Table
1
Hydrogenation
of
dehydroaminoesters
with
These results demonstrate the viability of a simple
P2Rh+OTfꢁcomplexes; all reactions went to completiona
2,20-diphosphinated biphenyl as the sole active ligand in an
asymmetric hydrogenation catalyst. The only previous
example involved BIPHEP and its sulfonated analogues
operating in an (L)-proline-derived ionic liquid solvent, giving
9 in up to 69% ee.18 With access to a range of BIPHEP
derivatives feasible now,19 these results extend the potential
for their practical application in asymmetric hydrogenation.
Support from the Schroedinger Foundation (MM), EPSRC
(CB, SG, EP/E036848/1), and UKIERI (TP) is gratefully
acknowledged. Johnson-Matthey kindly provided a loan of
rhodium salts. We thank Dr Amber Thompson and the CRL
X-Ray service for their input.
Entry
MeOH/mL Catalyst
Reactant Time/h
Ee (%) (S)
b
1
2
3
4
5
8a
8a
8a
8a
8a
8b
8a
8a
2.0
3
0.8
2.0
12
19
3.1
3.0
o1
20
27
84
85
17
6
5.0
5.0
6.0
5.0
2.5
1.5
5.0
4
15a
15b
15b
15b
15b
15c
5c
6d
7e
8f
a
80
Conditions: alkene (0.23 mmol), catalyst (0.0046 mmol), H2 1.8 atm,
b
RT, save run 5; assay by Chiralsil-Val GC. DPPBRh+ complex of
(S,S)-1. At 0 1C, 85% ee also by chiral shift analysis.16 d Half-scale;
ee was determined by HPLC with Chiralcel OD-H, 1 : 19 isopropanol :
c
e
hexane. 8c gave similar result. No equilibration of the catalyst
f
diastereomers before reaction. Ligand is 93% ee.
Notes and references
1 Original papers: (a) T. Hayashi, K. Ueyama, N. Tokunaga and K.
Yoshida, J. Am. Chem. Soc., 2003, 125, 11508–11509; (b) C.
Fischer, C. Defieber, T. Suzuki and E. M. Carreira, J. Am. Chem.
Soc., 2004, 126, 1628–1629.
2 Recent review: C. Defieber, H. Grutzmacher and E. M. Carreira,
Angew. Chem., Int. Ed., 2008, 47, 4482–4502.
3 J. W. Faller and J. C. Wilt, J. Organomet. Chem., 2006, 691,
2207–2212.
4 A. Miyashita, A. Yasuda, H. Takaya, K. Toriumi, T. Ito, T.
Souchi and R. Noyori, J. Am. Chem. Soc., 1980, 102, 7932–7934.
5 A. Uehara and J. C. Bailar, Jr., J. Organomet. Chem., 1982, 239,
1–10.
Scheme 4 Improved conditions for asymmetric hydrogenation with
catalyst 15b; as Table 1, entry 4 with 0.023 mmol (S,S)-1 added.
6 T. K. Miyamoto, Y. Matsuura, K. Okude, H. Ichida and Y.
Sasaki, J. Organomet. Chem., 1989, 373, C8–C12.
enantioselectivity (entry 1). Hydrogenation with the
a disappointing ee
7 K. Mikami, T. Korenaga, Y. Matsumoto, M. Ueki, M. Terada and
S. Matsukawa, Pure Appl. Chem., 2001, 73, 255–259.
8 (a) K. Mikami, K. Aikawa, Y. Yusa, J. J. Jodry and M.
Yamanaka, Synlett, 2002, 1561–1578; (b) P. J. Walsh, A. E. Lurain
and J. Balsells, Chem. Rev., 2003, 103, 3297–3344; (c) K. Mikami
and M. Yamanaka, Chem. Rev., 2003, 103, 3369–3400; (d) K.
Mikami and K. Aikawa, New Frontiers in Asymmetric Catalysis,
ed. K. Mikami and M. Lautens, John Wiley and Sons, New York,
2007, pp. 221–257.
