Cyclic alkenenitriles: synthesis, conjugate addition, and stereoselective annulation

O-Alkylation of unsaturated silyl cyanohydrins with DMSO-Ac2O triggers a rearrangement to methylthiomethyl-protected hydroxyalkenenitriles that are easily hydrolyzed for subsequent annulations with omega-chloroalkyl Grignard reagents. Deprotonating the gamma-hydroxyalkenenitriles with t-BuMgCl followed by addition of omega-chloroalkyl Grignard reagents triggers a conjugate addition-alkylation sequence leading exclusively to cis-octalins, hydrindanes, and decalins. Stereoelectronic control favors an axial conjugate addition leading to a particularly reactive conformer that rapidly cyclizes to cis-fused bicyclic nitriles, whereas generating the ring-flipped conformer, through a stepwise sequence, allows access to the diastereomeric trans-decalin. Collectively, the rearrangement-annulation sequence represents the first general annulation of alkenenitriles to assemble diverse bicyclic nitriles with complete control over the two newly installed stereocenters.     

Novel iminium ion equivalents prepared through C-H oxidation for the stereocontrolled synthesis of functionalized propargylic amine derivatives

Access to stereochemically complex, polyfunctionalized amine derivatives is made possible using novel oxathiazinane N,O-acetal starting materials. These heterocycles are prepared through intramolecular sulfamate ester C-H insertion with a Rh(2+)-carboxylate catalyst and PhI(OAc)(2) as the terminal oxidant. Such compounds function as unique iminium ion equivalents to which nucleophilic alkynylzinc reagents add smoothly in the presence of BF(3).OEt(2). The coupled products are isolated in high yield (63-92%) and with good levels of diastereoinduction (6 --> 20:1). The alkyne-substituted oxathiazinanes serve as versatile building blocks and may be further manipulated through nucleophilic ring-opening reactions of the sulfamate core. The efficient construction of 1,7,8-trihydroxyindolizidine in six steps and in 34% overall yield highlights the power of these combined methods for synthesis.     

Femtosecond pump-probe measurements of solvation by hydrogen-bonding interactions.

An additional ultrafast blue shift in the transient absorption spectra of hydrogen-bonding complexes of a strong photoacid, 8-hydroxypyrene 1,3,6-trisdimethylsulfonamide (HPTA), over the solvation response of the uncomplexed HPTA and also over that of the methoxy derivative of the photoacid (MPTA) in the presence of the hydrogen-bonding base was observed on optical excitation of the photoacid. The additional 55 +/- 10 fs solvation response was found to be about 35 % and 19% of the total C(t) of HPTA in dichloromethane (DCM) when it was hydrogen-bonded to dimethylsulfoxide (DMSO) and dioxane, respectively, and about 29% of the total C(t) of HPTA in dichloroethane (DCE) when it was hydrogen-bonded to DMSO. We have assigned this additional dynamic spectral shift to a transient change in the hydrogen bond (O-H...O) that links HPTA to the complexing base, after the electronic excitation of the photoacid.     

Alkenes from terminal epoxides using lithium 2,2,6,6-tetramethylpiperidide and organolithiums or grignard reagents

E-Alkenes (including arylated alkenes, dienes, and allylsilanes) are efficiently prepared by alpha-deprotonation of terminal epoxides using lithium 2,2,6,6-tetramethylpiperidide, followed by in situ trapping with organolithiums or Grignard reagents.     

Substituent Effects on the Photocleavage of Benzyl-Sulfur Bonds. Observation of the "Meta Effect"

Benzyl phenyl sulfide has been used to investigate the photocleavage mechanism for benzyl-sulfur bonds. Four experiments have shown that the reaction goes through a radical intermediate. First, the photoproducts observed can all be justified by radical mechanisms. Second, the radical intermediate was trapped with a five hexenyl tether. Third, UV analysis of analogs for the 4-NO(2) derivative indicate no exciplex or electron transfer pathway. Fourth, no strong correlation is observed between sigma values and the quatum yields for loss of substituted benzyl phenyl sulide. The effect of oxygen on quantum yields is best observed after samples are thoroughly outgassed with consecutive freeze-pump-thaw cycles. It is shown that oxygen diminishes the substituent effect. Upon photolysis of the outgassed samples, the meta-substituted derivatives showed more significant variances than the para derivatives. The meta derivatives are most efficiently cleaved in the following order: 3-CN > 3-NO(2) > 3-CF(3) > 3-CH(3) > 3-OCH(3). These findings are justified by an increase in electron density of the radical ipso to the forming benzyl radical for the 3-OCH(3) derivative and a decrease in the electron density of the radical ipso to the forming benzyl radical for the 3-CN derivative.     

