the most successful system involves the use of a CuCl-
chiral diamine based catalyst for aerobic oxidative couplings
of 3-carboalkoxy-2-naphthols.10
Oxovanadium(IV) complexes have been commonly uti-
lized as precatalysts for olefin epoxidation and sulfide
oxidation when combined with O2 or peroxide co-oxidants.11
By taking advantage of their electrophilic nature toward
oxygen functionality,12 we have recently unraveled the
catalytic activity of 2,2′-biphenol-based vanadyl complexes
toward Mukaiyama aldol additions.13 A diastereoselectivity
of up to 90/10 (anti/syn) was achieved in the preliminary
study. As part of our continuing search for asymmetric
variants of oxovanadium(IV) complexes along with the recent
finding of Uang on the use of V(O)(acac)2 in the aerobic
oxidative coupling of 2-naphthols,14 we focussed on develop-
ing a new type of tridentate chiral vanadyl complex specif-
ically active for both the aldol and the coupling processes.
We herein describe our preliminary findings for the first
successful application of these compounds in catalytic
asymmetric oxidative couplings of various 2-naphthol de-
rivatives.
Figure 1. Nine different vanadyl complexes derived from salicy-
laldehydes or hydroxynaphthaldehydes and R-amino acids
formation in view of its greater chance to induce the highest
level of asymmetric control under various conditions.20
The coupling reaction of 2-naphthol catalyzed by 10 mol
% of vanadyl complexes 1-9 was selected as a test system.
The model reactions were all carried out in CCl4 at ambient
temperature under an oxygen atmosphere, and the results are
compiled in Table 1.21 For all catalysts except 6d (entry 6),
Preliminary searches of chiral vanadyl complexes from a
wide variety of chiral templates were futile in terms of their
reactivity and turnover efficiency. These include BINAP,2
Evan’s and Nishiyama’s bis-oxazolines,15 O-acetyl mandelic
acid, N-tosyl R-amino acids,16 and several tridentate (Bolm’s11c)
or tetradentate (Jacobsen’s17) Schiff bases. We have subse-
quently discovered that tridentate Schiff bases derived from
functionalized salicylaldehydes and R-amino acids exhibit
unique catalytic attributes toward aerobic oxidative couplings
of 2-naphthol when combined with suitable vanadyl salts.18
Mass analyses of these complexes by the FAB technique
suggest that they are mainly composed of tetradentate
monomers and pentddentate dimers.19
Table 1. Effects of Catalyst Templates on the Catalytic
Asymmetric Couplings of 2-Naphthol
entry
catalyst
time (days)
yield, %
ee, %a
Seven different salicylaldehydes with varying steric and/
or electronic demands at the C3 and/or C5 positions and two
hydroxy-substituted naphthaldehydes were examined to gain
insights into their stereoelectronic influences on the reactivity
and enantioselectivity of the coupling process, Figure 1.
Valine was chosen as the test R-amino acid for the complex
1
2
3
4
5
6
7
8
9
1d
2d
3d
4d
5d
6d
7d
8d
9d
3
3
12
12
10
7
8
6
7
94
83
78
78
92
42
100
94
25
31
22
22
30
5
26
62
42
(10) (a) Nakajima, M.; Miyoshi, I.; Kanayama, K.; Hashimoto, S.-I.; Noji,
M.; Koga, K. J. Org. Chem. 1999, 64, 2264 and references therein. (b) A
catalytic cross-coupling protocol with a CuCl2-sparteine-AgCl based
system was first demonstrated by Smrcˇina and Kocˇovsky´ (ref 7b).
(11) (a) Nakajima, K.; Kojimo, M.; Fujita, J. Chem. Lett. 1986, 1483.
(b) Takai, T.; Yamada, T.; Mukaiyama, T. Chem. Lett. 1990, 1657. (c)
Bolm, C.; Bienewald, F. Angew. Chem., Int. Ed. Engl. 1995, 34, 2640.
(12) (a) Dichmann, K.; Hamer, G.; Nyburg, S. C.; Reynold, S. W. F. J.
Chem. Soc. D 1970, 1295. (b) For DMSO complexes, see: Agarwal, K.;
Singh, G. J. Indian Chem. Soc. 1986, 63, 926.
(13) Chen, C.-T.; Hon, S.-W.; Weng, S.-S. Synlett 1999, 816.
(14) Hwang, D. R.; Chen, C. P.; Uang, B. J. J. Chem. Soc., Chem.
Commun. 1999, 1207.
(15) (a) Evans, D. A.; Burgey, C. S.; Kozlowski, M. C.; Tregay, S. W.
J. Am. Chem. Soc. 1999, 121, 686 and references therein. (b) Nishiyama,
H.; Kondo, M.; Nakamura, T.; Itoh, K. Organometallics 1991, 10, 500.
(16) (a) Kiyooka, S.-I.; Kaneko, Y.; Komura, M.; Matsuo, H.; Nakano,
M. J. Org. Chem. 1991, 56, 2276. (b) Parmee, E. R.; Tempkin, O.;
Masamune, S. J. Am. Chem. Soc. 1991, 113, 9365. (c) Corey, E. J.; Cywin,
C. L.; Roper, T. D. Tetrahedron Lett. 1992, 33, 6907.
86
a Determined by HPLC analysis on a Chiralcel AD column.
the coupling reactions went to completion in 3-12 days,
providing scalemic (R)-BINOL in 74-100% yields.19 For
the 3-tert-butyl-substituted catalysts 1d-3d, appending an
electron-withdrawing substituent at C5 (e.g., R2 ) NO2)
reduces both coupling efficiency and enantioselectivity
(compare entries 1-3). For the 5-methyl- and 5-tert-butyl-
substituted catalysts 1d, 4d, and 5d, increasing the steric bulk
at C3 (e.g., R1 ) adamantyl) slows down the coupling
(17) (a) Annis, D. A.; Jacobsen, E. N. J. Am. Chem. Soc. 1999, 121,
4147. (b) Zhang, W.; Jacobsen, E. N. J. Org. Chem. 1991, 56, 2296.
(18) Theriot, L. J.; Carlisle, G. O.; Hu, H. J. J. Inorg. Nul. Chem. 1969,
31. 2841.
(20) Cogan, D. A.; Liu, G.; Kim, K.; Backes, B. J.; Ellman, J. A. J. Am.
Chem. Soc. 1998, 120, 8011.
(21) Among the six different solvent classes (chloroalkanes, ethers,
nitroalkanes, nitriles, alcohols, and arenes) examined, the best asymmetric
inductions were observed for coupling reactions conducted in CCl4.
(19) See the Supporting Information.
870
Org. Lett., Vol. 3, No. 6, 2001