Hydroboration allows for the functionalization of readily accessible 1,3-dienes to afford allylic and homoallylic boronate esters. These boronate esters provide outstanding opportunities for synthesis of enantiopure intermediates which can then be used in pharmaceutical and fine chemical synthesis. While investigating the mechanism of our recently discovered cobalt-mediated asymmetric hydrovinylation of 1,3-(E)-dienes, which provided expedient entry into enantiopure 1,4-skipped-dienes, we discovered a highly active cationic Co-(I) catalyst to effect a highly regio- and chemo- selective hydroboration of 1,3-(E)-dienes. The pre-catalyst LCo(I)X2 [L = P-P, P-N; X = Cl, Br, alkenyl] was treated with Lewis or Bronsted Acid [ex. NaBARF, B(C6F5)3, H(OEt2)2BARF] to generate the active Co(I) species. The treatment of 1,3-(E)-dodecadiene with dpppCoCl (5 mol%) [dppp = diphenylphosphine] and pinacol borane in methylene chloride at 0°C affords a mixture of homoallylic and allylic boronate esters in a 5.6:1 ratio. The (Z)-alkene, also characterized as the allylic boronate ester or 1,4-substituted product, was characterized as the major product via analysis of GC, HNMR, CNMR and mass spectrometry. Using this system, the ligand effects, solvent effects, counter-ion effects, temperature, and the presence or absence of an additive were studied to optimize the efficacy of Co(I). The boronate ester can then be oxidized into an alcohol which provides synthetic utility for pharmaceutical drugs or other chemical processes.

Cobalt-catalyzed hydroboration of 1,3-(E)-dienes

255th National Meeting (2018)

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