which showed heterolytic splitting of H
2
with TMP. It also
proved to be a good catalyst for the direct hydrogenation of
imines. Further detailed mechanistic investigations of FLP
with double Lewis acids are sought to eventually allow full
insight into the reaction course enabling ‘‘fine tuning’’ in the
search for even more functional bidentate Lewis acids.
Notes and references
ꢀ
z Crystal data for 1: space group P1, a = 10.5171(4), b = 11.9691(4),
˚
c = 14.0547(5) A, a = 69.240(3), b = 84.669(3), g = 66.772(4)1, V =
3
ꢀ1
˚
Scheme 2
1517.98(11) A , Z = 2, m = 0.189 mm , 17 221 reflections collected,
206 independent (Rint = 0.0386), R = 0.0448, wR = 0.0770 (for 3517
6
1
2
observed reflections with I Z 2s (I) and 505 refined parameters). CCDC
ꢀ
catalytic reaction the anion of 1 with ‘‘internal’’ H binding is
presumably not an intermediate, rather an ‘‘external’’ species
with the hydride connected to only one boron center and
facing the outside (Scheme 2). Consequently the intermolecular
hydride transfer to the iminium with formation of the amine
also occurs externally. Probing another internal mechanistic
possibility, we attempted the reaction of 2 with PhCHQNtBu,
which however did not insert into the B–H–B bond to generate
an amide complex, which demonstrated again that it is less
plausible to assume that the anion of hydride 2 is an inter-
mediate of the catalytic cycle of Scheme 2.
724802. Crystal data for 2: space group P2 /c, a = 28.2693(9), b =
˚
1
3
˚
16.9611(5), c = 18.6599(9) A, b = 96.118(3)1, V = 8896.1(6) A , Z = 8,
ꢀ1
m = 0.148 mm , 74894 reflections collected, 15793 independent
int = 0.1511), R = 0.0679, wR = 0.1414 (for 6368 observed
reflections with I Z 2s (I) and 1288 refined parameters). CCDC
731638. Crystal data for 3: space group P2 /n, a = 19.6245(2),
(
R
1
2
1
3
˚
˚
b = 24.8943(3), c = 22.7758(3) A, b = 98.977(1)1, V = 10990.6(2) A ,
ꢀ1
Z = 2, m = 0.243 mm , 113 766 reflections collected, 20 853 independent
int = 0.0395), R = 0.0696, wR = 0.1941 (for 13 444 observed
reflections with I Z 2s (I) and 1599 refined parameters). CCDC 724803.
(R
1
2
1
G. C. Welch, R. R. S. Juan, J. D. Masuda and D. W. Stephan,
Science, 2006, 314, 1124.
2 D. W. Stephan, Org. Biomol. Chem., 2008, 6, 1535.
3
4
5
D. W. Stephan, Dalton Trans., 2009, 3129.
G. C. Welch and D. W. Stephan, J. Am. Chem. Soc., 2007, 129, 1880.
¨
P. Spies, G. Erker, G. Kehr, K. Bergander, R. Frohlich, S. Grimme
and D. W. Stephan, Chem. Commun., 2007, 5072.
The final step of the catalytic cycle, the dissociation of the B–N
adduct to free the catalyst and the amines, is often considered to
27
be rate determining, however, this seems different for the
catalyses with 1. The quite congested geometries of the amine
adducts with 1 would not allow proper B–N interactions tight
enough to render under catalytic circumstances rate determining
dissociation. Therefore, in case the catalytic imine hydrogenation
with 1 is suppressed, this seems not to originate from a too tight
6 J. S. J. McCahill, G. C. Welch and D. W. Stephan, Angew. Chem.,
Int. Ed., 2007, 46, 4968.
7
8
P. A. Chase and D. W. Stephan, Angew. Chem., Int. Ed., 2008, 47, 7433.
D. Holschumacher, T. Bannenberg, C. G. Hrib, P. G. Jones and
M. Tamm, Angew. Chem., Int. Ed., 2008, 47, 7428.
9
V. Sumerin, F. Schulz, M. Nieger, M. Leskela
B. Rieger, Angew. Chem., Int. Ed., 2008, 47, 6001.
0 D. P. Huber, G. Kehr, K. Bergander, R. Fro
¨
, T. Repo and
amine–boron adduct rather from the too low kinetic H
2
affinity of
1
¨
hlich, G. Erker,
1. ‘‘Internal’’ access of H penetrating into the gap between both
2
S. Tanino, Y. Ohki and K. Tatsumi, Organometallics, 2008, 27, 5279.
11 S. J. Geier, T. M. Gilbert and D. W. Stephan, J. Am. Chem. Soc.,
008, 130, 12632.
