The structural theory and physical organic chemistry

Woolley's revolutionary proposal that quantum mechanics does not sanction the concept of “molecular structure”—which is but only a “metaphor”—has fundamental implications for physical organic chemistry. On the one hand, the Uncertainty Principle limits the precision with which transition state structures may be defined; on the other, extension of the structure concept to the transition state may be unviable. Attempts to define transition states have indeed caused controversy. Consequences for molecular recognition, and a mechanistic classification, are also discussed.     

Alternating hydrophilic and hydrophobic pockets in the channel structures of organostannoxane prismanes: preferential confinement of guest molecules

A hydroxyl-rich hexameric organooxotin prismane has been prepared by reaction of n-BuSn(O)OH with 9-hydroxy-9-fluorenecarboxylic acid. The supramolecular structure of this cage shows channels with hydrophobic and hydrophilic segments, which selectively entrap guest molecules.     

Synthesis of trisubstituted alkenes by reductive dehydroxylation of Baylis-Hillman adducts using polymethylhydrosiloxane (PMHS) and catalytic B(C6F5)3

B(C₆F₅)₃ as a catalyst and polymethylhydrosiloxane as a hydride source have been employed for the reductive dehydroxylation of Baylis-Hillman adducts wherein the hydride adds in an S_N2′ manner onto the unactivated allyl alcohol moiety with concomitant elimination of the hydroxy group along with double bond migration. The products formed were found to be E in the case of ester adducts and Z in the case of nitrile adducts.     

Determination of chromium by radiochemical displacement

A rapid and sensitive method has been developed for the determination of microgram amounts of chromium based on the radiochemical displacement of⁶⁵Zn from labeled Zn-[1-(2-pyridylazo-2-naphthol]₂ complex by chromium. The effect of pH on the displacement of⁶⁵Zn was studied. 5–65 g of chromium could be determined with great accuracy. The effect of various ions on the displacement of⁶⁵Zn by chromium was investigated and the method developed was utilized for the determination of chromium content present in geological water samples. The results were compared with values obtained by Atomic Absorption Spectrophotometry.     

Complementarity and the temporal Uncertainty principle. Insight into the dichotomous formulations of the Curtin-Hammett principle. A new formulation

By the temporal Uncertainty principle, chemical systems may be described in terms of ‘kinetic’ or ‘thermodynamic’ complementary formulations, based on rate or equilibrium constants, and free energy changes respectively. Thereby, the paradox of the dichotomous formulations of the Curtin-Hammett principle is resolved. A new formulation of the principle is suggested, and a fundamental conflict with transition state theory is indicated.     

Halogenated Squaraine Dyes as Potential Photochemotherapeutic Agents. Synthesis and Study of Photophysical Properties and Quantum Efficiencies of Singlet Oxygen Generation*

Abstract— We describe the synthesis and photophysical studies, including measurements of quantum yields of triplet excited states and singlet oxygen generation of bis(3,5-dibromo-2,4,6-trihydroxyphenyl)squaraine (2) and bis(3,5-diiodo-2,4,6-trihydroxyphenyl)squaraine (3). These dyes exist in solution in the protonated, neutral, single and double depro-tonated forms, depending on pH. The pK_a values of these dyes were found to be relatively lower than those of the parent bis(2,4,6-trihydroxyphenyl)squaraine (1). Only the single deprotonated forms (Sq) of 2 and 3 showed measurable fluorescence. In microheterogeneous media such as in the presence of β-cyclodextrin, cetyltrunethylammonium bromide and polyvinylpyrrolidone), bathochromic shifts in the absorption and emission spectra of Sq were observed with a substantial enhancement in their fluorescence yields. Triplet excited states are the main transient intermediates obtained upon 532 nm laser excitation of the various forms of 2 and 3 in methanol. These triplets have lifetimes in the range from 0.061 to 132 μs. The triplet quantum yields of double deprotonated forms are low (φT = <0.01), whereas the neutral and Sq^?forms of 2 (φr = 0.12 and 0.22) and 3 (φ_T= 0.24 and 0.5), respectively, exhibited significant triplet yields. Quantum yields of singlet oxygen generation by Sq^?forms of 2 and 3 were determined in methanol and were found to be 0.13 and 0.47, respectively, which are in good agreement with the triplet yields obtained in these systems.     

