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.

Synthesis and Reactivity of Silicon Transition Metal Complexes, 35 . — Metallo Silanols and Metallo Siloxanes, 9 . — Synthesis of the Diastereomerically Pure Ferrio Silanol (RS,SR)‐Cp(OC)(Ph3P)Fe‐Si(Me)(Ph)(OH) and Structure of (RR,SS)‐Cp(OC)(Ph3P)Fe‐Si(Me)(Ph)Cl Irradiation of the ferrio chlorosilane Cp(OC)2Fe‐Si(Me)‐(Ph)Cl (1) in the presence of Ph3P leads to Cp(OC)(Ph3P)‐Fe‐Si(Me)(Ph)Cl (2) which is formed as a mixture of diastereomers separated by crystallization. Pure (RR,SS)‐2 is converted highly stereoselectively into (RR,SS)‐Cp(OC)(Ph3P)‐Fe‐Si(Me)(Ph)H (3) with LiAlH4, and further with dimethyldioxirane into (RS,SR)‐Cp(OC)(Ph3P)Fe‐Si(Me)(Ph)(OH) (4). This ferrio silanol diasteromer is alternatively obtained by Cl/OH exchange with KOH from 2. Diastereomerically pure (RR,SS)‐2 is characterized by a single‐crystal structure analysis.

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The versatility of the iron tricarbonyl fragment as protecting group in the oxyfunctionalization of trienes has been examined. The photooxygenation of the unsaturated iron complexes 1−3 at low temperatures (−80 °C) afforded after Ph3P reduction the allylic alcohols 4, 4‘, 13, and 22 in good to excellent yields. The persistence of the intermediary allylic hydroperoxides is reflected in the distance between the iron fragment and the reacting free double bond. Only the hydroperoxides derived from the homoallylic (most distant) substrate 1 are persistent enough to undergo the titanium tetraisopropoxide-catalyzed oxygen transfer to β-hydroxy epoxides. Epoxidation of the complexes 1 and 2a with dimethyldioxirane could be optimized in boiling acetone to suppress oxidative destruction of the iron tricarbonyl ligand. The deprotection of the organic ligand with cerium(IV) ammonium nitrate (CAN) enables the convenient preparation of multiply functionalized cyclohexadienes. In contrast to these regio- and stereoselectively controlled transformations, the corresponding unprotected trienes 10 and 19a react with singlet oxygen and dioxirane at all double bonds without any particular preference.

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