Organometallics
Communication
reinvestigated our intramolecular Zr−P FLPs and specifically the
indenyl complex 6.
AUTHOR INFORMATION
Corresponding Author
Author Contributions
The manuscript was written through contributions of all authors.
The authors declare no competing financial interests.
■
Exposure of a solution of the substrate with 10 mol % catalyst 6
to dihydrogen led to hydrogenation and generation of the
corresponding amines (Table 3), although these particular
reactions are more sluggish and require higher temperatures to
observe appreciable conversion.
Notes
Table 3. Catalytic Hydrogenation of Enamines and Protected
Nitriles by 6
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.R.F. acknowledges the Engineering and Physical Sciences
Research Council (EPSRC) for financial support. S.R.F. and
O.M. thank the University of Bristol for financial support.
REFERENCES
■
(1) (a) Chapman, A. M.; Haddow, M. F.; Wass, D. F. J. Am. Chem. Soc.
2011, 133, 8826−8829. (b) Chapman, A. M.; Haddow, M. F.; Wass, D.
F. J. Am. Chem. Soc. 2011, 133, 18463−18478.
(2) For review articles and books, see: (a) Stephan, D. W.; Erker, G.
Angew. Chem., Int. Ed. 2010, 49, 46−76. (b) Erker, G. C. R. C. R. Chim.
2011, 14, 831−841. (c) Erker, G., Stephan, D. W., Eds. Topics in Current
Chemistry; 2013; p 334. (d) Stephan, D. W. J. Am. Chem. Soc. 2015, 137,
10018−10032. (e) Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed.
2015, 54, 6400−6441.
(3) Flynn, S. R.; Wass, D. F. ACS Catal. 2013, 11, 2574−2581.
(4) (a) Normand, A. T.; Richard, P.; Balan, C.; Daniliuc, C. G.; Kehr,
G.; Erker, G.; Gendre, P. L. Organometallics 2015, 34, 2000−2011.
(b) Xu, X.; Kehr, G.; Daniliuc, C. G.; Erker, G. J. Am. Chem. Soc. 2014,
136, 12431−12443. (c) Forrest, S. J. K.; Clifton, J.; Fey, N.; Pringle, P.
G.; Sparkes, H. A.; Wass, D. F. Angew. Chem., Int. Ed. 2015, 54, 2223−
2227.
a
Conversion to the corresponding amine based on 1H NMR
spectroscopy.
The necessity of an internal phosphine base for these
substrates suggests a different mechanism from that proposed
for imine substrates in that heterolytic cleavage of H2 across the
Zr−P pair is followed by proton and hydride transfer to the
substrate. It is noteworthy that heterolytic cleavage of H2 by 6 is
not observed by NMR spectroscopy in the absence of an external
hydrogen acceptor, suggesting that the H2 activation product is in
rapid equilibrium with 6 at 25 °C and the transfer of the
sequestered H2 to the enamine is the rate-determining step in
this case.
(5) Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem.,
Int. Ed. 2007, 46, 8050−8053.
(6) Stephan, D. W. Acc. Chem. Res. 2015, 48, 306−316 and references
therein.
(7) For representative examples, see: (a) Cuenca, T.; Flores, J. C.;
Royo, P. J. Organomet. Chem. 1993, 462, 191−201. (b) Paquette, L. A.;
Sivik, M. R.; Bzowej, E. I.; Stanton, K. J. Organometallics 1995, 14,
4865−4878. (c) Troutman, M. V.; Appella, D. H.; Buchwald, S. L. J. Am.
Chem. Soc. 1999, 121, 4916−4917. (d) Vassylyev, E.; Panarello, A.;
Khinast, J. G. Molecules 2005, 10, 587−619.
CONCLUSIONS
■
(8) (a) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc. 1992, 114,
7562−7564. (b) Willoughby, C. A.; Buchwald, S. L. J. Org. Chem. 1993,
58, 7627−7629. (c) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc.
1994, 116, 8952−8965.
In conclusion, the combination of a Lewis acidic zirconium
center with sterically hindered imines in the presence of
dihydrogen results in heterolytic cleavage of H2, generating a
zirconium hydride and an iminium salt. With suitable ancillary
ligands of Zr, the hydride can be delivered to the iminium,
leading to the catalytic hydrogenation of tert-butyl-substituted
imines under mild reaction conditions (1 bar dihydrogen, room
temperature) via an FLP-type mechanism. These mild
conditions compare favorably with similar chemistry using
main group FLPs (for example 5 atm H2, 120 °C reported by
Stephan and co-workers11). For the hydrogenation of enamines
and imines a further intramolecular phosphine Lewis basic
fragment is required, with the mechanism involving heterolytic
H2 cleavage by the Zr/P FLP prior to transfer of the proton/
hydride to the substrate.
(9) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116,
11703−11714.
(10) Metters, O. J.; Forrest, S. J. K.; Sparkes, H. A.; Manners, I.; Wass,
D. F. J. Am. Chem. Soc. 2016, 138, 1994−2003.
(11) Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Commun. 2008,
1701−1703.
(12) Hoskin, A. J.; Stephan, D. W. Organometallics 2000, 19, 2621−
2624.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
Experimental and spectroscopic details (PDF)
D
Organometallics XXXX, XXX, XXX−XXX