Page 676 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Allylic zinc reagents can be prepared in situ in aqueous solution in the presence of 651
aldehydes. 129 These reactions show a strong preference for formation of the more
branched product. This suggests that the reactions occur by coordination of the zinc SECTION 7.3
reagent at the carbonyl oxygen and that addition proceeds by a cyclic mechanism, Organometallic
Compounds of Group
similar to that for allylic Grignard reagents. The kinetic isotope of the reaction measured IIB and IIIB Metals
under these conditions is consistent with a cyclic mechanism. 130

OH
Zn dust
O CH 3 Br
+
H O-THF CH 3
2
90%
OH
CH 3 Cl Zn dust
(CH ) CHCH O + (CH 3 ) 2 CH CH 2
3 2
CH 3 H 2 O, NH Cl CH 3 CH 3
4
95%

An attractive feature of organozinc reagents is that many functional groups that
would interfere with organomagnesium or organolithium reagents can be present
in organozinc reagents. 131 132 Functionalized reagents can be prepared by halogen-
metal exchange reactions with diethylzinc. 133 The reaction equilibrium is driven to
completion by use of excess diethylzinc and removal of the ethyl iodide by distillation.
The pure organozinc reagent can be obtained by removal of the excess diethylzinc
under vacuum.

2 X(CH ) I + (C H ) Zn X(CH ) ZnC H [X(CH ) ] Zn + 2 C H I
2 5 2
2 n
2 5
2 n 2
2 n
2 5
n 2–5 X CH CO , (CH ) CCO , N C, Cl
2
3
3 3
2
These reactions are subject to catalysis by certain transition metal ions and with small
amounts of MnBr or CuCl the reaction proceeds satisfactorily with alkyl bromides. 134
2
5% MnBr
X(CH ) Br + (C H ) Zn 2 X(CH ) ZnBr
2 n
2 n
2 5 2
3% CuCl
n 3, 4; X C H O C, N C, Cl
2
2 5
Another effective catalyst is Ni acac . 135
2
129 C. Petrier and J.-L. Luche, J. Org. Chem., 50, 910 (1985).
130
J. J. Gajewski, W. Bocain, N. L. Brichford, and J. L. Henderson, J. Org. Chem., 67, 4236 (2002).
131 P. Knochel, J. J. A. Perea, and P. Jones, Tetrahedron, 54, 8275 (1998).
132
P. Knochel and R. D. Singer, Chem. Rev., 93, 2117 (1993); A. Boudier, L. O. Bromm, M. Lotz, and
P. Knochel, Angew. Chem. Int. Ed. Engl., 39, 4415 (2000); P. Knochel, N. Millot, A. L. Rodriguez, and
C. E. Tucker, Org. React., 58, 417 (2001).
133 M. J. Rozema, A. R. Sidduri, and P. Knochel, J. Org. Chem., 57, 1956 (1992).
134 I. Klemment, P. Knochel, K. Chau, and G. Cahiez, Tetrahedron Lett., 35, 1177 (1994).
135
S. Vettel, A. Vaupel, and P. Knochel, J. Org. Chem., 61, 7473 (1996).