6862
C. Monti et al. / Tetrahedron Letters 45 (2004)6859–6862
LaboratoriesÕ (MerckÕs Academic Development Pro-
gram Award to C.G.) and Universit a` degli Studi di Mi-
lano for financial support and for a postdoctoral
fellowship to C.M. (Assegno di ricerca). U.P. thanks
the Dipartimento di Chimica Organica e Industriale
14. (a) Alexakis, A.; Croset, K. Org. Lett. 2002, 4, 4147–4149;
b) Tissot-Croset, K.; Polet, D.; Alexakis, A. Angew.
(
Chem., Int. Ed. 2004, 43, 2426–2428; (c) Equey, O.;
Alexakis, A. Tetrahedron: Asymmetry 2004, 15,
1
5. Horibe, H.; Kazuta, K.; Kotoku, M.; Kondo, K.; Okuno,
531–1536.
1
1
(
Universityof Milano) for the hospitality.
H.; Murakami, Y.; Aoyama, T. Synlett 2003, 13,
2
047–2051.
6. For a different approach to a dynamic ligand library, see:
Tepfenhart, D.; Moisan, L.; Dalko, P. I.; Cossy, J.
Tetrahedron Lett. 2004, 45, 1781–1783.
References and notes
1
. For a recent review, see: Tang, W.; Zhang, X. Chem. Rev.
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. Pe n˜ a, D.; Minnaard, A. J.; de Vries, A. H. M.; de Vries, J.
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17. The use of chiral monodentate phosphites containing the
biphenol unit had recentlybeen described byXiao and
Chen in the rhodium catalysed hydrogenation of dimethyl
itaconate, albeit with moderate enantiomeric excess (ee
2
2
1
8
max. 75%). The use of a limited number of combinations
of different phosphites was also surveyed, but the results
were shown to be inferior to the corresponding homo-
combinations. However, these results are shadowed by
experimental problems associated with the purityof the
ligands. In fact, in our hands this class of compounds
3
. (a) Reetz, M. T.; Mehler, G. Angew. Chem., Int. Ed. 2000,
39, 3889–3890; (b) Claver, C.; Fernandez, E.; Gillon, A.;
Heslop, K.; Hyett, D. J.; Martorell, A.; Orpen, A. G.;
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M. T.; Sell, T. Tetrahedron Lett. 2000, 41, 6333–6336; (d)
Pe n˜ a, D.; Minnaard, A. J.; de Vries, J. G.; Feringa, B. L.
J. Am. Chem. Soc. 2002, 124, 14552–14553; (e) van den
Berg, M.; Minnaard, A. J.; Haak, R. M.; Leeman, M.;
Schudde, E. P.; Meetsma, A.; Feringa, B. L.; de Vries, A.
H. M.; Maljaars, C. E. P.; Willans, C. E.; Hyett, D.;
Boogers, J. A. F.; Henderickx, H. J. W.; de Vries, J. G.
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Zheng, Z.; Luo, H.; Bai, C.; Hu, X.; Chen, H. Org. Lett.
1
shows a single set of signals at room temperature by H,
1
3
31
C and P NMR, confirming their tropos nature. Only
upon complexation with Rh, two sets of signals can
1
9
possiblybe observed at low temperature ( ꢀ65°C). This
contrasts sharplywith the information contained in the
Xiao and Chen paper, where the ligands were described to
3
1
displaytwo singlets ( P NMR) with the same intensity
1
8
(1:1 ratio) at room temperature (without Rh). If the
Xiao and Chen compounds (phosphites) were not pure,
their hydrogenation results may be unreliable.
2003, 5, 4137–4139; (g) Hua, Z.; Vassar, V. C.; Ojima, I.
Org. Lett. 2003, 5, 3831–3834; (h) Huang, H.; Zheng, Z.;
Luo, H.; Bai, C.; Hu, X.; Chen, H. J. Org. Chem. 2004, 69,
18. Chen, W.; Xiao, J. Tetrahedron Lett. 2001, 42, 8737–8740.
19. Suarez, A.; Pizzano, A.; Fernandez, I.; Khiar, N. Tetra-
hedron: Asymmetry 2001, 12, 633–642.
2355–2361; (i) Hoen, R.; van den Berg, M.; Bernsmann,
H.; Minnaard, A. J.; de Vries, J. G.; Feringa, B. L. Org.
Lett. 2004, 6, 1433–1436.
20. Phosphites [P(O)
commerciallyavailable chiral alcohol with PCl
3
2
O 1–8] were prepared byreaction of the
4
. (a) Reetz, M. T.; Sell, T.; Meiswinkel, A.; Mehler, G.
Patent application DE-A 10247633.0 (11-10-2002); (b)
Reetz, M. T.; Sell, T.; Meiswinkel, A.; Mehler, G. Angew.
