Photoiodocarboxylation of Activated C=C Double Bonds with CO2 and Lithium Iodide

Article abstract of DOI:10.1021/acs.joc.8b02162

The photolysis at 254 nm of lithium iodide and olefins 1 carrying an electron-withdrawing Z-substituent in CO₂-saturated (1 bar) anhydrous acetonitrile at room temperature produces the atom efficient and transition metal-free photoiodocarboxylation of the C=C double bond. The reaction proceeds well for terminal olefins 1 to form the new C-I and C-C σ-bonds at the α and β-positions of the Z-substituent, respectively, and is strongly inhibited by polar protic solvents or additives. The experimental results suggest that the reaction channels through the radical anion [CO₂^?-] in acetonitrile, yet involves different intermediates in aqueous medium. The stabilizing ion-quadrupole and electron donor-acceptor interactions of CO₂ with the iodide anion play a crucial role in the reaction course as they allow CO₂ to penetrate the solvation shell of the anion in acetonitrile, but not in water. The reaction paths and the reactive intermediates involved under different conditions are discussed.

Full text of DOI:10.1021/acs.joc.8b02162

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Article
Photoiodocarboxylation of Activated C=C
Double Bonds with CO2 and Lithium Iodide
Rossella Mello, Juan Camilo Arango-Daza, Teresa Varea, and María Elena González-Núñez
J. Org. Chem., Just Accepted Manuscript • Publication Date (Web): 10 Oct 2018
Downloaded from http://pubs.acs.org on October 10, 2018
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The Journal of Organic Chemistry
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Photoiodocarboxylation of Activated C=C Double Bonds with
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Rossella Mello, Juan Camilo Arango-Daza, Teresa Varea, María Elena González-Núñez*
Departamento de Química Orgánica, Facultad de Farmacia, Universidad de Valencia. Avda. Vicente Andrés Estellés
s.n., 46100-Burjassot. Valencia. España
ABSTRACT: The photolysis at 254 nm of lithium iodide and olefins 1 carrying an electron-withdrawing Z-substituent in
CO2-saturated (1 bar) anhydrous acetonitrile at room temperature, produces the atom efficient and transition metal-free
photoiodocarboxylation of the C=C double bond. The reaction proceeds well for terminal olefins 1 to form the new C-I and
C-C -bonds at the  and -positions of the Z-substituent, respectively, and is strongly inhibited by polar protic solvents or
·-
additives. The experimental results suggest that the reaction channels through the radical anion [CO2 ] in acetonitrile, yet
involves different intermediates in aqueous medium. The stabilizing ion-quadrupole and electron donor-acceptor
interactions of CO2 with the iodide anion play a crucial role in the reaction course as they allow CO2 to penetrate the
solvation shell of the anion in acetonitrile, but not in water. The reaction paths and the reactive intermediates involved
under different conditions are discussed.
Introduction
e(OCO): 180º
Re(CO): 1.16 Å
e(OCO): 127  8º
Re(CO): 1.25 Å
pKa(HOCO): 2.3-3.4
DM: 0.27 D
O
C
O
O
C
O
+ e-
Eored = -2.14 V
The necessary transition from petroleum and natural gas
to renewable feedstocks for sustainable chemical
production1 has turned the research on the capture and
functionalization of carbon dioxide (CO2) into a major
innovation arena.2 The use of CO2 as a renewable C1-
feedstock has evolved along three main avenues, namely:
i) synthesis of carbamates and carbonates by reaction with
amines, alcohols or epoxides;3 ii) reduction of CO2 to CO,
formic acid and its derivatives, methanol or methane;4 iii)
CO2-fixation through the formation of a kinetically and
thermodynamically stable C-C -bond.5,6 While the CO2-
capture into carbonic acid derivatives and the CO2-
reduction to give useful fuels have become well developed
research areas,3,4 the carboxylation of organic substrates
under atmospheric CO2 and mild conditions has proven
more challenging.5,6 Recently, photoinduced electron
transfer reactions, generally coupled to transition metal
catalysis, have emerged as a powerful approach to C-C -
bond-forming reactions with CO2, which has provided
suitable CO2-fixation procedures.6
PA: 540.5 kJ mol-1
QM: -4.3 x 10-26 esu cm-2
C
O
+0.52 -0.76
e
e
charge density
spin density
68 %
16%
Figure 1. Data on CO2 and [CO2 ].7-11 e : bond angle, Re : bond
length, PA: proton affinity, QM: quadrupole moment, DM:
dipole moment; Eored: standard reduction potential vs. SCE in
water.
·-
CO2, which turns it into a strong reductant [Eo
red
·-
(CO2/CO2 ) = -2.14 V vs. SCE],10 a nucleophilic radical15 and
a Lewis base.11 The application of these properties to
synthesis requires suitable reaction conditions to generate
·-
the radical anion [CO2 ] and to channel it towards
products, while avoiding its deactivation and the side
reactions of the ensuing reactive intermediates. Our recent
report on the transition metal-free iodide-photocatalyzed
reduction of CO2 to formic acid with thiols16 has revealed
that the photolysis of the iodide anion in the presence of
CO2 provides a suitable entry to the chemistry of the
·-
radical anion [CO2 ] in the absence of transition metals.
The reactivity pattern of CO2, as determined by its
structure and bonding, is the basis of CO2 capture and
functionalization methodologies (Figure 1).7-11,12 The strong
quadrupole moment of CO2,9 with positive charge density
localized at the carbon atom, and its Lewis acid and weakly
basic character,13 determine the ability of CO2 to
coordinate Lewis bases14 and metal complexes,12 and to
react with nucleophilic species.3,5 This reactivity scope
further expands through the single electron reduction of
These results prompted us to further explore the
photochemistry of the iodide anion and CO2 in the
presence of olefins.
Herein we report that the photolysis at 254 nm of CO2-
saturated acetonitrile solutions of lithium iodide (LiI) and
olefins carrying electron-withdrawing substituents, at both
room temperature and atmospheric pressure, produces the
-iodo--carboxylation of the C=C double bond (Equation
1). The scope of the reaction and the involved reactive
intermediates are discussed.
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O
I
products (Entry 5, Table 1). Acrylic acid (1h) also gave 2-
iodopropanoic acid (4a) (37 %) (Entry 8, Table 1).
1
2
3
4
5
6
7
8
h 254 nm
O
Li
CO
(1 bar)
I
Li
+
+
2
(1)
CH3CN
3 h, r.t.
Methacrylamide (1i) reacted efficiently under our
reaction conditions, and reached 68 % conversion (Entry
9, Table 1). The reaction products readily decomposed in
aqueous solution in this case. Thus the reaction mixture
was analyzed by dissolving the solid materials with an
equimolar amount of trifluoroacetic acid and analyzing the
homogeneous solution by NMR with tert-butanol as the
external standard. The dehydrohalogenation reaction took
place in the -methyl group to exclusively give lithium 3-
carbamoylbut-3-enoate (3i).
Z
Z
1
2
Z
= CN, CONH2, CON(CH3)2, COOCH3,
COOCH2CH=CH2, COOH, COCH3, SO2Ph
Results
9
The photoiodocarboxylation reaction. Reactions
were performed by irradiating 0.04 M solutions of
substrates 1 and lithium iodide (1:LiI 1:1) in anhydrous
acetonitrile-d3 saturated with CO2 (1 bar) with a 36 W low
pressure mercury lamp ( = 254 nm) at room temperature
for 3 h. The initial CO2 concentration, estimated from the
solubility reported for carbon dioxide (1 atm) in
propionitrile at 25oC, was 0.245 M (initial molar ratio 1:CO2
1:3.5).17 The reaction mixtures were dark yellow
suspensions, with variable amounts of solid. Aliquots of
the decanted solutions and solid residues were analyzed by
1H NMR spectroscopy in acetonitrile-d3 and D2O, with tert-
butanol used as the external standard. Substrate
conversions and product yields were determined from the
integrals of their NMR signals and those of the external
standard. The results were confirmed in a series of
identical experiments in which the reaction mixtures were
diluted with a known amount of water to obtain a
homogeneous solution, and were analyzed by 1H NMR with
tert-butanol as the external standard. The results are
shown in Table 1. Preparative experiments were performed
in anhydrous acetonitrile as described above, and reaction
mixtures were decanted at 0oC. Solid residues were
isolated, washed with acetonitrile, dried under vacuum
and characterized. Detailed experimental procedures and
spectra are included in the Experimental Section and
Supplementary Material.
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11
12
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59
60
The substrates 1 carrying substituents at the -position
reacted less efficiently. Thus the substrate conversion
found for crotononitrile (1j) (mixture E:Z 1.9:1), the most
reactive substrate in this series, was 58 %, but was only 20-
30 % for (E)-crotonamide (1k), methyl (E)-crotonate (1l)
and methyl (E)-3-methoxyacrylate (1m). Adducts 2j-m
(mixtures of diastereomers, 1.8:1, 11:1, 4:1 and 2.1:1,
respectively) were the major products in all cases (Runs 10-
13, Table 1). Substrates 1j-m underwent isomerization
under these reaction conditions since the unreacted
materials were found as E:Z mixtures from 1:1 to 1.5:1.
