Chiral induction in the sysnthesis of 4,4-dimethyl-1-phenylpenta-1,2-diene (1-Ph-3-t-Bu-allene) catalyzed by chiral phosphine complexes of palladium

Abstract

4,4-Dimethyl-1-phenylpenta-1,2-diene (1-phenyl-3-t-butylallene) ( 2) has been prepared by a palladium-catalyzed cross-coupling reaction between either in situ prepared 4,4-dimethylpenta-1,2-dienylzinc chloride and iodobenzene (Route A) or 1-bromo-4,4-dimethylpenta-1,2-diene and phenylzinc chloride (Route B). Several palladium complexes with known chiral phosphine, phosphine-amine and diphosphine ligands were used as chiral catalysts. The highest enantiomeric excess (25% ee) was obtained via route A with catalysis by the 1 1 complex of palladium chloride and ( R,R)-2,3- O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane, “( R,R)-diop”. The enantiomeric excess (19% ee) when the complex Pd“( R,R)-diop” 2 was used appeared to be independent of the temperature, the amount of catalyst, and the ratio of the reagents. However, when a magnesium or copper reagent was used instead of the zinc reagent, the configuration was reversed (3–10% ee). The suggested intermediate PdIPh“( R,R)-diop” was prepared by treating one equivalent of ( R,R)-diop with PdIPh(tmeda) (tmeda = N,N,N′,N′,-tetramethylethanediamine) and studied by 31P NMR spectroscopy. PdIPh“( R,R)-diop” appeared to be present in solution as an equilibrium mixture of cis-chelating and trans-monodentate bonded species, with the ratio depending upon the concentration and the palladium/diphosphine ratio. Reaction of a slight excess of 4,4-dimethyl-1,2-pentadienyl-zinc chloride or -magnesium chloride with PdIPh“( R,R)-diop” gave 1-phenyl-3-t-butylallene 2 with the same configuration, though with a slightly smaller ee than that from the reactions catalyzed by PdCl 2“( R,R)-diop” or Pd“( R,R)-diop” 2. The results strongly suggest that reaction of the organometallic reagent with the palladium(II) intermediate PdIPh“( R,R)-diop” is the enantiodifferentiating step in the catalytic cycle.