Silyl cation mediated conversion of CO2 into benzoic acid, formic acid, and methanol

Article abstract of DOI:10.1002/anie.201107958

As you like it: The choice of solvents and substituents at the silicon atom determine what product is formed from carbon dioxide after electrophilic activation by silyl cations (see scheme). Benzoic acid as well as the C-1 building blocks formic acid and methanol are on the product tableau. Copyright

Full text of DOI:10.1002/anie.201107958

Angewandte
Chemie
DOI: 10.1002/anie.201107958
CO2 Reduction
Silyl Cation Mediated Conversion of CO2 into Benzoic Acid, Formic
Acid, and Methanol**
Andrꢀ Schꢁfer, Wolfgang Saak, Detlev Haase, and Thomas Mꢂller*
The use of CO2 as a renewable and environmentally friendly
C1 source for the synthesis of carboxylic acids and fuels such
as methanol and methane is a topic of current interest.[1–4] The
high thermodynamic stability of CO2 clearly calls for highly
efficient activation which must be combined with a strong
thermodynamic driving force to ensure irreversible fixation.
While in the past transition-metal chemistry played a domi-
nant role in CO2 conversion chemistry,[5] recent years have
seen the emergence of organocatalytic methods for CO2
reduction.[6–14] For example N-heterocyclic carbenes have
been applied for the nucleophilic activation of CO2, and
subsequent reduction of the resulting imidazolium carbox-
Scheme 1. Reduction of CO2 by [Ph3C][B(C6F5)4]/R3SiH in different
ylates by silanes yielded methanol.[9] In addition stoichiomet-
À
solvents (the B(C6F5)4 anion is omitted, 1a: R=Et, 1b: R=iPr).
Conditions: a) 0.1013 MPa CO2, 1 equiv Ph3C+, 2 equiv R3SiH, PhCl,
room temperature; b) 2 equiv Et3SiH, RT; c) H2O; d) 0.1013 MPa
CO2,1 equiv Ph3C+, 1 equiv Et3SiH, PhH, room temperature; e) sym-
collidine, PhH, temperature.
ric and catalytic reductions of CO2 have been reported which
utilize frustrated Lewis pairs (FLPs)[10] for the activation, and
dihydrogen, silanes, or ammonia borane as the hydrogen
source.[11–14] In view of the high electrophilic activity of silyl
cations[15] and their complexes with solvents and counter-
anions, we were intrigued by the possibility of exploiting this
extreme reactivity in CO2 activation. Silanes then would be
the logical hydrogen source for the reduction and would
provide the desired thermodynamic driving force through the
formation of siloxanes.
Here we report on a metal-free reduction of CO2 by
trialkylsilanes, R3SiH (R = Et, iPr), using stoichiometric
amounts of trityl borate [Ph3C][B(C6F5)4]. The reduction is
fast under ambient conditions, but different products—
depending on the applied solvent—are formed. In chloro-
benzene (PhCl) either disilylated formic acid 1 or the
disilylmethyl oxonium ion 2 is formed, depending on the
substituent R at the silane. Simple hydrolysis of these
compounds yields formic acid and methanol (Scheme 1). In
benzene (PhH), the reaction of CO2 with the preformed
silylbenzenium salt [Et3Si(C6H6)][B(C6F5)4] (3[B(C6F5)4])
leads to further functionalization of CO2.[8b] In this case the
benzylic cation 4 is formed, which can be easily transformed
either by hydrolysis into benzoic acid (PhCO2H, 6), or, by
careful deprotonation, into the silylester 5 (Scheme 1).
Reaction of triethylsilane with one equivalent of the trityl
borate [Ph3C][B(C6F5)4] in benzene yields benzenium ion 3,
identified by its characteristic 29Si NMR resonance at d(29Si) =
97.4 ppm[16] (Table 1). Also when PhCl is used as the solvent,
cationic complexes [R3Si(PhCl)]+ (7) are formed, as indicated
by the strongly deshielded 29Si NMR resonances at d(29Si) =
99.9 (R = Et, 7a) and 103.3 ppm (R = iPr, 7b; Table 1). Based
on the results of our quantum mechanical calculations,
however, we assign to these complexes the chloronium ion
structure 7 rather than the chloroarenium ion structure 8.
