Synthesis of beta-lactams from a N-rhenaimine: effect of the transition metal on the energetic profile of the Staudinger reaction

As depicted in the scheme, the alkylidenamido complex 1, a N-rhenaimine, reacts with ketenes to afford the beta-lactams 2-4, which possess a {Re(CO)3(bpy)} fragment as substituent at nitrogen. Clean demetalations using HOTf or MeOTf yield the free beta-lactams or N-methyl-beta-lactams along with [Re(OTf)(CO)3(bpy)]. DFT calculations help to rationalize why the reaction is faster than those of non transition metal N-substituted imines.     

NMR chemical shifts in the low–pH form of a-chymotrypsin. A QM/MM and ONIOM–NMR study

 The relationship between hydrogen bonding and NMR chemical shifts in the catalytic triad of low-pH α-chymotrypsin is investigated by combined use of the effective fragment potential [(2001) J Phys Chem A 105:293] and ONIOM–NMR [(2000) Chem Phys Lett 317:589] methods. Our study shows that while the His57 N^δ1−H bond is stretched by a relatively modest amount (to about 1.060 ?) this lengthening, combined with the polarization due to the molecular environment, is sufficient to explain the experimentally observed chemical shifts of 18.2 ppm. Furthermore, the unusual down-field shift of H^ɛ1 (9.2 ppm) observed experimentally is reproduced and shown to be induced by interactions with the C=O group of Ser214 as previously postulated. The free-energy cost of moving H^δ1 from His57 to Asp102 is predicted to be 5.5 kcal/mol. Received: 26 September 2001 / Accepted: 6 September 2002 / Published online: 21 January 2003 Contribution to the Proceedings of the Symposium on Combined QM/MM Methods at the 222nd National Meeting of the American Chemical Society, 2001 Correspondence to: J. H. Jensen e-mail: jan-jensen@uiowa.edu Acknowledgements. This work was supported by a Research Innovation Award from the Research Corporation and a type G starter grant from the Petroleum Research Fund. The calculations were performed on IBM RS/6000 workstations obtained through a CRIF grant from the NSF (CHE-9974502) and on supercomputers at the National Center for Supercomputer Applications at Urbana-Champaign. The authors are indebted to Visvaldas Kairys for help with the CHARMM program, and to Daniel Quinn for many helpful discussions.     

Self-assembly of [2]rotaxane exploiting reversible Pt(II)- pyridine coordinate bonds

A dinuclear self-assembled cationic macrocycle based on Pt(II)-N(pyridine) coordinative bonds and having competitive triflate anions, as metal counterions, is used in the construction of [2]rotaxane and [2]pseudorotaxane architectures assisted by hydrogen bonding. The kinetic lability of the Pt(II)-N(pyridine) coordinative bond controls the dynamics of the [2]rotaxane.     

Flow injection determination of bismuth in urine by successive retention of Bi(III) and tetrahydroborate(III) on an anion-exchange resin and hydride generation atomic absorption spectrometry

Bismuth as BiCl₄⁻ and BH₄⁻ ware successively retained in a column (150 mm × 4 mm, length × i.d.) packed with Amberlite IRA-410 (strong anion-exchange resin). This was followed by passage of an injected slug of hydrochloric acid resulting in bismuthine generation (BiH₃). BiH₃ was stripped from the eluent solution by the addition of a nitrogen flow and the bulk phases were separated in a gas–liquid separator. Finally, bismutine was atomized in a quartz tube for the subsequent detection of bismuth by atomic absorption spectrometry. Different halide complexes of bismuth (namely, BiBr₄⁻, BiI₄⁻ and BiCl₄⁻) were tested for its pre-concentration, being the chloride complexes which produced the best results. Therefore, a concentration of 0.3 mol l⁻¹ of HCl was added to the samples and calibration solutions. A linear response was obtained between the detection limit (3σ) of 0.225 and 80 μg l⁻¹. The R.S.D.% (n = 10) for a solution containing 50 μg l⁻¹ of Bi was 0.85%. The tolerance of the system to interferences was evaluated by investigating the effect of the following ions: Cu²⁺, Co²⁺, Ni²⁺, Fe³⁺, Cd²⁺, Pb²⁺, Hg²⁺, Zn²⁺, and Mg²⁺. The most severe depression was caused by Hg²⁺, which at 60 mg l⁻¹ caused a 5% depression on the signal. For the other cations, concentrations between 1000 and 10,000 mg l⁻¹ could be tolerated. The system was applied to the determination of Bi in urine of patients under therapy with bismuth subcitrate. The recovery of spikes of 5 and 50 μg l⁻¹ of Bi added to the samples prior to digestion with HNO₃ and H₂O₂ was in satisfactory ranges from 95.0 to 101.0%. The concentrations of bismuth found in six selected samples using this procedure were in good agreement with those obtained by an alternative technique (ETAAS). Finally, the concentration of Bi determined in urine before and after 3 days of treatment were 1.94 ± 1.26 and 9.02 ± 5.82 μg l⁻¹, respectively.     