Dehydrohalogenation product 3j was found as E:Z 1.9:1
(Entry 10, Table 1).
2-Cyclopentenone (1n) converted only 24 % to give
carboxylation products 2n (67 %) and 3n (13 %) (Entry 14,
Table 1), although other cyclic substrates with endocyclic
C=C double bonds, like 2-cyclohexenone (1o), N,N-
dimethyluracil (1p), and maleic anhydride (1q), were
unreactive under our standard reaction conditions (Entries
15-17, Table 1). Dimethyl maleate (1r) exclusively
underwent isomerization to give dimethyl fumarate (Entry
18, Table 1).
Styrene (1s) gave lithium cinnamate (3s) as the only
product, with 68 % conversion and a mass balance as low
as 44 % (Entry 19, Table 1). Isoprene (1t) reacted efficiently,
but no identifiable products were found (mass balance 23
%) (Entry 20, Table 1). Simple alkenes were non-reactive
under our reaction conditions.
The photolysis ( = 254 nm) of LiI and activated alkenes
1 in acetonitrile saturated with CO2 at atmospheric
pressure and room temperature led to the addition of the
iodine atom and CO2 to the  and -positions of the
substituted C=C double bonds, respectively (Table 1). No
polycarboxylation products were found in the reaction
mixtures. Adducts 2 underwent the partial elimination of
hydrogen iodide to give unsaturated compounds 3 under
our reaction conditions,27 or by standing for long periods
in aqueous medium. The extent of these side processes
depended on the substrate structure (Table 1). The
iodocarboxylation adducts 2 from acrylic acid derivatives
1a-c were partially converted into the corresponding
anhydrides in aqueous medium. The efficiency of the
photochemical reaction was hampered by the insolubility
of the reaction products in acetonitrile.
Control experiments. The reaction of methyl acrylate
(1a) and CO2 with potassium iodide (KI) in acetonitrile
(Entry 21, Table 1) gave results similar to those found with
LiI (Entry 1, Table 1).
The photoiodocarboxylation reactions of acrylamide (1b)
and acrylic acid (1h), KI and CO2 in D2O were performed
following the experimental procedure described above
(Entries 22 and 23, Table 1). The substrate 1 concentration
was 0.01 M in these cases, as solubility in water reported for
CO2 at atmospheric pressure and room temperature was
0.035 M (molar ratio 1:CO2 1:3.5).28 The reaction mixtures
1
were analyzed by H NMR, with tert-butanol used as the
Photoiodocarboxylation was efficient for the unsaturated
substrates carrying electron withdrawing substituents 1a-h
(Entries 1-8, Table 1), with conversions within the 53-86%
range, to give carboxylated products 2 and 3 as the major
products (over 90 % in most of these cases). Allyl acrylate
(1e) reacted exclusively at the carbonyl-substituted C=C
double bond to give 2e (84 %) and 3e (14 %) as the only
external standard.
The reaction of acrylamide (1b) in aqueous solution
proceeded with low substrate conversion (13 %) to give
carboxylation products 2b (40 %) and 3b (60 %) and a
quantitative mass balance (Entry 22, Table 1). Conversely,
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acrylic acid (1h) underwent complete conversion (>99 %)
1
2
3
4
5
6
7
8
9
Table 1. Photoiodocarboxylation of activated olefins 1 with LiI or LiK, and carbon dioxide.a
O
O
H
R
H
R
h 254 nm
R'
H
R
R'
R'
R'
H
O
2
Li
O
3
Li
CO2 LiI
,
+
+
Z
3 h, r.t.
Z
I
Z
Z
R
I
1
4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0
Run
1
Z
R
R’
Ered
Condi.
Conv.
(%)
Product distribution (%)
MB
(%)
(V)b
2
72
90
69
84
84
64
64
52
37
3 (E/Z)
22 (1:1.5)
10 (1:1.1)
27 (1.5:1)
14 (1:2)
14 (1.9:1)
20 (E)
4
6
1
1a
1b
1c
1d
1e
1f
COOCH3
CONH2
CON(CH3)2
CN
H
H
H
H
H
H
H
H
H
-2.1018
-2.0119
--
Stand.
Stand.
Stand.
Stand.
Stand.
Stand.
Stand.
Stand.
Stand.f
Stand.
Stand.
Stand.
Stand.
Stand.
Stand.
Stand.
Stand.
Stand.
Stand.
Stand.
KI
86
86
68
83c
52
60d
86e
60
68
58
30
28
20
24
13
90
91
2
H
--
4
3
H
85
93
4
H
-2.1418
--
2
5
COOAllyl
COCH3
SO2Ph
H
--
>99
97
66
74
80
77
6
H
-1.9120
-1.4921
-1.7619
--
1
7
1g
1h
1i
H
30 (1:1)
11 (1:1.7)
59g
8
COOH
H
37
4
9
CONH2
CN
CH3
H
H
10
11
1j
CH3
-2.5522
-2.7423
-2.4118
-2.6718
-2.2320
-2.1720
-2.0624
-0.9825
-1.8418
-2.6026
-2.7026
-2.1018
-2.0119
-1.7619
-2.1018
-2.1018
-2.1018
-2.1018
-2.1018
79h
99i
99j
99k
67
--
21 (1.9:1)
--
--
--
--
--
--
--
--
--
--
--
--
5
1k
1l
CONH2
COOCH3
COOCH3
COCH2-
COCH2-
CON(CH3)-
COO-
H
CH3
81
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
H
CH3
63
1m
1n
1o
1p
1q
1r
H
OCH3
--
96
83
H
-CH2-
33
H
-(CH2)2-
--
--
87
96
98
>99
44
23
H
-N(CH3)CO-
--
--
H
-CO-
--
--
--
COOCH3
Ph
H
COOCH3
68l
68
77
73
13
--
--
1s
H
H
H
H
H
H
H
H
H
H
H
--
>99
1t
CH=CH2
COOCH3
CONH2
COOH
CH3
H
--
--
1a
1b
1h
1a
1a
1a
1a
1a
70m
40
<1
25 (1.7:1)
60 (1:1.7)
--
82
>99
<1
H
D2O/KI
D2O/KI
H2On
CH3OHo
90 min
LiI 10%
LiI 30%
--
--
--
--
5
H
>99
6
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
H
63
--
84p
66q
64r
37 (1.3:1)
--
>99
93
H
--
H
66
18
11 (1.1:1)
30 (1:1.6)
31 (1:1.4)
91
H
4
85
80
H
53
5
a Standard procedure (Stand.): the reactions were performed by irradiating CO2-saturated (1 bar) acetonitrile-d3 solutions of 1
and LiI (1 equiv.), with a low pressure mercury lamp, for 3 h at room temperature; data from the 1H NMR analyses of the decanted
solution, and of the dried solid in D2O, with tert-butanol as external standard. MB: Mass balance. b Data vs. SCE, in acetonitrile or
dimethylformamide. c 2 % Lithium succinate derivative. d 16 % Lithium succinate derivative. e 5 % Lithium succinate derivative. f
g
Analysis of the homogeneous solution after addition of CF3COOH.
Lithium 3-carbamoylbut-3-enoate (3i), from
deshydrohalogenation at the CH3 group. h 2j: 1.8:1 mixture of diastereomers; 1j: starting material E:Z 1.9:1, unreacted material E:Z
i
j
1.5:1. 2k: 11:1 mixture of diastereomers; starting material (E)-1k, unreacted material E:Z 1:1.1. 2l: 4:1 mixture of diastereomers;
k
starting material (E)-1l, unreacted material E:Z 1:1.1. 2m: mixture of diastereomers 2.1:1; starting material E, unreacted material
E:Z 1.6:1. l Starting material (Z)-1r; unreacted material E:Z 2.1:1. m 2m 58 %, anhydride 12 %. n CH3CN:H2O 5:1; 2a 27%, anhydride
36%. o CH3CN:CH3OH 1:3 v:v. p 2a 44 %, anhydride 40 %. q 2a 34 %, anhydride 32 %. r 2a 43 %, anhydride 21 %.
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1
2
3
4
5
6
7
8
under the same conditions, yet no detectable products
significant amounts of 2-iodopropanoic acid (4h) (Entry 8,
Table 1). Minor amounts of products 4 were also found in
the reactions of methyl
were found in the reaction mixtures, which suggested the
involvement of radical side reactions (mass balance <1 %)
(Entry 23, Table 1). The reactions performed in acetonitrile
with water or methanol as cosolvents were also much less
Table 2. Photolysis of the iodocarboxylation adducts 2.a
efficient than the reactions in acetonitrile (<6
conversion) (Entries 24 and 25, Table 1).
%
O
O
H
H
H
The irradiation of the acetonitrile-d3 solutions of methyl
acrylate (1a), LiI and CO2 for 90 min (Entry 26, Table 1) led
to a lower conversion of 1a (66 %), a slightly better mass
balance, and a higher yield of iodocarboxylation adduct 2a
in relation to unsaturated derivatives 3a than the reactions
performed for 3 h (Entry 1, Table 1). This result suggests
CH3
O
2
Li
O
3
Li
h 254 nm
9
+
Z
+
3 h, r.t.
