Platinum-catalysed 1,4-diboration of 1,3-dienes

Abstract

The chiral diborane(4) compounds B₂[R,R-O₂CH(CO₂Me)CH(CO₂Me)]₂, B₂(S-O₂CH₂CHPh)₂, B₂(R,R-O₂CHPhCHPh)₂ and B₂(O₂C₂₀H₁₂)₂ (O₂C₂₀H₁₂ = 1,7′-bi-2-naphtholate) have been synthesized. All four compounds have been characterised by X-ray crystallography, the latter as a racemate. The B–B bond oxidative-addition reactions of the first three compounds with [Pt(PPh₃)₂(η-C₂H₄)] resulted in the platinum(II) bis(boryl) complexes cis-[Pt(PPh₃)₂{B[R,R-O₂CH(CO₂Me)CH(CO₂Me)]}₂], cis-[Pt(PPh₃)₂{B(S-O₂CH₂CHPh)}₂] and cis-[Pt(PPh₃)₂{B(R,R-O₂CHPhCHPh)}₂]; the former two were also characterised by X-ray crystallography. The platinum-catalysed diborations of a range of prochiral 1,3-dienes with the compounds B₂[R,R-O₂CH(CO₂Me)CH(CO₂Me)]₂, B₂(S-O₂CH₂CHPh)₂ and B₂(R,R-O₂CHPhCHPh)₂ were studied. Although these reactions were clean and quantitative, observed product d.e.s (measured by ¹H NMR spectroscopy) were low or non-existent indicating that chirality transfer from the diborane(4) diolate groups to the diene diboration product is not efficient in these cases.