
the formation of 3a over the course of 4 hours (Scheme 1).
This product is the result of a (4+1) cycloaddition.17 Heating
either crude or isolated samples of 3a at 100°C in C6D6 for 15
minutes results in the formation of 4a, a metallocyclobutane
derived from C–C σ-bond activation. The reaction also pro-
ceeds slowly at 25°C over the course of several days. The relief
of the ring strain and rearomatization provide a significant
thermodynamic driving force this reaction. 3a and 4a have
been characterized by single-crystal X-ray diffraction (Figure
2a). The reaction scope can be expanded to 2b-e. The range
of substrates demonstrates that aromatic substitution is not es-
sential for C–C σ-bond activation, as alkyl-substituted sub-
strates 2c and 2d react with 1 as does the parent methylidene
cyclopropane 2e. For trisubstituted alkenes 2b,d a single ste-
reoisomer of the product was observed. For 2c, allylic C–H
activation accompanies ring-opening.18
Reactions that break the strong C–C σ-bonds of hydrocar-
bons are essential for processing crude oil. The petrochemical
industry relies on catalysis to crack long-chain hydrocarbons
into shorter and more valuable building blocks. This transfor-
mation is challenging: C–C σ-bonds of hydrocarbons are
strong, sterically congested, and surrounded by C–H bonds -
which are often the first site to react. Common pathways for
C–C σ-bond activation with transition metal complexes in-
clude oxidative addition1 and β-alkyl elimination.2 These two
fundamental steps underpin numerous applications which in-
volve the transition metal catalysed functionalization of C–C
σ-bonds(Figure 1).3–5
Figure 1. C–C s-bond activation with transition metals.6
Examples of C–C σ-bond activation by main-group com-
pounds are limited in comparison to transition metal com-
plexes. For example, stoichiometric C–C σ-bond activation by
b-alkyl elimination has been observed during the thermolysis
of tris-neopentylaluminium at 200°C.7 Low-valent main-
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reaction of 1 with 2e support the notation that a closely related
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reaction sequence involving a (2+1) cycloaddition and a-mi-
gration becomes accessible (supporting information). The di-
rect oxidative addition of a C–C σ-bonds of strained three-
membered rings to 1 was also considered.19 A transition state
that directly connects 1 and 2e with 4e, corresponding to an
oxidative addition pathway, was found to be significantly
higher in energy than the corresponding α-migration pathway
(ΔG‡
= 35.3 kcal mol-1). Experimentally, 1 does not react
298K
9
with cyclopropylbenzene to form metallocyclobutane prod-
ucts even when heated at 100°C for one week in C6D6.
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Curious as to whether the C–C σ-bond activation chemis-
try could be expanded to alternative main group reagents, we
investigated the reaction of the magnesium(I) complex 620–23
with alkylidene cyclopropanes. Addition of 6 to 2a and 2b re-
sulted in the ring-opened 1,3-dimagnesio-3-butene products
7a and 7b after heating for 4h at 100°C and 1h at 25°C respec-
tively (Figure 3a). No reaction is observed with either alkyl-
substituted substrates 2c or 2d. Crystallization and isolation
was amenable through the preparation of their DMAP (4-di-
methylaminopyridine) adducts 7a•DMAP2 and 7b•DMAP2
(Figure 3b). 7b•DMAP2 forms as a single stereoisomer. The
mechanism for C–C σ-bond activation with 6 was again inves-
tigated using DFT calculations. Based on literature precedent
and by analogy to the aluminum reagent 1, it is highly likely
that this reaction is initiated by the 1,2-addition of the Mg–Mg
bond of 6 across the alkene to form a 1,2-dimagnesioethane
intermediate.24,25 In line with these expectations, Int-1 was
identified as an intermediate (ΔG°298K = –12.7 kcal mol-1) by
computational methods. From Int-1, C–C s-bond activation
Scheme 1. Reaction of alkylidene cyclopropanes with 1.