Z
OH
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Z
Z
I
H
1
5
Run
2
Solv.
Conv. Prod. distribu. (%) MB
(%)
(%)
1
3 (E:Z)
5
that iodocarboxylation adduct
2
may undergo
1
2b
2d
D2O
D2O
95
90
90
86
52 48 (1:1.6) --
59 41 (1.5:1) --
15 54 (1:1.4) 31
80
68
53
photochemical dehydrohalogenation under our reaction
conditions.27
2
3
4
2b ACN, AA
2d ACN, AA
This side reaction was also evident in the reactions of
methyl acrylate (1a) and CO2 performed with 10% and 30%
LiI (Entries 27 and 28, Table 1). The conversion of 1a
reached 18 % and 53 %, respectively, and gave higher
amounts of unsaturated products 3a compared to our
standard reaction conditions (Entry 1, Table 1). The fact
that carboxylation products 2a and 3a formed in larger
amounts than expected for the starting LiI in these
experiments suggested the possibility of designing an
iodide-photocatalyzed carboxylation of substrates 1, which
would proceed through the photoiodocarboxylation of 1,
followed by the photodeshydrohalogenation27 of adduct 2
to give unsaturated carboxylation product 3 and to deliver
the iodide anion back to the solution.
5
51 (1:1)
45 83
a
Reactions performed by irradiating solutions of 2 in D2O
or in mixtures of acetonitrile-d3 (ACN) and acetic acid (AA)
(100 equiv), with a low pressure mercury lamp (35 W), for 3 h
1
at room temperature; data from the H NMR analyses of the
solutions, with tert-butanol as external standard.
acrylate (1a), N,N-dimethylacrylamide (1c), allyl acrylate
(1e), methyl phenyl sulfone (1g) and methacrylamide (1i)
(Table 1). These results prompted us to explore the impact
of excess acetic acid (10 equiv) on the reactions of methyl
acrylate (1a), acrylamide (1b), acrylonitrile (1d), and (E)-
crotonamide (1k), with LiI in anhydrous acetonitrile, both
saturated with CO2 and in an argon atmosphere. The
In order to test the feasibility of the dehydrohalogenation
reaction27 under our reaction conditions, we carried out
the photolysis of iodocarboxylation adducts 2b and 2d
from acrylonitrile (1b) and acrylamide (1d) in an inert
atmosphere. The results are shown in Table 2. The
reactions in aqueous medium were very efficient to give ca.
1:1 mixtures of 1 and 3 (Entries 1 and 2, Table 2). No 2-
1
reaction mixtures were analyzed by H NMR. Table 3
provides the results.
The reactions with excess acetic acid run in a CO2
atmosphere (Entries 1, 3, 7 and 9, Table 3) led to lower
substrate 1 conversions than under our standard reaction
conditions (Table 1), which agrees with the negative impact
of the polar protic additives in the photoiodocarboxylation
reactions. The major products were the corresponding
hydrohalogenated derivatives 4 from methyl acrylate (1a),
acrylamide (1b), acrylonitrile (1d), and (E)-crotonamide
(1k), accompanied by small amounts of carboxylation
products 2 and 3. The reaction of (E)-crotonamide (1k) led
to the isomerization of the starting material (Entry 9, Table
3). These results indicate that the formation of
hydrohalogenated product 4h from acrylic acid (1h) (Entry
8, Table 1) derives from the acidic character of substrate 1h.
The formation of products 4 in other cases can be
attributed to the presence of acidic impurities in the
reaction medium.
hydroxysuccinic
acid
derivatives
5
from
the
photosolvolysis of the C-I -bond27 were observed as
products under these conditions. The reactions in
anhydrous acetonitrile were inefficient due to the low
solubility of adducts 2 in this medium. However, the
addition of acetic acid to improve the solubility of 2 in
acetonitrile (Entries 3 and 4, Table 2) led to significant
amounts of 2-hydroxypropanoic acid derivatives 5, which
indicated the opening of alternative reaction paths under
these conditions.
These results showed that the iodide photocatalyzed
carboxylation of substrates 1 may indeed be feasible,
provided that iodocarboxylation adduct 2 was soluble in
the reaction medium. The remarkable sensitivity of the
photoiodocarboxylation reaction to polar protic solvents
prevented an iodide photocatalyzed process from being
implemented for the carboxylation of substrates 1 under
our reaction conditions.
Conversely, the reactions performed in an inert
atmosphere produced the corresponding lactic acid
derivatives 5 as the only products, with substrate
conversions lower than for the reactions run with CO2 and
with satisfactory mass balances (Entries 2, 4, 8, Table 3).
(E)-Crotonamide (1k) underwent isomerization under
these conditions.
The
results
in
Table
1
show
that
the
photoiodocarboxylation of acrylic acid (1h) produced
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The reactions of acrylamide (1b) in the presence of excess
acetonitrile solutions of the LiI saturated with CO2 (1 bar)
at 22 oC, 0 oC and -20 oC displayed the bathochromic shifts
promoted by the increasing CO2 concentration in the
solution at these temperatures (Figure 2D), as well as the
characteristic hypsochromic shift of the CTTS band of
iodide associated with lowering temperature.29-33
1
acetic acid were also performed in aqueous solution with
KI (Entries 5 and 6, Table 3). The reactions underwent
similar substrate conversions to those found in
acetonitrile, yet no significant products were detected in
the reaction mixture.
2
3
4
5
6
7
8
Table 3. Reactions in the presence of excess acetic acid.a
9
O
O
H
H
H
I
h, Li
R'
H
H
R'
R'
R'
H
R'
+
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
O
CH3COOH
Li
+
+
Li
Z
3 h, r.t.
Z
I
Z
Z
Z
H
I
OH
1
2
3
4
5
Run
1
Cond.a
Conv. Prod. distribu. (%) MB
(%)
(%)
2
8
-
3
4
5
1
1a
1a
1b
1b
1b
CO2
Ar
43
23
46
19
14 78
-
86
2
3
4
5
-
-
100 84
75
100 85
CO2
Ar
34b 13 53
-
-
-
-
-
-
CO2/
D2O/KIc
Ar/
D2O/KIc
CO2
Ar
45
<1
-
55
6
1b
25
--
-- --
--
75
7
8
1d
1d
1k
1k
63
33
18d
20e
14 37 47
2
100
-
84
81
-
27
-
-
-
-
-
73
-
9
CO2
Ar
93
81
10
100
Figure 2. A) UV spectra of LiI (5.34 x 10-5 M) (red line) and
methyl acrylate (1a) (1.0 x 10-3 M) (black line) in anhydrous
acetonitrile (ACN). B) Changes in the UV spectra of LiI (5.34 x
10-5 M) in ACN (black line) upon addition of acrylonitrile (1d)
(from 5.55 x 10-3 M to 1.5 x 10-1 M). C) Changes in the UV spectra
of LiI (3.26 x 10-5 M) in ACN (black line) upon pressurization
with CO2 from 1-10 bar (ca. 0.25-4 M). D) Changes in the UV
spectra of LiI (5.34 x 10-5 M) in ACN at 22 oC, 0 oC and -20 oC
(black lines), upon saturation with CO2 (1 bar). E) Changes in
the UV spectra of LiI (5.34 x 10-5 M) in ACN (black line), upon
addition of water (from 5.5 x 10-2 M to 5.5 x 10-1 M). F) Changes
in the UV spectra of LiI (3.26 x 10-5 M) in water (black line),
upon pressurization with CO2 from 1-10 bar.
a Reactions were performed by irradiating acetonitrile-d3 of
1 ([1] = 0.04 M) LiI or KI (1 equiv.), and acetic acid (AA) (10
equiv.) saturated with CO2 (1 bar) or argon (Ar), with a low
pressure mercury lamp, for 3 h at room temperature; data
from the 1H NMR analyses of the reaction mixtures, with tert-
butanol as external standard; MB: mass balance. b Adduct 2b
10%, anhydride 24 %. c Substrate concentration [1] = 0.01 M. d
Starting material (E)-1k; unreacted material E:Z 1.9:1. e Starting
material (E)-1k; unreacted material E:Z 2.1:1.
UV spectra. The UV spectra of LiI (5.34 x 10-5 M) and
methyl acrylate (1a) (1.0 x 10-3 M) in anhydrous acetonitrile
(Figure 2A) showed that the main absorption at 254 nm
corresponded to the charge transfer to the solvent (CTTS)
band of the iodide anion (max = 247.2 nm).29 This band did
not significantly change upon the addition of increasing
amounts of acrylonitrile (1j) (up to 1.7 x 10-2 M) (Fig. 2B),
methyl acrylate (1a), and acrylamide (1b) to the acetonitrile
solution of LiI. The complete set of UV spectra can be
found in the Supplementary Material.
