Page 718 - Advanced Organic Chemistry Part B - Reactions & Synthesis

694 that esters and enones (Entry 13) are reactive to the catalytic processes involving
Grignard reagents. Entries 14 and 15 illustrate ketone syntheses from acyl chlorides
CHAPTER 8
and nitriles, respectively.
Reactions Involving
Transition Metals
8.1.2.5. Mixed Organocopper-Zinc Reagents. The preparation of organozinc reagents
is discussed in Section 7.3.1. Many of these reagents can be converted to mixed
66
copper-zinc organometallics that have useful synthetic applications. A virtue of these
reagents is that they can contain a number of functional groups that are not compatible
with the organolithium route to cuprate reagents. The mixed copper-zinc reagents are
not very basic and can be prepared and allowed to react in the presence of weakly
acidic functional groups that would protonate more basic organometallic reagents; for
example, reagents containing secondary amide or indole groups can be prepared. 67
They are good nucleophiles and are especially useful in conjugate addition. Mixed
68
zinc reagents can also be prepared by addition of CuCN to organozinc iodides. They
are analogous to the cyanocuprates prepared from alkyllithium and CuCN, but with
+
Zn 2+ in place of Li , and react with enones, nitroalkenes, and allylic halides. 69
In addition to the use of stoichiometric amounts of cuprate or cyanocuprate
reagents for conjugate addition, there are also procedures that require only a catalytic
70
amount of copper and use organozinc reagents as the stoichiometric reagent. Simple
organozinc reagents, such as diethylzinc, undergo conjugate addition with 0.5 mol %
CuO SCF in the presence of a phoshine or phosphite.
3 3
O O
CuO SCF , 0.5 mol %
3
3
+ (C H ) Zn
2 5 2
P(OC H ) , 1.0 mol %
2 5 3
H
C 2 5
100%
Ref. 71
In the presence of LiI, TMS-Cl, and a catalytic amount of CH CuCNLi , conjugate
3 2 2
addition of functionalized organozinc reagents occurs in good yield.
O O
LiI O
(CH ) SiCl
3 3
Zn(CH ) CPh O
+ CH 3 2 4
(CH ) Cu(CN)Li 2
3 2
) CPh
5 mol %, –78°C (CH 2 4
85%
Ref. 72
Either CuI or CuCN (10 mol %) in conjunction with BF and TMS-Cl catalyze addition
3
of alkylzinc bromides to enones.
66
P. Knochel and R. D. Singer, Chem. Rev., 93, 2117 (1993); P. Knochel, Synlett, 393 (1995).
67
H. P. Knoess, M. T. Furlong, M. J. Rozema, and P. Knochel, J. Org. Chem., 56, 5974 (1991).
68 P. Knochel, J. J. Almena Perea, and P. Jones, Tetrahedron, 54, 8275 (1998).
69 P. Knochel, M. C. P. Yeh, S. C. Berk, and J. Talbert, J. Org. Chem., 53, 2390 (1988); M. C. P. Yeh
and P. Knochel, Tetrahedron Lett., 29, 2395 (1988); S. C. Berk, P. Knochel, and M. C. P. Yeh, J. Org.
Chem., 53, 5789 (1988); H. G. Chou and P. Knochel, J. Org. Chem., 55, 4791 (1990).
70
B. H. Lipshutz, Acc. Chem. Res., 30, 277 (1997).
71 A. Alexakis, J. Vastra, and P. Mageney, Tetrahedron Lett., 38, 7745 (1997).
72
B. H. Lipshutz, M. R. Wood, and R. J. Tirado, J. Am. Chem. Soc., 117, 6126 (1995).