Ring current effects in crystals. Evidence from 13C chemical shift tensors for intermolecular shielding in 4,7-di-t-butylacenaphthene versus 4,7-di-t-butylacenaphthylene

13C chemical shift tensor data from 2D FIREMAT spectra are reported for 4,7-di-t-butylacenaphthene and 4,7-di-t-butylacenaphthylene. In addition, calculations of the chemical shielding tensors were completed at the B3LYP/6-311G** level of theory. While the experimental tensor data on 4,7-di-t-butylacenaphthylene are in agreement with theory and with previous data on polycyclic aromatic hydrocarbons, the experimental and theoretical data on 4,7-di-t-butylacenaphthene lack agreement. Instead, larger than usual differences are observed between the experimental chemical shift components and the chemical shielding tensor components calculated on a single molecule of 4,7-di-t-butylacenaphthene, with a root mean square (rms) error of +/-7.0 ppm. The greatest deviation is concentrated in the component perpendicular to the aromatic plane, with the largest value being a 23 ppm difference between experiment and theory for the 13CH2 carbon delta11 component. These differences are attributed to an intermolecular chemical shift that arises from the graphitelike, stacked arrangement of molecules found in the crystal structure of 4,7-di-t-butylacenaphthene. This conclusion is supported by a calculation on a trimer of molecules, which improves the agreement between experiment and theory for this component by 14 ppm and reduces the overall rms error between experiment and theory to 4.0 ppm. This intermolecular effect may be modeled with the use of nuclei independent chemical shieldings (NICS) calculations and is also observed in the isotropic 1H chemical shift of the CH2 protons as a 4.2 ppm difference between the solution value and the solid-state chemical shift measured via a 13C-1H heteronuclear correlation experiment.     

Direct observation of ion evaporation from a triply charged nanodroplet

Triply charged, highly solvated metal ions of the form [Ln(H(2)O)(n)](3+) can be generated using a commercial mass spectrometer, and CID studies on these highly charged metal-solvent clusters allow for the direct observation of a process best described as ion evaporation.     

Synthesis of the 2,4,5-tri-tert-butyl-1,3-diphospholide anion by phosphinidene elimination from 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene on treatment with the amide Li[NPh(SiMe3)

Treatment of the lithium amide Li[NPh(SiMe3)] with 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene, P(3)C(3)tBu(3), in a 1:2 ratio afforded equimolar amounts of the lithium salt of the five-membered 2,4,5-tri-tert-butyl-1,3-diphospholide anion, LiP(2)C(3)tBu(3) (isolated as its N,N,N',N'-tetramethylethylenediamine (TMEDA) adduct), and the tricyclic compound 6-[phenyl(trimethylsilyl)amino]-3,5,7-tri-tert-butyl-1,2,4,6-tetraphosphatricyclo[3.2.0.0(2,7)]hept-3-ene. Both compounds have been structurally characterised by single-crystal X-ray diffraction studies. The mechanism of this remarkable reaction has been elucidated by theoretical methods at the B3LYP/6-311+G** level of theory. The reaction involves a hitherto unobserved aminophosphinidene, which was formed by abstraction of a phosphorus atom from triphosphabenzene. The intermediate aminophosphinidene, which is further stabilised by the solvent THF, shows, in agreement with previous theoretical predictions, enhanced stability and reacts then with a second molecule of triphosphabenzene.     

Controlled synthesis of a structurally characterized family of sterically constrained heterocyclic alkoxy-modified titanium alkoxides

