Synthesis of tris(pyrazolyl)borate-stabilized vinyl, allyl, and homoallyl tungsten oxides and their selective oxyfunctionalization by singlet oxygen and dioxirane

The synthesis of new tris(pyrazolyl)borato-substituted alkenyltungsten complexes and their oxyfunctionalization was investigated. The derivatives 2a-e of the title compounds were prepared in 40-70% yield by Grignard reaction of [Tp*W(O)(2)Cl] (1) [Tp* = hydridotris(3,5-dimethyl-1-pyrazolyl)borato] and subsequent treatment with molecular oxygen. These alkenyltungsten complexes, except the homoallyl complex 2b with a monosubstituted double bond, displayed a high reactivity toward singlet oxygen (O-1(2)) to result in the corresponding allylic hydroperoxides by the Schenk ene reaction. While in the photooxygenation of the homoallyl complex 2a no special influence of the [Tp*W(O)(2)] fragment was observed, the allyl complexes 2c,d afforded stereoselectively the Z-configured products. Stereocontrol derives presumably from hyperconjugative stabilization by the W-C bond (beta effect) of the perepoxide-like transition state. The allylic hydroperoxide, derived from the O-1(2) ene reaction of the vinyl complex 2e, led by Hock-type cleavage to methacrolein and the tungstic acid derivative 9e. Upon photooxygenation in the presence of titanium tetraisopropoxide, only the homoallylic complex 2a underwent smoothly hydroxy-epoxidation. The other alkenyl complexes were sterically too hindered due to the bulky Tp* ligand. While the homoallyl and methallyl complexes 2a,d were quantitatively epoxidized by dimethyldioxirane (DMD), the vinyl complex 2e was again too sterically hindered because of the large [Tp*W(O)(2)] moiety, but its epoxide was obtained with m-chloroperbenzoic acid (mCPBA) as oxidant. In contrast, when equimolar amounts of methyl(trifluoromethyl)dioxirane (TFD) were used, the vinyl and methyl complexes 20e,f were regioselectively hydroxylated by C-H insertion into a pyrazolyl ring of the Tp* ligand. The present results demonstrate that the selective oxyfunctionalization of the alkenyltungsten complexes 2 by singlet oxygen and dioxiranes is controlled by electronic and steric factors of the [Tp*W(O)(2)] fragment.