Conjugate boryl/silyl

Finally, the fourth approach (path d) involves the generation of conjugated nitro olefins C from the starting AN by known methods. The latter are involved in the conjugated [1,4]-addition to give target nitronates. This process is of most importance for the synthesis of six-membered cyclic nitronates (see Section 3.2.1.2.2). This method was also used to prepare a small series of boryl-, silyl-and acyl nitronates (see Sections 3.2.3 and 3.2.4). It is beyond reason to believe that all possibilities of the approach (d) are exhausted by these examples. 

More recently, a copper-bifluoride complex has been described, starting once again from the alkoxide species. Subsequently, [Cu(NHC)2] [HF2] complexes were developed (Figure 8.2) [91]. Both complexes were highly air stable in the solid state and moderately stable in solution compared to their analog [Cu(F) (IPr)] [92]. Remarkably, catalytic activity was observed in several transformations such as the reduction of ketones, the 1,4-conjugated borylation and silylation, and the diastereoselective allylation. 

Different from the work of Oestreich and co-workers, the Sato group prepared chiral a-silyl amines with the aid of the simple diphenylethylenediamine ligand L3 (Scheme 20) [50]. L3 had already been used in conjugate borylation by the Shibasaki group [18]. Although levels of enantiomeric excess were somewhat lower than those obtained with Oestreich s protocol, a broad range of aldimines was converted successfully [96a-d- (/ )-97a-d]. Furthermore, the obtained... 

Analogously, copper(I) salts cleave diboron compounds [e.g., bis(pinacolato)diboron] to give, in the presence of an a,/3-enone, the corresponding conjugate boration product (eq 113). In contrast to the silylation procedure, reactions can be run at room temperature both on cyclic and acyclic enones. Subsequent oxidation of the borylated ketone provides the /3-hydroxyketone. 

Notwithstanding the extremely rich and diverse reactivity presented above, all the examples presented thus far feature the addition of an NHC catalyst onto an sp carbon. Several reactions have been developed in the last ten years describing the activation of heteroatom o bonds by NHCs [8cj. The concept can be well illustrated by the recent work of Hoveyda and coworkers, who reported boryl and silyl conjugate additions to a,P-unsaturated carbonyls (Scheme 18.35) [124]. In both cases, addition of the NHC onto a trivalent boron atom activated the B-heteroatom bond. The use of Cj-symmetric imidazolinium salt 169 in the sUyl conjugate addition led to P-silyl ketones, esters, and aldehydes in good to high enantioselectivity. 

Hydrosilylation, hydroboration, and hydrophosphination of conjugated enynes have been recently employed to access silylated, borylated, or phosphonylated 1,3-dienes with high stereoselectivities. 

Boryl and silyl conjugate additions, according to Hoveyda and coworkers. 

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