Chiral Induction in the synthesis 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 11 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 }3{}1{P 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.