Borylated methylenephosphonium salts: Precursors of elusive boryl(phosphino)carbenes

Article abstract of DOI:10.1021/ja103422b

The synthesis of a new family of boryl-substituted methylenephosphonium derivatives, the phosphorus analogues of iminium salts, has been developed. They were used in the preparation of the first stable boryl(phosphino)carbene, which has been fully characterized by NMR spectroscopy and X-ray crystallography. Density functional theory calculations indicate that these carbenes can be classified as push-pull carbenes with a relatively small singlet-triplet energy gap.

Full text of DOI:10.1021/ja103422b

Published on Web 06/10/2010
Borylated Methylenephosphonium Salts: Precursors of Elusive
Boryl(phosphino)carbenes
Florie Lavigne,† Eddy Maerten,† Gilles Alcaraz,‡ Nathalie Saffon-Merceron,§ Carles Acosta-Silva,|
Vicenc¸ Branchadell,| and Antoine Baceiredo*,†
UniVersite´ de Toulouse, UPS, and CNRS, LHFA, 118 route de Narbonne, F-31062 Toulouse, France, LCC,
UPR CNRS 8241, 205 route de Narbonne, F-31077 Toulouse, France, Structure Fe´de´ratiVe de Chimie, FR 2599,
118 route de Narbonne, F-31062 Toulouse, France, and Departament de Qu´ımica, UniVersitat Auto`noma de
Barcelona, 08193 Bellaterra, Barcelona, Spain
Received April 29, 2010; E-mail: baceired@chimie.ups-tlse.fr
Scheme 1. Synthesis of Ylide 2 and Methylenephosphonium Salts
3 and 4
Since the first stable carbenes were isolated by Bertrand and co-
workers1a in 1988 and by Arduengo et al.1b in 1991, tremendous
developments and substantial efforts have been devoted to understand-
ing their stabilization and tuning their reactivity.1c The stabilization
and chemical behavior of these species result from the degree of
carbene perturbation, which is directly dependent on the substitution
pattern at the electron-deficient carbon center.2 With the exception of
the cyclopropenylidene synthesis,3 all of the examples reported to date
involve one amino or phosphino π-donating (or push) substituent
attached to the carbenic center, defining the two main families of
stabilized carbenes. The second substituent determines the nature of
the carbene: push-push, push-pull, or push-spectator. The carbene
synthetic strategy depends strongly on the push group. The deproto-
nation route of imidazolium salts initially developed by Arduengo et
al. and later extended to iminium compounds allowed general access
to aminocarbenes.4 The synthetic approaches involving the weaker-
π-donor phosphino group are more limited, the syntheses being mostly5
restricted to the decomposition of phosphino diazomethane precursors.6
The challenging introduction of an electron-withdrawing group such
as a boryl substituent at the carbenic center should produce the best
model of a push-pull carbene version profiting from the ability of
boron to stabilize a lone pair in an adjacent position.7
was isolated in 73% yield (Scheme 1).10 The identity of isomer 4 was
unambiguously confirmed by NMR spectroscopy and X-ray diffraction
techniques (Figure 1).
As part of our program involving group-13-substituted electron-
deficient species, we report here the synthesis and characterization
of a new class of boryl(phosphino)carbocations that provide a
synthetic entry to novel and stable boryl(phosphino)carbenes.
Since boryl(phosphino)diazomethane species remain an elusive class
of diazo derivatives,8 we focused our attention on the possible
deprotonation of boryl-substituted methylene phosphonium derivatives,
the phosphorus analogues of iminium salts.9 Phosphino(boryl)methane
1 was synthesized by reaction of (lithiomethyl)dimesitylborane with
bis(diisopropylamino)chlorophosphine in tetrahydrofuran (THF). Sub-
sequent reaction of 1 with carbon tetrachloride in toluene afforded the
expected ylide 2 in 66% overall yield. A stoichiometric amount of
gallium trichloride was then used to abstract the chloride from 2 in
dichloromethane solution, generating methylenephosphonium salt 3.9b,c
Monitoring of the reaction by 31P NMR spectroscopy revealed the
complete formation of 3 (31P: 147 ppm), which appeared to be stable
only below -40 °C. Above this temperature, we observed the gradual
consumption of 3 with the concomitant formation of a new compound
exhibiting a signal at 153 ppm. After evaporation of the solvent and
workup, the methylenephosphonium isomer 4 resulting from an
exchange of substituents between the boron and phosphorus centers
Figure 1. Molecular structures of (a) methylenephosphonium salt 4 and
(b) boryl(phosphino)carbene 5.
Deprotonation of 4 was carried out in THF at -80 °C using a
stoichiometric amount of potassium hexamethyldisilazane (KHMDS)
(Scheme 2). After evaporation of the solvent and workup, the stable
boryl(phosphino)carbene 5 was isolated as yellow crystals from a cold
† Universite´ de Toulouse, UPS, and CNRS, LHFA.
‡ LCC, UPR CNRS 8241.
