Synthesis of Tris(pyrazolyl)borate-Stabilized Vinyl, Allyl, and Homoallyl Tungsten Oxides and Their Selective Oxyfunctionalization by Singlet Oxygen and Dioxirane

• Published in: Organometallics Vol. 15; no. 21; pp. 4586 - 4596
• Main Authors: Adam, Waldemar, Putterlik, Jürgen, Schuhmann, Rosemarie M, Sundermeyer, Jörg
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 15.10.1996; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Inorganic & Nuclear; Chemistry, Organic; Physical Sciences; Science & Technology; OXIDATION; D-METAL ACIDS; ENE REACTION; DIMETHYLDIOXIRANE; MULTIPLE BONDS; TIN-COMPOUNDS; MAIN-GROUP ELEMENTS; TRANSITION-METALS; OXO COMPLEXES; ATOM TRANSFER
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om960366d

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)2Cl] (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 (1O2) 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 (β effect) of the perepoxide-like transition state. The allylic hydroperoxide, derived from the 1O2 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.