Conversely, the CTTS band of LiI in acetonitrile
underwent progressive hypsochromic shifts upon the
stepwise addition of water34 (Figure 2E). Remarkably, the
progressive pressurization from 1-10 bar with CO2 at room
temperature of a 3.26 x 10-5 M aqueous solution of LiI (max
= 227.0 nm) did not significant displace the CTTS band
(Figure 2F).
The stepwise pressurization with CO2 from 1 to 10 bar of
a 3.26 x 10-5 M solution of LiI in anhydrous acetonitrile at
room temperature ([CO2] = 4 M at 10 bar and 25 C)
Discussion
o
The photolysis of LiI and activated olefins 1 in the CO2-
produced increasing bathochromic shifts of the CTTS band
of iodide (Figure 2C), which overcame the blue shifts
associated with the changes in density and pressure.30 The
CTTS band shift showed neither transient band changes
saturated
anhydrous
acetonitrile
led
to
the
iodocarboxylation of the C=C double bond, with the
formation of new C-I and C-C -bonds at the  and -
positions of the electron-withdrawing substituent,
respectively (Equation 1, Table 1). This reaction course
nor isosbestic points. The UV spectra of 5.34 x 10-5
M
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indicates the involvement of electron transfer processes
counterion.29,39 The photodetached electron can either
and radical-ion intermediates.35 As the main absorption at
254 nm corresponded to the iodide anion (Figure 2A), the
reaction would initiate with a photoinduced electron
transfer from iodide to some reducible species in solution.
Two alternative reaction paths (Scheme 1) differing in the
actual electron acceptor may account for the
photoiodocarboxylation reaction, namely the reduction of
the C=C double bond and the capture of the substrate
radical anion [1·-] by CO2 (path a, Scheme 1), or the
recombine with the iodine atom in the geminate pair or be
scavenged by electron acceptors in solution. Both the CTTS
band energy and bandwidth depend on the distinct
stabilization exerted by the solvent on the ground and the
excited state, the variety of configurations available for the
solvent molecules in the solvation shell, and the solvent
structure.29,39 The strong sensitivity of CTTS bands to the
structure and composition of the solvent has been applied
to explore the microscopic structure of solvent mixtures,
and the preferential solvation phenomena.40
1
2
3
4
5
6
7
8
9
·-
reduction of CO2 [Eored(CO2/CO2 ) -2.14 and -2.21 V vs. SCE
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
in water10 and dimethylformamide,36 respectively] and the
The blue shift of the CTTS band of lithium iodide in
acetonitrile observed upon the addition of water (Figure
2E) has been attributed29,39,40 to the progressive water
enrichment of the solvation shell of the iodide anion,
driven by the stronger ion-dipole and hydrogen-bonding
interactions provided by water compared to those exerted
by the polar, aprotic and less structured acetonitrile. These
interactions stabilize the ground state of the iodide anion
better than the more diffuse CTTS excited state. The
weaker impact observed for H-bond donors like acetic acid
and acrylamide (1b) (the spectra are shown in the
Supplementary Material) could be attributed to the solute
association.
·-
addition of the radical anion [CO2 ] to the activated C=C
double bond of substrate 1 (path b, Scheme 1), followed in
both cases by the non-rate-determining coupling of the
distonic radical anion [I·-] with the iodine atom to give
iodocarboxylation adduct 2.
Scheme 1. Starting hypothesis for the competitive
reaction paths towards photoiodocarboxylation
adducts 2.
path a
I
CO2
Z
h
1
O
I
1
O
I
Z
I
Accordingly, the progressive displacement of the CTTS
band of the iodide anion in acetonitrile observed upon the
intake of CO2 into the solution (Figures 2C,D) discloses the
progressive penetration of CO2 molecules into the
solvation shell of the iodide anion, devoid of efficient
solvation in the polar non protic acetonitrile, to provide
stabilizing ion-quadrupole and electron donor-acceptor
interactions. The formation of clusters [Hal(CO2)n]- in the
gas phase,41 and the ionization of the sensitive alkyl halides
in supercritical CO2,42 are further examples of the ability of
CO2 to coordinate halide anions. The bathochromic shift
of the CTTS band observed under these conditions
suggests that CO2 stabilizes the CTTS excited state better
than the ground state.
I
I
1
I
CO2
O
O
CO2
1
Z
Z
[
]
2
CO2
h
CO2
path b
I
1
Z
The competition of CO2 and substrates 1 as electron
acceptors was also discussed in relation to the
electrocarboxylation of substituted alkenes,5e which was
proposed to proceed through either path a or b (Scheme 1)
depending on the standard reduction potentials of the
37
substrate and CO2.5e,
Notwithstanding, the
photoiodocarboxylation of the substituted alkenes 1
described herein takes place through a photoinduced
electron transfer step which concomitantly delivers an
iodine atom to the solution,38 so that the actual reaction
paths could also depend on additional factors. The
inhibition of the reaction by polar protic solvents or
additives (Entries 22-25, Table 1, and Table 3), which
should actually facilitate the intermediacy of radical
anions, and the singular features of the CTTS spectra under
different conditions (Figure 2), are remarkable
observations that merit further consideration. The
experimental results and data available on the different
elementary steps of these reactions are discussed in the
paragraphs below.
The insensitivity to CO2 of the CTTS band of aqueous LiI
(Figure 2F) revealed that CO2 is unable to compete with the
ion-dipole and H-bonding interactions provided by the
water molecule to the iodide anion. Therefore, the results
suggest that CO2 displaces the solvent from the cybotactic
region of the iodide anion in acetonitrile, but not in water.
The addition of methyl acrylate (1a), acrylamide (1b), and
acrylonitrile (1d) to the acetonitrile solutions of lithium
iodide did no significantly modify the CTTS band of the
iodide anion (Figure 2B), which indicates the absence of
significant ion-molecule interactions in these cases.
Substrates 1 and CO2 are competitive scavengers of the
photodetached electron from the iodide anion (Eored (aq/e-
aq) = -3.11 V vs. SCE)10 under our standard conditions,
according to the data reported on the standard reduction
potentials (Table 1)18-26 and the rate constants for the
reaction with hydrated electrons (3.1 x 1010 M-1 s-1 and 7.7 x
109 M-1 s-1, for 1b and CO2, respectively).43,44 The rate
constants reported for selected reactions of acrylamide
(1b) from pulse radiolysis studies45,46 are shown in Scheme
The electron transfer step. The irradiation of the CTTS
band of the iodide anion is known29,39 to generate a short-
lived excited state that either deactivates back to the
ground state or evolves through solvent reorganization to
a photodetached electron that is in close proximity to the
iodine atom, held in a diffuse orbital bound by the
combined electrical fields of the solvent molecules and the
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The Journal of Organic Chemistry
2. However, CO2 has a singular competitive advantage over
Pulse radiolysis studies in aqueous medium have shown
that the radical anion [CO2 ] adds to acrylamide 1b (path
·-
1
2
3
4
5
6
7
8
substrates 1 in acetonitrile solution as it actually resides in
the solvation shell of the iodide anion. Indeed the effective
molarity of the reactants held in the proximity of its
reaction partner by non-covalent intermolecular forces
could reach values as high as 106 M,47 which is a key factor
in enzyme catalysis commonly applied to control chemical
reactivity.
b, Scheme 1) with a second-order rate constant of (4 + 1) x
107 M-1 s-1 without any significant competition of electron
transfer (Scheme 2).46 The addition of the radical anion
·-
[CO2 ] to methyl acrylate (1a) has also been demonstrated
in hydrated clusters in the gas phase.52 This reaction
proceeds with the rehybridization of only the -carbon
·-
atom of substrate 1 as the radical anion [CO2 ] already has
Conversely, polar protic solvents or additives displace the
CO2 molecules from the solvation shell of the iodide anion
a bent structure. Hence, the intermediacy of the distonic
radical anion [I·-] in the formation of the iodocarboxylation
adducts 2 in acetonitrile (Table 1) agrees well with the
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 2. Selected rate constants reported for
acrylamide (1b) from pulse radiolysis studies.
·-
chemistry reported for the radical anion [CO2 ] in aqueous
medium.46,52
CH3
H2N
Conversely, no data on the reaction of the substrate
radical anion [1·-] with CO2 (path a, Scheme 1) are reported
in the literature. Notwithstanding, it is known53 that
radical anions [1b·-] and [1h·-] from acrylamide (1b) and
acrylic acid (1h) in aqueous medium undergo reversible
and diffusion-controlled protonation at the oxygen atom
(pKa 7.9 and 7.0, for [1bH·] and ([1hH·] respectively), but
irreversible pH-dependent -protonation with pseudo-
first-order rate constants at pH 7 of 1.4 x 105 s-1 and 4.0 x 104
s-1, respectively (Scheme 2).53 The different magnitudes of
these rate constants have been attributed to the energy
penalty posed by the rehybridization of the terminal
carbon atom in the irreversible -protonation step.53
Accordingly, the reaction of the substrate radical anion [1·-
] with CO2 to give the distonic radical anion [I·-] (path a,
Scheme 1), which requires the rehybridization of both
reaction partners, is not expected to compete efficiently
with the proton transfer from the solvent in aqueous
medium.
