Substituent electronic effects on the persistence and absorption spectra of (Z)-o-xylylenols. A nanosecond laser flash photolysis study

[reaction: see text] A systematic investigation on a broad set of aldehydes reveals that the lifetimes of (Z)-photoenols can be modulated by variation of the substituents. We have found that the lifetimes of (Z)-enols (in benzene) can be varied by more than 1 order of magnitude with a judicious choice of the substituents that exert mesomeric and inductive effects as, for example, in the case of pentamethylbenzaldehyde (tau = 35 ns) and dicyanomesitaldehyde (tau = 760 ns). This study thus points to the fact that the electronic factors in conjunction with hydrogen bonding stabilization can considerably broaden the uni- as well as bimolecular chemistry based on photoenolization. Further, we have shown that the photoenols exhibit dramatic shifts in their absorption properties with variation of the substituents; although the photoenols have long been considered to be colored, their absorption properties have not been heretofore comprehensively examined.     

Solid-state diphotocyclization of iso- and terephthalaldehydes via dihalogen substitution

The supramolecular nonbonded C-H...X interactions between formyl hydrogens and ortho-halogen atoms (Br/Cl) have been exploited to achieve conformational control in the solid state of dimethyl-substituted iso- and terephthaladehydes (1-3) for unprecedented diphotocyclization. It is shown that the dihalogen substitution also contributes to the stability of the benzocyclobutenols relative to their precursor photoenols, so that the solid-state photolysis of dialdehydes 2b, 2c, and 3b leads to diphotocyclization to afford respectable yields of bis-benzocyclobutenols.     

Efficient photocyclization of o-alkylbenzaldehydes in the solid state: direct observation of E-xylylenols en route to benzocyclobutenols

The photocyclization to benzocyclobutenols of o-alkyl aromatic aldehydes that are predestined for gamma-hydrogen abstraction is found to occur efficiently in the solid state; in contrast, solution-phase photolysis is known to afford a mixture of several products. It is shown that mesitaldehyde, which is a liquid, also undergoes efficient cyclization when subjected to photolysis as a solid inclusion complex. The marginal energy differences in the relative energies of the E-enols and the corresponding cyclobutenols in the case of cyano-substituted mesitaldehydes has permitted direct observation, for the first time, of the E-enols en route to benzocyclobutenols. The AM1 calculations suggest that the cyano-substitution causes intrinsic stabilization of the E-enols relative to the corresponding cyclobutenols, while the bromo groups do the opposite. The lack of observation of the red color in bromo- and formyl-substituted aldehydes is attributed to rapid cyclization of the E-enols to the their respective cyclobutenols even at low temperatures.     

Highly diastereoselective tandem photoenolization-hetero-Diels-Alder cycloaddition reactions of o-tolualdehydes in the solid state

The (E)-photoenols generated in situ by photolysis of o-tolualdehydes 1-5 in the solid state react with the precursor aldehydes as dienophiles in a hetero-Diels-Alder cycloaddition fashion to afford trans-3-arylisochromanols in excellent yields and in a high diastereoselectivity. An examination of the reactivity of three different classes of rationally designed aldehydes shows that the tandem enolization-Diels-Alder cycloaddition occurs in the solid state efficiently for aldehydes whose (E)-photoenols (i) are more stable than their corresponding benzocyclobutenols and (ii) are not sterically congested. However, rapid cyclization to benzocyclobutenols is found to be the sole pathway for sterically encumbered (E)-enols derived from aldehydes 6-8. Given that the execution of heteromolecular reactions in the solid state is a challenge, the results obtained with simple crystalline o-tolualdehydes are remarkable and the strategy involving in situ generation of a dienol in the crystal lattice of a dienophile to achieve hetero-Diels-Alder reaction in a highly diastereoselective manner is heretofore unprecedented. In the context of enormous interest in tandem/domino reactions in contemporary synthetic organic chemistry, the results observed with o-tolualdehydes exemplify successful execution of tandem reactions in the solid state.     

