Even though hydrovinylations (HV) of 1,3-dienes have been extensively studied, a class
of substrates that has not received much attention are the 1,4-disubstituted derivatives.
The work described in this thesis addresses the gap in this area. A variety of 1,3-dienes
were prepared using existing methods starting from an a,b-unsaturated aldehyde. Thus,
Wittig reactions provided: (2E,4E)-deca-2,4-diene and (2Z,4E)-deca-2,4-diene, (4E,6E)-
dodeca-4,6-diene and (4Z,6E)-dodeca-4,6-diene, (1E,3E)-1-methoxynona-1,3-diene and(1Z,3E)-1-methoxynona-1,3-diene, and (1E,3E)-1- bromonona-1,3-diene and (1Z,3E)-1-bromonona-1,3-diene. The remaining substrates were synthesized from an intermediate alcohol (2E,4E)-hexa-2,4-dien-1-ol obtained via diisobutylaluminum hydride (DIBAL)reduction of commercially available ethyl sorbate. Protection of this alcohol with tbutyldimethylsilyl (TBS) and benzyl (Bn) groups provided substrates which proved viable under modified reaction conditions for cobalt-catalyzed asymmetric HV
established in our research group. This involves reduction of an (L’~L)CoX2 complex
[L’~L = 1, n-bis-diphenylphosphinoalkane, phosphinooxazoline (PHOX)] with Zn
followed by activation by sodium teterakis-1,3-bis-triflurophenylborate (NaBArF) to
prepare the active catalyst, and, subsequently adding the diene in the presence of
ethylene. An achiral ligand [1,3-bis(diphenylphosphino)propane (dppp)] was used
explore the feasibility of the catalytic HV and to identify the 4 possible chiral products (corresponding to 1,4-additions at each of the termini) by GC. Interestingly, the substrate functionalized at the C1 position with a halogen [(1E,3E)-1- bromonona-1,3-diene and (1Z,3E)-1-bromonona-1,3-diene] was unreactive. In the case of tert-butyl(((2E,4E)-hexa-2,4-dien-1-yl)oxy)dimethylsilane the reaction required additional optimization. Of the chiral ligands explored phenyl PHOX proved most effective giving enantiomeric excesses (>90%) in several instances even though the regioselectivity needs further improvement. Other chiral ligands explored [((2,2-dimethyl-1,3-dioxolane-4,5-diyl)bis(methylene))bis(diphenylphosphane)] (DIOP) and [((2S,4S)-pentane-2,4-
diyl)bis(diphenylphosphane)] (BDPP) effected the reaction only slowly and with only
modest yields and selectivities, sometimes with unacceptably high amounts of unwanted byproducts. The use of easily tunable PHOX ligands in these reactions is novel, and holds considerable promise for further development of a broadly applicable reaction.