Caesium fluoride-mediated hydrocarboxylation of alkenes and allenes: Scope and mechanistic insights

Article abstract of DOI:10.1039/c9sc02467k

A caesium fluoride-mediated hydrocarboxylation of olefins is disclosed that does not rely on precious transition metal catalysts and ligands. The reaction occurs at atmospheric pressures of CO₂ in the presence of 9-BBN as a stoichiometric reductant. Stilbenes, β-substituted styrenes and allenes could be carboxylated in good yields. The developed methodology can be used for preparation of commercial drugs as well as for gram scale hydrocarboxylation. Computational studies indicate that the reaction occurs via formation of an organocaesium intermediate.

Full text of DOI:10.1039/c9sc02467k

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DOI: 10.1039/C9SC02467K
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Caesium Fluoride-Mediated Hydrocarboxylation of Alkenes and
Allenes: Scope and Mechanistic Insights
Ashot Gevorgyan,a,† Marc F. Obst,b,† Yngve Guttormsen,a Feliu Maseras,c Kathrin H. Hopmann,*b
Received 00th January 20xx,
Accepted 00th January 20xx
Annette Bayer*a
DOI: 10.1039/x0xx00000x
A caesium fluoride-mediated hydrocarboxylation of olefins is disclosed that does not rely on precious
transition metal catalysts and ligands. The reaction occurs at atmospheric pressures of CO2 in the presence
of 9-BBN as a stoichiometric reductant. Stilbenes, β-substituted styrenes and allenes could be carboxylated
in good yields. The developed methodology can be used for preparation of commercial drugs as well as for
gram scale hydrocarboxylation. Computational studies indicate that the reaction occurs via formation of an
organocaesium intermediate.
generated organoboranes has not been reported. In the
following, we detail our findings of the CsF-mediated
Introduction
CO2 provides a sustainable source of carbon that increasingly is
being used in chemical synthesis.1 Construction of
anthropogenic chemical carbon cycles2 by valorisation of CO2
into chemicals, materials, and fuels, is a promising strategy for
replacing fossil carbon in the chemical industry.1,3 Various
studies have shown that transition metal-based catalysts are
able to selectively reduce CO2 into simple chemicals, such as
formic acid, methanol, alkanes, and CO.3a CO2 can also be
incorporated into carbonates, which are valuable starting
materials for polymer science.3b Use of CO2 in C-C bond forming
reactions opens new pathways towards value-added products
and pharmaceuticals from CO2.4
hydrocarboxylation of alkenes with CO2 (Scheme 1, C). A
detailed computational analysis indicates that the reaction
proceeds via formation of organocaesium intermediates. The
described transformation expands the repertoire of
carboxylation reactions that can be performed without the use
of transition metal catalysts.
(A) Hou, Sawamura, Skrydstrup
R2
1) (9-BBN)2, Solvent
R1
R2
R1
Cu ligand
2)
,
, base,
CO2H
2, 70-120oC, 12-24h
CO
(B) Sato, Cantat, Kondo
As part of our research interest to develop C-CO2 bond
forming reactions,5 we became interested in the copper-
catalysed hydrocarboxylation reactions reported by Hou,6a
Sawamura6b and Skrydstrup6c (Scheme 1, A). In these formal
hydrocarboxylations, an initial hydroboration with 9-
borabicyclo[3.3.1]nonane (9-BBN) transforms an alkene to an
organoborane, which in a subsequent copper-catalysed step is
carboxylated with CO2. In order to elucidate the mechanistic
R2
R3
R4
CO2H
CO
Base, Solvent,
2
Si
20-150oC, 2-24h
(C) Present approach
R2
1) (9-BBN)2, DME
R1
R2
R1
CO
2) CsF,
,
2
CO2H
120oC, 24h
Scheme 1. Previous works (A6, B7) and present study (C).