The reaction of [Ti(mu-ONep)(ONep)3]2 (ONep = OCH2C(CH3)3) with a series of heterocyclic methanol derivatives [tetrahydrofurfuryl alcohol (H-OTHF), thiophene methanol (H-OTPM), or 2-pyridylcarbinol (H-OPy)-collectively termed H-OR*], led to the isolation of a novel family of OR*-substituted titanium alkoxide precursors. Independent of the initial stoichiometry for the H-OTHF reaction, a monosubstituted, dinuclear species was isolated as [(ONep)3Ti(muc-OTHF)]2 (1). For 1, each Ti was octahedrally (Oh) bound by three terminal ONep ligands, one bidentate bridging OTHF ligand (muc-OTHF), and an oxygen from the other muc-OTHF ligand. For the OTPM derivatives, the product was identified as [(ONep)3Ti(mu-OTPM)]2 (2). For this ligand, the soft S atom does not bind to the Ti but the O atom does act as a bridge between the two trigonal bipyramidal bound Ti metal centers. The OPy system yielded (OPy)2Ti(OR)2 independent of the OR and the stoichiometry used [OR = ONep (3), OCHMe2 (4), OCMe3 (5)]. For 3-5, the two OPy ligands chelate to the Oh-bound Ti metal center with two terminal OR ligands. Compounds 1-5 were fully characterized using a variety of analytical techniques. An initial investigation of the proposed chemical stability of the '(OPy)2Ti' moiety of 3-5 to alcoholysis exchange pathways involving (i) alkyl alcohols, (ii) aryl alcohols, (iii) substituted phenols, (iv) H-OR* derivatives, and (v) silanols proved successful through the isolation of a novel family of structurally characterized (OPy)2Ti(OR')2 (7-24) compounds.     

X-ray structural characterization of imidazolylcobalamin and histidinylcobalamin: cobalamin models for aquacobalamin bound to the B12 transporter protein transcobalamin

The X-ray structures of imidazolylcobalamin (ImCbl) and histidinylcobalamin (HisCbl) are reported. These structures are of interest given that the recent structures of human and bovine transcobalamin prepared in their holo forms from aquacobalamin show a histidine residue of the metalloprotein bound at the beta-axial site of the cobalamin (Wuerges, J. et al. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 4386-4391). The beta-axial Co-N bond distances for ImCbl and HisCbl are 1.94(1) and 1.951(7) A, respectively. The alpha-axial Co-N bond distances to the 5,6-dimethylbenzimidazole are 2.01(1) and 1.979(8) A for ImCbl and HisCbl, respectively, and are typical for cobalamins with weak sigma-donor ligands at the beta-axial site. The corrin fold angles of 11.8(3) degrees (ImCbl) and 12.0(3) degrees (HisCbl) are smaller than those typically observed for cobalamins.     

Photodissociation dynamics of the reaction H2CO-->H+HCO via the singlet (S0) and triplet (T1) surfaces

We have explored the photodissociation dynamics of the reaction H(2)CO+hnu-->H+HCO in the range of 810-2600 cm(-1) above the reaction threshold. Supersonically cooled formaldehyde was excited into selected J(Ka,Kc) rotational states of six vibrational levels (1(1)4(1), 5(1), 2(2)6(1), 2(2)4(3), 2(3)4(1), and 2(4)4(1)) in the A((1)A2) state. The laser induced fluorescence spectra of the nascent HCO fragment provided detailed product state distributions. When formaldehyde was excited into the low-lying levels 1(1)4(1), 5(1), and 2(2)6(1), at E(avail)<1120 cm(-1), the product state distribution can be modeled qualitatively by phase space theory. These dynamics are interpreted as arising from a reaction path on the barrierless S0 surface. When the initial states 2(2)4(3) and 2(3)4(1) were excited (E(avail)=1120-1500 cm(-1)), a second type of product state distribution appeared. This second distribution peaked sharply at low N, Ka and was severely truncated in comparison with those obtained from the lower lying states. At the even higher energy of 2(4)4(1) (E(avail) approximately 2600 cm(-1)) the sharply peaked distribution appears to be dominant. We attribute this change in dynamics to the opening up of the triplet channel to produce HCO. The theoretical height of the barrier on the T1 surface lies between 1700 and 2100 cm(-1) and so we consider the triplet reaction to proceed via tunneling at the intermediate energies and proceed over the barrier at the higher energies. Considerable population was observed in the excited (0,0,1) state for all initial H(2)CO states that lie above the appearance energy. Rotational populations in the (0,0,1) state dropped more rapidly with (N,Ka) than did the equivalent populations in (0,0,0). This indicates that, although individual rotational states are highly populated in (0,0,1), the total v3=1 population might not be so large. Specific population was also measured in the almost isoenergetic Kc and J states. No consistent population preference was found for either asymmetry or spin-rotation component.     

Synthesis and chemistry of N‐arylated pyrano[2,3‐c]pyrazoles

Novel N2‐arylated pyrano[2,3‐c]pyrazol‐6‐ones 2 can be prepared in a selective manner by generating the anion of 1 ( R?H ) with lithium hexamethyldisilazide in DMF and quenching with activated aryl halides. Sterically demanding groups such as phenyl as in 5 reduce reactivity significantly while electronwithdrawing substituents such as trifluoromethyl and phenyl at C4 of the pyranone ring as in 10 and 15 render the pyranone carbonyl particularly susceptible to attack by nucleophiles resulting in ring‐opening to give novel crotonyl derivatives. Proof of structure required a variety of nmr methods involving proton, carbon, and nitrogen nuclei.     

