Difluorocarbene transfer from a cobalt complex to an electron-deficient alkene

Article abstract of DOI:10.1039/c7cc01418j

We report the synthesis of the trifluoromethyl cobalt(iii)tetraphenylporphyrinato complex [Co(TPP)CF₃], which loses fluoride upon one-electron reduction and transfers a difluorocarbene moiety to n-butyl acrylate to produce the corresponding gem-difluorocyclopropane. Catalytic CF₂ transfer from Me₃SiCF₃ to n-butyl acrylate becomes possible when directly using the divalent cobalt(ii) porphyrin catalysts in the presence of NaI.

Full text of DOI:10.1039/c7cc01418j

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Difluorocarbene transfer from a cobalt complex to an electron-
deficient alkene
Received 00th January 20xx,
Accepted 00th January 20xx
Monalisa Goswami, Bas de Bruin,* and Wojciech I. Dzik*
DOI: 10.1039/x0xx00000x
www.rsc.org/
We
report
the
synthesis
of
the
trifluoromethyl proposed
as
intermediates
in
palladium-catalysed
7
], which difluoromethylation of arylboronic acids. Ichikawa and co-
,8
cobalt(III)tetraphenylporphyrinato complex [Co(TPP)CF
3
loses fluoride upon one-electron reduction and transfers a workers reported that the yields of difluorocyclopropanation
difluorocarbene moiety to n-butyl acrylate to produce the of electron rich silyl dienol ethers with free difluorocarbene
corresponding gem-difluorocyclopropane. Catalytic CF
from Me SiCF to n-butyl acrylate becomes possible when directly with a CNC-type pincer ligand. The same group reported
using the divalent cobalt(II) porphyrin catalysts in the presence of catalytic cycloaddition of copper-difluorocarbene to silyl dienol
2
transfer were slightly improved in the presence of a nickel complex
9
3
3
1
0
NaI.
ethers to form difluorocyclopentenes. However, to our best
knowledge, no examples of catalytic CF transfer to electron
2
Metal-carbene complexes are key intermediates in many
organic reactions e.g. olefin metathesis or cyclopropanation
1
reactions. Their rich chemistry is well understood and many
deficient alkenes have been reported, likely due to the
electrophilic nature of the difluorocarbene moiety.
Traditional routes to gem-difluorocyclopropanes involve
recently developed catalytic transformations are based on the
intriguing and diverse reactivity of these species. However, the
chemistry of metallo-difluorocarbenes is thus far rather
reacting the in-situ generated free CF
2
carbene with an
The scope of difluorocyclopropanation
of electron deficient alkenes like acrylates is very limited, and
1
1,12
electron rich olefin.
2
1
3
underdeveloped. Only a handful of well-defined metal
rather harsh reaction conditions are required.
difluorocarbene complexes are reported that can mediate
stoichiometric formation of new carbon-carbon bonds.
Recently, the group of Baker disclosed some electron rich
F
F
F
F
M
3
4
EWG
Si(CH ) CF + [M]
EWG
cobalt(I) and nickel(0) difluorocarbene complexes containing
nucleophilic M=CF2 moiety. These species undergo
cycloaddition reactions with tetrafluoroethylene (C F ) or
-
F-
[M(CF3)]
a
3
3
3
2
4
difluorocarbene
producing
and
perfluorinated Scheme 1. Envisioned metal mediated difluorocarbene transfer to an electron deficient
3
b,4
3c
metallacyclobutanes
metallacyclopropanes,
olefin.
respectively.
A
related electrophilic cobalt(III) complex
Co-
We envisioned that by employing a transition metal centre
that can render the difluorocarbene moiety more nucleophilic,
mediated the insertion of difluorocarbene into
a
5
perfluoroalkyl bond. However, the stability of the thus formed
perfluoroalkyl metal complexes prohibited further reactivity.