details of the carboxylation step, we embarked on
a
computational study of the reaction. Surprisingly, our
computational analysis indicated the existence of a feasible
carboxylation pathway that does not involve the copper
Results and Discussion
complex. Our subsequent experiments confirmed that it is On basis of a preliminary computational investigation of the
possible to carboxylate in situ formed organoboranes in hydrocarboxylation of alkenes, we speculated that trans-
absence of copper. Related reports of carboxylations with CO2 stilbene 1a can be hydrocarboxylated via an organoborane
in absence of transition metals include fluoride-mediated intermediate in the absence of a transition metal catalyst, which
carboxylations of organosilanes7b-f and KOtBu-mediated is in contrast to previous reports.6 To test our hypothesis, we
carboxylations of benzylboronic esters (Scheme 1, B).7a used 9-BBN in dioxane to convert 1a into an organoborane
However, none of these reports addressed a possible role of the intermediate, which we attempted to carboxylate with CO2 in a
counteraction for the observed reactivity. To the best of our CsF-mediated transformation (Table 1). Gratifyingly, the
knowledge, a CsF-mediated hydrocarboxylation with in situ corresponding carboxylic acid 2a was obtained in 83% yield
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(Table 1, entry 2). In comparison, the previously reported 5), while other fluoride containing bases like KF and NaF gave
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copper-catalysed reaction6c gave the carboxylation product 2a inferior results (50% and 0% yield, entry 9DaOnId: 1100.1)0.3I9n/tCe9rSeCs0ti2n4g6l7yK,
in 78% yield (Table 1, entry 1). The higher yield in absence of also Cs2CO3 and K2CO3 gave good results (71% and 67% yield,
copper was observed for several substrates (ESI, Scheme S1 and entry 11 and 12) showing that not only the fluoride anion is
S2). This phenomenon may be explained by a copper-promoted important for the outcome of the reaction. On basis of the
decarboxylation reaction slowly consuming the product 2a.8 To reports by Hou,6a Sawamura6b and Schomaker7a we also
support this hypothesis, we mixed 2-phenylpropionic acid with attempted to employ alkoxides as base, but observed
the Cu complex under reaction conditions, which lead us to difficulties in our system. If mixed simultaneously, the reaction
recover only 95% of the starting acid, while in the absence of between alkoxide and CO2 lead to the corresponding
Cu, the recovery of acid was 99% (ESI, Scheme S1).
carbonates, and no carboxylation product was formed. Alkoxide
bases were effective only if the second reaction step was run
without CO2 for minimum 30 minutes at 20 C, followed by
addition of CO2, which provided a yield of 47% (Table 1, entry
13). Further screening related to the stoichiometry of reagents,
duration of the reaction and temperature showed that the best
conditions are 1 equiv. olefin and (9-BBN)2 and 3 equiv. CsF in
DME at 120 C for 24 h (Table 1, entry 5; for further details see
ESI, Table S1).
Table 1 Optimization of reaction conditions.a
1) (9-BBN)2 (1 equiv.),
Solvent, 70oC, 24h
2) Catalyst (mol%),
CO
Base (equiv.),
Solvent, oC, h
,
2
CO2H
2a
1a
Catalyst
(mol%)
Base
(equiv.)
Yield
Entry
Solvent
oC/h
b
%
With the optimized conditions at hand, we explored the
substrate scope of the reaction (Scheme 2; ESI Scheme S3).
Screening of different substrates showed that the CsF-mediated
hydrocarboxylation works only on systems where the in situ
hydroboration step (mediated by 9-BBN) generates benzylic or
allylic borane intermediates. Indeed, styrene and cyclohexene
were not reactive under optimal conditions (ESI Table S1). On
the other hand, stilbenes, β-substituted styrenes and allenes
were successful substrates. Neither the pinacol ester of
benzylboronic acid nor in situ-generated benzylic catechol
esters instead of the organoborane intermediate were reactive
in the CsF-mediated carboxylation (ESI Scheme S4).
The CsF-mediated hydrocarboxylation of stilbene
derivatives (1a-e) produced the corresponding carboxylic acids
2a-e with moderate to excellent yields (Scheme 2, A). The
conversion of (E)--methyl stilbenes (1d,1e) was regioselective
providing exclusive carboxylation at the sterically less hindered
-position resulting in formation of 2d and 2e, each as a mixture
of diastereomers. The observed regioselectivity is assumed to
be controlled by steric effects.6,9,10
The CsF-mediated hydrocarboxylation of β-substituted
styrenes (1f-l) gave the -carboxylated products 3a-g as the sole
product in moderate to good yields (Scheme 2, B). Interestingly,
whereas the selectivity of the 9-BBN-initiated hydroboration of
β-substituted styrenes is substrate-dependent and generally
gives a non-regioselective mixture of boranes,6,9,10 our base-
initiated carboxylation appears to convert only the benzylic
boranes, providing a single carboxylation product with excellent
regioselectivity for 3a-g (Scheme 2, B). In contrast, the Cu-
catalysed hydrocarboxylation does not differentiate between
the regioisomeric borane intermediates, giving a mixture of
carboxylic acids.6c For example, in the copper-catalysed
hydrocarboxylation of indene (1k), we observed a mixture of -
and β-regioisomers with a ratio of 4:1 (ESI, Scheme S2).