Kinetics measurements and DFT calculations were under-
taken to better understand the key C–C σ-bond activation
step. The transformation of 3a à 4a was found to be first-or-
der with respect to 3a. Eyring analysis over a 45-70°C range
gave activation parameters: ΔH‡ = 22.3 0.4 kcal mol-1 and
ΔS‡ = 28.9 5.5 cal K-1 mol-1. The negative entropy of activa-
tion is consistent with an ordered transition state. The Gibbs
activation energy is ΔG‡298K = 24.4 2.1 kcal mol-1. The initial
formation of the (4+1) cycloaddition intermediate 3a was cal-
culated to occur via a concerted pericyclic transition state, TS-
1. The modest energy barrier of TS-1, ΔG‡
= 14.1 kcal
occurs by b-alkyl migration via TS-3 to form 7a (ΔG‡
=
298K
298K
mol-1, is consistent with the observation that formation of 3a
occurs at 25°C and is not the rate-determining step of the C–
C s-bond activation sequence. From the (4+1) cycloaddition
intermediate 3a, TS-2 was located (ΔG‡298K = 25.8 kcal mol-1)
and connects directly to the product 4a. This key step breaks
the C–C σ-bond and involves an a-migration mechanism
(Figure 2b). While substrates 2a-b may both proceed through
an intermediate derived from a (4+1) cycloaddition, this path-
way is inaccessible for 2c-e. Further calculations on the
12.7 kcal mol-1, Figure 3c).
The discrepancy between experimentally observed reaction
conditions (4 h at 100°C) and calculated barriers (ΔG‡
=
298K
12.7 kcal mol-1) suggest that in this case, breaking of the C–C
bond is not rate limiting. While a transition state towards the
formation of the 1,2-dimagnesioethane intermediate could
not be located, the reaction proved sensitive to the steric de-
mands of the magnesium reagent and did not proceed with
bulkier analogues of 6.
Figure 2. (a) Structures from single crystal Xray diffraction experiments on 3a and 4a. (b) DFT calculated pathway for C–C s-bond activation via a
(4+1) intermediate (for the analogous pathway via a (2+1) intermediate see the supporting information).
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Figure 3. (a) C–C bond activation with magnesium(I) compound 6. (b) Structures for 7a•DMAP2 and 7b•DMAP2 from single crystal X-ray diffrac-
tion experiments. (c) DFT calculated pathway for C–C bond activation via a 1,2-dimagnesioethane intermediate. (d) Comparison of a-migration
and b-migration pathways.
The data show that the key factor for achieving C–C s-bond
activation is not the redox reactivity of the main group rea-
gents 1 and 6 but being able to install electropositive Al or Mg
atoms in the correct position of the hydrocarbon scaffold in
order to promote an a- or b-alkyl migration mechanism. Fur-
ther insight into the migratory mechanisms was provided by
NBO calculations. Second-order perturbation analysis impli-
cates the participation of the electrophilic main group site in
C–C s-bond activation in both mechanisms. Donor-acceptor
interactions involving electron donation from the breaking C–
C σ-bond into low-lying orbitals of aluminum(III) or magne-
sium(II) can be identified in both TS-2 and TS-3 (arrow-
pushing - Figure 3d, see supporting information for details).
Based on the advancement of our understanding, we envi-
sioned a new catalytic protocol for C–C σ-bond functionali-
zation. By combining the new b-alkyl migration step with es-
tablished s-bond metathesis and alkene insertion chemistry
of group 2 hydride catalysts the catalytic heterofunctionaliza-
tion of C–C bonds should be accessible (Figure 4a).26,27 Ini-
tially, each of the proposed steps of the catalytic cycle were in-
vestigated in stoichiometric reactions. The addition of the b-
diketiminate stabilized magnesium hydride 8 to 2e results in
Figure 4. (a) Proposed catalytic cycle for C–C bond hydrosilylation.
near quantitative formation of the ring-opened but-4-en-1-yl
magnesium species 9 over 24 hours at 25°C. 9 results from the
anti-Markovnikov insertion of the alkene into the Mg–H bond
of 8 followed by facile b-migration involving C–C σ-bond ac-
tivation. Subsequent addition of PhSiH3 (2 equiv.) to a solu-
tion of 8 and heating the resultant mixture at 80°C for 3 hours
afforded the known products 10a and 10b in a 5:3 ratio in
89% yield, along with reformation of 8. The former silane is
derived from a net hydrosilylation of the C–C s-bond, the lat-
ter forms from a second intramolecular hydrosilylation of 10a
(Figure 4b).
