Synthesis and selective oxidations of rhenium hydrotris(1-pyrazolyl) borate compounds

Reaction of (HBpz{dollar}sb3{dollar})ReO(OR){dollar}sb2{dollar} (R = methyl, ethyl, isopropyl, n-butyl, benzyl) (HBpz{dollar}sb3{dollar} = hydrotris(1-pyrazolyl) borate) with trimethylsilyl triflate (Me{dollar}sb3{dollar}SiOTf; OTf = OSO{dollar}sb2{dollar}CF{dollar}sb3{dollar}) yields (HBpz{dollar}sb3{dollar})ReO(OR)(OTf). Oxidation of (HBpz{dollar}sb3{dollar})ReO(OR)(OTf) with 2 equivalents of pyridine-N-oxide in CD{dollar}sb2{dollar}Cl{dollar}sb2{dollar} yields (HBpz{dollar}sb3{dollar})ReO{dollar}sb3{dollar} and aldehyde. A small amount of alcohol is generated due to the formation of HOTf. Reaction of OSMe{dollar}sb2{dollar} with (HBpz{dollar}sb3{dollar})ReO(OR)(OTf) gives a moderately stable adduct species, ((HBpz{dollar}sb3{dollar})ReO(OR)(OSMe{dollar}sb2{dollar})) (OTf), followed by further reaction to (HBpz{dollar}sb3{dollar})ReO{dollar}sb3{dollar}, aldehyde, and alcohol. Addition of silver triflate (AgOTf) to (HBpz{dollar}sb3{dollar})ReO(R)(Cl) (R = methyl, ethyl, n-butyl) in benzene yields (HBpz{dollar}sb3{dollar})ReO(R)OTf. Rapid oxidation of (HBpz{dollar}sb3{dollar})ReO(Et)OTf with 3 equivalents of pyridine-N-oxide or DMSO yields a quantitative conversion to (HBpz{dollar}sb3{dollar})ReO{dollar}sb3{dollar} and aldehyde. {dollar}sp1{dollar}H NMR spectroscopy shows the initial formation of an ylide species ((HBpz{dollar}sb3{dollar})ReO(OH){dollar}{lcub}{dollar}CH(L)CH{dollar}sb3{rcub}{dollar}) OTf (L = SMe{dollar}sb2{dollar}, pyridine) at {dollar}-{dollar}47{dollar}spcirc{dollar}C. Mechanistic studies show that both the oxidation of (HBpz{dollar}sb3{dollar})ReO(R)OTf and (HBpz{dollar}sb3{dollar})ReO(OR)OTf by an oxygen atom donor take place via an extremely reactive rhenium (VII) dioxo cation, (HBpz{dollar}sb3{dollar})ReO{dollar}sb2{dollar}(X){dollar}sp+{dollar} (X = R, OR). This is followed by abstraction of the hydrogen of the {dollar}alpha{dollar}-carbon of the alkyl or alkoxide by an electrophilic oxo ligand and then loss of the aldehyde. Reaction of ethylene oxide with (HBpz{dollar}sb3{dollar})ReO(X)OTf (X = alkyl, alkoxide, chloride) generates an adduct species (HBpz{dollar}sb3{dollar})ReO(X)(OCH{dollar}sb2{dollar}CH{dollar}sb2)sp+.{dollar} Addition of pyridine-N-oxide with (HBpz{dollar}sb3{dollar})ReO(Cl)OTf yields the neutral rhenium (VI) cis-dioxo species, (HBpz{dollar}sb3{dollar})ReO{dollar}sb2{dollar}Cl. The X-ray crystal structure indicates rhenium-oxo bond lengths of 1.797(11) and 1.741(10) A, and a O-Re-O angle of 107.1(4){dollar}spcirc{dollar}. The Re(VII)/Re(VI) couple is 0.93 V vs. Cp{dollar}sb2{dollar}Fe{dollar}sp+{dollar}/Cp{dollar}sb2{dollar}Fe in CH{dollar}sb3{dollar}CN. A number of rhenium imido complexes (HBpz{dollar}sb3{dollar})Re(Np-tol)(R)(I) (R = methyl, ethyl, isopropyl, n-butyl, benzyl) were generated. The electrochemistry of the compounds and oxidation with oxygen atom donors was examined.