Handbook of Functionalized Organometallics Applications in S

314
7 Polyfunctional Zinc Organometallics for Organic Synthesis
EtO2C(CH2) 3Cu(CN)ZnI + CO2Et EtO2C(CH2) 3Cu(CN)ZnI + CONH2 386
t-Bu
O
O H
OTMS
CO 2Me
+
EtO 2C
Zn
1) -60 ºC, 14 h
2) H 3O + ,-30ºC
TMSCl, rt, 18 h
THF
-30 ºC to0ºC, 2 h
CuBr·Me 2S
HMPA, Et 2O, THF
Scheme 7.101 Carbocupration of acetylenic carbonyl derivatives.
2
CO 2Et
TMS
CO2Et 385 :91%;100%Z
CO 2Et
t-Bu
O
CO 2Et
CO 2Et
384 :95%;100%E
CONH 2
387 :53%;100%E
O H
OTMS
388 :52%
O
CO2Me Functionalized allylic zinc reagents can also be generated in situ. The direct
cyclization of allenyl aldehydes such as 397 with diorganozincs in the presence of
Ni(cod) 2 (10±20 mol%) provides cyclic homoallylic alcohols such as 398 with good
diastereoselectivity [239]. The allylzincation of alkenylmagnesium reagents is a
very convenient synthesis of 1,1-bimetallic reagents of magnesium and zinc [240].
Thus, the addition of the ethoxy-substituted allylic zinc reagent (399) to the alkenylmagnesium
reagent 400 providing after the addition of Me 3SnCl (1 equiv) the
a-stannylated alkylzinc species 401 that is readily oxidized with O 2 leading to the
aldehyde 402 with an excellent transfer of the stereochemistry (Scheme 7.103)
[241].
The activation of primary zinc±copper reagents with Me 2Cu(CN)Li 2 allows to
carbocuprate weakly activated alkynes. The carbocupration of the alkynyl thioether
403 is leading to the E-alkene 404 with high stereoselectivity [242]. Although, primary
alkylzinc regents do not add to unactivated alkynes, the addition of the more
reactive secondary copper±zinc reagents affords the desired product 405 with high
E-stereoselectivity [242]. The intramolecular carbocupration proceeds also with
primary copper organometallics leading after allylation of the exo-alkenylidene to
cyclopentane derivative 406 in 60% yield (Scheme 7.104) [242]. The addition of
zinc malonates, such as 407 produces the carbometallation product 408 in 50%
yield (Scheme 7.104) [243].