Table 1: NMR parameters of cations 1–4 and 7 (at 305 K).[a]
[*] Dipl.-Chem. A. Schꢀfer, Dipl.-Chem. W. Saak, D. Haase,
Prof. Dr. T. Mꢁller
Compound
d(29Si)
d(13C)[b]
d(1H)[c]
1J(CH)
7a (R=Et)[d]
7b (R=iPr)[d]
3[e]
99.9
103.3
97.4
55.7
50.2
64.4
49.4
--
--
--
–
–
–
7.03
7.43
3.09
–
–
–
–
234
231
154
–
Institut fꢁr Reine und Angewandte Chemie
Carl von Ossietzky Universitꢀt Oldenburg
Carl von Ossietzky-Strasse 9–11, 26111 Oldenburg (Germany)
E-mail: thomas.mueller@uni-oldenburg.de
1a (R=Et)[d,f]
1b (R=iPr)[d]
2[d,f]
172.8
172.5
60.9
178.0
[**] This work was supported by the DFG (Mu-1440/7-1). The Center for
Scientific Computing at the Carl von Ossietzky Universitꢀt Old-
enburg is acknowledged for computer time and Prof. H. Ottosson,
Uppsala University, is thanked for stimulating discussions.
4[e]
[a] d in ppm, 1J in Hz. [b] Signal of the original CO2 C atom. [c] Signals for
the H atoms attached to the original CO2 C atom. [d] In [D5]chloro-
benzene. [e] In [D6]benzene. [f] 1a and 2a detected in the same sample.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107958.
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When chloronium ion 7b and a second equivalent of
triisopropylsilane in PhCl were treated with CO2 under
ambient conditions, disilylated formic acid 1b was detected
by NMR spectroscopy as the only product. Addition of excess
triisopropylsilane did not lead to further reduction of the
cation 1b. Simple hydrolysis of the reaction mixture with D2O
yielded deuterated formic acid, HCOOD, in 89–95% yield[17]
[d(1H) = 8.19 ppm, d(13C) = 167.4 ppm; 1J(CH) = 220 Hz] and
hexaisopropyldisiloxane (major product) and triisopropylsi-
lanol (minor product). When the less bulky triethylsilane was
applied in the reaction with CO2, a mixture of cations 1a and 2
was obtained. Hydrolysis of this mixture yields formic acid,
methanol [d(1H) = 3.11 ppm, d(13C) = 49.0 ppm; 1J(CH) =
143 Hz], and hexaethyldisiloxane. When a large excess
(more than 4 equiv) of triethylsilane was applied at room
temperature, methanol (87–98% yield)[17] and hexaethyldisi-
loxane were the sole products after hydrolysis.
Figure 1. a) 29Si NMR spectra of 3[B(C6F5)4] in C6D6 at 305 K under
argon atmosphere (top) and of a mixture of 3[B(C6F5)4] and 4[B(C6F5)4]
resulting from the reaction with CO2 after 4, 24, 36 h (lower spectra).
b) 13C NMR spectrum of [13C1]-4 obtained by reaction of 3 with 13CO2 in
13
À
*
*
CO2, [B(C6F5)4] anion).
C6D6 (
benzylic position as indicated by the strong increase of the
intensity of the 13C NMR signal at d(13C) = 178.0 ppm (Fig-
ure 1b). Further evidence for the formation of cation 4 in the
reaction of CO2 with silylarenium ion 3 came from its
independent synthesis by protonation of the silylester 5 and
by silylation of benzoic acid 6.[22] Finally, deprotonation of 4
with collidine allows the isolation of silylester 5, although only
in poor yield (11%). Alternatively, hydrolysis of 4 afforded
benzoic acid 6 in 51% yield.