9 Early papers: (a) K. Mikami, T. Korenaga, M. Terada, T.
Ohkuma, T. Pham and R. Noyori, Angew. Chem., Int. Ed., 1999,
´
38, 495–497; (b) J. J. Becker, P. S. White and M. R. Gagne, J. Am.
Chem. Soc., 2001, 123, 9478–9479; (c) O. Tissot, M. Gouygou, F.
Dallemer, J. C. Daran and G. G. A. Balavoine, Angew. Chem., Int.
Ed., 2001, 40, 1076–1078.
10 M. Mayr, C. J. R. Bataille, S. Gosiewska, J. A. Raskatov and J. M.
Brown, Tetrahedron: Asymmetry, 2008, 19, 1328–1332.
11 K. Mikami, S. Kataoka, Y. Yusa and K. Aikawa, Org. Lett., 2004,
6, 3699–3701.
12 For recent Rh–BINAP hydrogenations see: (a) J. M. Hopkins, S.
A. Dalrymple, M. Parvez and B. A. Keay, Org. Lett., 2005, 7,
3765–3768; (b) M. Alame, N. Pestre and C. de Bellefon, Adv.
Synth. Catal., 2008, 350, 898–908 and earlier papers.
13 O. Legrand, J. M. Brunel, T. Constantieux and G. Buono,
Chem.–Eur. J., 1998, 4, 1061–1067.
OMe-BIPHEP complex 15a gave
(entry 3). Much more promising results were obtained with
the OPri analogue 15b, improved only slightly at 0 1C and not
extended to the more slowly hydrogenating aromatic
dehydroamino ester (entries 4–6; see below for a more detailed
discussion). Starting from non-equilibrated catalyst 15b
(mixture of Rax and Sax-diastereomers) a low ee was obtained
(entry 7). The corresponding OBn complex 15c, which was
only 93% enantiomerically pure, also looked promising
(entry 8).
It was found that the enantioselectivity observed in hydro-
genation of 8 by catalyst 15b was strongly concentration
dependent. The ee in 0.5 mL MeOH was 46% (cf. Table 1,
entry 4); see ESI for details.wThis implied that high concentra-
tions of catalyst and substrate were detrimental. Higher
concentrations of reactant 8a, known to bind strongly to
rhodium during hydrogenation, could be responsible for
promoting partial or complete phosphine dissociation. Since
the ligand racemises easily this offers a possible explanation
for the unusual ee dependency.
14 T. D. Nelson and R. D. Crouch, Org. React., 2004, 63, 265–555.
15 E. Gorobets, B. M. M. Wheatley, J. M. Hopkins, R. McDonald
and B. A. Keay, Tetrahedron Lett., 2005, 46, 3843–3846.
16 T. Ema, D. Tanida and T. Sakai, J. Am. Chem. Soc., 2007, 129,
10591–10596.
Helpful insights came when the reaction of entry 4, Table 1
was repeated, but with in situ NMR monitoring. This demon-
strated that the original diene complex (S,S,Sax)-15b remained
largely intact on completion of hydrogenation! With this
evidence in hand, hydrogenation was now repeated in the
presence of excess (S,S)-1. A strong improvement in the ee of
the product was observed (Scheme 4). Hence free diene
provides an important additional function in regulating
the catalyst configuration and suppressing free ligand
racemisation,17 or the formation of a catalytically active but
less enantioselective by-product.
17 The barrrier to racemization for BIPHEP is 22 kcal molꢁ1
:
O. Desponds and M. Schlosser, Tetrahedron Lett., 1996, 37,
47–48; the barrier for the cationic BIPHEPRh+(CO)2 complex is
25 kcal molꢁ1 (ref. 3).
18 M. Schmitkamp, D. Chen, W. Leitner, J. Klankermayer and G.
Francio, Chem. Commun., 2007, 4012–4014.
19 (a) S. Doherty, J. G. Knight, C. H. Smyth, R. W. Harrington and
W. Clegg, Organometallics, 2007, 26, 6453–6461; (b) S. Doherty, J.
G. Knight, C. H. Smyth, R. W. Harrington and W. Clegg, Org.
Lett., 2007, 9, 4925–4928.
ꢀc
This journal is The Royal Society of Chemistry 2008
5094 | Chem. Commun., 2008, 5092–5094