§ Structure Fe´de´rative de Chimie, FR 2599.
| Universitat Auto`noma de Barcelona.
9
8864 J. AM. CHEM. SOC. 2010, 132, 8864–8865
10.1021/ja103422b  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Synthesis of Boryl(phosphino)carbene 5
Scheme 3. Synthesis of Phosphaalkene 6
(-40 °C) saturated pentane solution (52% yield) and fully characterized
by NMR spectroscopy and X-ray crystallography (Figure 1).
The 31P NMR spectrum of 5 shows a signal at high field (-36.7
ppm) that is typical for phosphinocarbenes.2b The 13C NMR chemical
shift for the carbene center appears as a well-resolved doublet at
through a 1,2-migration of the mesityl group from the phosphorus
to the central carbon atom (Scheme 3).
More than 20 years after the isolation of the first stable carbene,
the existence of a stable boryl(phosphino)carbene has been clearly
demonstrated. An original synthetic pathway starting from boryl-
substituted methylenephosphonium derivatives has been developed
and should provide a promising alternative to the classical phos-
phinodiazomethane precursors. Developments of new models
allowing a more complete understanding of this new family of
carbenes are under active investigation.
1
relatively low field (174.6 ppm, JPC ) 208.9 Hz), and the large
coupling constant is indicative of direct P-C connectivity. The X-ray
structures of 4 and 5 were determined at -80 °C (Figure 1 and Table
1). The phosphorus and boron atoms are both in planar environments,
and these planes are almost perpendicular for 5 (N2-P-B-N1 )
103.1°) and coplanar for 4 (N1-B-C1-P ) 172.6° and N2-P-C1-B
) 163.2°). In carbene 5, the P-C1 distance [1.563(13) Å] is shorter
than that in 4 [1.634(3) Å], and the value is consistent with those
reported in the previous phosphinocarbenes (1.53-1.56 Å).6a,c,d The
B-C1 bond undergoes a small shortening in going from 4 [1.568(4)
Å] to 5 [1.54(2) Å] but remains long for a BdC double bond.11 This
corrresponds to a slightly elongated N1-B bond in 5 [1.43(3) Å vs
1.406(3) Å in 4], indicative of a competition between the carbene and
nitrogen lone pairs to fill the vacant boron orbital. The large value of
the P-C1-B angle, which is wider in 5 [151.6(13)°] than in 4
[128.6(2)°], is reminiscent of that in phosphino(silyl)carbenes.12
Acknowledgment. This work was supported by the CNRS
(LEA 368), the Spanish Ministerio de Ciencia e Innovacio´n
(CTQ2007-61704/BQU), and Generalitat de Catalunya (2009SGR-
733 and XRQTC). Time allocated in the Centre de Supercomputacio´
de Catalunya (CESCA) is gratefully acknowledged.
Supporting Information Available: Experimental procedures,
selected NMR data, X-ray crystallographic files for 4 and 5 (CIF), and
computational details and Cartesian coordinates for the calculated
structures. This material is available free of charge via the Internet at
http://pubs.acs.org.
Table 1. Selected Geometrical Parameters for Experimental and
Optimized Structures of 4 and 5 (Distances in Å, Angles in deg)
References
(1) (a) Igau, A.; Grutzmacher, H.; Baceiredo, A.; Bertrand, G. J. Am. Chem.
Soc. 1988, 110, 6463. (b) Arduengo, A. J.; Harlow, R. L.; Kline, M. J. Am.
Chem. Soc. 1991, 113, 361. (c) Bourissou, D.; Guerret, O.; Gabbai, F. P.;
Bertrand, G. Chem. ReV. 2000, 100, 39.
5
5calcd
4
4calcd
P-C1
1.563(13)
1.54(2)
151.6(13)
1.569
1.498
160.2
1.634(3)
1.568(4)
128.6(2)
1.634
1.593
124.4
B-C1
(2) (a) Canac, Y.; Soleilhavoup, M.; Conejero, S.; Bertrand, G. J. Organomet.
Chem. 2004, 689, 3857. (b) Vignolle, J.; Cattoe¨n, X.; Bourissou, D. Chem.
ReV. 2009, 109, 3333. (c) Kato, T.; Maerten, E.; Baceiredo, A. Top.
Organomet. Chem. 2010, 30, 131.
P-C1-B
(3) (a) Lavallo, V.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G.
Science 2006, 312, 722. (b) Schoeller, W. W.; Frey, G. D.; Bertrand, G.
Chem.sEur. J. 2008, 14, 4711.
