Bridged-ring products from the acyloin-like cyclization of diterpenoid keto-esters

Bridged-ring structures are readily formed from acyloin-like cyclization of suitably constituted keto esters. thus reaction of the keto ester ent-seco-kauranoid derivatives 1 and 2 with Na-liquid NH₃ affords mixtures of ent-kaurane, ent-beyerane and dimeric products.     

Vapor-liquid equilibrium predictions of refrigerant mixtures from a cubic equation of state with a G-EoS mixing rule

A modified excess Gibbs energy model which is based on the local composition concept and assigns a single energy parameter per pair of components, is incorporated into the G^E—EoS thermodynamic formalism for vapor-liquid equilibrium (VLE) calculations of simple and complex refrigerant mixtures. One temperature set of data close to 273 K is used to obtain the model's parameters, which are used to extrapolate the VLE at other temperatures and pressures. A one-parameter form of the model based on the Wong-Sandler mixing rule is presented for several simple systems. The physical significance of the model's energy parameter is connected to the preference of the mixture for like to unlike interactions. The model is applied for VLE predictions of the ternary system R14-R23-R13, and the results are compared to calculations using the 3PWS model [H. Orbey. S.I. Sandler, Ind. Eng. Chem. Res. 34 (1995) 2520–2525] and the van der Waals mixing rule. Modelling of a few complex systems with only three data points given at each temperature is shown with a two parameter version of our model on the basis of the Huron-Vidal mixing rule.     

Using Stationary Points on Potential Energy Surfaces to Model Intermolecular Interactions and Retention in Gas Chromatography

The utility of electronic structure methods for studying and predicting interactions in gas chromatography is explored using a simplified model of polyethylene glycol with a homologous series of normal alcohols. Relative interaction energies were determined using stabilization energies taken at stationary points on the analyte/stationary phase potential energy surfaces using semi-empirical, ab initio, and density functional theory. Second order Mø øller-Plesset electronic structure method produced good qualitative agreement with experiment, clearly indicating the need for a model that includes weak dispersion forces.     

A novel palladium(0) catalysed tandem 1,3-allyl shift and Heck arylation

On treatment with Pd(PPh₃)₄ allyl vinyl ether (1) undergoes a Pd(0) catalysed 1,3 oxygen to carbon allyl shift to afford -allyl ketone (2). In the presence of both Pd(PPh₃)₄ and base the allyl vinyl ether undergoes a Pd(0) catalysed tandem 1,3 allyl shift and intramolecular Heck arylation to give the spiro indane (3). Mechanistic investigations suggest that the 1,3-allyl shift proceeds via a π-allyl palladium intermediate.     

Catalyst design in C–H activation: a case study in the use of binding free energies to rationalise intramolecular directing group selectivity in iridium catalysis

Remote directing groups in a bifunctional molecule do not always behave independently of one another in C–H activation chemistries. A combined DFT and experimental mechanistic study to provide enhanced Ir catalysts for chemoselective C–H deuteration of bifunctional aryl primary sulfonamides is described. This provides a pharmaceutically-relevant and limiting case study in using binding energies to predict intramolecular directing group chemoselectivity. Rational catalyst design, guided solely by qualitative substrate–catalyst binding free energy predictions, enabled intramolecular discrimination between competing ortho-directing groups in C–H activation and delivered improved catalysts for sulfonamide-selective C–H deuteration. As a result, chemoselective binding of the primary sulfonamide moiety was achieved in the face of an intrinsically more powerful pyrazole directing group present in the same molecule. Detailed DFT calculations and mechanistic experiments revealed a breakdown in the applied binding free energy model, illustrating the important interconnectivity of ligand design, substrate geometry, directing group cooperativity, and solvation in supporting DFT calculations. This work has important implications around attempts to predict intramolecular C–H activation directing group chemoselectivity using simplified monofunctional fragment molecules. More generally, these studies provide insights for catalyst design methods in late-stage C–H functionalisation.

In C–H activation chemistries, the interpretation of the influence of remote directing groups in a bifunctional molecule depends on the in silico method used to inform catalyst design.     