(b) Reaction of 8 with 2e and 9 with PhSiH3. Ar = 2,6-di-iso-
propylphenyl.
A catalytic procedure involving the reaction of 2e, PhSiH3
(2 eq.) and 10 mol% 8 led to the formation of 10a:10b in 73
% overall yield and a 4.6:1 ratio after 3h at 80°C. Further heat-
ing converted 10a into 10b in near quantitative yield. Simi-
larly, the substituted alkylidene cyclopropanes 2a and 2b un-
dergo catalytic C–C s-bond hydrosilylation with 8. In the
case of 2b a 1:1.1 mixture of E:Z-stereoisomers of the product
was obtained (Figure 5). The reaction does not proceed in the
absence of 8 at 80 °C.
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Bart, S. C.; Chirik, P. J. Selective, Catalytic Carbon−Carbon
Bond Activation and Functionalization Promoted by Late
1
Transition Metal Catalysts. J. Am. Chem. Soc. 2003, 125, 886–
887.
A distinction between “β-alkyl migration” and “β-alkyl elimina-
tion” has been made here. In both cases both the β-γC–C bond
is broken, however elimination results in two separate ligands
on the metal centre, where migration results in one contiguous
ligand.
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Pfohl, W. Metallorganische Verbindungen, XXXVII Alumin-
ium-tri-neopentyl. Liebigs Ann. 1960, 629, 207–209.
Wendel, D.; Porzelt, A.; Herz, F. A. D.; Sarkar, D.; Jandl, C.;
Inoue, S.; Rieger, B. From Si(II) to Si(IV) and Back: Reversi-
ble Intramolecular Carbon–Carbon Bond Activation by an
Acyclic Iminosilylene. J. Am. Chem. Soc. 2017, 139, 8134–
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Figure 5. Catalytic C–C bond hydrosilylation with 8.
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Liu, L. L.; Zhou, J.; Cao, L. L.; Andrews, R.; Falconer, R. L.;
In summary, we report the C–C s-bond activation of strained
alkylidene cyclopropanes by main group reagents. Analysis of
the mechanism through isolation of intermediates, kinetics
and DFT studies shows that C–C s-bond activation at main
group centers is possible by either a- or b-migration mecha-
nisms. This understanding was used to develop a magnesium
catalysed hydrosilylation of C–C bonds. We are continuing to
expand the scope of this catalytic methodology and to explore
the origin of stereochemistry.
Russell, C. A.; Stephan, D. W.
A Transient Vi-
nylphosphinidene via a Phosphirene–Phosphinidene Rear-
rangement. J. Am. Chem. Soc. 2018, 140, 147–150.
Hicks, J.; Vasko, P.; Goicoechea, J. M.; Aldridge, S. Reversible,
Room-Temperature C—C Bond Activation of Benzene by an
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Koshino, K.; Kinjo, R. Construction of σ-Aromatic AlB2 Ring
via Borane Coupling with
a Dicoordinate Cyclic (Al-
kyl)(Amino)Aluminyl Anion. J. Am. Chem. Soc. 2020, 142,
9057–9062.
ASSOCIATED CONTENT
Roy, A.; Bonetti, V.; Wang, G.; Wu, Q.; Klare, H. F. T.; Oes-
treich, M. Silylium-Ion-Promoted Ring-Opening Hydrosilyla-
tion and Disilylation of Unactivated Cyclopropanes. Org. Lett.
