Communication
ChemComm
NMR spectrum at +1.67 ppm and a d11B resonance at À14.7 like to acknowledge the use of the EPSRC UK National Service
consistent with a four coordinate boron centre. The accessi- for Computational Chemistry Software (NSCCS).
bility of boronium 7+ does not preclude catalytic (in trityl salt)
CO reduction,
Notes and references
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2 G. A. Olah, A. Goeppert and G. K. Surya Prakash, Beyond Oil and Gas:
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3 For recent reviews on CO reduction using transition metal catalysts
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1, 641; (b) M. M. West, A. J. Miller, J. A. Labinger and J. E. Bercaw,
as 7+ is presumably extremely sterically congested at boron
(confirmation by X-ray diffraction studies have been frustrated
by the lack of suitable crystals) and therefore may react with CO
Coord. Chem. Rev., 2011, 255, 881.
4 For a review on the use of frustrated Lewis pairs for the reduction of
main group oxides including CO see: D. W. Stephan and G. Erker,
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by a SN2 mechanism or via the borenium salt if reversible amine
dissociation generates a low concentrate of borenium salt. Activa-
tion of (EtiPr2N)BH3 with 0.5 equivalents of [Ph3C][B(C6F5)4] gener-
ated the hydride bridged dimer [((EtiPr2N)BH2)2(m-H)][B(C6F5)4],
8[B(C6F5)4], as previously reported.10c Degassing an o-DCB solution
of 8[B(C6F5)4] followed by the addition of ca. 2 atm. of CO led to no
immediate change in the 1H and 11B NMR spectra. On standing at
20 1C two new boron resonances were observed to increase in
intensity at d11B = +31.9 and À14.9, which after 24 h were the major
products with all 8[B(C6F5)4] consumed. The d11B chemical shifts
indicate formation of the boronium salt 7[B(C6F5)4] and 6 (eqn (10)),
with no intermediates observed. Thus the increased steric bulk of
EtiPr2N relative to Et3N is presumably destabilising the hydride and
oxo bridged cations leading to lower reaction barriers and complete
conversion to 6 at 20 1C whereas with the Et3N congeners this
requires heating to 100 1C. With the desired CO reductive cleavage
proceeding from 8[B(C6F5)4] to form 6 and 7[B(C6F5)4] attempts were
made to see if 7[B(C6F5)4] would be active for further CO reduction
cycles with additional amine borane. However, the addition of a
further 3 equivalents of (EtiPr2N)BH3 (and recharging with ca. 2 atm.
of CO) led to no further boroxine formation or consumption of
(EtiPr2N)BH3 even on heating to 60 1C for 24 h and to 100 1C for
24 h. Instead an unidentified resonance at +17.6 ppm was observed
in the 11B NMR spectrum as the major product.
¨
9 For reviews on borocation chemistry see: (a) P. Koelle and H. Noth,
Chem. Rev., 1985, 85, 399; (b) W. E. Piers, S. C. Bourke and K. D.
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Ingleson, Top. Organomet. Chem., 2015, 49, 39.
10 For select studies on the reactivity of [(amine)BH2]+ (or functional equiva-
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J. Jermaks, A. Borovika, J. W. Kampf and E. Vedejs, Organometallics, 2013,
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14 E. R. Clark, A. Del Grosso and M. J. Ingleson, Chem. – Eur. J., 2013,
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15 A. Berkefeld, W. E. Piers, M. Parvez, L. Castro, L. Maron and
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16 M. Horn, L. H. Schappele, G. Lang-Wittowski, H. Mayr and
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In conclusion, we report a metal free system that results in the
complete cleavage of the C–O bond in CO and reduction to B–CH3.
The high electrophilicity of weakly stabilized borocations is essential 17 G. Menard and D. W. Stephan, Dalton Trans., 2013, 42, 5447.
18 A. Prokofjevs, J. W. Kampf and E. Vedejs, Angew. Chem., Int. Ed.,
to activate the coordinated CO sufficiently to transform CO into a
strong Lewis acid towards hydride. We are currently investigating
2011, 50, 2098.
19 For solid state structure of relevant boronium salts see: (a) P. A. Fox,
the reactivity of other highly electrophilic borocations towards CO.
We are grateful to the European Research Council (FP7
Grant Agreement No. 305868), the Leverhulme Trust and the
Royal Society for funding this project. The authors also would
S. T. Griffin, W. M. Reichert, E. A. Salter, A. B. Smith, M. D. Tickell,
B. F. Wicker, E. A. Cioffi, J. H. Davis Jr., R. D. Rogers and
A. Wierzbicki, Chem. Commun., 2005, 3679; (b) O. J. Metters,
A. M. Chapman, A. P. M. Robertson, C. H. Woodall, P. J. Gates,
D. F. Wass and I. Manners, Chem. Commun., 2014, 50, 12146.
10906 | Chem. Commun., 2015, 51, 10903--10906
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