Activation of CO2 and SO2 by boryl(phosphino)carbenes

Article abstract of DOI:10.1002/anie.201108452

Dismembering molecules: The stable boryl(phosphino)carbene 1 can cleave small organic dioxide molecules. With CO₂ and SO₂, 1 gives, respectively, the phosphacumulene ylide [Mes(iPr₂N)B-O-P(CCO) (NiPr₂)Mes] (see

Full text of DOI:10.1002/anie.201108452

Angewandte
Chemie
DOI: 10.1002/anie.201108452
Metal-Free Activation
Activation of CO2 and SO2 by Boryl(phosphino)carbenes**
Florie Lavigne, Eddy Maerten,* Gilles Alcaraz, VicenÅ Branchadell, Nathalie Saffon-Merceron,
and Antoine Baceiredo*
Since the isolation of the first stable (phosphino)-
(silyl)carbene (PSC) in 1988,[1] followed three years later by
the publication of a structurally characterized N-heterocyclic
carbene (NHC),[2] important findings have regularly been
reported in the field of stable carbene chemistry. To better
understand the nature of these species, efforts have been
devoted to the design of new stabilization modes mainly by
modifying the substitution pattern at the carbene center and
thus delineating various subclasses of carbene reagents (PSCs,
NHCs, PHCs, CAACs, …) with a strong impact on their
intrinsic reactivity.[3] Recently, great progress has been
reported in using stable singlet (alkyl)(amino)carbenes for
stripping of both dioxides into the corresponding phosphoryl
ketenylidene and phosphoryl sulfine derivatives, respectively.
Reaction of boryl(phosphino)carbene 1, in pentane, with
a saturated CO2 atmosphere is carried out at room temper-
ature for five minutes. After evaporation of the solvent under
vacuum and workup, phosphacumulene ylide 2 (Scheme 1)
À
the activation of enthalpically strong E H (E = B, Si, P)
bonds and small molecules, such as CO, H2, NH3, or P4.[4] In
contrast, the area of the activation of carbon dioxide by
carbenes is mostly limited to NHCs and dominated by the
thermally reversible formation of the corresponding NHC–
CO2 adducts in which the structural integrity of both reagents
is preserved.[5] In the absence of any additional reagent,[6] the
imidazolium carboxylate is stable and does not evolve. The
only reported example of cleaving carbon dioxide with
a carbene results from the gas-phase splitting reaction of
CO2 with transient methylene.[7]
Scheme 1. Synthesis of 2.
was isolated as white crystals (57% yield) from a cold
(À308C) saturated pentane solution and fully characterized
by NMR and IR spectroscopy and by single-crystal X-ray
diffraction.[9]
The 31P{1H} NMR spectrum of 2 shows a singlet at d =
17.1 ppm. In the 13C{1H} NMR spectrum the carbon atoms of
the ketenyl moiety give rise to two characteristic doublets,
downfield (d = 141.6, 2JCP = 61.7 Hz) and upfield (d = 7.1,
1JCP = 259.3 Hz) with a large coupling constant indicative of
As a part of our program on the chemistry of Group-13-
substituted deficient species, we have recently disclosed the
isolation of the stable boryl(phosphino)carbene 1.[8] Herein
we report its reactivity towards CO2 and SO2 leading to the
À
direct P C bonding. In the IR spectrum of a THF solution of
2, a strong absorption at 2118 cmÀ1 confirms the presence of
the ketene fragment.[10]
The X-ray structure of 2 was determined at À808C
(Figure 1). The phosphorus atom is in a tetrahedral environ-
[*] Dr. F. Lavigne, Dr. E. Maerten, Dr. A. Baceiredo
Universitꢀ de Toulouse, UPS, and CNRS
LHFA UMR 5069, 31062 Toulouse (France)
E-mail: maerten@chimie.ups-tlse.fr
baceired@chimie.ups-tlse.fr
À
ment. The P1 C1 bond length is distinctly shorter
[11]
À
(1.644(3) ꢀ) than an average single P C bond (1.85 ꢀ),
and in the range expected for a phosphorus ylide bond.[12]
Homepage: http://hfa.ups-tlse.fr
Dr. N. Saffon-Merceron
Universitꢀ de Toulouse, UPS, and CNRS
ICT FR2599, 118 route de Narbonne, 31062 Toulouse (France)
Dr. G. Alcaraz
LCC, UPR CNRS 8241
205 route de Narbonne, 31077 Toulouse (France)
Prof. V. Branchadell
Departament de Quꢁmica, Universitat Autꢂnoma de Barcelona
08193 Bellaterra, Barcelona (Spain)
[**] This work was supported by the CNRS (LEA 368), Spanish
Ministerio de Ciencia e Innovaciꢃn (CTQ2010-15408), and Gen-
eralitat de Catalunya (2009SGR-733). Time allocated in the Centre
de Supercomputaciꢃ de Catalunya (CESCA) is gratefully acknowl-
edged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108452.
Figure 1. Molecular structure of 2. Thermal ellipsoids set at 30%
probability. H atoms are omitted for clarity.
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=
