Cobalt-Catalysed Asymmetric Hydrovinylation of 1,3-Dienes.

In the presence of bidentate 1,n--diphenylphosphinoalkane-CoCl complexes {ClCo[P~P]} and MeAl or methylaluminoxane, acyclic ()-1,3-dienes react with ethylene (1 atmosphere) to give excellent yields of hydrovinylation products. The regioselectivity (1,4- or 1,2-addition) and the alkene configuration (- or -) of the resulting product depend on the nature of the ligand and temperature at which the reaction is carried out. Cobalt(II)-complexes of 1,1-diphenylphosphinomethane and similar ligands with narrow bite angles give mostly 1,2-addition, retaining the -geometry of the original diene. Complexes of most other ligands at low temperature (-40 °C) give almost exclusively a single branched product, ()-3-alkylhexa-1,4-diene, which arises from a 1,4-hydrovinylation reaction. A minor product is the linear adduct, a 5-alkyl-hexa-1,4-diene, also arising from a 1,4-addition of ethylene. As the temperature is increased, a higher proportion of the major 1,4-adduct appears as the ()-isomer. The unexpectedly high selectivity seen in the Co-catalysed reaction as compared to the corresponding Ni-catalysed reaction can be rationalized by invoking the intermediacy of an η-[(diene)[P~P]CoH]-complex and its subsequent reactions. The enhanced reactivity of terminal -1,3-dienes over the corresponding -dienes can also be explained on the basis of the ease of formation of this η-complex in the former case. The lack of reactivity of the XCo(dppb) (X = Cl, Br) complexes in the presence of Zn/ZnI makes the MeAl-mediated reaction different from the previously reported hydroalkenylation of dienes. Electron-rich phospholanes, -oxazolines and -heterocyclic carbenes appear to be poor ligands for the Co(II)-catalysed hydrovinylation of 1,3-dienes. An extensive survey of chiral ligands reveals that complexes of DIOP, BDDP and Josiphos ligands are quite effective for these reactions even at -45 °C and enantioselectivities in the range of 90-99 % ee can be realized for a variety of 1,3-dienes. Cobalt(II)-complex of an electron-deficient Josiphos ligand is especially active, requiring only <1 mol% catalyst to effect the reactions.