Diels-Alder Reactions of 1,2-Azaborines.

Diels-Alder reactions employing 1,2-azaborine heterocycles as 1,3-dienes are reported. Carbocyclic compounds with high stereochemical and functional complexity are produced, as exemplified by the straightforward two-step synthesis of an amino allyl boronic ester bearing four contiguous stereocenters as a single diastereomer. Whereas electron-deficient dienophiles undergo irreversible Diels-Alder reactions, a reversible Diels-Alder reaction with the less electron-deficient methyl acrylate is observed. Both the N and the B substituent of the 1,2-azaborine exert significant influence on the [4+2] cycloaddition reactivity as well as the aromatic character of the heterocycle. The experimentally determined thermodynamic parameters of the reversible Diels-Alder reaction between 1,2-azaborines and methyl acrylate correlate with aromaticity trends and place 1,2-azaborines approximately between furan and thiophene on the aromaticity scale.

Reversible interconversion between a monomeric iridium hydroxo and a dinuclear iridium µ-oxo complex.

Treatment of the (PCsp(2)P)Ir(I)Cl complexes 2(R) (R = (i)Pr, (t)Bu) with cesium hydroxide in THF leads to the corresponding monomeric Ir(I) hydroxo complexes 5(R) in good to excellent yields of 70% (R = (i)Pr) and 92% (R = (t)Bu). The compounds are green in color and while they exhibit very similar (31)P NMR data to the chlorides 2, the (1)H NMR spectrum of each features a triplet ((3)JHP = 3.8 Hz) at 4.22 (R = (t)Bu) and 4.31 (R = (i)Pr) ppm that broadens in the presence of excess water and exchanges deuterium with D2O. Bands at 3642 and 3625 cm(-1) are observed in the IR spectrum for the νOH stretch. In the case of R = (i)Pr, a second product is observed in the crude reaction mixture and dominates when 5((i)Pr) is heated under vacuum and H2O is removed. This product is deep blue in color, and an X-ray crystal structure analysis reveals it to be the S4 symmetric dinuclear (PCsp(2)P)Ir-O-Ir(PCsp(2)P) complex 6((i)Pr), which features a µ-oxo ligand along an allene-like molecular core. Time-dependent DFT calculations with the M06 density functional show that a metal-to-ligand HOMO-LUMO excitation is mainly responsible for the blue color. Upon reaction of 6((i)Pr) with water, monomeric hydroxo complex 5((i)Pr) is quantitatively regenerated. Further, reaction of 6((i)Pr) with an excess of phenol smoothly yields the previously prepared (PCsp(2)P)IrOPh complex 3((i)Pr). Kinetic studies of the reaction indicated that it is first order in both [6((i)Pr)] and [HOPh] and exhibits a kH/kD of 1.9 when DOPh is employed. Eyring analysis is consistent with the bimolecular nature of the reaction, with ΔH(‡) = 13.1(5) kcal mol(-1) and ΔS(‡) = -13(2) cal K(-1). Finally, kobs is observed to increase when electron-withdrawing groups are incorporated in the para position of the phenol substrate and decrease when electron-donating groups are employed. These observations suggest that the rate-limiting step in this reaction is protonation of the µ-oxo ligand by the phenol substrate. This reaction serves as a model system for the reversible condensation of metal hydroxo ligands to form metal oxo moieties.

Dinitrogen functionalization at a ditantalum center. Balancing N2 displacement and N2 functionalization in the reaction of coordinated N2 with CS2.

The reaction of carbon disulfide (CS2) with the side-on end-on dinitrogen complex ([NPNSi]Ta)2(µ-η1:η2-N2)(µ-H)2 (1) (where [NPNSi] = [PhP(CH2SiMe2NPh)2]) has been studied and shown to generate two products, the ratio of which depends on the concentration of added carbon disulfide. At high concentrations of CS2, N-N bond cleavage and functionalization occur to generate a ditantalum complex with an isothiocyanato ligand N-bound to Ta along with bridging sulfido and nitrido moieties. At lower concentrations of CS2, less dinitrogen functionalization is observed; instead, N2 is displaced and the CS2 molecule is completely disassembled to generate a ditantalum derivative with bridging methylene and two sulfide moieties. Kinetic and DFT studies have been performed and provide clues about the product ratio and mechanistic information and shed light on why N2 functionalization is challenging.

Cationic mono and dicarbonyl pincer complexes of rhodium and iridium to assess the donor properties of PCcarbeneP ligands.

The donor properties of five different PCcarbeneP ligands are assessed by evaluation of the CO stretching frequencies in iridium(i) and rhodium(i) carbonyl cations. The ligands feature dialkyl phosphine units (R = (i)Pr or (t)Bu) linked to the central benzylic carbon by either an ortho-phenylene bridge, or a 2,3-benzo[b]thiophene linker; in the former, substituent patterns on the phenyl linker are varied. The carbonyl complexes are synthesized from the (PCcarbeneP)M-Cl starting materials via abstraction of the chlorides in the presence of CO gas. In addition to the expected mono carbonyl cations, products with two carbonyl ligands are produced, and for the rhodium example, a novel product in which the second carbonyl ligand adds reversibly across the Rh[double bond, length as m-dash]C bond to give an η(2) ketene moiety was characterized. The IR data for the complexes shows the 2,3-benzo[b]thiophene linked system to be the poorest overall donor, while the phenyl bridged ligands incorporating electron donating dialkyl amino groups para to the anchoring carbene are very strongly donating pincer arrays.