Compared to the dimesitylketene (Mes2CCO; C C
[13]
=
À
1.29(1) and C O 1.18(1) ꢀ), the C1 C2 distance
(1.234(4) ꢀ) is shorter, exhibiting a partial triple
À
bond character, whereas the C2 O2 bond
(1.204(4) ꢀ) is slightly elongated. The large value
of the P-C1-C2 angle (144.1(3)8) is indicative of
a markedly ylidic character and the geometrical
parameters on whole are reminiscent of ketenylide-
netriphenylphosphorane Ph3PCCO (Table 1).[10]
To shed more light on this special feature, DFT
calculations at the M05-2X/6-311 + G(d,p)//M05-2X/
6-31G(d) level have been performed on the reaction
pathway in pentane solution (see Figure 2). CO2
binding to carbene 1 to give phosphonium carbox-
ylate 3 is slightly endergonic (11.4 kcalmolÀ1). This
step is similar to the reactivity of NHCs with CO2
generating an imidazolium carboxylate as an end-
point. In our case, however, cyclization of 3 into
oxaphosphetane 4 can take place. This step is
thermodynamically favored (À26.6 kcalmolÀ1) and
Figure 2. Gibbs energy diagram (kcalmolÀ1) resulting in the formation of 2 (in
parentheses values relative to 1 +CO2).
Table 1: Selected geometrical parameters for 2, Ph3PCCO, and
Mes2CCO.[a]
Parameter
2
Ph3PCCO[10]
Mes2CCO[13]
P–C
C–C
C–O
P-C-C
C-C-O
1.644(3)
1.234(4)
1.204(4)
144.1(3)
177.2(4)
1.648(7)
1.210(10)
1.185(9)
145.5(7)
175.6(8)
1.29(1)
1.18(1)
Scheme 2. Synthesis of 6.
176(1)
[a] Bond lengths [ꢄ] and bond angles [8].
plays a major role in the overall transformation, illustrating
the crucial role of the strongly electrophilic phosphonium
moiety in the activation process of CO2. No transition state
could be found for the straightforward formation of 4 by
[2+2] cycloaddition to CO2. Subsequent rearrangement of 4
into boryl(phosphonio)ketene 5 in a Wittig-like fashion is also
thermodynamically favored (À16.5 kcalmolÀ1), involving
only a very low activation Gibbs energy (2.3 kcalmolÀ1).[14]
Finally, phosphacumulene ylide 2, which is computed to be
slightly more stable than 5 (À3.6 kcalmolÀ1), results from
a
1,3-boratropy with a
19.9 kcalmolÀ1 barrier process.
Unfortunately, intermediate 5 could not be detected by
NMR spectroscopy under the reaction conditions used.
Under similar conditions to the reaction with CO2,
1 reacts with SO2 leading selectively to sulfine 6 which is
only stable below À258C (Scheme 2).[15] The product was fully
characterized by NMR spectroscopy at low temperature.[16]
The structure of 6 was unambiguously determined by an
X-ray diffraction analysis at À508C, and agrees with the
spectroscopic data obtained (Figure 3).
In the case of SO2, theoretical calculations reveal that the
reaction follows a pathway comparable to that of CO2
(Supporting Information, Scheme S1). Sulfine 6 is thermody-
namically favored with respect to 1 + SO2 (À57.6 kcalmolÀ1).
In this case, the 1,3-boratropy is endergonic (9.1 kcalmolÀ1),
Figure 3. Molecular structure of 6. Thermal ellipsoids set at 30%
probability. H atoms are omitted for clarity.
explaining the absence of evolution of 6 into a phosphacumu-
lene ylide.
In conclusion, the original push–pull boryl(phosphino)-
carbene 1 exhibits unprecedented reactivity towards thermo-
dynamically stable small organic dioxides. Its unique elec-
tronic properties enable the challenging activation of carbon
dioxide and sulfur dioxide, transformations that are costly in
relation to the thermodynamics. This is one of the rare
examples of such a transformation with organic systems.[17]
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[9] CCDC 855932 (2) and 855933 (6) contain the supplementary
Development of new push–pull models is underway and will
allow better understanding of this type of reactivity.
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Received: November 30, 2011
Published online: January 27, 2012
[10] This absorption is very similar to the one seen for Ph3PCCO
(2080 cmÀ1): H. J. Bestmann, D. Sandmeier, Chem. Ber. 1980,
113, 274.
[11] Introduction to Phosphorus Chemistry (Ed.: H. Goldwhite),
Cambridge University Press, Oxford, 1981.
[12] O. I. Kolodiazhnyi, Phosphorus Ylides: Chemistry and Applica-
tion in Organic Synthesis, Wiley-VCH, Weinheim, 1999.
[13] J. Frey, Z. Rappoport, J. Am. Chem. Soc. 1996, 118, 5169.
[14] For similar Wittig-like process, see: J. D. Masuda, D. Martin, C.
Lyon-Saunier, A. Baceiredo, H. Gornitzka, B. Donnadieu, G.
Bertrand, Chem. Asian J. 2007, 2, 178.
Keywords: boron · carbenes · cumulenes · phosphorus · ylides
.
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