Handbook of Functionalized Organometallics Applications in S

262
Me 3Si
7 Polyfunctional Zinc Organometallics for Organic Synthesis
N3 I
1) n-BuLi, -100 ºC, 3 min
THF/ether/pentane (4/1/1)
2) ZnBr2,THF,-90ºC N3 ZnBr
45 46 :>85%
Scheme 7.20 Preparation of an alkenylzinc reagent bearing an azide function.
Br
48
1) t-BuLi (2 equiv.)
-78ºC
2) ZnCl 2
-78 ºC tort
Me 3Si
Me 3Si
ZnCl
Cl
Zn
O
MeCHO
Scheme 7.21 Preparation of a zinc/silicon bimetallic of type 47.
47
Me 3Si
Cl
Me 3
Si
OH
Zn O
Me
50 :41%;d.r.>9:1
The mixed 1,2-bimetallic Zn/Si-reagent 47 is a versatile species that reacts with
aldehydes in high diastereoselectivity [59]. It is prepared by a bromine±lithiumexchange
reaction starting from 48 followed by a transmetallation with ZnCl 2.
The reaction with acetaldehyde is leading initially to the alkenylzinc species 49,
which reacts with Me 3SiCl, providing the alkenylsilane 50 in 41% yield and a diastereoselectivity
> 9:1.
2-Lithiated oxazoles are unstable and readily undergo a ring opening to the tautomeric
isonitriles. This ring cleavage can be avoided by preparing the corresponding
2-zincated oxazole (51) that is much more stable toward a fragmentation reaction
(Scheme 7.22) [60]. The lithiation of the O-vinyl carbamate (52) with sec-BuLi
followed by transmetallation with zinc bromide provides the convenient acyl
anion derivative 53 which undergoes smooth Pd(0)-catalyzed cross-coupling reactions;
Scheme 7.22 [61].
This reaction sequence has been extended to lithiumenolates. The deprotonation
of the aminoester 54 with LDA followed by a transmetallation with zinc bromide
in ether furnishes a zinc enolate that readily adds to the double bond providing
the proline derivative 55 in high diastereoselectivity and enantioselectivity
(Scheme 7.23) [62].
Similarly, zincated hydrazone derivatives of type 56 undergo an intermolecular
carbozincation of strained cyclopropene rings such as 57 leading to the adduct 58
with 92% yield [63]. This type of addition can be extended to ethylene [63c]. It proceeds
with an excellent stereoselectivity allowing the enantioselective synthesis of
a-substituted ketones. Allylic zinc species also add to cyclopropenone acetals
49