Vibrationally controlled chemistry: mode- and bond-selected reaction of CH3D with Cl

Selective vibrational excitation controls the competition between C-H and C-D bond cleavage in the reaction of CH(3)D with Cl, which forms either HCl + CH(2)D or DCl + CH(3). The reaction of CH(3)D molecules with the first overtone of the C-D stretch (2nu(2)) excited selectively breaks the C-D bond, producing CH(3) exclusively. In contrast, excitation of either the symmetric C-H stretch (nu(1)), the antisymmetric C-H stretch (nu(4)), or a combination of antisymmetric stretch and CH(3) umbrella bend (nu(4) + nu(3)) causes the reaction to cleave only a C-H bond to produce solely CH(2)D. Initial preparation of C-H stretching vibrations with different couplings to the reaction coordinate changes the rate of the H-atom abstraction reaction. Excitation of the symmetric C-H stretch (nu(1)) of CH(3)D accelerates the H-atom abstraction reaction 7 times more than excitation of the antisymmetric C-H stretch (nu(4)) even though the two lie within 80 cm(-1) of the same energy. Ab initio calculations and a simple theoretical model help identify the dynamics behind the observed mode selectivity.     

Turning the [Ru(bpy)2dppz]2+ light-switch on and off with temperature

We report temperature-dependent excited-state lifetime measurements on [Ru(bpy)(2)dppz](2+) in both protic and aprotic solvents. These experiments yield a unifying picture of the excited-state photophysics that accounts for observations in both types of solvent. Our measurements support the notion of bpy-like and phz-like states associated with the dppz ligand and show that the ligand orbital associated with the bright state is similar in size to the corresponding orbital in the (3)MLCT state of [Ru(bpy)(3)](2+). In contrast to the current thinking, the experiments presented here indicate that the light-switch effect is not driven by a state reversal. Rather, they suggest that the dark state is always lowest in energy, even in aprotic solvents, and that the light-switch behavior is the result of a competition between energetic factors that favor the dark state and entropic factors that favor the bright (bpy) state.     

The Use of Modified Zr-nPropoxide in the Consolidation of Calcite: A Preliminary Study Focused into the Conservation of Cultural Heritage

The monomeric precursor Zr-npropoxide was modified using chelating agents. The resulting sols were used to compose a consolidant sol that included coupling agents selected based on previous studies done by ourselves, the hydrophobic agent trimethyl methoxysilane (TMS) and mesitylene as drying control chemical additive (DCCA). The sols were studied by Fourier Transform Infrared Spectroscopy (FTIR). In a later stage calcite powder was impregnated with the sols and studied by Diffuse Reflectance Infrared Spectroscopy (DRIFT). According to the DRIFT results new bands appeared, that indicate interactions between the coupling agents and calcite. The Kratky plots obtained from the sols indicate that in all cases fiber-like oligomeric structures were obtained, having a fractal dimension value 1.60.     

Determination of the effective charge on muonium during its reaction in aqueous solution

The rate of reaction of muonium atoms with solutes of either charge is unaffected by the addition of a high concentration of an inert salt, therefore the effective charge on the muonium at the point of reaction is essentially zero.     

Cellulose crystallites

This article discusses advances in understanding the structural and physicochemical characteristics of suspensions of cellulose crystallites prepared by acid hydrolysis of natural cellulose fibres. Consideration of recent developments in visualization of crystallite ultrastructure may provide clues to suspension behavior. In addition, novel applications in a diverse range of fields are presented, from iridescent pigments to biomolecular NMR studies.