[H ]
10 M-1 s-1
H2O
1.4 x 10
5 s-1
O
1.8 x 10
IIb
[
]
[e-]aq (3.1 x 10
)
10 M-1 s-1
H2N
H2N
9 M-1 s-1
Ox (0.7 x 10
)
O
O
1b
1b
]
[
4
-1 s-1
x 107 M
H2O (fast)
[CO2
]
O
H2N
H2N
O
]
OH
O
Ib
IIIb
]
[
[
Ox: 2-sulfoanthraquinone (Eored = -0.634 vs SCE) .45,46
and, thereby, hinder the formation of the radical anion
·-
[CO2 ]. Water and methanol do not scavenge hydrated
electrons efficiently (1.9 x 101 M-1 s-1, and < 1 x 10-4 M-1 s-1,
respectively),43 yet unionized acetic acid and the [H3O+]
ions do (2.0 x 108 M-1 s-1 and 1.9 x 1010 M-1 s-1, respectively),43
and these reactions deliver H-atoms43 to the solution.
Therefore, the data set discussed in this section suggests
that the photolysis of the CO2-saturated solutions of LiI
and substrates 1 preferentially channels through the radical
It is noteworthy that all the steps proposed for the
photoiodocarboxylation reaction (Scheme 1), except the
capture of CO2 by the radical anion [1·-], have been proven
·-
efficient in aqueous medium, even the addition of [CO2 ]
to 1.45,46,48,52,54, Therefore, the formation of the radical anion
·-
·-
[CO2 ] in water is probably marginal under our reaction
anion [CO2 ] in acetonitrile, but involves different reactive
conditions, otherwise the photoiodocarboxylation adduct
would also be produced in this medium.
species in aqueous medium.
·-
The carboxylation step. The radical anions [CO2 ] and
The distonic radical anion [I·-]. The impact of the
solvent on the photoiodocarboxylation reaction could
alternatively be attributed to the fragmentation of the
[1·-] are strong reductants that undergo electron transfer
reactions with oxidants at rates close to diffusion [0.7-5 x
109 M-1 s-1, for different species [1·-],45 and 0.33 x 1010 M-1 s-1,
·-
distonic radical anion [I·-] to give [CO2 ] and 1 (path b,
·- 48
for [CO2 ] , in the reaction with 2-sulfoanthraquinone (E0
Scheme 1). This decarboxylative process, which has been
disclosed in the ion cyclotron mass spectrometry study of
= -0.634 V vs. SCE)] (Scheme 2). Therefore, the
photoinduced electron transfer reaction will not proceed
towards the iodocarboxylation adducts 2 unless the
ensuing steps are efficient enough to compete with the
electron transfer to the oxidizing iodine species in the
reaction medium (Scheme 1).
·-
the addition of [CO2 ] to methyl acrylate (1a) in hydrated
clusters in the gas phase,52 could be more efficient in water
than in acetonitrile given the better solvation of the radical
·-
anion [CO2 ] in the polar protic solvent. In other words,
the carboxylation equilibrium could be strongly displaced
to the left-hand side in aqueous medium (path b, Scheme
1).
Iodine atoms are not expected to play a key role in the
photoiodocarboxylation reaction as they react with excess
iodide anion with a rate constant of 2.5 + 0.4 x 1010 M-1 s-1 to
·-
The results in Table 2 provide information on the
reactivity of the intermediate [I·-]. Indeed the photolysis of
iodocarboxylation adducts 2b and 2d in water (Table 2),
which proceeds through the radical pair [I·-, I·],27 gave a ca.
give the radical anion [I2 ], which is able to produce
additional negatively charged species under our reaction
conditions.49,50 However, we will herein consider the iodine
atoms as actual intermediates for simplicity reasons.
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and -protonation of the radical anion [1·-]53 (path a,
1:1 mixtures of starting materials 1b and 1d, and maleic and
1
2
3
4
5
6
7
8
fumaric acid derivatives 3b and 3d (Table 2). These results
Scheme 4), and the second through one electron reduction
of the carboxylic acid moiety43 coordinated to the iodide
anion, and addition of the resulting H-atom to the C=C
double bond of 1 (path b, Scheme 4),43 followed in both
cases by coupling of the resulting C-radical [II·] with the
iodine atom. Note that these reaction paths require that
either the substrate 1 (path a, Scheme 4) or the carboxylic
acid (path b, Scheme 4) efficiently compete with CO2 as
electron acceptors in the electron transfer step, which
contradicts some of the observations discussed above.
·-
establish that, first, the addition of the radical anion [CO2 ]
to the substrates 1 is reversible, in agreement with the
conclusions drawn from the ion cyclotron mass
spectrometry study;52 and second, the radical anion [I·-]
undergoes irreversible H-atom transfer to the iodine atom
to give the unsaturated products 3 (path b, Scheme 3), in
competition with decarboxylative fragmentation followed
·-
by electron transfer from [CO2 ] to the iodine atom to give
9
the starting materials (path a, Scheme 3).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 3. Reaction paths proposed for the distonic
radical anion [I·-].27,52
Scheme 4. Possible reaction paths leading to the
hydrohalogenated products 4 in the presence of
acids.
path a
I
path a
Z
1
I
RCOOH
CO2
CO2
O
I
O
RCOO
Z
1
h
I
1
Z
I
O
H+-transfer
H•-transfer
O
]
1
h
I
I
1
CH3
CH3
O
3
HI
Z
Z
Z
RCOOH
I
Z
Z
I
2
I
[
O
I
I
path b
II
4
[
]
RCOOH
h
RCOO
[RCOOH
]
path b
Z
I
1
The most salient feature in Table 3 is, however, the
formation of 2-iodopropanoic acid derivatives 4 as the
major product in a CO2 atmosphere, but of lactic acid
derivatives 5 in an inert atmosphere, which indicates the
involvement of different intermediates for each reaction
condition. Indeed if substrate 1 was able to compete with
acetic acid and CO2 for the photodetached electron, and if
the radical anion [1·-] gave 2-iodopropanoic acid derivatives
4 through path a in Scheme 4, then the reaction in an inert
atmosphere would also produce 4. The same conclusion
would apply if acetic acid competed with CO2 and
substrate 1 as an electron acceptor to give H-atoms, which
would lead to 4 through path b in Scheme 4. The reaction
paths towards lactic acid derivatives 5 will not be discussed
herein.
The photolysis of acrylic acid (1h), KI and CO2 in aqueous
medium (Entry 23, Table 1) gave neither iodocarboxylation
adducts 2h, nor maleic and fumaric acids 3h, which should
accompany the decarboxylative fragmentation of the
intermediate [Ih·-]. The reaction of acrylamide (1b) under
the same conditions (Entry 22, Table 1) gave only a small
amount of 3b, which indicates that the fragmentation of
[Ib·-] back to the starting materials does not play a key role
in preventing this reaction, otherwise the low substrate
conversion would be accompanied by a greater formation
of 3b than that actually found. Similar results were found
for the photoiodocarboxylation reaction of methyl acrylate
(1a) when water and methanol were used as cosolvents
(Entries 24 and 25, Table 1). Therefore, neither the
photolysis of iodocarboxylation adduct
2
nor an
unfavorable carboxylation equilibrium, satisfactorily
accounts for the failure of the photoiodocarboxylation
reaction in polar protic media.
Actually, the results in Table 3 show that products 4 arise
only when CO2 and acetic acid are both present in the
reaction medium, which reveals that the radical anion
·-
[CO2 ] is involved in the reaction path to 4. Since pulse
Impact of acids on the reaction course. The results in
Table 3 revealed that the presence of excess acetic acid in
the reaction medium actually opens up alternative paths in
the photoiodocarboxylation reaction. The reactions took
place with low substrate conversions (Table 3), as expected
from the partial displacement of CO2 from the solvation
sphere of the iodide anion by the polar protic additive.
Notwithstanding, CO2 still competed with acetic acid for
the photodetached electron to eventually give significant
amounts of carboxylation products 2 and 3 (Entries 1, 3, 7
and 9, Table 3).
radiolysis studies55,11a,b have established that the radical
·-
anion [CO2 ] does not generate H-atoms even at pH < 3,
the formation of 4 under these conditions is better
accounted for through the radical anion intermediate [1·-]
(path a, Scheme 4), generated by electron transfer from the
·-
radical anion [CO2 ] to 1. 56 Accordingly, the radical anion
·-
[CO2 ] would react with substrates 1 through either
addition to the C=C double bond to give [I·-], or electron
transfer to give [1·-], which agrees with its dual character as
a nucleophilic radical15 and a strong reductant. The fact
·-
that the electron transfer step from [CO2 ] to 1 went
The formation of 2-iodopropanoic acid derivatives 4
under these conditions (Table 3) could be tentatively
accounted for by two alternative reaction paths (Scheme
4). The first proceeds through one electron reduction of 1
unnoticed in acetonitrile under our standard reaction
conditions suggests that the radical anion [1·-] most
probably deactivates through electron transfer to the
oxidizing iodine species.