The geometries of phosphino(boryl)carbene 5 and methylenephos-
phonium 4 were optimized at the M05-2X/6-31G(d) level (see the
Supporting Information). Except for the too-large P-C1-B angle
computed for 5, as in the case of other P-C-Si carbene analogues,13
the other geometrical parameters were reproduced accurately, and the
optimized geometries of 4 and 5 are in good agreement with the
experimental structures (see Table 1). The bonding nature can be
rationalized using natural bond order (NBO) analysis. For carbene 5,
the analysis shows a Pt C triple bond, with the π orbital in the
P-C1-B plane (π′) strongly polarized to C1 (88% C vs 12% P). As
expected, the perpendicular 2p atomic orbital at boron [LP*(B)] in 4
and 5 has a relatively large occupancy resulting from π donation of
the nitrogen lone pair (0.429e in 5 and 0.396e in 4), with a slight
increase upon deprotonation despite the more-localized lone pair at
the N1 atom (1.706e in 5 vs 1.623e in 4). Indeed, the analysis of the
donor-acceptor interactions in the NBO basis estimated from second-
order perturbation theory shows that in the case of carbene 5, the
donations LP(N1) and π′(C1-P) to LP*(B) are of comparable
magnitude. This feature is illustrated in 5 by a rotation of the boron
plane that favors the interaction between the carbene lone pair
[π′(C1-P)] and the vacant LP*(B) orbital at boron. Finally, the
variations of the Wiberg bond indices around the carbenic center in
going from 4 to 5 (P-C1, +0.33; C1-B, +0.25) confirm that 5 can
be classified as a push-pull carbene. As expected for this type of
carbene, the singlet-triplet energy gap was predicted to be relatively
small (17.7 kcal/mol).2b
(4) N-Heterocyclic Carbenes in Transition Metal Catalysis; Glorius, F., Ed.; Topics
in Organometallic Chemistry, Vol. 21; Springer: Berlin, 2007.
(5) Martin, D.; Baceiredo, A.; Gornitzka, H.; Schoeller, W. W.; Bertrand, G.
Angew. Chem., Int. Ed. 2005, 44, 1700.
(6) (a) Soleilhavoup, M.; Baceiredo, A.; Treutler, O.; Ahlrichs, R.; Nieger,
M.; Bertrand, G. J. Am. Chem. Soc. 1992, 114, 10959. (b) Treutler, O.;
Ahlrichs, R.; Soleilhavoup, M. J. Am. Chem. Soc. 1993, 115, 8788. (c)
Buron, C.; Gornitzka, H.; Romanenko, V.; Bertrand, G. Science 2000, 288,
834. (d) Despagnet, E.; Gornitzka, H.; Rozhenko, A. B.; Schoeller, W. W.;
Bourissou, D.; Bertrand, G. Angew. Chem., Int. Ed. 2002, 41, 2835. (e)
Despagnet-Ayoub, E.; Gornitzka, H.; Bourissou, D.; Bertrand, G. Eur. J.
Org. Chem. 2003, 2039.
(7) (a) Pelter, A.; Singaram, B.; Wilson, J. W. Tetrahedron Lett. 1983, 24,
631. (b) Cooke, M. P. J. Org. Chem. 1994, 59, 2930. (c) Pelter, A. Pure
Appl. Chem. 1994, 66, 223.
(8) Only a few boryldiazomethane derivatives have been described. See: (a)
Arthur, M. P.; Baceiredo, A.; Bertrand, G. J. Am. Chem. Soc. 1991, 113,
5857. (b) Weber, L.; Wartig, H. B.; Stammler, H.-G.; Neumann, B.
Organometallics 2001, 20, 5248.
(9) (a) Igau, A.; Baceiredo, A.; Gru¨tzmacher, H.; Pritzkow, H.; Bertrand, G.
J. Am. Chem. Soc. 1989, 111, 6853. (b) Gru¨tzmacher, H.; Pritzkow, H.
Angew. Chem., Int. Ed. 1991, 30, 709. (c) Heim, U.; Pritzkow, H.;
Scho¨nberg, H.; Gru¨tzmacher, H. Chem. Commun. 1993, 673.
(10) Species 3 and4 can be regarded as electron-deficient R-boryl carbocations.
Calculations at the M05-2X/6-31G(d) level revealed that isomer 4 is 29.9
kcal/mol more stable than isomer 3, suggesting increased stabilization due
to π donation from LP(N1) to LP*(B) in 4.
(11) (a) Horchler von Locquenghien, K.; Baceiredo, A.; Boese, R.; Bertrand,
G. J. Am. Chem. Soc. 1991, 113, 5062. (b) Chiu, C.-W.; Gabba¨ı, F. P.
Angew. Chem., Int. Ed. 2007, 46, 6878.
(12) Kato, T.; Gornitzka, H.; Baceiredo, A.; Savin, A.; Bertrand, G. J. Am. Chem.
Soc. 2000, 122, 998.
(13) (a) Martin, D.; Illa, O.; Baceiredo, A.; Bertrand, G.; Ortuno, R. M.;
Branchadell, V. J. Org. Chem. 2005, 70, 5671. (b) Lecea, B.; Ayerbe, M.;
Arrieta, A.; Coss´ıo, F. P.; Branchadell, V.; Ortun˜o, R. M.; Baceiredo, A.
J. Org. Chem. 2007, 72, 357.
Carbene 5 is perfectly stable in the solid state and slowly
rearranges in solution into the corresponding phosphaalkene 6
JA103422B
9
J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 8865