1
2
IPrCuI(5)c
CsF(3)
CsF(3)
Dioxane 120/24
Dioxane 120/24
78
83
61
67
87
0
-
-
-
-
-
-
-
-
3
CsF(3)
THF
120/24
4
CsF(3)
Diglyme 120/24
5
CsF(3)
DME
120/24
6
CsF(3)
DMA
120/24
7
CsF(3)
Toluene 120/24
70
0
8
CsF(3)
MeCN
DME
DME
DME
DME
DME
DME
DME
DME
120/24
120/24
120/24
120/24
120/24
120/24
120/24
80/24
9
KF(3)
50
0
10
11
12
13
14
15
16
NaF(3)
Cs2CO3(3)
K2CO3(3)
KOtBu(3)d
CsF(2)
-
-
-
-
-
-
71
67
47
57
59
85
CsF(3)
CsF(3)
120/28
a Reaction conditions: 1) 1a (0.444 mmol), (9-BBN)2 (1 equiv.), Solvent (3 mL), 70oC,
24h. 2) (IPrCuI (5 mol%)), base (2-3 equiv.), CO2 120 mL, 80-120oC, 24-28h.
Isolated yields. The active catalyst was prepared in situ (IPr = 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene). d The reaction mixture was run at 20oC for
30 min before addition of CO2.
b
c
We proceeded to establish the optimum reaction conditions
of the base-mediated carboxylation reaction. Screening of
different solvents revealed that the reaction works well in
ethers. The best yield was observed in dimethoxyethane (DME,
87%, Table 1, entry 5). The screening of different bases
indicated that the optimal base is CsF (87% yield; Table 1, entry
Allenes also proved to be suitable substrates for CsF-
mediated hydrocarboxylation (Scheme 2, C). Both aliphatic and
aromatic allenes could be transformed to carboxylic acids 5a-f
with good yields. The regioselectivity of the reaction was
strongly dependent on the nature of the allene substituents.
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Allenes with aliphatic substituents gave the internal allylic
carboxylic acid as a single product 5a-c (Scheme 2, C).
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DOI: 10.1039/C9SC02467K
R1
1) (9-BBN)2 (1 equiv.),
R1
DME, 70oC, 24h
R3
R2
R3
R2
2) CsF (3 equiv.), CO2,
CO2H
120oC, 24h
1a-l
Me
2a-e, 3a-g
(A) Stilbenes
Me
Cl
Me
Me
CO2H
CO2H
CO2H
CO2H
CO2H
2b 91%
F
Cl
Me
2d 70%
dr 10:1
2e 55%
dr 12:1
2a 87%
(B) -Substituted styrenes
2c 51%
F
OMe
OMe
CO2H
CO2H
Ph
O
Me
Me
Ph
Me
CO2H
O
CO2H
3e 51%
CO2H
CO2H
3d 52%
CO2H
3f 51%
3g 92%
3a 94%
3b 75%
3c 63%
1) (9-BBN)2 (0.7 equiv.),
DME, 20-50oC, 24h
(C) Allenes
R2
R2
CO2H
R2
R1
and/or
R1
R1
2) CsF (3 equiv.), CO2,
CO2H
120oC, 24h
4a-f
5a-f
CO2H
CO2H
CO2H
Me
Me
CO2H
Me
HO2C
CO2H
5b 63%
CO2H
5a 76%
5c 65%
5da
5db
72% 10:1.5:4.5
CO2H
5dc
CO2H
CO2H
Me
CO2H
Me
Me
Me
Me
CO2H
Me
Me
Me
5ea
5eb
68% 10:1.8:4.3
5ec
5fa
5fb
5fc
53% 10:1.1:4.1
(D) Preparation of Butetamate
1) SOCl2 (1.2 equiv.),
DMF (0.5 equiv.),
DCM, 20oC, 3h
1) (9-BBN)2 (1 equiv.),
DME, 70oC, 24h
Et Me
Me
Me
2) CsF (3 equiv.), CO2,
2) Alcohol (1.4 equiv.),
BuLi (1.4 equiv.),
THF, 0-20oC, 6h
N
120oC, 24h
CO2H
Et
O
O
1f
3a 94%
Butetamate
cough suppressant 6a 91%
(E) Preparation of Butibufen
(F) Scaling up the reaction
Me
1) (9-BBN)2 (1 equiv.),
Diglyme, 70oC, 24h
1) (9-BBN)2 (1 equiv.),
Me
DME, 70oC, 24h
Me
Me
Me
2) CsF (3 equiv.),
2) CsF (3 equiv.), CO2,
Me
CO2 balloon, 120oC, 24h
120oC, 24h
CO2H
2a 1.427g 76%
CO2H
Butibufen
anti-inflammatory drug 6b 64%
1m
1a 1.5g
Scheme 2. Substrate scope of the CsF-mediated hydrocarboxylation.
