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Alkyne/Methylene Coupling Reactions at Adjacent Rh/Os Centers:  Stepwise Transformations from C1- through C4-Bridged Species

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journal contribution
posted on 2005-12-19, 00:00 authored by James R. Wigginton, Amala Chokshi, Todd W. Graham, Robert McDonald, Michael J. Ferguson, Martin Cowie
Reaction of the methylene-bridged complex [RhOs(CO)3(μ-CH2)(dppm)2][CF3SO3] (dppm = μ-Ph2PCH2PPh2) with dimethyl acetylenedicarboxylate or hexafluorobutyne results in alkyne insertion into the Rh−CH2 bond, yielding [RhOs(CO)3(μ-η11-RCC(R)CH2)(dppm)2][CF3SO3] (R = CO2Me (5), CF3 (6)). Carbonyl removal from each of these species results in a Rh-bound carbonyl being replaced by a triflate ion. Reaction of 5 with diazomethane results in CH2 insertion into the Rh−C(R) bond with accompanying C−H activation of the original osmium-bound CH2 group, yielding [RhOs(CO)3(μ-η11-CH2C(R)C(R)CH)(μ-H)(dppm)2][CF3SO3] (R = CO2Me). Reaction of the alkyne-bridged species [RhOs(CO)3(μ-η11-RCCR)(dppm)2][CF3SO3] (R = CO2Me) with diazomethane at −78 °C yields [RhOs(CO)3(μ-η11-CH2C(R)CR)(dppm)2][CF3SO3] (12), which upon warming to ambient temperature, in the absence of diazomethane, undergoes alkyne deinsertion to yield [RhOs(CO)32-RC⋮CR)(μ-CH2)(dppm)2][CF3SO3] (13). This species and the inserted product (12) appear to be in equilibrium, so reaction of 13 with diazomethane proceeds via 12 to give [RhOs(CO)311-CH2CH2C(R)CR)(dppm)2][CF3SO2] (14), in which the hydrocarbyl fragment is chelating to Os and interacting with Rh via an agostic interaction with one of the Os-bound CH2 hydrogens. Upon standing for weeks, or more rapidly upon refluxing, compound 14 transforms into an isomer (15) in which the C4 unit has migrated from sites on Os adjacent to Rh to sites on Os remote from Rh. An X-ray structure determination of compound 15 as the BF4- salt confirms the geometry of this final species. Reaction of the hexafluorobutyne-bridged species [RhOs(CO)3(μ-η11-RCCR)(dppm)2][CF3SO3] (R = CF3) with diazomethane does not give a product analogous to that described for dimethyl acetylenedicarboxylate, but yields the isomer 6. These carbon−carbon bond forming processes are discussed.