Deboronation and acylation (no glovebox needed) then
provided the first member of a new class of ligands,
HPN-6. Ligand HPN-7 was prepared analogously.
Scheme 4. Hydrophosphinations of Alkynylimidazolines
Figure 3. Asymmetric hydrogenations.
in good agreement with related platinum complexes.7a,c,8
Reaction of the same intermediate with the spin 1/2
nucleus 103Rh gave complex [15N]-17b. The CCS values
for this complex were ꢀ30 and þ85 ppm (15N, 31P). Both
nuclei again showed coupling to the metal, with 1J values
of 11 and 169 Hz for 15N and 31P, consistent with known
rhodium complexes.9 The combination of pronounced
CCS values and significant scalar couplings establishes a
bidentate chelation mode of these new PꢀN ligands.
HPN-6 and HPN-7 bear a resemblance to BIPI ligands
which have been used for highly enantioselective hydro-
genations,10 with the important exception of their central
alkene moiety. It was unclear whether the presence of the
olefin would prove detrimental to catalysis. The cationic
iridium BArF complexes of each ligand were prepared, in
concert with the pioneering work of Pfaltz et al. on iridium
complexes.11 Asymmetric hydrogenations (AH) of several
different olefin substrates demonstrated the competency
of these ligands in catalytic transformations. Reduction of
R-methylstilbene with the Ir complex of HPN-6 under just
1 bar of H2 gaveadduct 18in a promising 70% ee, while the
HPN-7 Ir complex furnished the target (100%) with a high
selectivity of 95% ee under the same conditions (Figure 3).
Figure 2. [15N]-Labeled metal complexes.
For applications of the new PꢀN species to catalysis, it
was necessary to establish whether they could achieve a
bidentate chelation mode to transition metals. The syn-
thesis of aminophosphine borane E-14 was thus repeated
(Scheme 3) with [15N]-aniline to give the isotopically
labeled species [15N]-E-14. This compound was then de-
boronated and the free aminophosphine exposed to 1 equiv
of 195Pt, a spin 1/2 nucleus, to give the metal complex [15N]-
ꢀ
(8) (a) Pazderski, L.; Tousek, J.; Sitkowski, J.; Kozerski, L.; Szzyk, E.
17a (Figure 2). The complex was analyzed by 15N and 31
NMR. The coordination chemical shifts (CCS: δ complex
P
ꢀ
Magn. Reson. Chem. 2007, 45, 1059–1071. (b) Schenetti, L.; Mucci, A.;
Longato, B. J. Chem. Soc., Dalton Trans. 1996, 3, 299–303. (c) Schenetti,
L.; Bandoli, G.; Dolmella, A.; Trovo, G.; Longato, B. Inorg. Chem.
1994, 33, 3169–3176.
δ free ligand) of 15N and 31P wereꢀ41and þ74 ppm, respec-
tively. The shielding of 15N and deshielding of 31P are well-
precedented phenomena in metal complexes.7 Both 15N
and 31P of complex 17a showed coupling to platinum, with
1J values of 298 and 3791 Hz, respectively. These values are
ꢀ
(9) Pazderski, L.; Tousek, J.; Sitkowski, J.; Kozerski, L.; Marek, R.;
Szzyk, E. Magn. Reson. Chem. 2007, 45, 24–36.
ꢀ
(10) (a) Busacca, C. A.; Qu. B.; Gret, N.; Fandrick, K. R.; Saha, A. K.;
Marsini, M.; Reeves, D.; Haddad, N.; Eriksson, M.; Wu, J.-P.; Grinberg,
N.; Lee, H.; Li, Z.; Lu, B.; Chen, D.; Hong, Y.; Senanayake, C. H. Adv. Syn.
Catal. 2013, 355, in press. (b) Busacca, C. A.; Lorenz, J. C.; Saha, A. K.;
Cheekoori, S.; Haddad, N.; Reeves, D.; Lee, H.; Li, Z.; Rodriguez, S.;
Senanayake, C. H. Cat. Sci. Technol. 2012, 2, 2083–2089. (c) Busacca, C. A.;
Lorenz, J. C. Electronically tuned ligands for asymmetric hydrogenation.
U.S. Pat. 7,994,335, 2011. (d) Lorenz, J. C.; Busacca, C. A.; Feng, X.; Grinberg,
N.; Haddad, N.; Johnson, J.; Kapadia, S.; Lee, H.; Saha, A.; Sarvestani, M.;
Spinelli, E. M.; Varsolona, R.; Wei, X.; Zeng, X.; Senanayake, C. H. J. Org.
Chem. 2010, 75, 1155–1161. (e) Busacca, C. A.; Lorenz, J. C.; Grinberg, N.;
Haddad, N.; Lee, H.; Li, Z.; Liang, M.; Reeves, D.; Saha, A.; Varsolona,
R.; Senanayake, C. H. Org. Lett. 2008, 10, 341–344.
(7) Nitrogen: (a) Pazderski, L.; Szyzk, E.; Sitkowski, J.; Kamienski, B.;
ꢀ
Kozerski, L.; Tousek, J.; Marek, R. Magn. Reson. Chem. 2006, 44, 163–170.
ꢀ
(b) Pazderski, L.; Tousek, J.; Sitkowski, J.; Kozerski, L.; Szzyk, E. Magn.
Reson. Chem. 2007, 45, 1059–1071. (c) Busacca, C. A.; Grossbach, D.;
Campbell, S. J.; Dong, Y.; Eriksson, M. C.; Harris, R. E..; et al. J. Org.
Chem. 2004, 69, 5187–5195. (d) Schenetti, L.; Mucci, A.; Longato, B.
J. Chem. Soc., Dalton Trans. 1996, 3, 299–303. (e) Schenetti, L.; Bandoli,
G.; Dolmella, A.; Trovo, G.; Longato, B. Inorg. Chem. 1994, 33, 3169–3176.
Phosphorus: (f) 31P and 13C NMR of Transition Metal Phosphine Complexes;
Pregosin, P. S., Kunz, R. W., Eds.; Springer-Verlag: Berlin, 1979. (g) Power, W. P.;
Wasylishen, R. E. Inorg. Chem. 1992, 31, 2176–2183.
(11) For a review, see: Pfaltz, A.; Blankenstein, J.; Hilgraf, R.;
Hormann, E.; McIntyre, S.; Menges, F.; Schlonleber, M.; Smidt, S. P.;
Wustenberg, B.; Zimmermann, N. Adv. Syn. Catal. 2003, 345, 33–44.
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