Recently, the first example of catalytic olefin cross-metathesis
of tetrafluoroethylene or other gem-difluoroolefins with enol
2
the transfer of CF to an electron deficient olefin would be
facilitated (Scheme 1). Reaching this goal could enable
catalytic gem-difluorocyclopropanation of electron deficient
double bonds. Cobalt(II) porphyrin complexes are efficient
catalysts for cyclopropanation of electron deficient alkenes
6
ethers was reported. Despite the highly inert character of the
Ru=CF intermediate, turnover numbers of up to 13.4 could be
2
1
4
using (stabilized) diazo esters as carbene precursors. This
contrasts with the Fischer-type reactivity of the majority of
cyclopropanation catalysts, which have a general preference
for electron rich alkenes. The observed ‘umpolung’ of the
reactivity of the cobalt-carbenoid species as compared to
other metallo-carbenes is caused by the transfer a discrete
unpaired electron to the coordinated (Fischer-type) carbene
ligand. This renders the carbene moiety more nucleophilic and
reached. Palladium-difluorocarbene species were recently
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9
weakens the metal-carbon bond. We hypothesized that this acrylate in 12% yield as determined by F NMR (Scheme 3).
feature might also promote the formation of Additionally, C was formed as a side product. In the absence
difluorocyclopropanes from related cobalt difluorocarbenoid of CoCp* no reaction took place.
2 4
F
2
intermediates.
In
this
perspective
(Me SiCF
the
)
use
as
of
trimethyl(trifluoromethyl)silane
3
3
the
difluorocarbene source is attractive, as it can operate at mild
1
temperatures.
2
To evaluate the feasibility of difluorocarbene transfer from
cobalt to an olefin we first decided to investigate whether a
3
difluorocarbene cobalt(II) complex can be formed by one- Scheme 3. Reaction of [Co(TPP)(CF )] with n-butyl acrylate upon one-electron
reduction.
electron reduction of the novel trifluoromethyl cobalt(III)
III
Co (TPP)(CF
complex
[
3
)]
(TPP
=
meso-tetraphenyl-
The formation of the desired difluorocyclopropane suggests
porphyrinato). Formation of a cobalt difluorocarbene complex
by reduction and subsequent dissociation of fluoride from a
cobalt trifluoromethyl complex has been reported by Baker
III
that upon reduction of [Co (TPP)(CF )]
a
cobalt-
difluorocarbene complex is formed, which is apparently
nucleophilic enough to facilitate CF transfer to the
electrophilic C=C bond of the acrylate. The [Co (TPP)(CF
3
3
a
2
and co-workers. Thus, we anticipated that the release of F¯
II
II
]− complex could lead to
)]
2
from the anionic [Co (TPP)(CF
formation of a [Co(TPP)(CF ] species, potentially capable of
CF transfer to acrylates under mild reaction conditions.
The trifluoromethyl cobalt(III) porphyrin complex
)] required for these studies was obtained in
0% yield (see ESI for characterisation) by reacting
3
)
intermediate could not be observed with EPR spectroscopy
though (See ESI for details).
2
)
2
The feasibility of the stoichiometric reaction shown in Scheme
III
3 triggered us to evaluate whether catalytic CF
transfer from
2
[
Co (TPP)(CF
3
Me SiCF to n-butyl acrylate is possible when directly using a
3
3
8
III
divalent cobalt(II) porphyrin catalyst in the presence of NaI as
II
the activator. Indeed, with 5 mol% of [Co (TPP)] in THF
[
Co (TPP)(Cl)] with Me
3
SiCF
3
and CsF as an initiator (Scheme
2).
catalytic formation (TON
difluorocyclopropanated acrylate was observed (Table 1, entry
), albeit in low yield (12%). Besides the desired cyclopropane,
= 2.4) of the desired gem-
1
19
the only other F-NMR detected fluorine containing side
product was C F , presumably formed by free carbene
2
4
dimerization. The TMSCF /NaI combination can transfer CF to
3
2
1
2
3c
electron rich alkenes and metals. Control experiments show
that in the absence of cobalt (Table 1, entry 2) no gem-
difluorocyclopropane product was formed, which makes it
3
Scheme 2. Synthesis of [Co(TPP)(CF )].
