Reactions of cis- and trans-(.eta.5-C5H5)Mo(CO)2[P(OPh)3]CH2OCH3 with electrophiles

Abstract

Reactions of trans- and cis-CpMo(CO)[sub 2][P(OPh)[sub 3]]CH[sub 2]OCH[sub 3] (1a, b) with a variety of protonic acids and with Me[sub 3]SiOTf have been examined. The composition of product mixtures has been shown to be dependent upon the strength of the electrophile and the presence of nucleophiles such as Br or PPh[sub 3] or the methylene acceptor Ph[sub 3]SiH and is proposed to be governed by complex equilibria involving transient oxonium ions as well as methylidene cations. Neither type of transient intermediate can be observed in low-temperature reactions. Methoxymethylidene cations, trans- and cis-CpMo(CO)[sub 2][P(OPh)[sub 3]] (=CHOCH[sub 3])[sup +], are generated in some low-temperature reactions with electrophiles. Independent synthesis of these has shown that they decompose in solution at room temperature to metallacycle 2, Cp[ovr Mo(CO)[sub 2][P(OPh)[sub 2](o-OC[sub 6]H[sub 4]C]H[sub 2])], and CpMo(CO)[sub 3][P(OPh)[sub 3]][sup +]. Reactions of trans- and cis-CpMo(CO)[sub 2][P(OPh)[sub 3]]CH[sub 2]OC(O)CF[sub 3] (6a,b) with CF[sub 3]COOH indicate that metallacycle formation occurs directly from the cis isomer (6b) in an acid-catalyzed process. Thermolysis studies of 6a,b indicate that trans to cis isomerization is facile and that 6b is readily converted to 2. Thermolysis of trans-CpMo(CO)[sub 2][P(OPh)[sub 3]]CH[sub 2]Br (10b) yields 2 and its ring-opened derivative as major products. Metallacycle 2more » has been characterized by X-ray crystallography: a = 14.457(3) [angstrom], b = 10.729(2) [angstrom], c = 15.611(3)[angstrom], [beta] = 103.65(2)[degrees], Z = 4, d[sub c] = 1.53 g cm[sup 3], space group P2[sub 1]/n, and R = 0.024. 23 refs., 1 fig., 3 tabs.« less