ChemComm
Communication
donors in 1 and 2 combined with the demonstration of catalytic
imine hydrogenation by 3 urges future study of these com-
pounds for selective FLP processes.
In summary, we have prepared an N-boryl substituted NHC-
borane (1) with hydride bridging the two boron centers. This
species is shown to react with I
borenium-borate’’ 2. Treatment of 1 with [Ph
2
to give the ‘‘zwitterionic
6 5 4
C][B(C F ) ]
3
cleanly abstracts hydride to provide facile access to the cationic
bidentate Lewis acid 3. Compound 3 readily catalyzes the room-
temperature hydrogenation of N-benzylidene-tert-butylamine.
The further use of these compounds in FLP chemistry and their
structural modification through judicious choice of borane
Fig. 3 POV-ray depiction of 3 N: blue; C: black; B: yellow-green. F: pink. and N-heterocycle are key areas of on-going examination. The
H-atoms are omitted for clarity.
results of these studies will be reported in due course.
NSERC of Canada is thanked for financial support. DWS is
grateful for the award of a Canada Research Chair.
the suggested formulation (Fig. 3). Both boron centers are three-
Notes and references
coordinate with the sums of bond angles about B and B found to
N C
be 358.61 and 359.31, respectively. A B–N bond length of 1.468(5) Å
and a B–CNHC bond length of 1.587(5) Å were observed. The
latter bond length is similar to B–CNHC bond lengths of other
1
G. Erker and D. W. Stephan, Frustrated Lewis Pairs I & II, Springer,
Berlin, Heidelberg, 2013.
D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 46.
D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2015, 54, 6400.
D. W. Stephan, Org. Biomol. Chem., 2008, 6, 1535.
2
3
4
+
51,54
[NHC–BBN] ions.
The short B–N bond length observed for 3
together with small CNHC–N–B–CBBN dihedral angles (e.g. a 13.21
dihedral angle measured in 3 greatly contrasts with a 89.41
dihedral angle measured in 2) infer significant resonance con-
tribution from a non-aromatic structure bearing a BN double
bond (Scheme 2). This assessment is further supported by the
significant discrepancy between the N-heterocycle N–CB bond
lengths of 3, which are 1.367(5) Å and 1.334(5) Å, respectively.
These data indicate a more localized bonding character than
anticipated for a pseudo-aromatic description. In order to probe
5 D. W. Stephan, Dalton Trans., 2009, 3129.
6
D. W. Stephan, S. Greenberg, T. W. Graham, P. Chase, J. J. Hastie,
S. J. Geier, J. M. Farrell, C. C. Brown, Z. M. Heiden, G. C. Welch and
M. Ullrich, Inorg. Chem., 2011, 50, 12338.
7 G. C. Welch, R. R. S. Juan, J. D. Masuda and D. W. Stephan, Science,
006, 314, 1124.
K. Maruoka, Pure Appl. Chem., 2002, 74, 123.
2
8
9 W. E. Piers, G. J. Irvine and V. C. Williams, Eur. J. Inorg. Chem., 2000,
2131.
1
1
0 J. D. Wuest, Acc. Chem. Res., 1999, 32, 81.
1 V. C. Williams, W. E. Piers, W. Clegg, M. R. J. Elsegood, S. Collins
and T. B. Marder, J. Am. Chem. Soc., 1999, 121, 3244.
the utility of 3 as a Lewis acid in FLP chemistry, we attempted the 12 T. Ooi, T. Miura and K. Maruoka, Angew. Chem., Int. Ed., 1998,
6
3
37, 2347.
FLP catalyzed hydrogenation of an imine. To our delight, the use
1
3 T. Ooi, M. Takahashi and K. Maruoka, J. Am. Chem. Soc., 1996,
of 5 mol% 3 as a catalyst effected the room-temperature hydro-
118, 11307.
genation of substrate N-benzylidene-tert-butylamine to N-benzyl- 14 D. J. H. Emslie, W. E. Piers and M. Parvez, Angew. Chem., Int. Ed.,
2
003, 42, 1252.
tert-butylamine. A 49% conversion to product was observed by
1
1
5 H. E. Katz, J. Am. Chem. Soc., 1985, 107, 1420.
6 M. Melaimi and F. P. Gabba ¨ı , J. Am. Chem. Soc., 2005, 127, 9680.
1
H-NMR spectroscopy after 24 h reaction time in CH
02 atm H
fashion to previously reported NHC-borenium catalyzed hydro-
2 2
Cl under
1
2
. Presumably the reaction proceeds in an analogous 17 N. Asao, P. Liu and K. Maruoka, Angew. Chem., Int. Ed., 1997,
6, 2507.
8 S. N. Kessler, M. Neuburger and H. A. Wegner, Eur. J. Org. Chem.,
011, 3238.
19 T. W. Hudnall, C.-W. Chiu and F. P. Gabba ¨ı , Acc. Chem. Res., 2009,
2, 388.
3
1
51,54
genations
wherein the bulky Lewis basic imine substrate and
2
the borenium ion comprise an FLP that heterolytically cleaves H
2
.
4
The iminium ion generated is subsequently attacked by boron-
bound hydride to give the reduced amine and the regenerated
2
2
0 M. Hirai and F. P. Gabba ¨ı , Angew. Chem., Int. Ed., 2015, 54, 1205.
1 D. F. Shriver and M. J. Biallas, J. Am. Chem. Soc., 1967, 89, 1078.
borenium salt. As expected based on this mechanism, a similar 22 W. Uhl, J. Grunenberg, A. Hepp, M. Matar and A. Vinogradov,
Angew. Chem., 2006, 118, 4465.
3 W. Uhl, A. Vinogradov and S. Grimme, J. Am. Chem. Soc., 2007,
reaction carried out using 5 mol% of the neutral compound 1 as a
2
catalyst in place of 3 failed to show any conversion of the substrate
129, 11259.
1
to hydrogenated product by H-NMR spectroscopy after 24 h 24 M. H. Lee and F. P. Gabba ¨ı , Inorg. Chem., 2007, 46, 8132.
2
5 T. W. Hudnall, Y.-M. Kim, M. W. P. Bebbington, D. Bourissou and
F. P. Gabbai, J. Am. Chem. Soc., 2008, 130, 10890.
6 5 2
reaction time in C D Br under 102 atm H at room temperature.
The easily accessible compound 3 illustrates the synthetic
2
6 H. Zhao and F. P. Gabba ¨ı , Org. Lett., 2011, 13, 1444.
feasibility of cationic diboryl-N-heterocycles as bidentate Lewis 27 H. Zhao and F. P. Gabba ¨ı , Nat. Chem., 2010, 2, 984.
2
2
3
8 C. Jiang, O. Blacque and H. Berke, Chem. Commun., 2009, 5518.
9 X. Zhao and D. W. Stephan, Chem. Commun., 2011, 47, 1833.
0 M. J. Sgro, J. D ¨o mer and D. W. Stephan, Chem. Commun., 2012,
48, 7253.
acids. The divergent bonding arrangements of hydride and
iodide when coordinated to this Lewis acid are also striking.
A consistent challenge in FLP chemistry is to overcome the poor
selectivity of highly electrophilic boranes for small molecules of
interest in the presence of competing donor functional groups.
Therefore, the divergent bonding arrangements of anionic
3
3
1 Z. Lu, H. Hausmann, S. Becker and H. A. Wegner, J. Am. Chem. Soc.,
2015, 137, 5332.
2 M. H. Holthausen, J. M. Bayne, I. Mallov, R. Dobrovetsky and
D. W. Stephan, J. Am. Chem. Soc., 2015, 137, 7298.
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Chem. Commun.