Activation of alkanes with organotransition metal complexes.

Alkanes, although plentiful enough to be considered for use as feedstocks in large-scale chemical processes, are so unreactive that relatively few chemical reagents have been developed to convert them to molecules having useful functional groups. However, a recently synthesized iridium (lr) complex successfully converts alkanes into hydridoalkylmetal complexes (M + R-H --> R-M-H). This is a dihydride having the formula Cp(*)(L)lrH(2), where Cp(*) and L are abbreviations for the ligands (CH(3))(5)C(5) and (CH(3))(3)P, respectively. Irradiation with ultraviolet light causes the dihydride to lose H(2), generating the reactive intermediate Cp(*)lrL. This intermediate reacts rapidly with C-H bonds in every molecule so far tested (including alkanes) and leads to hydridoalkyliridium complexes Cp(*)(L)lr(R)(H). Evidence has been obtained that this C-H insertion, or oxidative addition, reaction proceeds through a simple three-center transition state and does not involve organic free radicals as intermediates. Thus the intermediate Cp(*)lrL reacts most rapidly with C-H bonds having relatively high bond energies, such as those at primary carbon centers, in small organic rings, and in aromatic rings. This contrasts directly with the type of hydrogen-abstraction selectivity that is characteristic of organic radicals. The hydridoalkyliridium products of the insertion reactions can be converted into functionalized organic molecules-alkyl halides-by treatment with mercuric chloride followed by halogens. Expulsion (reductive elimination) of the hydrocarbon from the hydridoalkyliridium complexes can be induced by Lewis acids or heat, regenerating the reactive intermediate Cp(*)lrL. Oxidative addition of the corresponding rhodium complexes Cp(*)RhL to alkane C-H bonds has also been observed, although the products formed in this case are much less stable and undergo reductive elimination at -20 degrees C. These and other recent observations provide an incentive for reexamining the factors that have been assumed to control the rate of reaction of transition metal complexes with C-H bonds-notably the need for electron-rich metals and the proximity of reacting centers.

Gas-Phase Rates of Alkane C-H Oxidative Addition to a Transient CpRh(CO) Complex.

The gas-phase irradiation of CpRh(CO)(2) (Cp = eta(5)-C(5)H(5)) was examined in order to study the rates of reaction of the 16-electron intermediates presumed to be involved in the C-H oxidative addition of alkanes. "Naked" (unsolvated) CpRh(CO) was detected, and direct measurements of the rates of reaction of this very short-lived complex with alkane C-H bonds were made. Activation of C-H bonds occurs on almost every collision for alkanes of moderate size, and intermediates in which the alkanes are bound to the metal centers, without their C-H bonds being fully broken, are implicated as intermediates in the overall reaction.

Regioselective [2+2] and [4+2] cycloaddition reactivity in an asymmetric niobium(bisimido) moiety towards unsaturated organic molecules.

The asymmetric bis-imido structure and the lability of the diethyl ether linkage in complex 1 provide a niobium complex that undergoes regioselective [4+2] cycloaddition reactions with an α,ß-unsaturated ketone and cycloaddition reactions that involve bond formation to the MAD ligand (MonoAzabutaDiene). DFT calculations have been used to support an initial azametallacyclobutene intermediate in the alkyne reaction.

Zirconium-mediated metathesis of imines: a study of the scope, longevity, and mechanism of a complicated catalytic system.

By kinetically stabilizing imidozirconocene complexes through the use of a sterically demanding ligand, or by generating a more thermodynamically stable resting state with addition of diphenylacetylene, we have developed transition metal-catalyzed imine metathesis reactions that are mechanistically analogous to olefin metathesis reactions catalyzed by metal carbene complexes. When 5 mol % of Cp*Cp(THF)Zr=N(t)Bu is used as the catalyst precursor in the metathesis reaction between PhCH=NPh and p-TolCH=N-p-Tol, a 1:1:1:1 equilibrium mixture with the two mixed imines p-TolCH=NPh and PhCH=N-p-Tol is generated in C(6)D(6) at 105 degrees C. The catalyst was still active after 20 days with an estimated 847 turnovers (t(1/2) 170 m; TON = 1.77 h(-1)). When the azametallacyclobutene Cp(2)Zr(N(Tol)C(Ph)=C(Ph)) is used as the catalyst precursor under similar reaction conditions, a total of 410 turnovers are obtained after 4 days (t(1/2) 170 m; TON = 4.3 h(-1)). An extensive kinetic and equilibrium analysis of the metallacyclobutene-catalyzed metathesis of PhCH=N-p-Tol and p-F-C(6)H(4)CH=N-p-F-C(6)H(4) was carried out by monitoring the concentrations of imines and observable metal-containing intermediates over time. Numerical integration methods were used to fit these data to a detailed mechanism involving coordinatively unsaturated (16-electron) imido complexes as critical intermediates. Examination of the scope of reaction between different organic imines revealed characteristic selectivity that appears to be unique to the zirconium-mediated system. Several zirconocene complexes that could generate the catalytically active "CpCp'Zr=NAr" (Cp' = Cp or Cp*) species in situ were found to be effective agents in the metathetical exchange between different N-aryl imines. N-Alkyl aldimines were found to be completely unreactive toward metathesis with N-aryl aldimines, and metathesis reactions involving the two N-alkyl imines TolCH=NPr and PhCH=NMe gave slow or erratic results, depending on the catalyst used. Metathesis was observed between N-aryl ketimines and N-aryl aldimines, but for N-aryl ketimine substrates, the catalyst resting state consists of zirconocene enamido complexes, generated by the formal C-H activation of the alpha position of the ketimine substrates.

Monomeric rhodium(II) catalysts for the preparation of aziridines and enantioselective formation of cyclopropanes from ethyl diazoacetate at room temperature.

A family of bis(oxazoline) complexes of coordinatively unsaturated monomeric rhodium(II) (2a,b, 3a,b) are described. These complexes serve as catalysts for cyclopropanation of olefins by ethyl diazoacetate, giving excellent yields (66-94%). Enantioselectivities for the cis product isomers are good (61-84%). The reaction shows an unusual preference for formation of the cis isomers. Catalytic aziridination of N-aryl imines with ethyl diazoacetate is also described.

Reactivity of a titanium dinitrogen complex supported by guanidinate ligands: investigation of solution behavior and a novel rearrangement of guanidinate ligands.

The titanium dinitrogen complex, [[(Me(2)N)C(N(i)Pr)(2)]( 2)Ti](2)(N(2)) (2), was synthesized by reduction of the dichloride precursor, [(Me(2)N)C(N(i)Pr)(2)](2)TiCl(2) (1). The dinitrogen complex reacts with phenyl azide to yield the titanium imido complex, [(Me(2)N)C(N(i)Pr)(2)](2)TiNPh (3). The fluxional behavior of the guanidinate ligands in compounds 1-3 was investigated using variable temperature and two-dimensional NMR techniques; guanidinate ligand rotation and racemization reactions were observed. Rearrangement of the guanidinate ligand to an asymmetrical bonding mode utilizing the dimethylamino and amide-nitrogen atoms is observed in the bridging oxo and sulfido derivatives (4 and 5). These compounds are formed by the reactions of 2 with pyridine N-oxide and propylene sulfide, respectively. The ligand rearrangement was observed to be reversible for the bridging sulfido complex 5; the structure of this compound is sensitive to temperature and solvent. The solid-state and solution structures of compounds 1-5 are discussed.