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In aqueous medium, however, carboxylic acids release
O
into the solution [H3O+]-ions which react efficiently with
hydrated electrons (2.3 x 1010 M-1 s-1).43,57 The reaction, then,
probably channels through the radical intermediates
generated from the addition of H-atoms to 1, which
proceeds in aqueous medium with rate constants of 3.3 x
1010 M-1 s-1 and 1.8 x 1010 M-1 s-1 for 1h and 1b, respectively
(Scheme 2).43 Indeed the photolysis of acrylic acid (1h) with
KI and CO2 in water failed to give products from the -
protonation of the radical anion [1h·-]53 and proceeded with
a low mass balance (Entry 23, Table 1), suggesting radical
side processes. Non-acidic acrylamide (1b) did not undergo
such a destructive reaction under the same conditions
(Entry 22, Table 1), but it did in the presence of acetic acid
(Entries 5 and 6, Table 3). These data suggest that the
radical anion [1·-] either does not form in aqueous medium
or deactivates through electron transfer to the oxidizing
iodine species.50
1
2
3
4
5
6
7
8
O
2
I
I
O
I
Z
I
h
O
O
I
I
H
Z
Z
1
[
]
O
I
h
I
CO
CO
[ ]
2
[
,
]
Z
I
2
3
CO2
I
I
Z
9
1
Z
10
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1
I
Z
CH3
CH3
1
H+
Z
Z
I
4
II
[
]
·-
The nucleophilic radical anion [CO2 ] efficiently adds to
substrates 1 with reduction potentials that fall within the
range of ca. -2.50 to -2.00 V vs. SCE (Table 1). For substrates
1 with a reduction potential less negative than -2.00 V, the
reaction channels preferentially through electron transfer
(Scheme 5), as shown by the isomerization of dimethyl
The scope of the reaction. Scheme 5 summarizes the
major features of the photoiodocarboxylation reaction in
anhydrous acetonitrile discussed herein, namely: i) CO2 is
the main electron acceptor in the photoinduced electron
maleate
[(Z)-1r]
observed
under
the
·-
transfer from the iodide anion; ii) the radical anion [CO2 ]
photoiodocarboxylation conditions (Entry 18, Table 1), and
in agreement with the results from the pulse radiolysis
studies on dimethyl maleate [(Z)-1r] and dimethyl
fumarate [(E)-1r].57 The absence of polycarboxylation18
reacts with substrate 1 through either reversible addition
to the C=C double bond to give the distonic radical anion
[I·-] or electron transfer to give [1·-]; iii) the intermediate [I·-
] reacts with the iodine atom through either radical
coupling to give adduct 2 or H-atom transfer to give
unsaturated carboxylation product 3; iv) the radical anion
[1·-] either deactivates back to 1 through electron transfer
to an oxidizing iodine species or, in the presence of acids,
undergo irreversible -protonation and coupling of the
resulting C-radical [II·] with the iodine atom to give
product 4. However, the reaction in aqueous medium
channels through alternative intermediates because CO2,
unable to displace water from the solvation sphere of the
iodide anion, loses its competitive advantage as an electron
acceptor. The results obtained for the different substrates
1 (Table 1) can now be discussed in light of the reaction
paths outlined in Scheme 5.
·-
products from the reaction of [CO2 ] with unsaturated
product 3 under our reaction conditions (Table 1) is the
result of this unproductive path. The insolubility of lithium
carboxylates 2 and 3 in acetonitrile contributes to prevent
further carboxylation or photolysis reactions of these
products under our reaction conditions.
For substrates 1 with similar or more negative reduction
potentials than for CO2, the reaction course depends on
the efficiency of the carboxylation step, and the
competitive reactions of the distonic radical anion [I·-], i.e.
fragmentation back to the reactants, and coupling or
deshydrohalogenation with the iodine atom (Scheme 5).
The
results
in
Table
1
show
that
the
photoiodocarboxylation is more efficient for acrylic acid
derivatives 1a-e than for crotonic acid derivatives 1j-l and,
in general, for substrates 1 with substituents at the -
position. This observation cannot only be attributed to the
more negative reduction potentials of substrates 1 with
electron-releasing substituents at -position because
cyclopentenone (1n), cyclohexenone (1p), and N,N-
dimethyluracil (1o), with reduction potentials that fall
within the optimal range, do not react significantly under
our conditions (Entries 14-16, Table 1). Instead these results
Scheme 5. Reaction paths proposed for the
photoiodocarboxylation reaction in anhydrous
acetonitrile.
·-
evidence that the addition of [CO2 ] to substrates 1 is
sensitive to steric hindrance at the -position.15b,c
·-
The approach trajectory of [CO2 ] towards 1 aligns the C-
CO2 -bond parallel to the p orbital of the -carbon radical
[I·-] in the plane perpendicular to the C=C double
bond,15b,c,58 which is the optimal orientation for the reverse
reaction. The steric or electronic factors that prevent the
intermediate [I·-] to escape this reactive conformation are
expected to favor the path back to the starting materials.
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Moreover, the carboxylation reaction occurs with Conclusions
Page 10 of 16
1
2
3
4
5
6
7
8
rehybridization of the -carbon atom of 1, and those factors
that prevent this geometrical change are expected to
hinder the formation of the new C-C -bond. The
restricted conformational mobility of the cyclic distonic
radical anions [In-o·-] would then contribute to the poor
performance of substrates 1n-o, regardless of their
favorable reduction potentials (Table 1).
The photolysis at 254 nm of the CO2-saturated
acetonitrile solutions of LiI and olefins 1 carrying an
electron withdrawing Z-substituent led to the addition of
iodine and CO2 to the C=C double bond. This reaction
proceeded efficiently with terminal C=C double bonds to
give new C-I and C-C -bonds at the  and -positions of
the Z-substituent, respectively. The UV spectra of the
reaction components under different conditions have
revealed that CO2 is able to penetrate the solvation sphere
of the iodide anion in acetonitrile, but not in water, as the
intermolecular ion-quadrupole interactions provided by
CO2 to the halide anion cannot successfully compete with
H-bonding. This fact determined the negative impact of
polar protic solvents on reaction efficiency. This
transformation has been interpreted in terms of a
photoinduced electron transfer reaction from the iodide
anion to reducible species in solution, which leads to
different reactive intermediates, depending on the reaction
conditions. The results suggest that the reactions in
The isomerization of the unreacted substrates observed
for the crotonic acid derivatives 1j-l, with more negative
standard reduction potentials than CO2 (Table 1), could be
attributed to the contributions made from the reversible
formation of the distonic radical anion [I·-]52 and electron
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·-
transfer from [CO2 ]. The isomerization of (E)-
crotonamide (1k) observed in the presence of acetic acid
under inert atmosphere (Entry 10, Table 3) suggests that
the addition to the C=C double bond of intermediate H-
atoms may also be reversible.
The reactions of the radical anion [I·-] with the iodine
atom are crucial for the formation of carboxylation
products 2 and 3 (paths a and b, Scheme 5). The coupling
of these intermediates, though not rate-determining, is
sensitive to steric and electronic factors, as indicated by the
remarkable diastereselectivity observed for substrates 1j-l,
whose intermediates [Ij-l·-] have stereogenic -carbon
atoms (Table 1). The H-atom transfer from [I·-] to the
iodine atom is less favorable as it involves the breaking of
a C-H bond, which has to be aligned in parallel to the p
orbital holding the unpaired electron in order to be
activated.27,58 With methacrylamide (1i) (Entry 9, Table 1),
the H-atom transfer occurred preferentially from the -
methyl group of [Ii·-], which provides stereoelectronically
and statistically more favored C-H -bonds than the -
carbon atom. The trends on the selectivity for this reaction
are, however, obscured by the possible isomerization of
products 3 through reversible electron transfer, the
photolysis of adducts 2 under our reaction conditions,27 the
impact of the -substituents on the lability of the C-I -
bond of adduct 2, and the possible formation of transient
carbocation intermediates by electron transfer from [I·-] to
the iodine atom.27 For instance, the reaction of styrene (1s)
gave only unsaturated carboxylation product 3s with a low
mass balance (Entry 19, Table 1). In this case, the labile
benzylic C-I -bond of adduct 2s could open up alternative
polar or radical reaction paths and lead to alternative
products or polymeric materials.
·-
acetonitrile involve the radical anion [CO2 ] as a key
intermediate, which reacts with substrate 1 through
reversible addition or electron transfer, depending on the
reduction potential and the steric hindrance at the -
position of 1. Conversely, CO2 is not an efficient electron
acceptor in water and the reaction proceeds through
alternative paths, which probably involve H-atoms as
intermediates.
In summary, the present report describes the transition
metal-free and atom-efficient photoiodocarboxylation of
activated olefins 1 with CO2 and LiI in acetonitrile, and
discusses the reaction course and the reactive
intermediates involved.
