Nozaki-Hiyama-Kishi reaction chromium complexes

Considering the unique features that the Nozaki-Hiyama-Kishi reaction possesses and its undoubted potential in the synthesis of complex natural products, the development of an efficient enantioselective version to control the absolute stereochemical outcome for a range of processes was highly desirable. However, because of the difficulties such as ligand coordination and specificity combined with the tendency of chromium(II) to form dimers or clusters with polydentate ligands, considerable effort has been devoted to the development of enantioselective variants. These studies have resulted in the expansion of the NHK to now include an impressive array of carbon-carbon, bond-forming processes (Scheme 12.6). 

Berkessel et al. have used an asymmetric Nozaki-Hiyama-Kishi reaction for the synthesis of several laulilamide analogs. (/ ,/ )-DlANANE ligand 14 was used for the key NHK coupling of the macrocyclic aldehyde 61 and iodide 62. Ten mol% of the chromium(ll) complex of 14 overcame the substrate selectivity and afforded the desired configuration in a dr of 78 22 and yield of 43% (Scheme 12.40). 

Usanov and Yamamoto recently found that catalytic amounts of Co(TPP) 367 led to a dramatic rate acceleration of Nozaki-Kishi-Hiyama reactions catalyzed by chromium complex 368 (Fig. 101) [460]. The authors attributed the rate enhancement to initial reduction of 367 to a Co(I) complex. The latter is able to undergo an Sn2 substitution at propargyl bromide 366 giving an allenylCo(ffl) species. It was proposed that its homolysis leads to allenyl radical 366A, which couples to Cr(II) complex 368. The resulting allenyl Cr(III) complex adds in an SN2 process... 

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