In contrast, allenes possessing aromatic substituents yielded
We further tested the possibility of asymmetric
the carboxylic acids 5d-f as isomeric mixtures, with the terminal hydrocarboxylation using the (−)-isopinocampheylborane
carboxylic acids as the major product (Scheme 2, C). Although TMEDA complex – a chiral analogue of 9-BBN – in the initial
borane-mediated hydroboration of allenes has been hydroboration step (ESI Scheme S5).12 Even though the
described,11 the selectivity is not well understood, and hydroboration-oxidation of trans-β-methylstyrene gave the
equilibria of internal and terminal allylic boranes have been corresponding alcohol with 36% ee (ESI, Scheme S5, A), the
proposed. Recently, Chida and Sato showed that the hydroboration-carboxylation using our conditions led to
hydroboration of allenes in deuterated THF occurs racemic product (ESI, Scheme S5, B). The observed racemisation
predominantly at the terminal double bond.11f Carboxylation of may be explained by the structural instability of intermediate
alkyl allenes may then proceed from the terminal allylic borane organometallic compounds, such as the organoborane or an
with an allyl shift, or involve the internal allylic borane organocaesium (vide infra) at elevated temperatures.13
generated through equilibration. For aryl allenes the direct In order to show the versatility of the developed CsF-
carboxylation of the terminal allylic borane is preferred as the mediated hydrocarboxylation reaction, we applied our strategy
system is less likely to rearrange due to conjugation.
in the synthesis of the commercial drugs butetamate 6a and
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butibufen 6b from β-substituted styrenes (Scheme 2, D, E). clear preference to interact with the cesium centre (Fig. 2; see
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DOI: 10.1039/C9SC02467K
Although in case of butetamate, four steps are required also ESI, Fig. S9), in contrast to other computational studies
(hydroboration, carboxylation, preparation of acid anhydride, predicting CO2-Cs interactions.16 However, for b2, a preference
and esterification), only two isolations were needed, providing for a weak CO2-Cs interaction is seen (ESI, Fig. S10). The reason
almost quantitative yields. Similarly, butibufen was obtained in may be that the Cs atom experiences stronger interactions with
64% yield using the direct hydrocarboxylation of β-substituted the two phenyl rings of b1 than with the single aromatic ring in
styrene 1m (Scheme 2, E).
b2, making additional CO2-Cs interactions preferable for b2.
Importantly, the hydrocarboxylation reaction can be scaled
up (Scheme 2, F). For this we changed the solvent from DME to
diglyme (2-methoxyethyl ether), which has a higher boiling
point, allowing the reaction to be performed in simple flasks
using a CO2 balloon. Starting from 1.5 g of stilbene, we could
prepare 1.427 g of the corresponding acid 2a (Scheme 2, F). The
yield at gram scale (76%, Scheme 2) is slightly larger compared
to the small scale (67%, Table 1, entry 4), probably due to better
recovery of material during work-up at larger scale.
B
F
CsF
CO2
Cs
O
O
R
B
B
F
Cs
Mechanism B
R
R'
R
R'
R'
i0
i1
p1
Mechanism C
The computational analysis of the CsF-mediated
carboxylation of in situ generated organoboranes provided
insights into the mechanistic steps. Three boranes were
included in the theoretical study (Figure 1): b1 and b2, derived
from the experimentally reactive alkenes trans-stilbene (1a)
and trans-β-methylstyrene (1f), and b3, corresponding to the
non-reactive alkene cyclohexene (1o). Three possible reaction
mechanisms (referred to as A, B and C) were found by an
automated search of the potential energy surface with the AFIR
method.14 Mechanism A (ESI, Figure S6) is characterized by a
nucleophilic attack of the reactive carbon of the borane on a
CO2 molecule, followed by a transmetalation with CsF. This
mechanism is considered not viable, as all the evaluated
boranes show a computed Gibbs free activation energy of >50
kcal/mol for the first step (ESI, Table S2).