Experimental Section
General. Lithium iodide (LiI) and potassium iodide (KI)
o
were dried under vacuum for 3 h at 60 C and stored in a
desiccator. Solvents and reagents were degassed by
standard freeze-pump-thaw procedures and stored under
argon atmosphere. Carbon dioxide at 40 oC and 60 bar was
passed through a Drierite® column prior to depressurizing
and dosing into the reaction mixtures. The reactions were
performed with a 45 cm long compact twin-tube low
pressure mercury lamp (254 nm) Phillips TUV PL-L
36W/4P (power 36 W, UVC radiation 12 W). 13C NMR
spectra for reaction products 2 in D2O were recorded with
the minimum acquisition times required for observing the
signals, in order to minimize the decomposition of product
2. The UV spectra of the anhydrous acetonitrile solutions
under argon or CO2 atmosphere, and at different
temperatures, were measured by using a fiber optic probe;
the UV spectra under CO2 pressure were measured on a 25
mL stainless steel view cell with sapphire windows (optical
path 3.53 cm). HRMS measurements were carried out on a
quadrupole-time of flight (Q-TOF) tandem mass
spectrometer by using Electrospray (ESI) as ionization
technique. The accurate mass measurements obtained
Finally, it is worth mentioning that ion pairing could be
a significant factor for the reactions carried out in
acetonitrile. For instance, the interactions of the lithium
cation with the polar and ionic groups of the negatively
charged intermediates could impact the electron capture
by CO2, the conformational equilibrium of the
intermediates [I·-], the carboxylation step and the coupling
of [I·-] with the iodine atom. The change of LiI to KI did not
significantly alter the reaction course (Entries 1 and 21,
Table 1), but this issue deserves a detailed study, which is
beyond the scope of the present work.
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The Journal of Organic Chemistry
W) placed parallel 2 cm above the quartz tube, for 3 h, at
from ESI in positive mode (ESI+) correspond to the
protonated 2-substituted-2-iodocarboxylic acids (2HH+).
room temperature under rotation and magnetic stirring.
The reaction mixture was a dark yellow suspension which
was allowed to decant for 30 min at 0 oC protected from the
light. The supernatant solution was removed and the solid
residue was washed with cold anhydrous acetonitrile (3 x 5
mL), at 0 oC and protected from the light. The final residue
was dried under vacuum to give 57 mg (90 % yield) of crude
lithium 3-iodo-4-methoxy-4-oxobutanoate (2a), with
minor amounts of lithium (E)- and (Z)- 4-methoxy-4-
oxobut—2-enoate [(Z)- and (E)-3a].
1
2
3
4
5
6
7
8
Photoiodocarboxylation reactions and control
experiments. General procedures.
(A) Photolysis of lithium iodide, substrates 1, and CO2
(Table 1). An acetonitrile-d3 solution (3 mL) of lithium
iodide (0.12 mmol) and methyl acrylate (1a) (0.12 mmol)
was placed into a Suprasil® quartz test tube under CO2
atmosphere. The tube was capped with a rubber septum,
connected through a syringe needle to a CO2-reservoir at
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59
60
o
ca. 20 psi, and cooled to 0 C for 20 min under magnetic
(C) Photolysis of iodocarboxylation adducts 2 (Table 2).
A water-d2 solution (2.1 mL) of lithium 4-amino-3-iodo-4-
oxobutanonate (2b) (0.044 mmol) was placed into a
Suprasil® quartz test tube under argon atmosphere. The
tube was capped with a rubber septum, fixed to a rotor
unit, and irradiated at 254 nm with a low-pressure mercury
lamp (36 W) placed parallel 2 cm above the quartz tube,
for 3 h, at room temperature under rotation and magnetic
stirring. A 0.6 mL aliquot of the yellow solution was diluted
with 0.2 mL of a 0.020 M water-d2 solution of tert-butanol
stirring. The quartz tube was firmly capped with a new
rubber septum at 0 oC under CO2 at atmospheric pressure,
and fixed to a rotor unit. The sample was irradiated at 254
nm with a low-pressure mercury lamp (36 W) placed
parallel 2 cm above the quartz tube, for 3 h, at room
temperature under rotation and magnetic stirring. The
reaction mixture was a dark yellow suspension which was
allowed to decant at room temperature protected from the
light. A 0.8 mL aliquot of the supernatant was diluted with
0.1 mL of a 0.049 M acetonitrile-d3 solution of tert-butanol
1
1
as external standard, and the sample was analyzed by H-
as external standard, and the sample was analyzed by H-
NMR. The substrate conversion and the products
distribution were determined from the integrals of their
NMR. The remaining of the reaction mixture was
evaporated to dryness under vacuum. The solid residue
was dissolved in 0.8 mL of a 0.042 M solution of tert-
1
respective H-NMR signals compared to those of the
1
standard. The reaction products were identified as lithium
(Z)- and (E)-4-amino-4-oxobut-2-enoate [(Z)- and (E)-3b],
and acrylamide (1b).
butanol in water-d2, and the sample was analyzed by H-
NMR. The substrate conversion and the reaction products
were quantified from both samples by comparing the
integrals of their respective 1H-NMR signals to those of the
standard. The reaction products were identified as lithium
3-iodo-4-methoxy-4-oxobutanoate (2a), lithium (Z)- y (E)-
4-metoxi-4-oxobut-2-enoato de litio [(Z)- and (E)-3a], and
methyl 2-iodopropanoate (4a).
The reactions in the presence of acetic acid were
performed by irradiating acetonitrile-d3 solutions (2.1 mL)
of 2b (0.044 mmol) and acetic acid (4.4 mmol), following
the same procedure C. A 0.8 mL aliquot of the yellow
solution was diluted with 0.1 mL of a 0.038 M acetonitrile-
d3 solution of tert-butanol as external standard, and
analyzed by 1H NMR.
The results were confirmed with a series of identical
experiments which were analyzed by diluting the reaction
mixtures with 0.4 mL of ultrapure water. A 0.8 mL aliquot
of the homogeneous solution was diluted with 0.1 mL of a
0.049 M acetonitrile-d3 solution of tert-butanol as external
standard. The samples were analyzed by 1H-NMR and the
results compared with those obtained through procedure
A. The samples showed variable amounts of 2-iodosuccinic
anhydride.
(D) Reactions in the presence of acetic acid (Table 3). An
acetonitrile-d3 solution (3 mL) of lithium iodide (0.12
mmol), methyl acrylate (1a) (0.12 mmol), and acetic acid
(12 mmol) was placed into a Suprasil® quartz test tube
under either argon or CO2 atmosphere and irradiated,
following the procedure A. The yellow reaction mixture
was diluted with 0.2 mL of water-d2. A 0.8 mL aliquot of
the homogeneous solution was diluted with 0.1 mL of a
0.035 M acetonitrile-d3 solution of tert-butanol as external
standard, and the sample was analyzed by 1H NMR.
The reactions in water were performed with 0.01 M
solutions of KI and substrate 1 in water-d2, following the
same procedures. An aliquot (0.8 mL) of the homogeneous
reaction mixture was diluted with 0.1 mL of a 0.042 M
water-d2 solution of tert-butanol as external standard, and
analyzed by 1H NMR.
The reactions in aqueous medium were performed by
irradiating a solution of acrylamide (1b) (0.03 mmol),
potassium iodide (0.03 mmol) and acetic acid (0.3 mmol)
in water-d2 (3 mL), either under argon or saturated with
CO2, following the procedure A. A 0.8 mL aliquot of the
homogeneous solution was diluted with 0.1 mL of a 0.020
M water-d2 solution of tert-butanol as external standard,
and the sample was analyzed by 1H NMR.
Lithium 3-iodo-4-methoxy-4-oxobutanoate (2a). A1H
NMR (300 MHz, D2O): δ 2.92 (dd, J = 6.3, 16.6 Hz, 1H), 3.14
(dd, J = 9.8, 16.6 Hz, 1H), 3.79 (s, 3H), 4.70 (dd, J = 6.3, 9.8
Hz, 1H). 13C {1H} NMR (75 MHz, D2O): δ 12.8, 43.1, 52.5,
(B) Preparative procedure. An acetonitrile solution (6
mL) of lithium iodide (0.24 mmol) and methyl acrylate (1a)
(0.24 mmol) was placed into a Suprasil® quartz test tube
under CO2 atmosphere. The tube was capped with a rubber
septum, connected through a syringe needle to a CO2-
reservoir at ca. 20 psi, and cooled to 0 oC for 20 min under
magnetic stirring. The quartz tube was firmly capped with
o
a new rubber septum at 0 C under CO2 at atmospheric
pressure, and fixed to a rotor unit. The sample was
irradiated at 254 nm with a low-pressure mercury lamp (36
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The Journal of Organic Chemistry
Page 12 of 16
173.8, 177.2. HRMS (ESI-Q-TOF) m/z: [MH+H]+ Calcd for
C5H8IO4 258.9467; Found 258.9462.
δ 18.5, 28.5, 47.7, 177.0, 180.5. HRMS (ESI-Q-TOF) m/z:
[MH+H]+ Calcd for C5H9INO3 257.9627; Found 257.9622.