Carbocation 4 is only marginally stable at room temper-
ature and decomposes slowly also at temperatures around
58C. This instability is one reason for the low yields of isolated
benzoic acid 6 and silylester 5 and it severely hampers all
crystallization attempts. For example, from a C6F6 solution of
4[B(C6F5)4] heterocycle 10 was isolated in low yield (17%).
Compound 10 was formed by degradation of the anion. In its
molecular structure the original CO2 unit is preserved and this
provides pictorial evidence for the CO2 fixation (Figure 2).[22]
A mechanistic proposal for the CO2 fixation by silyl-
arenium ions, outlined in Scheme 2 for 3, starts with a pre-
equilibrium between benzenium ion 3 and the silyl–CO2
complex 11a. As a result of the activation by the Lewis acid
Et3Si+, the electrophilicity of the central carbon atom in
complex 11a is enhanced and in a subsequent step it
1
Cations 1 and 2 were identified by H, 13C, 29Si, and two-
1
1
dimensional H,29Si and H,13C NMR spectroscopy. Charac-
teristic for disiloxycarbenium ions 1 are the doublets in the
proton-coupled 13C NMR spectra at relatively low field
[d(13C) = 172.8, 172.5 ppm; 1J(CH) = 234, 231 Hz] and the
correlated signals for the methine protons at d(1H) = 7.03 and
7.43 ppm. These values can be favorably compared with those
reported for dimethoxycarbenium tetrafluoroborate,
[(MeO)2CH][BF4] [d(13C) = 181.4 ppm, d(1H) = 8.9 ppm].[18]
The 29Si resonances of cations 1 and 2 [d(29Si) = 50.2–
64.4 ppm] are found in the typical region for silylated
oxonium ions [i.e. d(29Si) = 46.7 (tBu3SiOH2 ), 59.0
+
((Me3Si)2OEt+), 66.9 ppm (Me3SiOEt2 )].[19,20]
+
The reaction of triethylsilyl benzenium borate 3[B(C6F5)4]
with CO2 in benzene at room temperature yielded the
perfluorinated tetraphenyl borate of protonated benzoic
ester 4 in almost quantitative yield, as judged from NMR
spectroscopy (Scheme 1). The reaction was complete in
minutes even at room temperature under ambient CO2
pressure. In contrast, when the reaction was conducted in
a Young NMR tube without stirring, the reaction was slow
and could be monitored conveniently by 29Si NMR spectros-
copy over hours (see Figure 1a). The protonated silylester 4
was identified by its characteristic NMR chemical shifts (see
Table 1). In particular, the 29Si NMR resonance at d(29Si) =
49.4 ppm is of high diagnostic value as it is shifted to lower
field by Dd29Si = 23.0 ppm compared to the neutral silylester
5. The benzylic carbon atom gives rise to a signal in the
13C NMR spectrum at d(13C) = 178.0 ppm, which is close to
the NMR chemical shift value reported for the protonated
benzoic methyl ester 9 in SO2ClF [d(13C) = 181.9 ppm for C+
in 9].[20] When 13C-labeled CO2 was applied in the reaction
with 3[B(C6F5)4], the 13C atom was found exclusively in the
Figure 2. Ellipsoid presentation of the molecular structure of the
heterocyclic compound 10 in the crystal (50% probability, only the H
2
+
ipso
À
atom attached to O is shown). Pertinent bond lengths [pm]: C
C
+
À
À
À
146.12(25), C O1 127.63(12), B O1 150.42(17), B O2 152.13(14).
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Scheme 2. Proposed reaction course for the formation of cation 4
from CO2.
undergoes an electrophilic addition to the benzene forming
the Wheland intermediate 12. The terminating step consists of
a deprotonation–protonation sequence to give the protonated
silylester 4 (Scheme 2). Silyl–CO2 complexes 11 are also
central for the CO2 reduction in chlorobenzene (Scheme 3).
