102307-86-8Relevant articles and documents
Reactions of ML4Cl2 (M = Mo, W; L = PMe3, PMePh2) with epoxides, episulfides, CO2, heterocumulenes, and other substrates: A comparative study of oxidative addition by oxygen atom, sulfur atom, or nitrene group transfer
Hall, Keith A.,Mayer, James M.
, p. 10402 - 10411 (2007/10/02)
A comparative survey of the reactivity of the divalent molybdenum and tungsten chloro-phosphine complexes ML4Cl2 (M = Mo, W; L = PMe3, PMePh2) toward oxidation by a variety of oxygen atom, sulfur atom, and nitrene donors is presented. In general, reactions result in net two-electron oxidation of the metal center, producing metal oxo, sulfido, and imido complexes. The reactions can also be described as oxidative addition reactions, in many cases oxidative addition of C=X double bonds. Reactions are apparently thermodynamically driven by the propensity of Mo and W to form strong multiple bonds with oxygen, sulfur, and nitrogen. ML4Cl2 compounds react with ethylene oxide and ethylene sulfide to produce oxo and sulfido tris(phosphine) species, M(E)L3Cl2 (E = O, S), in equilibrium with oxo and sulfido ethylene species M(E)(CH2=CH2)L2Cl2. Isocyanates (RN=C=O; R = tBu, p-tolyl) and tBuN=C=NtBu react to form imido tris(phosphine) and imido carbonyl or imido isonitrile complexes, respectively. Phosphine sulfides are desulfurized forming sulfido complexes, but phosphine oxides are unreactive. The π-acids formed in these reactions - for instance, CO from cleavage of RNCO - bind more strongly to the tungsten(IV) versus the molybdenum(IV) oxo, sulfido, and imido products. Similarly, the equilibria for π-acid coordination are more favorable when the ligand is PMePh2 than when L = PMe3. For all of the complexes, reactions are slowed by free phosphine, consistent with a mechanism involving an initial dissociation of a phosphine ligand followed by trapping of the coordinatively unsaturated species by the oxidizing substrate. Ligand loss from ML4Cl2 is rapid for L = PMePh2 at ambient temperatures but slower for L = PMe3, with half-lives for PMe3 loss of 18 min at 24°C for Mo(PMe3)4Cl2 and 6 min at 69°C for W(PMe3)4Cl2. For the molybdenum complexes MoL4Cl2 (L = PMe3, PMePh2), dimerization to the known Mo(II) quadruply bound species Mo2L4Cl4 is competitive with oxidation at the metal center. In reactions involving stronger oxidants (SO2, DMSO, and N2O), the formation of trivalent species ML3Cl3 is often observed, indicating that chlorine atom transfer processes also occur.
On the mechanism of oxygen atom or nitrene group transfer in reactions of epoxides and aziridines with tungsten(II) compounds
Atagi, Lauren M.,Over, Diana E.,McAlister, Donald R.,Mayer, James M.
, p. 870 - 874 (2007/10/02)
The tungsten(II) complexes WCl2(PMePh2)4 (1) and WCl2(CH2=CH2)2(PMePh 2)2 (2) react with epoxides and aziridines to form tungsten(IV)-oxo and -imido complexes. The relative reactivities of epoxides with 2 have been determined from competition experiments. More substituted epoxides are harder to deoxygenate: the reactivities of ethylene, isobutylene, and tetramethylethylene oxides fall in the geometric progression 100:10:1. cis-2-Butene oxide is deoxygenated faster than its trans isomer. Reaction occurs with predominant (≥85%) retention of configuration (e.g. cis epoxides to cis olefins). The reaction of 2 with ethylene-d4 oxide yields W(O)Cl2(CH2=CH2)(PMePh2)2 (4) and uncoordinated CD2=CD2. The data suggest that de-epoxidation occurs via oxygen atom abstraction, and not via an oxametallacyclobutane which rearranges to an oxo-ethylene complex. Similarly, the tungsten center is suggested to attack the nitrogen atom of the aziridines, rather than react by initial oxidative addition of a C-N bond. This is indicated by the observation of an N-bound complex of aziridine and by the much slower rates of reaction for aziridines with bulky substituents on the nitrogen. The reactivities of para-substituted styrene epoxides are not strongly affected by the nature of the substituent (a ρ+ value of -0.5 is calculated with Hammett σ+ parameters) indicating that the transition state is not very polar, although there appears to be some conjugation between the phenyl ring and the epoxide. In sum, the data are most consistent with either concerted oxygen or nitrene transfer to tungsten or a mechanism involving a short-lived radical intermediate.
