Aryloxide ligands in metathesis of olefins and olefinic esters: catalytic behaviour of W(OAr) 2Cl 4 complexes associated with alkyl-tin or alkyl-lead cocatalysts; alkylation of W(OAr) 2Cl 4 by SnMe 4, Sn(n-Bu) 4, Pb(n-Bu) 4, MgNp 2: synthesis of W(OAr) 2Cl 2(CHCMe 3)(OR 2) and W(OAr) 2Cl(CHCMe 3)(CH 2CMe 3)(OR 2)

Abstract

Metathesis of internal and terminal olefins, as well as olefinic esters, can be achieved with the complexes W(OAr) 2Cl 4 (OAr = O-2,6-C 6H 3Me 2, O-2,6-C 6H 3(C 6H 5) 2, O-2,6-C 6H 3Br 2, O-2,6-C 6H 3Cl 2 and O-2,6-C 6H 3F 2) associated with MR 4 (M = Sn, Pb; R = Me, n-Bu). For cis-2-pentene metathesis, many parameters play a role in the activity: the electron-withdrawing property of the aryloxide ligand, the nature of the cocatalyst and the time of interaction between the precursor and the cocatalyst. For a given cocatalyst and a given time of interaction, the activity varies with the nature of the substituent in o, o'-position on the aryloxide (X = CH 3 < C 6H 5 < F < Cl < Br). For a given precursor complex and a given time of interaction, the activity increases with the nature of the cocatalyst in the following order: SnMe 4< Sn(n-Bu) 4 < Pb(n-Bu) 4. For a given catalyst and cocatalyst, there is an optimum time of interaction before introducing the olefin. The decline of activity which is observed if the olefin is introduced after this optimum value can be sup-pressed if diethyl ether is added. The stereochemical results obtained with W(OAr) 2Cl 2 and Pb(n-Bu) 4 show an increase of the stereoselectivity with the nature of the aryloxide substituents in the following order F ⋍ Cl < Br ⋍Me < Ph. For metathesis of olefinic esters, the precursors W(O-2,6-C 6H 3X 2)Cl 4 (X = Cl, Br), associated with MR 4 (M = Sn, Pb) gave higher activities than the homogeneous systems reported previously in the literature. Alkylation of W(OAr) 2Cl 4 by MR 4 (M = Sn, Pb; R = Me, n-Bu) has been achieved in order to determine the coordination sphere of the catalyst. The results obtained with M = Sn and OAr = O-2,6-C 6H 3Br 2 or O-2,6-C 6H 3Cl 2 suggest a process of double alkylation, α-H elimination and reductive elimination of alkane leading to SnR 3Cl and W(OAr) 2Cl 2(CHR') + RH. Side reactions of β-H elimination and coupling of carbene to yield olefin are also observed respectively for R = n-Bu and R = CH 3. Similar compounds can be obtained in pure form by alkylation of W(OAr) 2Cl 4 by MgNp 2 in diethyl ether (OAr = O-2,6-C 6H 3Me 2, O-2,6-C 6H 3(C 6H 5) 2, O-2,6-C 6H 3Br 2, O-2,6-C 6H 3Cl 2, O-2,6-C 6H 3F 2). Depending on the Mg/W ratios, one obtains W(OAr) 2Cl 2(CHR)(Et 2O) and W(OAr) 2Cl(CHR)(CH 2R)(Et 2O) (R = CMe 3). The isolation of such compounds is further proof of the role of aryloxide ligands and coordinated ether in the two-component catalysts.