Figure 3. Calculated reaction coordinates for the reaction of CO2 with
silylarenium ion 3 (a) and chloronium ion 7a (b). The labeling of the
reaction coordinate for (b) is incomplete for a clearer presentation.
The reaction coordinate is computed for [C19H36ClO2Si2]+ starting from
7a+CO2 +Et3SiH and ending at 1a+PhCl. (at B3LYP/6-311+G(d,p)).
Free enthalpies at 298 K (G298) are given relative to the starting
materials.
Scheme 3. Proposed reaction course for the formation of cations 1 and
2 from CO2 (a: R=Et, b: iPr).
lying hydrogen-bridged intermediate 14 for this step was
located. It is, however, separated from the product 1a only by
a small activation barrier of 6.3 kJmolÀ1 which suggests that
this intermediate cannot be detected under the applied
experimental conditions. The calculation further shows that
the following step (Scheme 3), the formation of silylated
formaldehyde 13, is slightly endergonic but this is overruled
by the thermodynamically extremely favored formation of
oxonium ion 2 ((DG298(13) =+ 8.9 kJmolÀ1 and DG298 (2) =
À131.5 kJmolÀ1 for the formation of compounds 13 and 2
from cation 1a according to Scheme 3).
In this case the electrophilic activation of CO2 is not sufficient
for its addition to the relatively electron-poor chlorobenzene.
However, the reduction of CO2 is facilitated by additional
silane to give the bissilylated formic acids 1. Clearly, this step
shows parallels to the known hydrosilylation of carbonyl
compounds.[23–25] When iPr3SiH is applied in this reaction, no
further conversion of cation 1 is observed. In contrast, with an
excess of the less bulky Et3SiH further hydrosilylation of the
partial CO double bond in cation 1 occurs to give the silylated
formaldehyde 13. It seems reasonable to assume that the
highly activated cation 13 adds another equivalent of
triethylsilane to yield the ultimately observed oxonium ion 2.
The results of computations at the hybrid DFT B3LYP/6-
311 + G(d,p) level of theory support these mechanistic
proposals.[22,26] In detail, they show that the reaction of
silylarenium ion 3 with CO2 to give the benzylic cation 4 is
exergonic by 52.5 kJmolÀ1 (see Figure 3a). Although the
formation of silylated CO2, 11, from silylarenium ion 3 is
thermodynamically disfavored by 16.5 kJmolÀ1, the barrier
for the nucleophilic attack to give arenium ion 12 is small
enough for the reaction to proceed at room temperature at
reasonable rates. Finally, it is the high exothermicity of the
terminating formal 1,3-H shift to give the benzylic cation 4
which guarantees the quasi irreversibility of the complete
reaction sequence (Figure 3a).
For the reaction in chlorobenzene the calculations predict
a similar reaction profile. While the computational results
indicate that the chloronium ion 7a is more stable than the
isomeric arenium ion 8a by 24.1 kJmolÀ1, the formation of the
silylated CO2 11a is even less favored than in the case of the
reaction of arenium ion 3 with CO2 (Figure 3b). The barrier
for the nucleophilic attack by the second Et3SiH molecule is,
however, significantly smaller and on the G298 surface the
disilylated formic acid 1a is produced in practically one
extremely exergonic step (DG298 = À179.8 kJmolÀ1). A high-
Received: November 11, 2011
Revised: December 22, 2011
Published online: February 6, 2012
Keywords: CO2 reduction · density functional calculations ·
.
Lewis acids · NMR spectroscopy · silyl cations
[1] E. A. Quadrelli, G. Centi, ChemSusChem 2011, 4, 1179 – 1181.
[2] M. Aresta, A. Dibenedetto, Dalton Trans. 2007, 2975 – 2992.
[3] T. Sakakura, J.-C. Choi, H. Yasuda, Chem. Rev. 2007, 107, 2365 –
2387.
[4] T. J. Marks, et al., Chem. Rev. 2001, 101, 953 – 996.