Photoenolization of 2-(2-methyl benzoyl) benzoic acid, methyl ester: effect of E photoenol lifetime on the photochemistry

[reaction: see text] Photolysis of 3 in argon-saturated 2-propanol led to formation of 5 via intermolecular H-atom abstraction followed by lactonization. Irradiation of 4 in 2-propanol gave compounds 6 and 7 that also come from intermolecular H-atom abstraction. In contrast, photolysis of an oxygen-saturated solution of 3 in 2-propanol yields products 8, 9, and 10, which were all formed from intramolecular H-atom abstraction and trapping of the corresponding biradical with oxygen. Laser flash photolysis of 3 in methanol showed formation of biradical 3BR (lambda(max) 330 nm, and tau = 50 ns) via intramolecular H-atom abstraction as the main photoreactivity of 3. Biradical 3BR decayed into photoenols 3Z and 3E (lambda(max) 390 nm, tau = 6.5 micros and tau = 162 micros, respectively). In comparison, laser flash photolysis of 4 yielded photoenols 4Z and 4E (lambda(max) 390 nm, tau = 15 micros and tau = 3.6 ms, respectively). Thus photoenol 3E is unusually short-lived, and therefore it does not undergo the intramolecular lactonization as we have observed for the analogous photoenol 1E. Photoenol 3Z decays back to 3 via an intramolecular 1,5-H shift, whereas photoenol 3E reforms 3 efficiently via the solvent with the aid of the ortho ester group. The intramolecular lactonization of photoenols 1E and 3E must be a slow process, presumably because the photoenols are rigid and the hydroxyl group is inhibited, by intramolecular hydrogen bonding, from acquiring the correct geometry for lactonization. Thus only photoenols that are resistant to reformation of their ketone via the solvent are long-lived enough to undergo lactonization and release the alcohol moiety.     

Remote stereocontrol using allylstannanes: reversal in stereoselectivity using indium(III) and bismuth(III) halides as promoters

Allyl-indium(III) and -bismuth(III) dihalides, generated by transmetallation of 5-benzyloxy-4-methylpent-2-enyl(tributyl)stannane 1, react with aldehydes with useful levels of 1,5-stereocontrol, a 93:7 ratio of 1,5-epimers in favour of the 1,5-anti-(E)-stereoisomers 7 and 11 typically being obtained using bismuth(III) iodide. The 4-benzyloxypent-2-enylstannane 4 similarly gives the 1,5-syn-(E)-hex-3-enols 13 also with ca. 93:7, stereoselectivity.     

NMR structural elucidation of photochromic quinolone photoproducts

UV irradiation of 3-benzoyl-2-benzyl-6,7-difluoro-1-propyl-1H-quinolin-4-one leads to the formation of a mixture of photoproducts, with different evolutions. The structure of each of them has been obtained by one- and two-dimensional multinuclear NMR experiments. Two photoenols, which are thermally reversible, have been identified. All of the other photoproducts have been assigned to degradation. Based on the structural identification and the photochemical and thermal evolution of samples, a plausible mechanism is proposed.     

Partially O-alkylated thiacalix[4]arenes: synthesis, molecular and crystal structures, conformational behavior

NMR spectroscopy, X-ray diffraction analysis, and quantum chemical calculations were used for conformational behavior study of partially alkylated thiacalix[4]arenes bearing methyl (1), ethyl (2), or propyl (3) groups at the lower rim. The conformational properties are governed by two basic effects: (i) stabilization by intramolecular hydrogen bonds, and (ii) sterical requirements of the alkoxy groups at the lower rim. While the monosubstituted derivatives 1a and 3a adopt the cone conformation in solution, distally disubstituted compounds 1b, 1'b, 2b, 2'b, 3b, and 3'b exhibit several interesting conformational features. They prefer pinched cone conformation in solution, and, except for 3'b, they form also 1,2-alternate conformation, which is flexible and undergoes rather fast transition between two identical structures. The crystal structures of the compounds 1b, 2b, 2'b, and 3b revealed yet quite rare 1,2-alternate conformation forming molecular channels held together by pi-pi interactions. Different channels-with hexagonal symmetry, 0.26 nm wide-are formed in the crystal structure of the pinched cone conformation of 3b. An uncommon hydrogen bonding pattern was found in dimethoxy and dipropoxy derivatives 1'b and 3'b that adopt distorted cone conformations in crystal. Trialkoxy-substituted compounds 1c and 3c adopt the partial cone conformation in solution. A higher mobility of methyl derivative 1c enables also existence of the cone conformer.     