2020, 22, 1213–1216.
Morton, J. G. M.; Dureen, M. A.; Stephan, D. W. Ring-Open-
ing of Cyclopropanes by “Frustrated Lewis Pairs.” Chem. Com-
mun. 2010, 46, 8947–8949.
Zhang, Z.-Y.; Liu, Z.-Y.; Guo, R.-T.; Zhao, Y.-Q.; Li, X.; Wang,
X.-C. B(C6F5)3-Catalyzed Ring Opening and Isomerization of
Unactivated Cyclopropanes. Angew. Chem., Int. Ed. 2017, 56,
4028–4032.
Cui, C.; Roesky, H. W.; Schmidt, H.-G.; Noltemeyer, M.; Hao,
H.; Cimpoesu, F. Synthesis and Structure of a Monomeric Alu-
minum(I) Compound [{HC(CMeNAr)2}Al] (Ar=2,6–
IPr2C6H3): A Stable Aluminum Analogue of a Carbene. Angew.
Chem., Int. Ed. 2000, 39, 4274–4276.
Zhong, M.; Sinhababu, S.; Roesky, H. W. The Unique β-
Diketiminate Ligand in Aluminum(I) and Gallium(I) Chem-
istry. Dalton Trans. 2020, 49, 1351–1364.
Bakewell, C.; Garçon, M.; Kong, R. Y.; O’Hare, L.; White, A. J.
P.; Crimmin, M. R. Reactions of an Aluminum(I) Reagent
with 1,2-, 1,3-, and 1,5-Dienes: Dearomatization, Reversibility,
and a Pericyclic Mechanism. Inorg. Chem. 2020, 59, 4608–
4616.
Bakewell, C.; White, A. J. P.; Crimmin, M. R. Reversible Alkene
Binding and Allylic C–H Activation with an Aluminium(I)
Complex. Chem. Sci. 2019, 10, 2452–2458.
Jain, S.; Vanka, K. The Unusual Role of Aromatic Solvent in
Single-Site AluminumI Chemistry: Insights from Theory.
Chem. Eur. J. 2017, 23, 13957–13963.
The Supporting Information is available free of charge on the
ACS Publications website. X-ray crystallographic data for3a, 4a-
c, 7a.DMAP2, 7b.DMAP2, and 9 are available from the Cam-
bridge Crystallographic Data Centre (CCDC 1984848-
1984854) and as a .cif file, full details of the experiments and cal-
culations are available as a .pdf . NMR spectra and computational
coordinates are available at DOI: 10.14469/hpc/7252.
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Corresponding Author
*m.crimmin@imperial.ac.uk
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Funding Sources
No competing financial interests have been declared.
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ACKNOWLEDGMENT
We are grateful to the ERC (FluoroFix: 677367) for generous
funding. We thank Imperial College London for the award of a
President’s Scholarship (RYK).
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oxidative
addition
CH3
CH3
CH3
LnM
+
LnM
1
CH3
2
3
4
5
β-alkyl
elimination
γ
CH3
LnM
CH3
LnM
+
6
7
α
β
8
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[Al]
C
H
[Al]
C
C
TS-1
H
Ph
C
TS-2
14.1
Ph
C
N
Al
N
Ph
[Al]
Ph
1
2
3
4
5
6
7
8
N
11.6
C
Al
N
C
ΔG‡298K
C
0.0
25.8
[Al]
kcal mol-1
H
3a
–14.2
Gibbs
Energy
Ph
3a
4a
(kcal mol-1)
Al–N = 1.9038(13)
to 1.9124(13) Å
4a
–36.5
Al–N = 1.9139(17)
1.9134(16) Å
N–Al–N = 97.43(7) º
(4+1) cycloaddition
α-migration
N–Al–N = 96.96(6) to 96.77(6) º
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Me2N
Ph
[Mg]
Ph
Journal of the American Chemical Society
Page 8 of 12
Me2N
(a)
(c)
Mes Mes
1
2
0.0
0.0
[Mg]
25 – 100 °C
N
TS-3
N
N
N
3
+
Mg Mg
6 + 2a
[Mg]
N
C6D6
1 – 4 h,
then DMAP
1
3
1
2
3
4
5
6
R1 R2
N
4
Mes Mes
2
[Mg]
2a-b
Gibbs
Energy
Ph
[Mg]
7a
Ph
[Mg]
R1 R2
6 = [Mg]–[Mg]
4
Int-1
a: R1 = R2 = Ph; 100°C, 4 h, 70 %
b: R1 = Ph, R2 = H; 25°C, 1 h, 57 %
(kcal mol-1)
–12.7
–16.0
.