Cs
CO2
B
R
R'
F
i2
Scheme 3. Computed reaction mechanisms B and C.
Figure 2. Optimized geometries for b1 (Mechansim C) the
organocaesium intermediate i2 (left) and the C-CO2 bond
formation TS (TSi2-p1, right)
B
B
B
For boranes b1 and b2, the rate-limiting step of mechanism
C is the cleavage of the boron-carbon bond with overall barriers
of 34.0 kcal/mol for borane b1 (derived from trans-stilbene) and
36.7 kcal/mol for b2 (derived from trans-β-methylstyrene).
Mechanism C is thus the preferred pathway for boranes b1 and
b2. The full energy profile for carboxylation of b1 via
mechanism C is shown in Figure 3.
For borane b3 (derived from cyclohexene), the rate-limiting
step of mechanism C is the C-CO2 bond formation with an
overall barrier of 51.5 kcal/mol, which is not feasible. The lowest
computed barrier for borane b3 is thus observed with
mechanism B (Figure 3), which at 44.4 kcal/mol is not feasible
at the experimental temperature, in line with the
experimentally observed lack of reactivity of cyclohexene.
Our computational and experimental results are in good
agreement, indicating that the carboxylation of benzylic
boranes occurs via reaction mechanism C, which features an
organocaesium intermediate i2. The benzylic boranes b1 and b2
are able to stabilize the organocaesium intermediate i2 via
delocalization of the negative charge and via cation-
interactions between caesium and the aromatic substituents on
the organoborane. Similar Cs- interactions have been
observed in related computational studies.17 The cost of
b1
b2
b3
Figure 1. Computationally investigated boranes.
Reaction mechanism B (Scheme 3) occurs through two
steps: First, the formation of a B-F bond between the borane i0
and a CsF molecule yielding intermediate i1, and second, the
nucleophilic attack of intermediate i1 on CO2. The latter step is
characterized by a concerted formation of the C-CO2 bond and
the cleavage of the B-C bond, releasing F-(9-BBN) and forming
the product p1. The overall barrier computed for the different
boranes with mechanism B (ESI, Table S3) is significantly lower
than with mechanism A (Table S2). However, with values of 44.4
kcal/mol (cyclohexane-derived borane b3) to 52.3 kcal/mol
(trans-β-methylstyrene-derived borane b2), the barriers are too
high to be overcome at the reaction temperature of 120 C.15
The first step of mechanism C (Scheme 3) is the same as for
B, the formation of intermediate i1. In the next step, the boron-
carbon bond is cleaved, releasing a F-(9-BBN) molecule and
forming the organocaesium intermediate i2 (Figure 2). In the
final step, i2 undergoes a nucleophilic attack on a CO2 molecule.
Interestingly, at the insertion TS for substrate b1, CO2 shows no
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forming i2 is only 7.4 kcal/mol for b1 and 12.7 kcal/mol for b2. caesium fluoride-mediated carboxylation was effective for in
View Article Online
DOI: 10.1039/C9SC02467K
The cyclohexyl borane b3 lacks these stabilizing effects resulting situ generated benzylic and allylic organoboranes derived from
in a relative energy of 37.2 kcal/mol for the i2 intermediate. We stilbenes, β-substituted styrenes and allenes, providing the
therefore suggest that the stability of the organocaesium corresponding carboxylic acids with good yields and excellent
intermediate i2 is the factor determining the reactivity of olefins regioselectivities. The developed methodology was
in the CsF-mediated hydrocarboxylation.
demonstrated at gram-scale and was used for the production of
commercial drugs. Computational studies indicate that benzylic
organoboranes are transformed to organocaesium
intermediates, which then undergo a nucleophilic attack on
CO2. Stabilisation of the organocaesium intermediate by the
aromatic substituent account for the observed selectivity
towards benzylic organoboranes.