Chem., Int. Ed. 2003, 42, 790–793; (c) Reetz, M. T.;
Mehler, G. Tetrahedron Lett. 2003, 44, 4593–4596.
. Pe n˜ a, D.; Minnaard, A. J.; Boogers, J. A. F.; de Vries, A.
H. M.; de Vries, J. G.; Feringa, B. L. Org. Biomol. Chem.
(THF,
room temperature), followed byaddition of a mixture of
2
1
the appropriate biphenol and triethylamine in THF.
0
0
0
2,2 -Dihydroxy-3,3 ,5,5 -tetra-tert-butylbiphenyl was syn-
thesised byoxidation of 2,4-di- tert-butylphenol with
2
2
5
6
potassium manganate. Phosphoramidites [P(O)
16] were synthesised by treatment of the appropriate chiral
secondaryamine with PCl in the presence of triethylam-
2
N 9–
2003, 1, 1087–1089.
3
. See also: (a) Yuan, Y.; Zhang, X.; Ding, K. Angew. Chem.
Int. Ed. 2003, 42, 5478–5480; (b) Ding, K.; Du, H.; Yuan,
Y.; Long, J. Chem. Eur. J. 2004, 10, 2872–2884, and
references cited therein.
ine (toluene, 70°C, 6h), followed bya solution of biphenol
23
in toluene at ꢀ78°C. Bis-[(S)-1-naphthalen-1-yl-ethyl]-
amine and bis-[(R)-1-naphthalen-1-yl-ethyl]-amine were
2
4
prepared via a reported one-pot procedure. (2S,S)-2,5-
Diphenylpyrrolidine and (2R,R)-2,5-diphenylpyrrolidine
7
. The hetero-complex homo-complex ratios usuallyexceed
the statistical value, see: Duursma, A.; Hoen, R.; Schup-
pan, J.; Hulst, R.; Minnaard, A. J.; Feringa, B. L. Org.
Lett. 2003, 5, 3111–3113.
2
5
were prepared as reported. Generally, the ligands were
purified via flash-chromatography, to give the products as
3
1
P
white foamysolids. All the ligands showed a singlet by
NMR (CDCl ): 1-P(O) O d = 152.8; 2-P(O) O d = 152.8;
3-P(O) d = 147.7; 4-P(O) d = 151.5; 5-P(O)
d = 145.4; 6-P(O) O d = 153.4; 7-P(O) O d = 146.7; 8-
8
. For a recent review on combinatorial libraries of chiral
ligands for enantioselective catalysis, see: Gennari, C.;
Piarulli, U. Chem. Rev. 2003, 103, 3071–3100.
. For an account on atropos/tropos ligands, see: Mikami,
K.; Aikawa, K.; Yusa, Y.; Jodry, J. J.; Yamanaka, M.
Synlett 2002, 1561–1578.
3
2
2
2
O
2
O
2
O
2
2
9
P(O) O d = 147.0; 9-P(O) N d = 147.6; 10-P(O) N
2
2
2
d = 147.6; 11-P(O)
2
N d = 149.6; 12-P(O)
2
N d = 149.6; 13-
P(O)
d = 149.2; 16-P(O)
2
2
N
d = 150.1; 14-P(O)
2
N
2
d = 150.1; 15-P(O) N
1
0. (a) Alexakis, A.; Rosset, S.; Allamand, J.; March, S.;
Guillen, F.; Benhaim, C. Synlett 2001, 9, 1375–1378; (b)
Alexakis, A.; Benhaim, C.; Rosset, S.; Humam, M. J. Am.
Chem. Soc. 2002, 124, 5262–5263.
1. Dieguez, M.; Ruiz, A.; Claver, C. Tetrahedron: Asymme-
try 2001, 12, 2895–2900.
2. Reetz, M. T.; Neugebauer, V. Angew. Chem., Int. Ed.
N d = 149.2. All the ligands gave
1
13
consistent H and C NMR spectral data, which are
available from the corresponding authors upon request.
21. Hua, Z.; Vassor, V. C.; Ojima, J. Org. Lett. 2003, 5,
3831–3834.
1
1
1
22. Barton, D. H. R.; Choi, S.; Hu, B.; Smith, J. A.
Tetrahedron 1998, 54, 3367–3378.
1
999, 38, 179–181.
23. Rimkies, A.; Sewald, N. Org. Lett. 2003, 5, 79–80.
24. Alexakis, A.; Gille, S.; Prian, F.; Rosset, S.; Ditrich, K.
Tetrahedron Lett. 2004, 45, 1449–1451.
25. Aldous, D. J.; Dutton, W. M.; Steel, P. G. Tetrahedron:
Asymmetry 2000, 11, 2455–2462.
3. Buisman, G. J. H.; van der Veen, L. A.; Klootwijk, A.; de
Lange, W. G. J.; Kamer, P. C. J.; van Leeuwen, P. W. N.
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references cited therein.