III
To evaluate the potential of [Co (TPP)(CF
3
)] to form a
unlikely that the cyclopropanation proceeds via free CF or the
2
difluorocarbene complex we investigated its electrochemistry
free CF ¯ anion.
3
under reductive conditions. The cyclic voltammogram (CV) of
III
complex [Co (TPP)(CF
3
)] in THF reveals a reversible reduction
2 3 3
Table 1. Catalyst screening for CF transfer from Me SiCF to n-butyl acrylate (for
wave at
wave at
–1.71 V, followed by an irreversible one with the peak
+
2.13 V. vs Fc/Fc (See ESI, Figure S7). The first wave
further screening, see Table S1 in the ESI).
–
can be attributed to the one-electron reduction of the
II
III
Co (TPP)(CF
–
[
3 3
)] to form the anionic complex [Co (TPP)(CF )] .
The subsequent irreversible reduction leads to clean formation
I
of [Co (TPP)] as evidenced by an independent measurement
–
II
of an original [Co (TPP)] sample (See ESI, Figure S8). These
o
a
b
#
1
2
3
4
5
6
Catalyst
Solvent
THF
T ( C)
Yield
12%
0%
TON
III
results show that one-electron reduction of [Co (TPP)(CF
II
[Co (TPP)]
50
2.4
3
)]
–
II
leads to formation of the anionic complex [Co (TPP)(CF
None
THF
50
-
-
3
)] ,
II
[Co (acac)
2
]
THF
50
0%
which is stable on the CV timescale (scan rate 100 mV/sec).
–
II
[Co (salophen)]
THF
50
0%
-
The second reduction step leads to rapid (net) loss of the CF
anion from the cobalt centre.
3
II
[Co (TPPF20)]
THF
50
24%
40%
4.8
8
c
II
Knowing that [Co (TPP)(CF
–
[Co(TPPF20)]
THF
50
3
)] can be selectively accessed
using reductants with a reduction potential in the
+
–
1.8–2.0 V Reaction conditions: 0.5 mmol of n-butyl acrylate; 2 mL of solvent; 5 mol%
a
19
range (vs Fc/Fc ), we next investigated its propensity to catalyst and 4 equiv. of TMSCF
3
, 18 h. with respect to n-butyl acrylate using
F
b
NMR spectroscopy and fluorobenzene as an internal standard. Turnover
generate a metallodifluorocarbene species on an extended
c
3 3
numbers. A second batch of 4 equiv. of Me SiCF was added after 4 hours.
timescale and at elevated temperatures. Interestingly, the
III
intended reduction of [Co (TPP)(CF
3
)] with one equivalent of
+
The low turnover numbers obtained using [Co(TPP)] as the
catalyst suggests that the catalyst quickly deactivates under the
applied reaction conditions. Hence, in an attempt to increase the
TONs we explored a few additional catalysts (Table 1). The
0
2
decamethylcobaltocene (CoCp* E = –1,91 vs Fc/Fc ) in the
presence of n-butyl acrylate at 65°C indeed led to the
formation of the desired gem-difluorocyclopropanated
2
| J. Name., 2012, 00, 1-3
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II
complexes [Co (acac)
II
and [Co (salophen)] proved inactive porphyrin complexes without meso-substituents, are 65% for
2
]
(entries 3-4), but the perfluorinated porphyrin complex cobalt and only 23% for the carbene carbon atom (Scheme 4).
[Co(TPPF20)] (TPPF20
=
meso-tetra(pentafluorophenyl)-
porphyrinato) indeed proved to be a somewhat more robust
catalyst (entry 5) than [Co(TPP)], producing the desired gem-
difluorocyclopropanated acrylate in 24% yield (TON = 4.8).
3 3
Changing the initiator, solvent and the amount of Me SiCF did
not improve the yield of the difluorinated cyclopropane (see
ESI, Table S1). However, when an additional batch of 4
equivalents of Me
entry 6), the yield increased to 40% (TON = 8). Any further
addition of Me SiCF did not lead to an increased yield.
