4
Tetrahedron
Table 4. Palladium-catalyzed reaction of phenylboronic acid to aromatic
Conclusion
aldehydes
In conclusion, a series of new chiral benzimidazolium salts
have been prepared and the single-crystal X-ray diffraction result
further confirmed the molecular structure of NHC−Rh complex
8. Their applicability in the asymmetric additions of arylboronic
acids to aromatic aldehydes has been demonstrated, and the
corresponding diarylmethanols were obtained with high yields
and moderate enantiomeric excesses (up to 56%). It is worth
mentioning that the palladium complex catalyst was also active
in this reaction by addition of a catalytic amount of chloroform to
the reaction mixture. Further work is in progress to utilize these
benzimidazolium salts in asymmetric Rh-catalyzed 1,2-addition
reactions of arylboronic acids with ketones.
Entrya
Ar1
Solvent
additive
−
Yield (%)b
−
48 7ba
−
−
ee (%)c
1
2
3
4
5
6
7
8
9
1-Naphthyl 5b Toluene
1-Naphthyl 5b Toluene
1-Naphthyl 5b DME
1-Naphthyl 5b CH2Cl2
1-Naphthyl 5b THF
1-Naphthyl 5b Xylene
2-Naphthyl 5a Toluene
2-MeOPh 5c
4-CF3Ph 5d
−
5
−
−
−
−
4
7
6
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
−
trace
37 7aa
39 7ca
34 7da
Toluene
Toluene
Acknowledgments
a Reaction condition: Pd(OAc)2 (3 mol %), ligand (3 mol %), NaOtBu (2
equiv), arylboronic acids (2 equiv), N2, 100 oC, 24 h.
b Isolated yields.
We are grateful to the National Natural Science Foundation of
China (81302668) and Hangzhou Science and Technology
Information Institute of China (20150633B45).
c Determined by chiral HPLC (CHIRALCEL OD Column) analysis.
The X-ray crystal structure of NHC−Rh. We attempted to
prepare rhodium(I) complex of 1e using a mild transmetalation
method developed by Wang and Lin.12 Reaction of
benzimidazolium precursors with Ag2O in CH2Cl2 at room
temperature in the darkness gave rather unstable silver
complexes, observed by NMR spectroscopy, which decomposed
too quickly to be isolated. Direct addition of [Rh(COD)Cl]2 to a
freshly prepared solution of silver complexes yielded upon
workup the corresponding chiral complex 8, which were purified
by chromatography on silica gel (Scheme 3). The complex 8 is
air stable in solid state and X-ray-quality crystals of 8 was easily
grown by layering a CH2Cl2 solution of 8 with hexane. The
structure of the new chiral NHC−Rh complex 8 was confirmed
by X-ray diffraction (Fig. 1).
Supplementary Material
Supplementary data associated with this article can be found,
in the online version, at http://
References and notes
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Figure 1. ORTEP diagram of 8. Selected bond lengths (Å) and angles (o):
Rh(1)-C(8) 2.003(14), Rh(1)-C(2) 2.098(12), Rh(1)-C(3) 2.113(11),
Rh(1)-C(9) 2.201(11), Rh(1)-C(1) 2.234(15), Rh(1)-Cl(1) 2.394(3);
C(8)-Rh(1)-C(2) 94.1(5), C(8)-Rh(1)-C(3) 89.9(5), C(2)-Rh(1)-C(3)
38.7(6), C(2)-Rh(1)-C(9) 82.1(5), C(3)-Rh(1)-C(9) 98.6(5), C(2)-Rh(1)-
C(1) 89.2(6), C(3)-Rh(1)-C(1) 81.2(6), C(9)-Rh(1)-C(1) 38.2(5), C(8)-
Rh(1)-Cl(1) 90.0(4), C(2)-Rh(1)-Cl(1) 162.1(4), C(3)-Rh(1)-Cl(1)
158.9(4), C(9)-Rh(1)-Cl(1) 88.2(4), C(1)-Rh(1)-Cl(1) 92.4(5).