Cobalt—boron bonds reactions with

In this first example, a cobalt(I)-catalyzed Diels-Alder reaction employing alkynyl boron derivatives 2 and 1,3-diene 1 for preparation of dihydroaromatic building blocks 4 is disclosed. Ensuing, dihydroaromatic boronic ester intermediates 3 were used as useful synthetic scaffolds in an unusual thermal Diels-Alder reaction with different dienophiles, generating highly substituted and functionalized products in acceptable to very good yields by recrystallization after four synthetic steps. Five new C—C bonds from four simple components were formed in this process. 

The Nicholas reaction was used to synthesize the p-lactam precursor of thienamycin in the laboratory of P.A. Jacobi and thereby accomplish its formal total synthesis. The necessary p-amino acid was prepared by the condensation of a boron enolate (derived from an acylated oxazolidinone) with the cobalt complex of an enantiopure propargylic ether. The resulting adduct was oxidized with ceric ammonium nitrate (CAN) to remove the cobalt protecting group from the triple bond, and the product was obtained with a 17 1 anti.syn selectivity and in good yield. 

Acetylenic cobalt complexes greatly facilitate the heterolytic cleavage of adjacent alcohols or ethers. On treatment with Lewis acids, these complexes afford cobalt stabilized carbenium ions, which can be captured by nucleophiles such as enolates. Jacobi and Zheng have employed chiral boron enolates of Evans s oxa-zolidinone 6.91 (R = i-Pr). After removal of the chiral auxiliary, they obtained anti adds 11.43 with a high selectivity [1677] (Figure 11.9). The reaction can be extended to the boron enolates of related oxazolidinones and to a-branched propargyl derivatives. This reaction has been applied to the synthesis of P-aminoacids after Curtius rearrangement and oxidation of the triple bond [1677]. 

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