On the planarity of tetracoordinate carbon enclosed by annulene perimeters

Proposals that planar tetracoordinate carbon might be achieved by enclosure in an annulene perimeter have been examined by means of MNDO calculations. Unfortunately, the lowest energy forms of 1–3, and 5 are indicated to distort as far away from planarity as possible, consistent with the strain demands of the carbon skeletons. Even in hypothetical stereomutation transition states, planar tetracoordination around the central carbon should be energetically difficult to achieve in these hydrocarbons, which, at best, are expected to be unstable strained polyolefins. Planar tetracoordinate carbon is more likely to be achieved when both π and σ systems provide effective stabilization through multicenter bonding. Several illustrative proposals for experimental examination are presented.     

A distorted cubic tetranuclear copper(II) phosphonate cage with a double-four-ring-type core

The reaction of Cu2(O2CMe)(4).2H2O with tert-butylphosphonic acid and 3,5-di-tert-butylpyrazole in the presence of triethylamine leads to a high-yield synthesis of the tetranuclear compound [Cu2(3,5-t-Bu2PzH)2(t-BuPO3)2]2 (1). The latter has a distorted cubic cage structure and its core resembles the D4R (double-four-ring) motif found in zeolites. The phosphonate, [t-BuPO3]2-, functions as a dianionic tridentate ligand, while the pyrazole ligands are neutral and are monodentate. The coordination geometry at each copper atom is distorted square planar with a 3O,1N coordination environment. Magnetic measurements on 1 reveal that the chiT product continuously decreases to reach a value very close to zero at 1.8 K, indicating dominant antiferromagnetic interactions between Cu(II) ions that leads to an S=0 ground state. The tetranuclear cage 1 functions as a very effective artificial nuclease in the presence of an external oxidant, magnesium monoperoxyphthalate.     

Synthesis, structure, magnetism and nuclease activity of tetranuclear copper(II) phosphonates containing ancillary 2,2'-bipyridine or 1,10-phenanthroline ligands

The reaction of cyclohexylphosphonic acid (C(6)H(11)PO(3)H(2)), anhydrous CuCl(2) and 2,2'-bipyridine (bpy) in the presence of triethylamine followed by a metathesis reaction with KNO(3) afforded [Cu(4)(mu-Cl)(2)(mu(3)-C(6)H(11)PO(3))(2)(bpy)(4)](NO(3))(2) (1). In an analogous reaction involving Cu(OAc)(2).H(2)O, the complex [Cu(4)(mu-CH(3)COO)(2)(mu(3)-C(6)H(11)PO(3))(2)(2,2'-bpy)(4)](CH(3)COO)(2) (2) has been isolated. The three-component reaction involving Cu(NO(3))(2).3H(2)O, cyclohexylphosphonic acid and 2,2'-bipyridine in the presence of triethylamine afforded the tetranuclear assembly [Cu(4)(mu-OH)(mu(3)-C(6)H(11)PO(3))(2)(2,2'-bpy)(4) (H(2)O)(2)](NO(3))(3) (3). Replacing 2,2'-bipyridine with 1,10-phenanthroline (phen) in the above reaction resulted in [Cu(4)(mu-OH)(mu(3)-C(6)H(11)PO(3))(2)(phen)(4)(H(2)O)(2)](NO(3))(3) (4). In all the copper(II) phosphonates (1-4) the two phosphonate ions bridge the four copper(II) ions in a capping coordination action. Each phosphonate ion bridges four copper(II) ions in a mu(4), eta(3) coordination mode or 4.211 of the Harris notation. Variable-temperature magnetic studies on reveal that all four complexes exhibit moderately strong intramolecular antiferromagnetic coupling. The DNA cleavage activity of complexes 1-4 is also described. Compounds 1 and 3 were able to completely convert the supercoiled pBR322 DNA form I to nick form II without any co-oxidant. In contrast, 50% conversion occurred with and 40% with 4. In the presence of magnesium monoperoxyphthalate all four compounds achieved rapid conversion of form I to form II.     

Spirastrellolide B: the synthesis of southern (C9-C25) region

A combination of "chiron" and "asymmetric" approaches is utilized to construct the southern (C 9-C 25) region of marine natural product spirastrellolide B. The key functionalities are derived from d-glucose and Sharpless asymmetric epoxidation and dihydroxylation.