7a-b DMAP2
1,2-insertion
β-migration
7
8
9
(b)
β
(d)
N
α-migration
α
N
4e
7a
[Al]
Mg
N
C
10
Mg
Mg
C
C
11
12
13
14
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C
Ph
Mg
[Mg]
N
N
C
Ph
β
β-migration
N
α
N
N
C
[Mg]
γ
.
.
7b DMAP2
7a DMAP2
ACS Paragon Plus Environment

PageJ9ouorfn1a2l of the American Chemical Society
(a)
E
LnM H
E = SiR3
E–H
1
2
3
4
5
6
7
8
9
σ-bond
metathesis
insertion
γ
LnM
α
M
Ln
α
β
β
β-alkyl
migration
γ
10
11
(b)
Ar
Ar
25 °C
24 h
2
1
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
N
N
N
4
1
3
+
Mg H
Mg
C6D6
N
3
H
H
2
4
Ar
Ar
8
9
2e
9
excess
PhSiH3
PhH2Si
PhHSi
+
80 °C,
C6D6
10a : 10b
3h, 56 % 33%
18h, 0 % : 81%
:
ACS Paragon Plus Environment

Journal of the American ChemicaPl Saogceie1t0yof 12
10 mol % 8
PhH2Si
PhHSi
+
1
2
3
4
5
6
7
8
9
PhSiH3
2 equiv.
H
H
10a : 10b
3h, 60 % 13%
26h, 3 % : 91%
:
10 mol %8
R1
R2
PhH2Si
PhSiH3
2 equiv.
R1 R2
H
11, R1 = R2 = Ph, 18 h, 80 ºC, 100 %
(E:Z)-12, R1 = Ph, R2 = H, 48 h, 80 ºC, 93 %, 1:1.1 E:Z
10
11
12
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ACS Paragon Plus Environment

PageJ1o1uornfa1l2of 2the American Chemical Society
time, temp
C6D6
Ar
1
Ar
1
N
3
N
2
+
Al
Al
a R1 = R2 = Ph
R1 4 R2
N
N
b R1 = Ph, R2 = H
c R1 = R2 = -(CH2)5-
d R1 = Cy, R2 = H
e R1 = R2 = H
1
2
3
3
4 R2
Ar
Ar
4a-e
R1
1= [Al]
2a-e
4
5
6
7
isolated (4+1) intermediate
[Al]
[Al]
[Al]
H
Δ
Ph
8
H
Ph
Ph
Ph
9
3a
4a
4b
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
25 ºC, 4 h
93 % yield
100 ºC, 15 min
84 % yield
100 ºC, 15 min
62 % yield
[Al]
[Al]
[Al]
H
Cy
H
H
4d
25 ºC, 4 h
61 % yield
4e
4c
25 ºC, 5 min
98 % yield
100 ºC, 10 h
47 % yield
ACS Paragon Plus Environment

[Si]
Journal of the American ChemicalPSaogceie1t2y of 12
LnM
H
Stoichiometric
C–C σ-bond activation
[Si]–H
1
2
Catalysis
R'
3 R
4
M = Mg
LnM
L
M
n
[M] = [Al], [Mg]–[Mg]
ACS Paragon Plus Environment
5
6
7