Handbook of Functionalized Organometallics Applications in S

Cu(CN)ZnI
Cu(CN)ZnI
EtO 2C(CH 2) 3Cu(CN)ZnI
NO 2
CO 2Me
+
+
MeS
+
Ph
SMe
380
NO 2
NO 2
EtO 2C(CH 2) 3Cu(CN)ZnI
7.3 Reactions of Organozinc Reagents
THF
-30 ºC, 0.5 h
1) THF, 0 ºC
2) PhSeBr, 0 ºC
3) H2O2,rt 1) 0 ºC, 1 h
2) O3, CH2Cl2 -78 ºC, 3 h
3) Me2S, -78 ºC
Ph
381 :85%
NO 2
O
NO 2
382 :64%
CO 2Et
CO 2Me
CO2Et 383 :87%
Scheme 7.100 Reaction of functionalized zinc±copper reagents with polyfunctional nitroolefins.
carbocupration of ethyl propiolate at ±60 C to ±50 C produces the syn-addition
product 384 after protonation at low temperature. Interestingly, if the reaction
mixture is warmed up in the presence of an excess of TMSCl, an equilibration
occurs and the C-silylated unsaturated product 385 is obtained [48d]. The presence
of acidic hydrogens of the amid group does not interfere with the carbocupration
reaction. Thus, the unsaturated amide 386 affords, after addition, the E-unsaturated
amide 387 in 53% as 10% E-isomer (Scheme 7.101) [11].
A formal [3+2]-cycloaddition can be accomplished by adding bis-(2-carbethoxyethyl)
zinc to acetylenic esters [232c]. This reaction allows the construction of complex
cyclopentenones, such as 388 which is a precursor of (±)-bilobalide (Scheme
7.101) [232c]. The allylzincation of trimethylsilylacetylenes can be performed
intramolecularly providing a functionalized alkenylzinc that cyclizes in the presence
of Pd(PPh 3) 4 at 25 C within 3.5 h leading to the bicyclic product 389 in 84%
yield (Scheme 7.102) [233]. The addition of allylic zinc halides to various alkynes
occurs in the absence of copper salts. The related addition to 1-trimethylsilyl
alkynes [234], unsaturated acetals [235] and cyclopropenes [236] occurs readily.
Functionalized allylic zinc reagents can be prepared by the carbozincation of a dialkoxymethylenecyclopropane
390 with dialkylzincs. Thus, the reaction of 390 with
Et 2Zn provides the allylic zinc reagent 391. After the addition of an electrophile,
the desired adduct 392 is obtained in good yield. [237].
Allylic zinc reagents are highly reactive reagents that are prone to undergo carbozincation
of weakly activated alkenes. [143]. Thus, the addition of the mixed
methallyl(butyl)zinc 393 with the vinylic boronate 394 provides the intermediate
zinc reagent 395. After the addition of an extra equivalent of ZnBr 2, CuCN ´ 2LiCl
and allyl bromide, the reaction mixture was worked up oxidatively, providing the
alcohol 396 in 83% yield. [238].
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