Conclusions
We report a CsF-mediated hydrocarboxylation of alkenes
proceeding via a hydroboration with 9-BBN followed by a CsF-
mediated carboxylation of the resulting organoboranes. The
Figure 3. Computed Gibbs free energy profile (DLPNO-CCSD(T)//ω-B97XD) of the preferred reaction pathways, mechanism C for b1 (black
solid line) and mechanism B for b3 (blue dashed line).
at 20 C was added CsF (3 equiv.). The pressure tube was closed
with the cap and removed from the glove box. Afterwards CO2
Methods
(120 mL) was added via a syringe, which was followed by stirring
of the reaction mixture at 120 C for 24h. Next, the reaction
mixture was diluted with 30 mL Et2O and transferred into a 500
mL separating funnel. The resulting mixture was extracted with
30 mL saturated basic (NaHCO3, 1M KOH) solution (3 times). The
resulting basic solution was washed with 15 mL Et2O (once),
acidified (50-55 mL 6M HCl) and extracted with 30 mL Et2O (3
times). The resulting solution of Et2O was distilled to dryness to
give the corresponding acid (in case of 5c the final solution of
Et2O was dried using Na2SO4, which was followed by careful
evaporation of solvents).
Computational Methods. Density functional theory (DFT)
calculations were performed with the ω-B97XD hybrid
functional,18 as implemented in Gaussian 16, Revision B.01.19
Geometries were optimized with the SDD ECP and basis set for
Cs and the 6-31+G* basis set for all other elements. Initial guess
Experimental and computational details are given in the
electronic supporting information (ESI). The ESI includes
experimental procedures and analytical data, an example input
for DLPNO-CCSD(T) calculations, computed energies for the full
reaction pathways for b1, b2 and b3, and a comparison of
computed C-CO2 TS structures. A separate .xyz file contains all
optimized coordinates in a format that allows easy visualization
with Mercury.
General procedure for metal-free hydrocarboxylation of
stilbenes, β-substituted styrenes and allenes. Inside the glove
box, a 45 mL pressure tube was charged with the corresponding
olefin or allene (1.5 mmol), (9-BBN)2 (1 equiv. for olefins or 0.7
equiv. for allenes) and dry DME (7 mL). The flask was closed with
a suitable cap, removed from the glove box and heated to 70 C
(olefin) or 50 C (allene) for 24h. Afterwards, the pressure tube
was transferred back to the glove box. To the reaction mixture
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M. Borjesson, T. Moragas and R. Martin, AngeVwie.wCAhrteicmle O. nInlinte.
structures for the transition states were obtained through linear
transit calculations and through artificial force induced reaction
modelling (AFIR) as implemented in GRRM.14 Solvation effects
were included in the final geometry optimizations via the
IEFPCM model (1,4-dioxane). Explicit solvent molecules may
bind to specific points in the system, but we do not expect them
to affect the overall mechanistic picture,17 and because of this
they were omitted from the calculation. Vibrational, entropic,
and temperature corrections were computed at 393.15 Kelvin,
with the same level of theory as geometry optimizations.
Electronic energies were obtained with DLPNO-CCSD(T)20 using
ORCA 4.1.1.21 The ZORA operator as well as the basis sets SARC-
ZORA-TZVPP (for Cs) and ZORA-def2-QZVPP (all other elements)
were employed. The final Gibbs free energies (ΔGDPLNO-CCSD(T)//ω-
B97XD) in the main text correspond to the DLPNO-CCSD(T)
electronic energies combined with the DFT-based vibrational,
entropic and temperature corrections, and the standard state
(SS, 393.15 K) conversion in case of a change in the number of
moles:22 ΔGDPLNO-CCSD(T)//ω-B97XD = ΔGω-B97XD/IEFPCM - ΔEω-B97XD/IEFPCM
+ ΔEDPLNO-CCSD(T) + SS. All ORCA and Gaussian calculations were
performed on the Norwegian supercomputer Stallo at UiT,
whereas GRRM calculations were perfomed on the computer
cluster at ICIQ. More information on the computational details
and example inputs as well as additional DFT energies are given
in the ESI.
Ed., 2018, 57, 15948-15982.
DOI: 10.1039/C9SC02467K
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgments
This work has been performed with support from NordForsk (Grant
No. 85378 to KHH and AB), the Research Council of Norway (Centre
of Excellence Grant No. 262695, KHH), the Tromsø Research
Foundation (Grant No. TFS2016KHH to KHH), and Notur - The
Norwegian Metacenter for Computational Science (CPU Grant No.
nn9330k to KHH) and the Artic Centre for Sustainable Energy (ARC)
at UiT. We gratefully acknowledge the Faculty of Science and
Technology at UiT for a travel grant to MFO. We thank M. K. Langer
for HPLC analysis.
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Chemical Science
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DOI: 10.1039/C9SC02467K
A caesium fluoride-mediated regioselective hydrocarboxylation
of alkenes and allenes is described. Computational studies
reveal that the stability of an organocaesium intermediate
accounts for the observed reactivity.