New silica-immobilized chiral amino alcohol for the enantioselective addition of diethylzinc to benzaldehyde

[reaction: see text] A readily available chiral amino alcohol has been immobilized on silica by sol-gel synthesis and grafting. The solid prepared according to the latter method led to the best enantioselectivity (77% ee) described for the asymmetric addition of diethylzinc to benzaldehyde using inorganic solids.     

“Meta”-rigid motions and frames of reference

We define the meta-rigid motions as particular classes of time-like congruences which are solutions of intrinsically defined partial differential equations that generalize Born's conditions. We consider in particular two hierarchies of such congruences. The first one is a geometrically motivated direct generalization of the symmetry concept inherent in Born congruences. The second one is an indirect generalization based on the conditions which guarantee the existence of a particular class of adapted coordinates of space, named quo-harmonic coordinates, whose definition is akin to the definition of harmonic coordinates but which differs from it in an essential point.     

The curvature of the conical intersection seam: an approximate second-order analysis

We present a method for analyzing the curvature (second derivatives) of the conical intersection hyperline at an optimized critical point. Our method uses the projected Hessians of the degenerate states after elimination of the two branching space coordinates, and is equivalent to a frequency calculation on a single Born-Oppenheimer potential-energy surface. Based on the projected Hessians, we develop an equation for the energy as a function of a set of curvilinear coordinates where the degeneracy is preserved to second order (i.e., the conical intersection hyperline). The curvature of the potential-energy surface in these coordinates is the curvature of the conical intersection hyperline itself, and thus determines whether one has a minimum or saddle point on the hyperline. The equation used to classify optimized conical intersection points depends in a simple way on the first- and second-order degeneracy splittings calculated at these points. As an example, for fulvene, we show that the two optimized conical intersection points of C2v symmetry are saddle points on the intersection hyperline. Accordingly, there are further intersection points of lower energy, and one of C2 symmetry--presented here for the first time--is found to be the global minimum in the intersection space.     

Triplet-state formation along the ultrafast decay of excited singlet cytosine

We address the possibility of populating the lowest triplet state of cytosine by an "intrinsic"mechanism, namely, intersystem crossing (ISC) along the ultrafast internal conversion pathway of the electronically excited singlet species. For this purpose, we present a discussion of the ISC process and triplet-state reactivity based on theoretical analysis of the spin-orbit strength and the potential energy surfaces for the relevant singlet and triplet states of cytosine. High-level ab initio computations show that ISC is possible in wide regions of the singlet manifold along the reaction coordinate that controls the ultrafast internal conversion to the ground state. Thus, the ISC mechanism documented here provides a possibility to access the triplet state, which has a key role in the photochemistry of the nucleic acid bases.     

The Ni-mediated cyclocarbonylation of allyl halides and alkynes made catalytic. Evidence supporting the involvement of pseudoradical Ni(i) species in the mechanism

We report here on a highly efficient catalytic method to synthesize intermolecularly the cyclopentane skeleton from starting products as simple as allyl halides, alkynes, and carbon monoxide under very mild reaction conditions by means of a substoichiometric amount of iron, acetone, and a catalytic amount of Ni(II) iodide.     

Theoretical study of benzotriazole UV photostability: ultrafast deactivation through coupled proton and electron transfer triggered by a charge-transfer state

A theoretical CASSCF study of the reaction path for excited-state intramolecular proton transfer (ESIPT) for a model system derived from the UV absorber 2-(2'-hydroxyphenyl) benzotriazole without the fused benzo ring on the triazole has been carried out. A planar reaction path can be optimized but is shown to have no physical significance. The true reaction path involves twisted geometries. Adiabatic proton transfer is triggered by a charge-transfer from the phenol to the triazole group, and is followed by radiationless decay at the keto form. Along the nonplanar reaction path, there is a coupled proton and electron transfer in a manner similar to tryptophan. This rationalizes unexpected experimental results on the effect of electron withdrawing substituent groups on the photostability. The coupled proton and electron transfer is followed by a barrierless relaxation in the ground state to recover the enol form. An alternative photostabilization pathway from a phenyl localized state has also been documented and is similar to the channel 3 decay pathway in benzene photochemistry. Additionally, a long-lived intermediate for a twisted intramolecular charge-transfer (TICT) state has been identified as the species potentially responsible for the increase of blue fluorescence in strongly polar media.     