Theory of late-transition-metal alkyl and heteroatom bonding: analysis of Pt, Ru, Ir, and Rh complexes

Density functional and correlated ab initio methods were used to calculate, compare, and analyze bonding interactions in late-transition-metal alkyl and heteroatom complexes (M-X). The complexes studied include: (DMPE)Pt(CH(3))(X) (DMPE = 1,2-bis(dimethylphosphino)ethane), Cp*Ru(PMe(3))(2)(X) (Cp* = pentamethylcyclopentadienyl), (DMPE)(2)Ru(H)(X), (Tp)(CO)Ru(Py)(X) (Tp = trispyrazolylborate), (PMe(3))(2)Rh(C(2)H(4))(X), and cis-(acac)(2)Ir(Py)(X) (acac = acetylacetonate). Seventeen X ligands were analyzed that include alkyl (CR(3)), amido (NR(2)), alkoxo (OR), and fluoride. Energy decomposition analysis of these M-X bonds revealed that orbital charge transfer stabilization provides a straightforward model for trends in bonding along the alkyl to heteroatom ligand series (X = CH(3), NH(2), OH, F). Pauli repulsion (exchange repulsion), which includes contributions from closed-shell d(π)-p(π) repulsion, generally decreases along the alkyl to heteroatom ligand series but depends on the exact M-X complexes. It was also revealed that stabilizing electrostatic interactions generally decrease along this ligand series. Correlation between M-X and H-X bond dissociation energies is good with R(2) values between 0.7 and 0.9. This correlation exists because for both M-X and H-X bonds the orbital stabilization energies are a function of the orbital electronegativity of the X group. The greater than 1 slope when correlating M-X and H-X bond dissociation energies was traced back to differences in Pauli repulsion and electrostatic stabilization.     

Ferrocenyl hetaryl thioketones: A computational study of their conformational stability

In this work, the conformational behavior of ferrocenyl- and hetaryl-functionalized thioketones was studied by means of computational quantum chemical methods. Four hetaryl substituents (furan-2-yl, thiophen-2-yl, selenophen-2-yl, and N-methylpyrrol-2-yl) were taken into account. The conformational space of the four ferrocenyl hetaryl thioketones was explored, and all found conformers were characterized using density functional (B3LYP) and wave function (SCS-MP2) theories. Their stability was explained in terms of intramolecular interactions. Such interactions were described using the methods of natural bond orbitals, “atoms in molecules,” noncovalent interaction index, and localized molecular orbital energy decomposition analysis. The identified conformations essentially differ in the arrangement of hetaryl heteroatom relative to the thiocarbonyl sulfur atom. The furan-2-yl substituent favors an s-trans-like arrangement of its heteroatom, while the remaining hetaryl substituents tend to adopt an s-cis-like arrangement. Such a conformational preference mainly results from the π → π* stabilization between the CS group and the hetaryl ring. Weak intramolecular hydrogen bonding of C H⋯O type was detected in the preferred conformer of ferrocenyl furan-2-yl thioketone. Low-polarity solvents, such as toluene, chloroform, and tetrahydrofuran, have a small effect on the preferred conformers of the four thioketones.     

Photochromism of 2-benzyl-3-benzoyl-4(1H)-quinolone derivatives

A series of 2-benzyl-3-benzoyl-4(1H)-quinolone derivatives was investigated. The UV–vis spectra of all photochromic derivatives and the corresponding colored photoenols are similar and almost solvent independent. In contrast, the stability of the photoenols depends strongly on the substituents at the quinolone moiety and solvent. We conclude that conversion of the photoinduced forms into the initial quinolones occurs through ionization rather than usual for photochromic compounds thermal relaxation or sigmatropic H-shift. The experimental observations are in good agreement with the results of quantum mechanical calculations.     

