Counterion and solvent effects on the dilute solution viscosity of polystyrene ionomers

We present an analysis of data on the intrinsic viscosity [η] of sulfo-polystyrene ionomers in several solvents for a variety of sulfonation levels and counterions. For solvents of low dielectric constant, 2 < ε < 18, [η] decreases from the base polymer value [η]₀ with increasing substitution level. This behavior was attributed to intramolecular association of ionic dipoles. The ratio [η]/[η]₀ was found to depend on a single reduced variable α_Aα_Sx, where x is the fractional substitution, α_A depends only on the counterion, and α_S ∝ ε^?1 depends only on the solvent. For solvents of high dielectric constant, 36 < ε < 47, [η] increases approximately as x³, and counterion effects are small. This behavior was attributed to ionic dissociation, giving rise to a polyelectrolyte effect. Implications of the low ε results are discussed in relation to association-induced gelation behavior and possible generalizations of the reduced variables approach.     

Development of an effective chiral auxiliary for hydroxyalkyl radicals

The development of an effective chiral auxiliary for hydroxyalkyl radicals is delineated. Both the 2-tetrahydropyranyl (THP) and tri-O-benzyl-2-deoxy-alpha-D-glucopyranosyl (GLU) auxiliaries resulted in diastereoselective radical additions to methyl acrylate at -78 degrees C (ds = 6/1 and 11/1, respectively). The developing stereochemistry at the radical center was completely under auxiliary control. Correlation experiments showed that the D-GLU auxiliary led to attack on the radical Si-face. The selectivity of these radical additions dropped-off considerably when the more reactive 2-nitropropene trap was employed. Computational studies suggested that the observed facial selectivity was due primarily to entropic factors in the transition state but that a smaller temperature-dependent enthalpic contribution was also involved. It was hypothesized that incorporation of a quaternary center at C-6 (THP numbering) would restore the facial selectivity with more reactive radical traps by restricting the orientations available to the incoming alkene. In the event, the trans-6-tert-butyltetrahydropyranyl (tBu-THP) auxiliary resulted in very good diastereoselection with 2-nitropropene (ds = 35/1 at -78 degrees C, 15/1 at 0 degrees C, and 8/1 at RT) as did the tri-O-benzyl-6,6-dimethyl-2-alpha-D-deoxyglucopyranosyl (diMe-GLU) auxiliary during additions to ethyl alpha-trifluoroacetoxyacrylate (ds = 10/1 at 0 degrees C). A protocol for recovery of the sugar-derived chiral auxiliaries was also established. This work sets the stage for the development of a novel approach to 1, 3, 5.(2n + 1) polyols based on iterative radical homologation as well as the application of these pyranosidic auxiliaries to other synthetically important reactions.     

Molecular sieving of lambda phage DNA in polyacrylamide solutions as a function of the molecular weight of the polymer.

Electrophoresis of lambda phage DNA was carried out in solutions at various concentrations of uncrosslinked polyacrylamide of 0.6, 1, 5 and 9 x 10(6) molecular weight (Mw) with narrow Mw distribution. By inspection of mobilities in the various concentration ranges, it appears that mobilities decrease, and retardation increases, with increasing Mw. The relation between electrophoretic retardation and the Mw of the polymer was also interpreted (i) in the manner previously applied to nonlinear Ferguson plots and compatible with the Ogston model; and (ii) empirically, on the basis of the first derivatives of the functions describing the Ferguson plots at the polymer concentrations used. Interpretation (i) shows that the retardation increases linearly in the order of 0.6, 1, 5 and 9 x 10(6) Mw of polyacrylamide. Interpretation (ii) shows a nonlinear increase of retardation in the Mw range 5 to 9 x 10(6), and a decrease in retardation as Mw is raised from 0.6 to 5.0 x 10(6). Hypothetically, interpretation (ii) can be explained mechanistically by a progressive change, as the polymer size is increased, from a collision with the surface of the polymer fiber to one occurring after permeation in the interior of a random-coiled fiber. Interpretation (i) may fail to detect that change due to the large difference between DNA mobility in solutions of the smallest polymer and the free mobility. DNA peak detection in all of the four size classes of polyacrylamide in solution is limited to relatively narrow ranges of polymer concentration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Photolithographic synthesis of cyclic peptide arrays using a differential deprotection strategy

We report here a strategy for the photolithographic synthesis of diverse, spatially addressable arrays of cyclic peptides which employs a differential deprotection strategy for the combinatorial addition of side chains to a pre-fabricated cyclic core.     