Highly active/selective and adjustable zirconium polymerization catalysts stabilized by aminopyridinato ligands

This paper describes a substantial enhancement of the aminopyridinato ligand stabilized early transition metal chemistry by introducing the sterically very demanding 2,6-dialkylphenyl substituted aminopyridinato ligands derived from (2,6-diisopropylphenyl)-[6-(2,6-dimethylphenyl)-pyridin-2-yl]-amine (1a-H, ApH) and (2,6-diisopropylphenyl)-[6-(2,4,6-triisopropylphenyl)-pyridin-2-yl]- amine (1b-H, Ap^∗H). The corresponding bis aminopyridinato zirconium dichloro complexes, [Ap₂ZrCl₂] (3a) and [Ap₂^∗ZrCl₂] (3b) and the dimethyl analogues, [Ap₂ZrMe₂] (4a) and [Ap₂^∗ZrMe₂] (4b) (Me = methyl) were synthesized, using standard salt metathesis routes. Single-crystal X-ray diffraction was carried out for the dichloro derivatives. Both zirconium metal centers have a distorted octahedral environment with a cis-orientation of the chloride ligands in 3a and a closer to trans-arrangement in 3b. The dimethyl derivatives are proven to be highly active ethylene polymerization catalysts after activation with [R₂N(Me)H][B(C₆F₅)₄] (R = C₁₆H₃₃-C₁₈H₃₇). During attempted co-polymerizations of α-olefins (propylene) and ethylene high activity and selectivity for ethylene and nearly no co-monomer incorporation was observed. Increasing the steric bulk of the ligand going from (2,6-dimethylphenyl) to (2,4,6-triisopropylphenyl) substituted pyridines, switches the catalyst system from producing long chain α-olefins to polymerization of ethylene in a living fashion. In contrast to the dimethyl complexes only [Ap₂^∗ZrCl₂] in the presence of MAO at elevated temperature gave decent polymerization activity. NMR investigations of the reaction of dichloro complexes with 25 equiv. of MAO or AlMe₃ at room temperature revealed, that [Ap₂ZrCl₂] decomposes under ligand transfer to aluminum and formation of [ApAlMe₂], while [Ap₂^∗ZrCl₂] remains almost unreacted under the same conditions. The aminopyridinato dimethyl aluminum complexes, [ApAlMe₂] (5a) and [Ap^∗AlMe₂] (5b) were synthesized independently and structurally characterized. The aluminum complexes 5a and b show no catalytic activity towards ethylene, when “activated” with[R₂N(Me)H][B(C₆F₅)₄].     

Carbonylative ring opening of terminal epoxides at atmospheric pressure

The carbonylative opening of terminal epoxides under mild conditions has been developed using Co2(CO)8 as the catalyst. Under 1 atm of carbon monoxide and at room temperature in methanol, propylene oxide is converted to methyl 3-hydroxybutanoate in up to 89% yield. This transformation is general for many terminal epoxides bearing alkyl, alkenyl, aryl, alkoxy, chloromethyl, phthalimido, and acetal functional groups. The opening takes place without epimerization at the secondary stereocenter.     

Trisannulated benzene derivatives by Acid catalyzed aldol cyclotrimerizations of cyclic ketones. Methodology development and mechanistic insight

Several factors that contribute to the success of aldol cyclotrimerizations have been clarified as part of an effort to shed light on the inner workings of this century old reaction. The use of 4,7-di-tert-butylacenaphthenone (11) as a mechanistic probe molecule has led to intriguing discoveries about temperature, solvent, and solubility effects. Solvents that are both polarizable and somewhat polar, e.g., o-dichlorobenzene (ODCB), work best for the aromatic ketones examined. Certain Br?nsted acids were found to work better than Lewis acids as catalysts for the archetypal aldol cyclotrimerization of indanone (2) in aprotic solvents, and a strong dependence on the pKa of the acid was observed. A standardized protocol, using p-toluenesulfonic acid monohydrate, is shown to work well in a number of test cases.