Regularity of solutions and interfaces of a generalized porous medium equation inR N

Summary We consider the Cauchy problem for the generalized porous medium equation u_t=(u) where u=u(x, t), xRⁿ and t>0, and the initial datum u(x, 0) is assumed to be nonnegative, integrable mid to nave compact support. The nonlinearity (u) is a C¹ function defined for uO which grows like a power of u. Our assumptions generalize the porous medium case, (u)=u^m, m>1, and also include the equation of the Marshak waves. This problem has finite speed of propagation. We estimate the rate of growth of the support of the solution with precise estimates for t 0 and t. Our main result deals with the regularity of the solutions. We show that after a certain time t₀ the pressure, defined by v=(u), with (u)=(u)/u and (0)=0, is a Lipschitz-continuous function of x and t and the interface is a Lipschitz-continuous surface in R^N+1; the solution u is Hölder continuous for all times t> 0.Both authors partially supported by CAICYT, Project 2805-83. The second author also supported by USA-Spain Joint Research Grant CCB-8402023.     

Shock fluctuations in asymmetric simple exclusion

Summary The one dimensional nearest neighbors asymmetric simple exclusion process in used as a microscopic approximation to the Burgers equation. We study the process with rates of jumpsp>q to the right and left, respectively, and with initial product measure with densities < to the left and right of the origin, respectively (with shock initial conditions). We prove that a second class particle added to the system at the origin at time zero identifies microscopically the shock for all later times. If this particle is added at another site, then it describes the behavior of a characteristic of the Burgers equation. For vanishing left density (=0) we prove, in the scale t^1/2, that the position of the shock at timet depends only on the initial configuration in a region depending ont. The proofs are based on laws of large numbers for the second class particle.     

A moment problem associated to rational Szegő functions

Leta ₁,...,a _p be distinct points in the finite complex plane ?, such that |a _j|>1,j=1,..., p and let \(b_j = 1/\bar \alpha _j ,\) j=1,..., p. Let μ₀, μ _π ^(j) , ν _π ^(j) j=1,..., p;n=1, 2,... be given complex numbers. We consider the following moment problem. Find a distribution ψ on [?π, π], with infinitely many points of increase, such that $$\begin{array}{l} \int_{ - \pi }^\pi {d\psi (\theta ) = \mu _0 ,} \\ \int_{ - \pi }^\pi {\frac{{d\psi (\theta )}}{{(e^{i\theta } - a_j )^n }} = \mu _n^{(j)} ,} \int_{ - \pi }^\pi {\frac{{d\psi (\theta )}}{{(e^{i\theta } - b_j )^n }} = v_n^{(j)} ,} j = 1,...,p;n = 1,2,.... \\ \end{array}$$ It will be shown that this problem has a unique solution if the moments generate a positive-definite Hermitian inner product on the linear space of rational functions with no poles in the extended complex plane ?^* outside {a ₁,...,a _p,b ₁,...,b _p}.     

Optimal allocation of replicas in parallel tempering simulations

We have studied the efficiency of parallel tempering simulations for a variety of systems including a coarse-grained protein, an atomistic model polypeptide, and the Lennard-Jones fluid. A scheme is proposed for the optimal allocation of temperatures in these simulations. The method is compared to the existing empirical approaches used for this purpose. Accuracy associated with the computed thermodynamic quantities such as specific heat is also computed and their dependence on the trial-exchange acceptance rate is reported.     

Palladium-catalyzed cascade reaction of alpha,beta-unsaturated sulfones with aryl iodides

Unlike traditionally used acyclic 1,2-disubstituted alkenes, the reaction of alpha,beta-unsaturated phenyl sulfones with aryl iodides under Heck reaction conditions (Pd(OAc)(2) as catalyst, Ag(2)CO(3) as base in DMF at 120 (0)C) takes place mainly by a cascade process, involving one unit of the alkene and three units of the aryl iodide, to afford a substituted 9-phenylsulfonyl-9,10-dihydrophenanthrene. The dominant formation of this 3:1 coupling product, instead of the Heck trisubstituted olefin, shows that aromatic C-H bond activation processes can compete with the usually fast syn beta-hydrogen elimination step in the Heck arylation of an acyclic olefin. The structural scope of this palladium-catalyzed cascade arylation of alpha,beta-unsaturated sulfones has proved to be wide with regard to substitution at the beta-position (alkyl, aryl, or alkenyl substitution), substitution at the sulfone unit (alkyl or phenyl sulfones), and configuration at the CdoublebondC bond (trans or cis). Moreover, although less favored than in the case of the arylation of alpha,beta-unsaturated sulfones, similarly substituted 9,10-dihydrophenanthrenes have also been obtained in the case of alpha,beta-unsaturated phosphine oxides and alpha,beta-unsaturated phosphonate esters. A Pd(0)-Pd(II)-Pd(IV) mechanistic pathway involving the successive formation of highly electrophilic sigma-alkylpalladium intermediates and palladacycles is proposed for this multicomponent arylation.