Characterization of the photoisomers from cis- and trans-chlordanes, trans-nonachlor and heptachlor epoxide

Irradiation of acetone solutions of trans-chlordane (4a) and trans-nonachlor (4b) with UV light produces new half-cage photoisomers (5a or 6a and 5b or 6b, respectively) with bridging that differs from that of the photoisomers (2 and 8a) obtained from cis-chlordane (1b) and heptachlor epoxide (7). A new photoisomer (10) obtained from heptachlor epoxide is transformed into photoisomer 8a on further irradiation. Detailed PMR and ¹³C-NMR studies establish the structures of the new photoisomers and permit a decision between alternative formulations for the structures of the half-cage photoisomers of heptachlor epoxide and cis-chlordane,     

The μ3-cyclopentadienylidene ligand: X-ray structure of [Ru4(CO)5 {P(OMe)3}(μ3-C5H4)2(η-C5H5)2]

UV irradiation of [Ru₂(CO)₄(η-C₅H₅)₂] yields the tri- and tetra-ruthenium complexes [Ru₂(CO)₄(η-C₅H₅){η-C₅H₄Ru(CO)₂(η-C₅H₅)}] and [Ru₄(CO)₆(μ₃-C₅H₄)₂(η-C₅H₅)₂]. The μ₃-C₅H₄ ligand in the latter has been characterised through an X-ray diffraction study on [Ru₄(CO)₅{P(OMe)₃}(μ₃-C₅H₄)₂(η-C₅H₅)₂].     

C-C bond formation at polynuclear metal centers

Studies on C-C bond formation between simple hydrocarbon species such as CH₂, C=CH₂, CH=CH₂, CH₂=CH₂, CH₂=C=CH₂ and CHCH at a diruthenium center suggest that the process is promoted when the dimetal center can readily compensate for the two electrons lost in the formation of the new C-C bond. Thus, whereas -CH₂ and ethene combine only under forcing conditions, the combination of -CH₂ with allene or ethyne, which have additional -electrons available for coordination, occurs readily at room temperature. Likewise, the availability of uncoordinated -electrons in -C=CH₂ allows vinylidene to link rapidly with ethene at room temperature. Alkyne complexes [Ru₂(CO)(-RCCR)(-C₅H₅)₂] (R=CF₃ or Ph) react only under vigorous conditions with additional alkyne to give [Ru₂(CO)(-C₄R₄) (-C₅H₅)₂], but give these same species at room temperature in the presence of acid, shown to be due to the intermediacy of highly reactive 30-electron -vinyl cations. Thermally, alkyne linking proceedsvia three-alkyne species [Ru₂(-C₆R₆)(-C₅H₅)₂] to a four-alkyne complex [Ru₂(-C₈R₈)(-C₅H₅)₂], containing an unprecedented C₈ ligand composed of a C₆ ring with a C₂ tail. Treatment of [Ru₂(CO)(-RCCR)(-C₅H₅)₂] with unsaturated metal fragments gives trimetal complexes such as [Ru₃(CO)₅(₃-CF₃CCCF₃) (-C₅H₅)₂]. The MeCN derivative of this species undergoes unusual linking processes on reaction with additional alkyne to giveinter alia [Ru₃(CO)₃(₃-CCF₃){₃-C₃(CF₃)₃}(-C₅H₅)₂], arising from alkyne cleavage, and [Ru₃(CO)₃{₃-C₄(CF₃)₂(CO₂Me)₂}(-C₅H₅)₂], a closo-pentagonal bipyramidal Ru₃C₄ cluster.     

The inverse hull of the free semigroup on a set X

In this paper the inverse hull of the free semigroup on a non-empty set X is characterized. It is shown that if F^O is a certain semilattice and (G,*) is the free group on X, then is isomorphic to a Rees factor semigroup of a subsemigroup of (F^O x G, #), where (A,w)#(B,u) = ((A*w B)*w^–1, w*u). Several properties of , including Green's relations on are also determined.