Reaction of substituted furan-2-carboxaldehydes and furo[b]pyrrole type aldehydes with hippuric acid

4-Heteroarylidene-2-phenyl-1,3-oxazol-5(4H)-ones were prepared by reactions of hippuric acid with substituted furan-2-carboxaldehydes or furo[b]pyrrole type aldehydes. The reactivity of various furan-2-carboxaldehyde derivatives in this reaction is discussed. The effect of microwave irradiation on some condensation reactions was compared with "classical" conditions. The results show that microwave irradiation shortens the reaction times while affording comparable yields. Elementary analysis, UV, IR and 1D NMR proved the structure of new synthesised compounds. 2D NMR spectroscopic measurements confirmed that the configuration at the carbon-carbon double bond corresponds to the pure E isomers of the products.     

Conformer-independent ureidoimidazole motifs--tools to probe conformational and tautomeric effects on the molecular recognition of triply hydrogen-bonded heterodimers

Linear arrays of hydrogen bonds are useful for the reversible assembly of “stimuli‐responsive” supramolecular materials. There is thus an ongoing requirement for easy‐to‐synthesise motifs that are capable of presenting hydrogen‐bonding functionality in a predictable manner, such that high‐affinity and high‐fidelity recognition occurs. The design of linear arrays is made challenging as a consequence of their ability to adopt multiple conformational and tautomeric configurations; with each additional hydrogen‐bonding heteroatom added to an array, the available tautomeric and conformational space increases and it can be difficult to anticipate where unproductive conformers/tautomers will arise. This paper describes a detailed study on the complementary ureidoimidazole donor–donor–acceptor (DDA) array ( 1 ) and amidoisocytosine donor–acceptor–acceptor (DAA) array ( 2 ). A specific feature of 1 is that two degenerate, intramolecular hydrogen‐bonded conformations are postulated, both of which present a DDA array that is complementary to appropriate DAA partners. 1D and 2D ¹H NMR spectroscopy, isothermal titration calorimetry, and ab initio structure calculations confirm 1 interacts with 2 (K_a≈33000 M ^?1 in CDCl₃) in a conformer‐independent fashion driven by enthalpy. Comparison of the binding behaviour of 1 with hexylamidocytosine ( 4 ) and amidonaphthyridine ( 5 ) provides insight on the role that intramolecular hydrogen‐bonding plays in mediating affinity towards DAA partners.     

UV irradiation enhances the bonding strength between citric acid-crosslinked gelatin and stainless steel

The effect of ultraviolet ray (UV) irradiation on the bonding strength between low carbon stainless steel 316 (SUS316L) and trisuccinimidyl citrate (TSC)-crosslinked alkali-treated gelatin (AlGelatin-TSC) was investigated. The UV irradiation effectively generated hydroxyl groups on the surface of SUS316L. The bonding strength between AlGelatin-TSC and SUS316L before UV irradiation was 0.345±0.007 MPa, and upon UV irradiation it increased to 0.750±0.069 MPa. In order to explain this enhanced bonding strength, the surface of SUS316L was examined using its water contact angle and X-ray photoelectron spectroscopy. Furthermore, the N 1s peaks derived from the TSC succinimidyl group were assigned to the surface of SUS316L after the immobilization of the TSC. This indicates that ester bond formation between the TSC active esters and the SUS316L hydroxyl groups contributed to the enhanced bonding strength. Therefore, UV irradiation and subsequent TSC immobilization is a simple way to functionalize biometal surfaces with various structures. This has practical applications for medical devices such as drug-eluting stents, dental implants, and metallic artificial bone.     

1-alkoxyallene as an effective precursor for regio- and stereocontrolled allylation reactions with aliphatic aldehydes via bis-stannylation

Bis-stannylation of 1-(methoxy)methoxyallene yields (E)-(3-(methoxymethoxy)prop-2-ene-1,2-diyl) bis-tributylstannane (2) as a highly reactive allylation reagent. Stereoselective reactions of functionalized stannane 2 with a variety of aldehydes provide good yields of 1,2-diol derivatives. Reactions with alpha-alkoxyaldehydes proceed with chelation-controlled addition to afford efficient constructions of 3,4-anti-4,5-syn-relationships in stereotriads bearing three differentiated oxygen (alkoxy and hydroxy) substituents. Mechanistic considerations are presented, and derivatives for NMR experiments support the stereochemical assignments. An X-ray diffraction study is included.     

