Rhenium(VII) oxide catalyzed heteroacylative ring-opening dimerization of tetrahydrofuran.

Re(2)O(7), which is known primarily as a strong oxidant, was found to be a highly selective Lewis acid catalyst that affects the heteroacylative dimerization of THF at room temperature. This multicomponent reaction, which involves THF, trifluoroacetic anhydride (TFAA), and a carboxylic acid, produces a nonsymmetrical diester, RCO(2)(CH(2))(4)O(CH(2))(4)OCOCF(3), in high yields. The reaction is quite general with respect to the carboxylic acid but is highly selective for unsubstituted THF in preference to other cyclic ethers. It is also highly selective for TFAA in preference to other anhydrides. Isotope labeling experiments indicate that two of the five oxygen atoms in the product originate from THF; one comes from rhenium oxide, and the two carbonyl oxygens originate from the carboxylic acid and from TFAA. The catalytic cycle, which is proposed on the basis of these experiments, involves a multistep sequence of nucleophilic attacks, starting with an attack of a rhenium oxo ligand on a coordinated THF, then attack of the resultant alkoxide ligand on a second coordinated THF, nucleophilic addition of the resultant alkoxide ligand to the coordinated carboxylic acid (an intramolecular metal-oxygen bond metathesis), and, finally, electrophilic cleavage of the other coordinated alkoxide by TFAA to produce the nonsymmetrical diester. This synthetically useful reaction highlights the unique, frequently avoided Lewis acidity of transition-metal oxides.

Proton walk in the aqueous platinum complex [TpPtMeCO] via a sticky sigma-methane ligand.

Both experimental and theoretical evidence suggest that the proton exchange between water and the methyl group in [TpPt(CO)CH(3)] (1, Tp=hydridotripyrazolylborate) involves the formation and deprotonation of a "sticky" sigma-methane ligand. The efficiency of this nontrivial process has been attributed to the spatial orientation of functional groups that operate in concert to activate a water molecule and then achieve a multistep proton walk from water to an uncoordinated pyrazolyl nitrogen atom, to the methyl ligand, and then back to the nitrogen atom and water. The overall proton-exchange process has been proposed to involve an initial attack of water at the CO ligand in 1 with concerted deprotonation by the uncoordinated pyrazolyl nitrogen atom. The pyrazolium proton is then transferred to the Pt--CH(3) bond, leading to a sigma-methane intermediate. Subsequent rotation and deprotonation of the sigma-methane ligand, followed by reformation of 1 and water, result in scrambling of the methyl protons with the hydrogen atoms of water. An alternative two-step process that involves oxidative addition and reductive elimination has also been considered. The two competing mechanistic routes from 1 into [D(3)]-1, as well as the conversion of 1 into [TpPt(CH(3))H(2)] (2), have been examined by density functional theory (DFT) using a variety of exchange-correlation methods, primarily PW6B95, which was recently shown to be highly accurate for evaluating reactions of late-transition-metal complexes. The key role played by the free pyrazolyl nitrogen atom, acting as a proton carrier that abstracts a proton from water and transfers the proton to the Pt--CH(3) bond, is reminiscent of the dual functionality of histidine in the catalytic triad of natural serine proteases.

TpPtMe(H)(2): why is there H/D scrambling of the methyl group but not methane loss?

The reactivity of TpPtMe(H)(2) (Tp = hydrido-tris(pyrazolyl)borate) was investigated. This complex is remarkably resistant to methane loss; heating it in methanol at 55 degrees C does not lead to either methane or hydrogen loss. When CD(3)OD is used, reversible H/D scrambling of the hydrides and the methyl hydrogens occurs. This reactivity was investigated by density functional theory (DFT) methods at the mPW1k/LANL2DZ+P//mPW1k/LANL2DZ level. It was found that methane loss cannot occur due to the rigidity of the Tp ligand, which does not allow the trans geometry which would be required for the product of methane elimination, TpPtH. The resulting complex is very high in energy, and therefore the loss of methane is unfavorable. On the other hand, H/D scrambling of the methyl ligand is relatively facile. It proceeds through an eta(2-CH)-CH(4) complex, even though methane loss is not observed. The model system, [(NH(3))(3)PtMe(H)(2)](+) was examined to verify that the cause of the observations is the rigidity of the Tp system. The reaction was investigated at a number of levels of DFT. It was concluded that investigations of similar sized systems should be examined at the above level of theory or the mPW1k/SDB-cc-pVDZ//mPW1k/SDD level for improved accuracy of the energetic calculations.

TpPt(IV)Me(H)(2) forms a sigma-CH(4) complex that is kinetically resistant to methane liberation.

Complex 1 undergoes H/D scrambling in methanol without concomitant liberation of either methane or dihydrogen (k(H)/k(D) = 0.76, 55 degrees C). The measured isotope effect was proposed to directly relate to the initial reductive coupling step in reductive elimination reactions.

Regioselective Reduction of NAD+ Models with [Cp*Rh(bpy)H]+ : Structure-Activity Relationships and Mechanistic Aspects in the Formation of the 1,4-NADH Derivatives.

The hydride transfer mechanism of the NAD+ model compound 1 to its 1,4-NADH derivative 3 [Eq. (1)] is proposed to be a consequence of the critical role of the carbonyl group of the amide to coordinate to the ring-slipped η5 - to η3 -Cp*Rh metal center of the catalyst [Cp*Rh(bpy)H]+ , prepared in situ from 2, while a steric effect of a substituent in the 3 position, for example, C(O)NEt2 , was found to totally inhibit this regioselective reduction. bpy=2,2'-bipyridine, Cp*=C5 Me5 , OTf=trifluoromethanesulfanate.