Page 727 - Advanced Organic Chemistry Part B - Reactions & Synthesis

chiral ligands have been investigated to determine the degree of enantioselectivity 703
that can be achieved. The combination of diethylzinc and cyclohexenone has been
studied extensively, and several amide and phosphine ligands have been explored. SECTION 8.1
Enantioselectivity can also be observed using Grignard reagents with catalytic amounts Organocopper
Intermediates
of copper. Scheme 8.7 shows some examples of these reactions using various chiral
ligands.
CH 3
L = Ph N(CH ) Ph
N 3 2 L = CH 3 N
N –
O – CH 3

Ref. 87 Ref. 88

CH 3 Ph

N CH PPh 2 L = N O
2
L = (CH ) CH P
3 2
(CH ) N O
3 2
)
N(CH 3 2
Ref. 89 Ref. 90

Enantioselective catalysis of S 2 alkylation has been achieved. 91 A BINOL-
N
phosphoramidite catalyst (o-methoxyphenyl analog) similar to that in Entry 3 in
Scheme 8.7 gave good results.
S
CO 2 Cu C H
2 5
83% yield
H )Zn
+ (C 2 5
Ph Br BINOL– Ph 96:4
2':S 2
phosphoramidite S N N
catalyst 91% e.e.
8.1.2.9. Aryl-Aryl Coupling Using Organocopper Reagents. Organocopper interme-
diates are also involved in several procedures for coupling of two aromatic reactants
to form a new carbon-carbon bond. A classic example of this type of reaction is
the Ullman coupling of aryl halides, which is done by heating an aryl halide with a
copper-bronze alloy. 92 Good yields by this method are limited to halides with EWG
substituents. 93 Mechanistic studies have established the involvement of arylcopper

87 E. J. Corey, R. Naef, and F. J. Hannon, J. Am. Chem. Soc., 108, 7144 (1986).
88
N. M. Swingle, K. V. Reddy, and B. L. Rossiter, Tetrahedron, 50, 4455 (1994); G. Miao and
B. E. Rossiter, J. Org. Chem., 60, 8424 (1995).
89 M. Kanai and K. Tomioka, Tetrahedron Lett., 35, 895 (1994); 36, 4273, 4275 (1995).
90
A. Alexakis, J. Frutos, and P. Mageney, Tetrahedron: Asymmetry, 4, 2427 (1993).
91
K. Tissot-Croset, D. Polet, and A. Alexakis, Angew. Chem. Int. Ed. Engl., 43, 2426 (2004).
92 P. E. Fanta, Chem. Rev., 64, 613 (1964); P. E. Fanta, Synthesis, 9 (1974).
93
R. C. Fuson and E. A. Cleveland, Org. Synth., III, 339 (1955).