Handbook of Functionalized Organometallics Applications in S

6.3 Nucleophilic Additions
a-hydroxycarbonyl compounds and amines, respectively. The rhodium-catalyzed
conjugated addition of aryl- and vinylstannanes to a,b-unsaturated ketones and
esters has been targeted also to prepare arylated ketones and esters [174,175]. This
reaction was used for the synthesis of natural and unnatural amino acid derivatives
in air and water [176] (Scheme 6.19).
SnMe 3
Scheme 6.19
+
O
6.3
Nucleophilic Additions
O
N
OEt
O
Rh 2 (cod) 2 Cl 2
H 2 O/air
sonication
6.3.1
Nucleophilic Addition onto Carbonyl Compounds
6.3.1.1 Introduction
O
82%
O
N
OEt
O
The addition of organotins to carbonyl compounds is a reaction of major importance
in organic synthesis, especially with functionalized allyltins. There are several
advantages such as (i) the easy preparation and stability of the organometallic
reagents even under an enantiopure fashion, (ii) their great reactivity once the carbonyl
partner is activated, (iii) the possibility to reach an excellent regio- and
stereocontrol of the addition. Importantly, the organotin reagents authorize several
mechanisms involving different transition states, so that the selectivity of the
reaction appears to be closely dependent on the experimental conditions.
6.3.1.2 Functionalized Allyltins
6.3.1.2.1 Activation of the Carbonyl Compounds
The first activation of the carbonyl substrate by a Lewis acid was reported in
1979 [177] and initiated the increasing use of organotin reagents. The reaction proceeds
via an open transition state, as the tin does not compete with the Lewis acid
in the coordination of the carbonyl. The diastereoselectivity of the reaction can be
induced either by the organometallic or the carbonyl partners.
Diastereoselectivity induced by c-substituted allyltins: the reaction leads to an excellent
syn diastereoselectivity. First reported by Maruyama and coworkers [178], this
was explained by proposing an antiperiplanar transition state for the syn selectivity
whatever the nature (E or Z) of the crotylstannane. Contrastingly, it was also
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