Handbook of Functionalized Organometallics Applications in S

140
4 Polyfunctional Magnesium Organometallics for Organic Synthesis
tive, [117] producing a mixture of diastereoisomers of type 217, although this
method provides an efficient synthesis of tri- and tetra-substituted alkenes 217a±d
[118, 119] (Scheme 4.44).
Ph
SO 2Ph
Ph
OH
R1 Y i-PrMgBr R1 Y E R1 Y
R2 Br
THF, -40 ºC
1-5h
R2 MgBr
R2 E
+
215 216 217
Y = CN, SO 2Ph, CO 2t-Bu, CONEt 2
t-BuO2C Me
Me
O
Br
Me
Me
CONEt 2
217a: 67% 217b: 81% 217c: 72% 217d: 77%
Scheme 4.44 Functionalized alkenylmagnesium compounds
bearing an electron-withdrawing group in a-position. The
dotted lines indicate the new carbon±carbon bond formed.
Similarly, the exchange reaction on readily available dibromoester 218 [120] furnishes,
after reaction with iPrMgCl (1 equiv) Grignard reagent 219 that reacts
with cyclopentanone to butenolide 220 in 71% yield (Scheme 4.45) [121].
Br Br
1) iPrMgCl
(1 equiv) Br MgCl O Br
O
Ph CO2Et -50 ºC, ether
15 min
Ph CO2Et rt, 4h
Ph
O
218 219
220: 71%
Br Br
Ph
CO 2Et
1) iPrMgCl
(2 equiv) iPr MgCl allyl bromide
-50 ºC, ether
15 min
Ph
CO 2Et
218 221
222: 75%
O
Pr
rt, 4h
Scheme 4.45 Selective functionalization of geminal dibromoalkenyl compounds.
H
Ph
CN
iPr
Ph
CO 2Et
The addition of 2 equivalents of iPrMgCl leads, after exchange of one bromine and
1,2-migration at the carbenoid center, to the new substituted Grignard reagent 221.
This can further react with an electrophile, furnishing, for example, product 222 in
75% yield (Scheme 4.45) [121]. Recently, Satoh introduced a sulfoxide±magnesium
exchange reaction for the synthesis of magnesium carbenoids [122]. The 1-chlorovinylidene
223, reacts readily with EtMgCl leading to the carbenoid 224 that furnishes
after addition of benzaldehyde to the alcohol 225 in 66% yield (Scheme 4.46) [122a].