Nucleophiles heteroatom, conjugate

Conjugate Addition of Heteroatom Nucleophiles and Subsequent Nef Reaction... 

Nucleophilic Attack on Ring Atoms Adjacent to Heteroatom or Conjugated with 428... 

Nucleophilic attack on ring atoms adjacent to a heteroatom or conjugated with a... 

Substitution of complexed dienols (244) or dienol acetates with carbon or heteroatom nucleophiles, in the presence of a Lewis acid, occurs with retention of configuration (Scheme 69). (Alkyl aluminum reagents act as both nucleophile and Lewis acid in this process). This reaction is believed to proceed via stereospecific ionization, with anchimeric assistance from the iron, to generate the transoid pentadienyl cation (247) followed by attack of the weak nucleophile on the face opposite to iron. The cross-conjugated pentadienyl cation can also be generated the substitution of (2-acetoxymethyl-l,3-butadiene)Fe(CO)3 (193) has previously been discussed (Section 6.1.1). 

In the palladium-catalyzed 1,4-oxidations of conjugated dienes described so far, only heteroatom nucleophiles have been employed. There is an intrinsic problem in using free carbanions in an oxidation reaction since the oxidant can readily remove an electron and oxidize the carbanion to a radical. Furthermore, in the procedure associated with the best selectivity, i.e., the benzoquinone-based process, acid is required to regenerate the Pd(0)-benzoquinone complex to Pd(II) and hydroquinone. 

This commoner type of reaction involves the attack of carbon or heteroatom nucleophiles onto carbonyl compounds, by direct or conjugate addition, and onto imines. Because we have just dealt with boranes we shall start with the reaction of allylic boron compounds with such electrophiles. You might strictly not call this nucleophilic attack. 

The term Michael addition has been used to describe 1,4- (conjugate) additions of a variety of nucleophiles including organometallics, heteroatom nucleophiles such as sulfides and amines, enolates, and allylic organometals to so-called Michael acceptors such as a,p-unsaturated aldehydes, ketones, esters, nitriles, sulfoxides, and nitro compounds. Here, the term is restricted to the classical Michael reaction, which employs resonance-stabilized anions such as enolates and azaenolates, but a few examples of enamines are also included because of the close mechanistic similarities. 

The organocatalytic asymmetric conjugate addition of heteroatom nucleophiles to different electrophilic olefins has become a very popular reaction during the last few years. Different nitrogen, oxygen, sulfur, and selenium nucleophilic species have been successfully used leading to enantiomerically emiched heterofunctionalized derivatives. 

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