The reactions reported in Table 1 serve as a proof-of-principle,
clearly showing the feasibility of cobalt-catalysed CF transfer
from Me SiCF to electron-deficient alkenes. However, the
3 3
SiCF was added after 4 hours (Table 1,
3
3
2
3
3
currently applied catalysts do not seem to be robust enough to
obtain the cyclopropane product in high yields exceeding 40%.
Hence, additional catalyst screening is clearly a subject of
follow-up research.
Figure 1. DFT calculated pathway for difluorocyclopropanation of methyl acrylate by a
cobalt(II) porphyrinato complex.
However, despite the dominant metalloradical character of the
[
2
Co(por)(CF )] species, the computed carbene transfer to
methylacrylate proceeds via a stepwise radical process, just
like for other cobalt(II) porphyrin catalysed carbene and
nitrene transfer reactions. All attempts to find the transition
state for a concerted addition of acrylate failed and converged
to the stepwise pathway. Addition of the difluorocarbene
2
Scheme 4. Spin density plot and resonance structures of [Co(por)(CF )].
1
5
species
transition state (TS1
the [Co(por)(CF -CH
unpaired electron primarily localized at the
alkyl’ moiety. Hence, despite its significant Fischer-type
character in the ground state, the reactivity of [Co(por)(CF )]
A to the olefin proceeds via a relatively low energy
Due to the redox non-innocent behaviour of carbenes, the
electronic structure of oxidation state +II group 9 transition
metal carbene complexes is best described as metal(III)
‡
-1
,
ꢀ
G
298K = +14.4 kcal mol ), generating
-CH•COOMe)] species , which has its
-carbon of the
2
2
B
γ
‘
carbene radical’ species in which the unpaired electron
resides mainly in the p-orbital of the carbene moiety (the
SOMO being the metal d - carbene p antibonding * molecular
‘
2
π
follows the characteristics of a ‘carbene radical’ species. The
transition state TS2 for ring closure leading to formation of the
cyclopropane product involves simultaneous C–C bond
formation and homolytic cleavage of the Co–C bond. The
orbital, which is strongly polarized to carbon). This
phenomenon has been reported for various transition metal
carbene complexes, and can lead to unique reactivity of the
1
5-16
carbene moiety.
group 9 nitrene complexes. However, as pointed out by
Woodcock and co-workers, cobalt porphyrinato
difluorocarbene complexes actually might not behave as so-
A similar behaviour has been reported for
‡
-1
1
7
barrier for this process is higher (ꢀG 298K = +20.7 kcal mol )
and seems to be the rate limiting step of the overall reaction.
We also investigated an alternative pathway in which the
difluorocarbene generated on the cobalt centre dissociates to
react with the acrylate outside the metal coordination sphere.
The release of free difluorocarbene from cobalt is energetically
1
8
called ‘carbene radicals’. According to their NBO analysis
DFT, M06-L functional) the spin density is mainly located on
the metal (71%) rather than on the carbon atom (28%) and uphill (
thus [Co(por)(CF )] should have a substantial Fischer-type barrierless (See ESI, Figure S9). This energy is comparable to
carbene character. Hence, unlike all other reported carbene the energy barrier for coupling of the cobaltocarbene with
(
-1
ꢀG°298K = +16.3 kcal mol ) and the reverse reaction is
2
1
5,16,18
methyl acrylate. Formation of free CF in this manner, as well
2
adducts of cobalt(II) porphyrins,
not behave as a genuine ‘carbene radical’. Thus, we wondered
if the CF carbene adduct reacts with alkenes via a stepwise
typical for ‘carbene radicals’) or a concerted (typical for
the CF
2
adduct might
as the coupling of two [Co(por)(CF
responsible for the observed formation of C2F4 as a side
product in the reaction. However, free CF carbene formation
2
)] species, could be
2
2
(
is unlikely associated with formation of the cyclopropane
product, as the addition of free difluorocarbene to methyl
Fischer carbenes) mechanism. Therefore we performed
additional DFT calculations to shed light on the mechanistic
‡
-1
acrylate has a much higher barrier (ꢀG 298K = 28.0 kcal mol )
details concerning the observed CF
the [Co(por)(CF )] complex to the electron deficient acrylate.