2 M. Ullrich, A. J. Lough and D. W. Stephan, J. Am. Chem. Soc.,
009, 131, 52.
13 P. Spies, G. Kehr, K. Bergander, B. Wibbeling, R. Fro
G. Erker, Dalton Trans., 2009, 1534.
4 A. Ramos, A. J. Lough and D. W. Stephan, Chem. Commun., 2009,
118.
5 S. J. Geier and D. W. Stephan, J. Am. Chem. Soc., 2009, 131(10), 3476.
boron centers, the ‘‘super Lewis acidic activation pathway’’,
apparently possesses a higher barrier than the ‘‘external’’
access of H2 approaching just one boron center from the
outside. Therefore in catalytic reactions 1 behaves obviously
as one-centered and is related to the corresponding imine–
B(C F ) FLPs. This was further substantiated by a com-
2
1
2
¨
hlich and
1
1
6
5 3
1
parison of the hydrogenation activities of 1 and B(C
various imines displaying somewhat better performance for
B(C , but overall both types of reaction were kinetically in
the same range (Table S1, ESIw). We then tested the H
pressure dependence of the reaction of PhCHQNCHPh with
B(C and found that under identical conditions as for 1,
the TOF curve initially increased linearly with pressure and
6 5 3
F ) with
16 P. A. Chase, G. C. Welch, T. Jurca and D. W. Stephan, Angew.
Chem., Int. Ed., 2007, 46, 8050.
6 5 3
F )
1
7 P. Spies, S. Schwendemann, S. Lange, G. Kehr, R. Fro
G. Erker, Angew. Chem., Int. Ed., 2008, 47, 7543.
18 V. Sumerin, F. Schulz, M. Atsumi, C. Wang, M. Nieger,
M. Leskela, T. Repo, P. Pyykko and B. Rieger, J. Am. Chem.
Soc., 2008, 130(43), 14117.
9 H. Wang, R. Frohlich, G. Kehr and G. Erker, Chem. Commun.,
2008, 5966.
20 W. E. Piers, G. J. Irvine and V. C. Williams, Eur. J. Inorg. Chem.,
000, 2131.
¨
hlich and
2
2
6
F
5
)
3
¨
¨
1
¨
ꢀ1
reached a TOF of 20 h at 5 bar of H2.
Attempts to trace any interaction of H with the double
2
1
19
2
Lewis acid 1 via H and F NMR spectroscopy at tempera-
tures as low as 193 K were not successful, which might suggest
that the formation of a more stable and observable internal
2
2
2
1 H. E. Katz, J. Am. Chem. Soc., 1985, 107, 1420.
2 H. E. Katz, J. Org. Chem., 1985, 50, 5027.
3 J. D. Hoefelmeyer and F. P. Gabbaı
9054.
4 H. O. House, D. G. Koepsell and W. J. Campbell, J. Org. Chem.,
972, 37, 1003.
¨
, J. Am. Chem. Soc., 2000, 122,
1
1
–H
–H
2
adduct has a high barrier to overcome, while the external
6 5 3
adduct is relatively unstable—similar to the B(C F )
2
2
2
1
cases—and too short-lived to be identified with conventional
analytical methodologies. Nevertheless H adduct formation
5 R. L. Letsinger, J. A. Gilpin and W. J. Vullo, J. Org. Chem., 1962,
27, 672.
2
and splitting must occur in transient catalytic intermediates,
otherwise hydrogenation catalysis could not be envisaged.
In summary, we could prepare the novel bidentate
Lewis acid 1,8-bis(dipentafluorophenylboryl)naphthalene,
26 D. J. Parks, W. E. Piers and G. P. A. Yap, Organometallics, 1998,
7, 5492.
7 P. A. Chase, T. Jurca and D. W. Stephan, Chem. Commun., 2008, 1701.
8 T. A. Rokob, A. Hamza, A. Stirling and I. Papai, J. Am. Chem.
Soc., 2009, 131, 2029.
1
2
2
´
5
520 | Chem. Commun., 2009, 5518–5520
This journal is ꢂc The Royal Society of Chemistry 2009