1
2
3
4
5
6
7
8
1
Lithium 4-amino-3-iodo-4-oxobutanoate (2b). H NMR
(500 MHz, D2O): δ. 2.87 (dd, J = 7.4, 16.3 Hz, 1H), 3.08 (dd,
Lithium 3-iodo-4-methoxy-2-methyl-4-oxobutanoate (2l,
mixture of two diastereoisomers). 1H NMR (500 MHz, D2O):
δ 1.15 (d, J = 6.9 Hz, 3H), 1.33 (d, J = 7.2 Hz, 3H), 2.81 (dq, J
= 10.7, 7.3 Hz, 1H), 3.75 (s, 3H), 3.79 (s, 3H), 4.48 (d, J = 10.8
Hz, 1H). 13C {1H} NMR (125 MHz, D2O) (major
diastereoisomer): δ 17.9, 25.2, 47.8, 53.3, 174.9, 180.8. HRMS
(ESI-Q-TOF) m/z: [MH+H]+ Calcd for C6H10IO4 272.9624;
Found 272.9618.
13
J = 8.3, 16.3 Hz, 1H), 4.71 (t, J = 7.9 Hz, 1H). C {1H} NMR
(125 MHz, D2O): δ 17.1, 44.1, 176.8, 178.0. HRMS (ESI-Q-
TOF) m/z: [MH+H]+ Calcd for C4H7INO3 243.9471; Found
243.9465.
Lithium 4-(dimethylamino)-3-iodo-4-oxobutanoate (2c).
1H NMR (300 MHz, D2O): δ 2.85-3.15 (m, 2H), 2.93 (s, 3H),
3.11 (s, 3H), 4.97 (dd, J = 6.5, 8.8 Hz, 1H). 13C {1H} NMR (75
MHz, D2O): δ 14.6, 36.4, 37.8, 44.4, 173.2, 178.4. HRMS (ESI-
Q-TOF) m/z: [MH+H]+ Calcd for C6H11INO3 271.9784;
Found 271.9778.
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57
58
59
60
Lithium 3-iodo-2,4-dimethoxy-4-oxobutanoate (2m,
mixture of two diastereisomers). 1H NMR (500 MHz, D2O):
δ 3.40 (s, 3H), 3.44 (s, 3H), 3.78 (s, 3H), 3.79 (s, 3H), 3.98 (d,
J = 8.3 Hz, 1H), 4.63 (d, J = 8.3 Hz, 1H), 4.93 (d, J = 5.7 Hz,
1H). 13C {1H} NMR (125 MHz, D2O): δ 18.5, 53.68, 53.73, 57.6,
57.7, 58.3, 84.9, 85.0, 172.1, 175.3, 175.4. HRMS (ESI-Q-TOF)
m/z: [MH+H]+ Calcd for C6H10IO5 288.9573, found
288.9573.
1
Lithium 3-cyano-3-iodopropanoate (2d). H NMR (300
MHz, D2O): δ 3.06 (qd, J = 7.2, 16.3 Hz, 2H), 4.70 (t, J = 7.2
13
Hz, 1H). C {1H} NMR (75 MHz, D2O): δ -10.3, 44.6, 121.1,
176.2. HRMS (ESI-Q-TOF) m/z: [MH+H]+ Calcd for
C4H5INO2 225.9365; Found 225.9360.
Lithium (E)-4-methoxy-4-oxobut-2-enoate ((E)-3a) [2756-
87-8].59 1H NMR (300 MHz, D2O): δ 3.81 (s, 3H), 6.51 (d, 1H,
3J = 16 Hz), 6.92 (d, 1H, 3J = 16.0 Hz). 13C {1H} NMR (75 MHz,
D2O): δ 52.9, 128.2, 141.3, 169.3, 173.4.
Lithium 4-(allyloxy)-3-iodo-4-oxobutanoate (2e). 1H NMR
(500 MHz, D2O): δ 2.92 (dd, J = 16.6, 6.2 Hz, 1H), 3.14 (dd, J
= 16.6, 10.0 Hz, 1H), 4.66-4.75 (m, 3H), 5.33 (d, J = 10.5 Hz,
1H), 5.45 (d, J = 17.3 Hz, 1H), 6.00 (ddd, J = 16.2, 10.7, 5.4 Hz,
Lithium (Z)-4-methoxy-4-oxobut-2-enoate ((Z)-3a) [3052-
50-4].59 1H NMR (300 MHz, D2O): δ 3.74 (s, 3H), 5.87 (d, 1H,
3J = 12.1 Hz), 6.64 (d, 1H, 3J = 12.1 Hz). 13C {1H} NMR (75 MHz,
D2O): δ 52.5, 118.8, 141.6, 168.4, 175.2.
Methyl 2-iodopropanoate (4a) [56905-18-1].60 1H RMN
(300 MHz, CD3CN): δ 1.89 (d, 3H, 3J = 7.0 Hz), 3.68 (s, 3H),
4.57 (q, 1H, 3J = 7.0 Hz). 13C {1H} NMR (75 MHz, CD3CN): δ
14.0, 23.8, 53.4, 173.4.
13
1H). C {1H} NMR (125 MHz, D2O): δ 14.1, 44.1, 66.8, 118.7,
131.2, 173.4, 178.2. HRMS (ESI+): HRMS (ESI-Q-TOF) m/z:
[MH+H]+ Calcd for C7H10IO4 284.9624; Found 284.9618.
Lithium 3-iodo-4-oxopentanoate (2f). 1H NMR (400 MHz,
D2O): δ 2.47 (s, 3H), 2.84 (dd, J = 16.5, 6.5 Hz, 1H), 3.09 (dd,
13
J = 16.5, 9.2 Hz, 1H), 4.93 (dd, J = 9.1, 6.6 Hz, 1H). C {1H}
NMR (100.5 MHz, D2O): δ 25.5, 25.8, 43.3, 178.5, 208.8.
HRMS (ESI-Q-TOF) m/z: [MH+H]+ Calcd for C5H8IO3
242.9518; Found 242.9513.
Lithium 3-iodo-3-(phenylsulfonyl)propanoate (2g). 1H
NMR (400 MHz, D2O): δ 2.80 (dd, J = 15.6, 10.9 Hz, 1H), 3.18
(dd, J = 15.6, 3.9 Hz, 1H), 5.55 (dd, J = 10.9, 3.9 Hz, 1H), 7.73
– 7.70 (m, 2H), 7.84 (t, J = 7.5 Hz, 1H), 7.99 (d, J = 7.6 Hz,
2H). 13C {1H} NMR (100 MHz, D2O): δ 37.7, 42.7, 129.4, 129.7,
134.8, 139.3, 175.2. HRMS (ESI-Q-TOF) m/z: [MH+H]+ Calcd
for C9H10IO4S 340.9344; Found 340.9339.
ASSOCIATED CONTENT
Supporting Information. Details on the experimental setup,
UV spectra, 1H and 13C NMR spectra of the reaction products
1
2, and H NMR spectra of the reaction mixtures for the
experiments reported in Tables 1-3. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
1
Lithium 3-carboxy-3-iodopropanoate (2h). H NMR (500
Corresponding Author
MHz, D2O): δ 2.81 (dd, J = 16.1, 9.1 Hz, 1H), 3.04 (dd, J = 16.1,
6.5 Hz, 1H), 4.51 (dd, J = 9.1, 6.5 Hz, 1H). 13C {1H} NMR (125
MHz, D2O): δ 23.2, 45.2, 178.7, 178.7. HRMS (ESI-Q-TOF)
m/z: [M-] Calcd for C4H4IO4 242.9154; Found 242.9160.
* elena.gonzalez@uv.es
Notes
The authors declare no competing financial interests.
Lithium 3-cyano-3-iodo-2-methylpropanoate (2j, mixture
1
of two diastereoisomers). H NMR (300 MHz, D2O): δ 1.30
ACKNOWLEDGMENT
(d, J = 7.0 Hz, 3H), 1.35 (d, J = 7.1 Hz, 3H), 2.76 – 2.64 (m,
1H), 2.92 – 2.77 (m, 1H), 4.66 (d, J = 7.8 Hz, 1H), 4.75 (d, J =
Financial support from the Spanish Ministerio de Economía,
Industria y Competitividad (CTQ2016-80503-P), and Fondos
FEDER is gratefully acknowledged. We thank the SCSIE
(Universidad de Valencia) for access to its instrumental
facilities.
13
6.1 Hz, 1H). C {1H} NMR (75 MHz, D2O): δ 0.0, 1.6, 17.7,
18.1, 48.5, 49.8, 121.5, 121.6, 180.0, 180.3. HRMS (ESI-Q-TOF)
m/z: [M-] Calcd for C5H5NO2I 237.9365; Found 237.9371.
Lithium 4-amino-3-iodo-2-methyl-4-oxobutanoate (2k,
mixture of two diastereoisomers). 1H NMR (400 MHz, D2O):
δ 1.11 (d, J = 7.1 Hz, 3H), 1.30 (d, J = 7.2 Hz, 3H), 2.76 (dq, J =
10.5, 7.2 Hz, 1H), 4.35 (d, J = 11.6 Hz, 1H), 4.47 (d, J = 10.6 Hz,
1H). 13C {1H} NMR (100 MHz, D2O) (major diastereoisomer):
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