In agreement with the results reported by Woodcock, the
2
transfer reactions from
than those associated with the metal-mediated pathway (TS1
and TS2 in Figure 1). Lewis-acid activation of the acrylate also
seems unlikely, as in a separate control experiment with
2
1
8
DFT (BP86, def2-TZVP) calculated NBO spin populations of [Zn(TPP)] only traces (< 1%) of the gem-difluorocyclopropane
Co(por)(CF
)], which is a simplified model of the experimental product was formed (See ESI, Table S1).
[
2
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2
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1
1
Scheme 5. Proposed catalytic cycle for the difluorocyclopropanation of n-butyl acrylate
(a) R. Csuk, L. Eversmann, Tetrahedron 1998, 54, 6445–6456.
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Based on the above considerations we propose a mechanism
−
in which the CF
3
anion (generated by the reaction of Me
and NaI) coordinates to [Co(TPP)] to form the anionic complex
3
SiCF
3
−
[
[
Co(TPP)(CF
Co(TPP)(CF
3
2
)] , which produces the neutral carbene adduct
)] upon loss of a fluoride anion (Scheme 5). The
1
2
F. Wang, T. Luo, J. Hu, Y. Wang, H. S. Krishnan, P. V Jog, S. K.
Ganesh, G. K. S. Prakash and G. Olah, Angew. Chem. Int. Ed., 2011,
cobalt difluorocarbene complex subsequently undergoes a
stepwise radical-type addition to the acrylate double bond,
forming an alkylcobalt(III) intermediate with the unpaired
5
0, 7153–7157.
1
3
a) S. Eusterwiemann, H. Martinez, W. R. Dolbier, J. Org. Chem.
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1
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n-butyl acrylate forming a gem-difluorocyclopropane. The
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showing the feasibility of cobalt-catalysed CF transfer from 1452–1460. (b) W. I. Dzik, J. N. H. Reek, B.de Bruin, Chem. Eur. J.
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(a) A. Penoni, R. Wanke, S. Tollari, E. Gallo, D. Musella,
2
3
3
that the current cobalt(II) porphyrin catalysts quickly and B. de Bruin, J. Am. Chem. Soc., 2010, 132, 10891–10902; (d) H.
deactivate under the applied reaction conditions (maximum Lu, W. I. Dzik, X. Xu, L. Wojtas, B. de Bruin and X. P. Zhang, J. Am.
TON = 8), thus suggesting that future investigations aimed at Chem. Soc., 2011, 133, 8518–8521. (e) S. K. Russell, J. M. Hoyt, S. C.
the development of efficient protocols for catalytic CF
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focus on using catalysts that are more stable in the presence of Vyushkova and V. M. Iluc, Chem. Sci., 2015, 6, 4570–4579. (g) P. Cui
2
Bart, C. Milsmann, S. C. E. Stieber, S. P. Semproni, S. DeBeer and P.
3
3
−
the reactive, free CF and CF
the applied reaction conditions.
2
3
intermediates generated under and V. M. Iluc, Chem. Sci., 2015, 6, 7343–7354. (h) D. A. Sharon, D.
Mallick, B. Wang, and S. Shaik, J. Am. Chem. Soc., 2016, 138, 9597–
9
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We thank Netherlands Organization for Scientific Research (NWO-CW Inorg. Chem., 2011, 50, 9896–9903.
1
6
(
a) Das, B.G.; Chirila, A.; Tromp, M.; Reek, J.N.H; de Bruin, B., J.
VICI project 016.122.613 (MG and BdB) and VENI grant 722.013.002
WID)) for funding.
Am. Chem. Soc., 2016, 138, 8968−8975. (b) Paul, N.D.; Mandal, S.
Otte, M. Cui. X. Zhang, X.P.; de Bruin, B., J. Am. Chem. Soc., 2014,
(
1
36, 1090–1096. (c) Paul, N.D.; Chirila, A.; Lu, H.; Zhang, X.P.; de
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TOC figure
A Co(II) porphyrin complex catalyses difluorocarbene transfer to an acrylate using TMSCF
source of CF2
3
as the