Handbook of Functionalized Organometallics Applications in S

TsO
284
Me OP(O)(OEt) 2
Zn
N
OiPr
Cl
+
Zn
Scheme 7.82 Catalytic S N2¢-allylic substitutions.
+
2
2
c-Hex
N
Fe
7.3 Reactions of Organozinc Reagents
1) 283 (10 mol %)
CuCN (10 mol %)
-78 ºC, 48 h
2) KOH, aq EtOH, 80 ºC
O
283
H
N
NH 2
Bn
O
t-Bu
286 (10 mol %)
NHn-Bu
t-Bu
CuBr ·SMe 2 (1 mol %)
82 %
HO
Me
96 % ee
S N2' : S N2=98:2
Me
Me
285 :82%;82%ee
Sterically very hindered diorganozincs like dineopentylzinc react enantioselectively
with allylic chlorides in the presence of the chiral ferrocenylamine 286 with
up to 98% ee (Scheme 7.82) [183]. The addition of zinc±copper organometallics to
unsaturated cationic metal complexes derived for example from pentadienyliron
and pentadienylmolybdenum cations affords the corresponding dienic complexes.
Thus, the addition of a zinc±copper homoenolate to the cationic g 5 -cycloheptadienyliron
complex 287 leads to the polyfunctional iron-dienic complex 288 in 65%
yield [184]. This chemistry has been extensively developed by Yeh and coworkers
[185]. The addition of allylic and benzylic zinc reagents to (g 6 -arene)-Mn(CO) 3 cations
of type 289 provides with excellent stereoselectivity the neutral (g 5 -cyclohexadienyl)Mn(CO)
3 complexes such as 290 in 92% yield. Less-reactive functionalized
alkylzinc compounds show a more complicated reaction pathway due to an isomerization
of the organozinc species (Scheme 7.83) [186]. Rigby and Kirova-Snover
[187] have applied these reactions to the synthesis of several natural products.
Thus, the alkylation of the cationic tropyliumcomplex 291 with the functionalized
zinc reagent 292 furnishes the chromium complex 293 that was an intermediate
for the synthesis of b-cedrene (Scheme 7.83).
299