Mononuclear nitrogen/sulfur-ligated zinc methoxide and hydroxide complexes: investigating ligand effects on the hydrolytic stability of zinc alkoxide species

The synthesis and properties of mononuclear zinc methoxide ([(ebnpa)Zn-OCH3]ClO4) (1) and hydroxide ([(ebnpa)Zn-OH]ClO4) (2) complexes of a new mixed nitrogen/sulfur ligand (ebnpa = N-2-(ethylthio)ethyl-N,N-bis(6-neopentylamino-2-pyridylmethyl)amine) are reported. The structures of 1 and 2 were determined by X-ray diffraction. Each possesses a single zinc-coordinated anion (methoxide or hydroxide) and exhibits an overall trigonal bipyramidal geometry. Structural and spectroscopic studies indicate the presence of two hydrogen-bonding interactions involving the oxygen atom of the zinc-bound anion in each complex. Treatment of [(ebnpa)Zn-OH]ClO4 with CH3OH results in the formation of an equilibrium mixture of 1 and 2. 1H NMR spectroscopic methods were used to examine the equilibrium as a function of temperature, yielding KMe (304 K) = 0.30(8), DeltaHMe = -0.9(1) kcal/mol, and DeltaSMe = -5(1) eu. The negative enthalpy indicates that spontaneous zinc alkoxide formation from a hydroxide precursor occurs in this system at low temperature. Using the experimentally determined DeltaHMe value, we found the homolytic Zn-O bond dissociation energy (BDE) in the Zn-OCH3 unit to be approximately -14 kcal/mol relative to the Zn-O BDE in the Zn-OH unit.     

Asymmetric synthesis and applications of beta-amino Weinreb amides: asymmetric synthesis of (S)-coniine

Conjugate addition of lithium (S)-N-benzyl-N-alpha-methylbenzylamide to a range of alpha, beta-unsaturated Weinreb amides proceeds with high levels of diastereoselectivity (>95% de). The beta-amino Weinreb amide products may be transformed into beta-amino ketones via reactions with Grignard reagents, while treatment with DIBAL-H furnishes beta-amino aldehydes. Trapping of the aldehyde via Wadsworth-Emmons reaction and subsequent manipulation offers an efficient route to homochiral delta-amino acid derivatives and 2-substituted piperidines. The application of this methodology for the synthesis of (S)-coniine is demonstrated.     

Muonium formation and the “missing fraction” in vapors

The vapor phase fractional polarizations of positive muons thermalizing as the muonium atom (P _M) and in diamagnetic environments (P _D) has been measured in H₂O, CH₃OH, C₆H₁₄, C₆H₁₂, CCl₄, CHCl₃, CH₂Cl₂ and TMS, in order to compare with the corresponding fractions measured in the condensed phases. There is a marked contrast in every case, with the vapor phase results being largely understandable in terms of a charge exchange/hot atom model. Unlike the situation in the corresponding liquids, there is no permanent lost fraction in the vapor phase in the limit of even moderately high pressures (1 atm); at lower pressures, depolarization is due to hyperfine mixing and is believed to be well understood. For vapor phase CH₃OH, C₆H₁₄, C₆H₁₂, and TMS therelative fractions are found to be pressure dependent, suggesting the importance of termolecular hot atom (or ion) reactions in the slowing down process. For vapor phase H₂O and the chloromethanes, the relative fractions are pressure independent. For CCl₄,P _M=P _D0.5 in the vapor phase vs.P _D=1.0 in the liquid phase; fast thermal reactions of Mu likely contribute significantly to this difference in the liquid phase. For H₂O,P _M 0.9 andP _D0.1 in the vapor phase vs.P _D 0.6 andP _M0.2 in the liquid phase. Water appears to be the one unequivocal case where the basic charge exchange/hot atom model is inappropriate in the condensed phase, suggesting, therefore, that radiation induced spur effects play a major role.