Structural analysis of a beta-helical protein motif stabilized by targeted replacements with conformationally constrained amino acids

Here we study conformational stabilization induced in a beta-helical nanostructure by position-specific mutations. The nanostructure is constructed through the self-assembly of the beta-helical building block excised from E. coli galactoside acetyltransferase (PDB code 1krr , chain A; residues 131-165). The mutations involve substitutions by cyclic, conformationally constrained amino acids. Specifically, a complete structural analysis of the Pro-Xaa-Val sequence [with Xaa being Gly, Ac 3c (1-aminocyclopropane-1-carboxylic acid) and Ac 5c (1-aminocyclopentane-1-carboxylic acid)], corresponding to the 148-150 loop region in the wild-type (Gly) and mutated (Ac 3c and Ac 5c) 1krr , has been performed using Molecular Dynamics simulations and X-ray crystallography. Simulations have been performed for the wild-type and mutants of three different systems, namely the building block, the nanoconstruct and the isolated Pro-Xaa-Val tripeptide. Furthermore, the crystalline structures of five peptides of Pro-Xaa-Val or Xaa-Val sequences have been solved by X-ray diffraction analysis and compared with theoretical predictions. Both the theoretical and crystallographic studies indicate that the Pro-Ac n c-Val sequences exhibit a high propensity to adopt turn-like conformations, and this propensity is little affected by the chemical environment. Overall, the results indicate that replacement of Gly149 by Ac 3c or Ac 5c significantly reduce the conformational flexibility of the target site enhancing the structural specificity of the building block and the nanoconstruct derived from the 1krr beta-helical motif.     

Photokinetics of two novel photochromic diarylethenes derived from benzothiophene

Two new unsymmetrically substituted photochromic diarylethenes, namely 3‐[3, 3,4,4,5,5‐hexafluoro‐2‐(2‐methylbenzothien‐3‐yl)cyclopent‐1‐en‐1‐yl]‐2‐methyl‐6‐methoxy‐1‐ benzothiophene and 3‐[3,3,4,4,5,5‐hexafluoro‐2‐(2‐methylbenzothien‐3‐yl)cyclopent‐1‐en‐1‐yl]‐2‐methyl‐6‐methoxy‐7‐nitro‐1‐benzothiophene, were synthesized. Their optical properties and kinetics of cyclization and cycloreversion were determined in four solvents and compared with the symmetrical diarylethene (3,3,4,4,5,5‐hexafluorocyclopent‐1‐en‐1,2‐diyl)bis(2‐methyl‐1‐benzothiophene). While the UV spectra of the three compounds are almost insensitive to solvent changes, the rates of the ring‐opening and ring‐closure reactions exhibit interesting kinetic behavior that differs from other diarylethenes reported in the literature. The cycloreversion for the three compounds follows first‐order kinetics, whereas the cyclization cannot be described by a simple kinetic law. Different approaches were tested, and a complex exponential equation could be derived that fits the experimental data. The unusual presence of two ring‐opened conformational isomers, one of them nonphotochromic, is proposed for the ring‐opened compounds to explain the derived kinetic law and observed solvent effects. These results are of interest in relation to the application of the novel diarylethenes as potential optical materials. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 736–744, 2012     

Formaldehyde Formation on Vanadium Oxide Surfaces V2O3(0001) and V2O5(001): How does the Stable Methoxy Intermediate Form?

Hydroxy‐mediated methoxy formation or stabilization is probably an important process in many methanol adsorption systems. Hydrogen atoms originating from the scission of the methanol O? H bond react with the substrate and form water. This process may result 1) in the production of additional surface defects as reactive centers for methoxy formation and 2) in the stabilization of methoxy groups by suppression of methanol formation.

     

Benzaldehyde lyase-catalyzed enantioselective carboligation of aromatic aldehydes with mono- and dimethoxy acetaldehyde

[reaction: see text] Benzaldehyde lyase from the Pseudomonas fluorescens catalyzes the reaction of aromatic aldehydes with methoxy and dimethoxy acetaldehyde and furnishes (R)-2-hydroxy-3-methoxy-1-arylpropan-1-one and (R)-2-hydroxy-3,3-dimethoxy-1-arylpropan-1-one in high yields and enantiomeric excess via acyloin linkage. Aromatic aldehydes and benzoins are converted into enamine-carbanion-like intermediates prior to carboligation.