Handbook of Functionalized Organometallics Applications in S

7.3 Reactions of Organozinc Reagents
duces the highly Lewis-acidic species RZnOCOCF 3 (226). This reaction proceeds
smoothly in CH 2Cl 2 and furnishes the a-C-glycoside 227 in 58% yield.
Hex
AcO
AcO
BnO
BnO
O O
O
t-Bu
OAc
O
OBn
225
O
OAc
Et 2Zn
TMSOTf
-78 ºC, 2 h
Hex
O O
Et
t-Bu
223 :100%
Cl(CH2) 4ZnI
AcO
O
TMSOTf
. or BF3 OEt2
AcO
+
Cl
226
224 :63%;α/β =9:1
ZnOCOCF 3
CH 2Cl 2
0-20 ºC, 5 h
Scheme 7.66 Reaction of zinc reagents with acetals and related compounds.
Cl
BnO
BnO
O
OH
OBn
227 :58%
Although alkylzinc derivatives add only slowly to aldehydes, alkenylzinc derivatives
display a higher reactivity. Thus, the addition of vinylzinc chloride 228 to the
amino-aldehyde 229 provides the allylic alcohol 230 in 60% yield [156]. The addition
rate can be increased by performing the reaction in the presence of a Lewis
acid. Thus, the addition of the homoenolate 216 to the amino-aldehyde 231 provides
the aminoalcohol 232 with good diastereoselectivity (Scheme 7.67) [157].
Reactive benzylic or related zinc reagents, such as 233 smoothly add to aldehydes
providing the allylic alcohol 234 in almost quantitative yield (Scheme 7.67) [158].
In a noncomplexing solvent, such as dichloromethane, functionalized alkylzinc
halides add to a-functionalized aldehydes leading to the addition product 235
again with a remarkable diastereoselectivity (Scheme 7.67) [158].
Benzylic acetates are unreactive toward organozinc compounds. However, various
ferrocenyl acetates, such as 236 react with alkylzinc halides in the presence
of BF 3´OEt 2 with retention of configuration leading to the chiral ferrocenyl derivatives
like 237 (Scheme 7.68) [159].
The reactivity of zinc organometallics can be dramatically increased by adding
polar solvents like N-methylpyrrolidinone (NMP). Under these conditions various
diorganozincs add to a range of Michael acceptors like a,b-unsaturated ketones
aldehydes, nitriles or nitro derivatives (Scheme 7.69) [160]. The preparation of
mixed diorganozincs bearing nontransferable Me 3SiCH 2-groups allows a more efficient
transfer of the functionalized group to the Michael acceptor (Scheme 7.70) [98].
289
Cl