Quenching of tryptophan (1)(pi,pi*) fluorescence induced by intramolecular hydrogen abstraction via an aborted decarboxylation mechanism

CASSCF computations show that the hydrogen-transfer-induced fluorescence quenching of the (1)(pi,pi*) excited state of zwitterionic tryptophan occurs in three steps: (1) formation of an intramolecular excited-state complex, (2) hydrogen transfer from the amino acid side chain to the indole chromophore, and (3) radiationless decay through a conical intersection, where the reaction path bifurcates to a photodecarboxylation and a phototautomerization route. We present a general model for fluorescence quenching by hydrogen donors, where the radiationless decay occurs at a conical intersection (real state crossing). At the intersection, the reaction responsible for the quenching is aborted, because the reaction path bifurcates and can proceed forward to the products or backward to the reactants. The position of the intersection along the quenching coordinate depends on the nature of the states and, in turn, affects the formation of photoproducts during the quenching. For a (1)(n,pi*) model system reported earlier (Sinicropi, A.; Pogni, R.; Basosi, R.; Robb, M. A.; Gramlich, G.; Nau, W. M.; Olivucci, M. Angew. Chem., Int. Ed. 2001, 40, 4185-4189), the ground and the excited state of the chromophore are hydrogen acceptors, and the excited-state hydrogen transfer is nonadiabatic and leads directly to the intersection point. There, the hydrogen transfer is aborted, and the reaction can return to the reactant pair or proceed further to the hydrogen-transfer products. In the tryptophan case, the ground state is not a hydrogen acceptor, and the excited-state hydrogen transfer is an adiabatic, sequential proton and electron transfer. The decay to the ground state occurs along a second reaction coordinate associated with decarboxylation of the amino acid side chain and the corresponding aborted conical intersection. The results show that, for (1)(pi,pi*) states, the hydrogen transfer alone is not sufficient to induce the quenching, and explain why fluorescence quenching induced by hydrogen donors is less general for (1)(pi,pi*) than for (1)(n,pi*) states.     

Mechanism of an Exceptional Class of photostabilizers: a seam of conical intersection parallel to excited state intramolecular proton transfer (ESIPT) in o-hydroxyphenyl-(1,3,5)-triazine

We present a detailed CASSCF study of the mechanism of excited-state intramolecular proton transfer (ESIPT) in the o-hydroxyphenyl triazine class of photostabilizers. The valence-bond analysis of the ground state and the two pipi* excited states permits a simple chemical interpretation of the mechanistic information. Our results show that the barrier to enol-keto tautomerism on the ground-state adiabatic surface is high. Following photoexcitation to the charge-transfer state, the ESIPT is predicted to take place without a barrier. Radiationless decay to the ground state is associated with an extended seam of conical intersection, with a sloped topology lying parallel to the ESIPT path, which can be accessed at any point along the reaction path. Our results show that the triazine class of photostabilizers has the photochemical and photophysical qualities associated with exceptional photostability.     

On Linear Configurations in Subsets of Compact Abelian Groups,and Invariant Measurable Hypergraphs

We prove an arithmetic removal result for all compact abelian groups, generalizing a finitary removal result of Král’, Serra, and the third author. To this end, we consider infinite measurable hypergraphs that are invariant under certain group actions, and for these hypergraphs we prove a symmetry-preserving removal lemma, which extends a finitary result of the same name by the second author. We deduce our arithmetic removal result by applying this lemma to a specific type of invariant measurable hypergraph. As a direct consequence of our removal result, we obtain the following generalization of Szemerédi’s theorem: for any compact abelian group G, any measurable set \({A \subseteq G}\) with Haar probability \({\mu(A) \geq \alpha > 0}\) satisfies \({\int_{G}\int_{G}1_{A}(x)1_{A}(x+r)...1_{A}(x+(k-1)r)\, {\rm d} \mu(x)\, {\rm d} \mu(r) \geq c}\), where the constant \({c=c(\alpha, k) > 0}\) is valid uniformly for all G. This result is shown to hold more generally for any translationinvariant system of r linear equations given by an integer matrix with coprime \({r \times r}\) minors.     

Competitive Cd2+/H+ complexation to polyacrylic acid described by the stepwise and intrinsic stability constants

Stepwise constants can be used to describe competitive proton and metal binding to macromolecules with a large number of sites. With the aim of accessing information on the microscopic binding model, we report an expression that connects the stepwise constants to the site-specific metal constants. This expression holds for a very general complexation model including heterogeneity, interactions, and chelate complexation. Assuming bidentate binding of the Cd ions to adjacent carboxylate groups in poly(acrylic acid), stepwise and intrinsic stability constants for proton and cadmium binding were estimated from the experimental data. Intrinsic values were split into specific and electrostatic contributions (by means of the Poisson-Boltzmann equation under cylindrical geometry). Free of the electrostatic contribution, the remaining Cd binding energy showed almost no dependence on the coverage and ionic strength, and the corresponding average values allowed for a reasonable reproduction of raw binding data. Small systematic discrepancies from the homogeneous behavior are critically discussed.