[5] M. Cokoja, C. Bruckmeier, B. Rieger, W. A. Herrmann, F. E.
Kꢀhn, Angew. Chem. 2011, 123, 8662 – 8690; Angew. Chem. Int.
Ed. 2011, 50, 8510 – 8537.
[6] L. Gu, Y. Zhang, J. Am. Chem. Soc. 2010, 132, 914 – 915.
[7] V. Nair, V. Varghese, R. R. Paul, A. Jose, C. R. Sinu, R. S.
Menon, Org. Lett. 2010, 12, 2653 – 2655.
[8] a) Y. Kayaki, M. Yamamoto, T. Ikariya, Angew. Chem. 2009, 121,
4258 – 4261; Angew. Chem. Int. Ed. 2009, 48, 4194 – 4197;
b) C. D. N. Gomes, O. Jacquet, C. Villiers, P. Thuꢁry, M.
Ephritikine, T. Cantat, Angew. Chem. 2012, 124, 191 – 194;
Angew. Chem. Int. Ed. 2012, 51, 187 – 190.
[9] S. N. Riduan, Y. Zhang, J. Y. Ying, Angew. Chem. 2009, 121,
3372 – 3375; Angew. Chem. Int. Ed. 2009, 48, 3322 – 3325.
[10] D. W. Stephan, G. Erker, Angew. Chem. 2010, 122, 50 – 81;
Angew. Chem. Int. Ed. 2010, 49, 46 – 76.
Angew. Chem. Int. Ed. 2012, 51, 2981 –2984
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2983

.
Angewandte
Communications
[11] C. M. Mçmming, E. Otten, G. Kehr, R. Frçhlich, S. Grimme,
D. W. Stephan, G. Erker, Angew. Chem. 2009, 121, 6770 – 6773;
Angew. Chem. Int. Ed. 2009, 48, 6643 – 6646.
[18] U. Pindur, C. Flo, Synth. Commun. 1989, 19, 2307.
[19] Z. Xie, R. Bau, C. A. Reed, J. Chem. Soc. Chem. Commun. 1994,
2519.
[12] A. E. Ashley, A. L. Thompson, D. OꢂHare, Angew. Chem. 2009,
121, 10023 – 10027; Angew. Chem. Int. Ed. 2009, 48, 9839 – 9843.
[13] G. Mꢁnard, D. W. Stephan, J. Am. Chem. Soc. 2010, 132, 1796 –
1797.
[14] A. Berkefeld, W. E. Piers, M. Parvez, J. Am. Chem. Soc. 2010,
132, 10660 – 10661.
[15] a) H. F. T. Klare, M. Oestreich, Dalton Trans. 2010, 39, 9176 –
9184; b) T. Mꢀller, Adv. Organomet. Chem. 2005, 53, 155 – 215.
[16] J. B. Lambert, S. Zhang, S. M. Ciro, Organometallics 1994, 13,
2430 – 2443.
[17] The yield was determined relative to employed [Ph3C][B(C6F5)4]
using 1H NMR spectroscopy and dioxane as the internal
standard; see the Supporting Information for further details.
[20] M. Kira, T. Hino, H. Sakurai, J. Am. Chem. Soc. 1992, 114, 6697.
[21] G. A. Olah, P. W. Westerman, D. A. Forsyth, J. Am. Chem. Soc.
1975, 97, 3419 – 3427.
[22] See the Supporting Information for details.
[23] M. Kira, T. Hino, H. Sakurai, Chem. Lett. 1992, 555.
[24] D. J. Parks, J. M. Blackwell, W. E. Piers, J. Org. Chem. 2000, 65,
3090 – 3098.
[25] K. Mꢀther, M. Oestreich, Chem. Commun. 2011, 47, 334 – 336.
[26] All calculations were performed using the Gaussian03
(Rev.C.02) and the Gaussian09 (Rev.B.01) package. Calcula-
tions using the MPW1K and M05-2X functionals gave very
similar results.
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