The reduction ring-opening reaction of imidazoline

A new reduction ring-opening reaction of 2-imidazoline with sodium borohydride is reported, and the effect of reaction condition on the yield, reaction mechanism and the use of the new reaction in synthesis of ethylenediamine derivatives are discussed. A new method for the preparation of unsymmetrical substituted ethylenediamine via the reduction ring-opening reaction of imidazoline is provided.     

Tributylphosphine: a remarkable promoting reagent for the ring-opening reaction of aziridines

Tributylphosphine was found to be an effective promoting reagent for ring opening of a variety of aziridines and nucleophiles to produce anti-bifunctional products in good to excellent yield. The study showed that the reaction is initiated through the attack of tributylphosphine as a nucleophile at the carbon atom of the aziridine ring.     

The photochemical reaction cycle of retinal reconstituted bacteriorhodopsin

The function of three types of bacteriorhodopsins was compared: the wild-type, the bleached and retinal reconstituted and retinal deficient bacteriorhodopsin after retinal addition. The apparent pK(a) of the proton acceptor group for the bleached BR and retinal deficient BR shifted toward higher pH values compared to the wild-type BR. Fitting the photocycle model to the absorption kinetic signals for all three proteins showed the existence of the same intermediates, but the time-dependent concentration of the intermediates was different. Although measurements were made at pH 7, the absorption kinetics and photoelectric signals in both retinal reconstituted samples acted as wild-type bacteriorhodopsin at significantly higher pH. Below pH 3 the retinal deficient and reconstituted sample bleached. These results suggested that the added retinal was not able to rebind in the same position in the protein as in native bacteriorhodopsin. This points out that care should be taken, when bleached bacteriorhodopsin is reconstituted with different retinal analogs.     

The Dewar benzene radical cation and its ring-opening reaction

The radical cation of Dewar benzene, 1*+, has been generated and observed by optical spectroscopy in cryogenic matrices. 1*+ distinguishes itself by a charge resonance band at 600 nm, very similar in shape and position to that observed for the related radical cation of norbornadiene. This coincidence indicates that in ground-state 1*+ the odd electron is also located in a pi-MO. The energy of the charge resonance transition, which is very sensitive to the dihedral angle between the four-membered rings in 1*+, is predicted consistently too low by TD-DFT and CASPT2. Probably this angle is too large in the B3LYP and CASSCF geometries. As 1*+ can be observed at 77 K, it must be separated by a barrier of at least 7-8 kcal/mol from its very exothermic decay to the radical cation of benzene, 2*+. An analysis shows that the ring-opening of 1*+ is a multistep process involving two avoided crossings between potential surfaces of different symmetry and electronic nature. Owing to the orbital symmetry-forbidden nature of the process, the energy of 1*+ starts by increasing steeply on stretching the central C-C bond, but then the system undergoes a crossing to a 2A1 surface which leads adiabatically to an excited state of 2*+. Therefore, another avoided crossing must be transited before the molecule can decay on the ground-state surface of 2*+. The rearrangement of 1*+ to 2*+ is an example of a "pseudodiabatic" thermal reaction that transits between potential surfaces representing very different electronic structures.     

Stereochemistry as a probe for photochemical reaction mechanisms

In this publication we have reviewed examples derived from our photochemical investigations where stereochemistry provides information allowing elucidation of the mechanistic details of electronically excited state transformations. The reactions discussed include unimolecular rearrangements of both singlet and triplet excited state species.     

Metal-carbene unit: a universal tool towards high-efficient photochemical reaction in solution

<正>Topochemically-controlled stereoselective [2+2] photocycloadditions are now well-established in the literatures.The construction of cyclobutane rings has attracted considerable attention due to the existence of this structural motif in natural products and synthetic medicines. Furthermore, cyclobutane derivatives appended with pyridyl or carboxylic groups exhibit intriguing properties in some ad-     

Photochemistry of 1,3-dimethyluracil. A novel photochemical ring-opening leading to an enamine

Besides known reduction and solvent addition products, the photolysis of 1,3-dimethyluracil in methanol resulted in formation of the enamine 2-methoxycarbonyl-N-methyl-3-methylaminopropenamide (5) $\text{via}$ a novel ring-opening reaction. A mechanism for the formation of 5 is suggested and its structure was confirmed by an independent synthesis.     

Stereoselectivity reversal of a photochemical reaction in the solid state

The stereoselectivity of Norrish type II cyclobutanol formation resulting from photolysis of α-adamantyl-p-methoxyacetophenone is altered in favor of the more hindered cis isomer as the reaction medium is changed from isotropic liquid phases (benzene or acetonitrile) to the pure crystal. Based on the reactant X-ray crystal structure, it is suggested that the intermediate 1,4-biradical in the solid state is born in, and restricted to, a conformation which is ideal for direct closure to the more hindered product. In the relatively unrestricted solution environment, however, conformational isomerism of the biradical is faster than closure, thus leading to a predominance of the less hindered trans cyclobutanol.     

The phase-dependent photochemical reaction dynamics of halooxides and nitrosyl halides

Recent progress in understanding the phase-dependent reactivity of halooxides and nitrosyl halides is outlined. Halooxide reactivity is represented by the photochemistry of chlorine dioxide (OClO) and dichlorine monoxide (ClOCl). The gas phase photochemical dynamics of OClO are contrasted with the dynamics in condensed environments. The role of excited-state symmetry in defining the reaction dynamics and the observation of photoisomerization resulting in the production of ClOO are discussed. The current understanding of the excited-state reaction dynamics of ClOCl and evidence for photoisomerization of this species resulting in the production of ClClO are outlined. Finally, the photochemical reaction dynamics of the nitrosyl halide ClNO are presented. The main difference between the gas and condensed phase reaction dynamics of this species is that whereas photodissociation to form Cl and NO dominates the gas phase reaction dynamics, photoisomerization resulting in ClON production occurs to an appreciable extent in condensed environments. The observation of photoisomerization for OClO, ClOCl and ClNO suggests that this process is a general feature of the condensed phase reaction dynamics for smaller halooxides and nitrosyl halides. Finally, future areas for study in both halooxide and nitrosyl halide photoreactivity are outlined.     

Light-driven transformation of ZnS-cyclohexylamine nanocomposite into zinc hydroxysulfate: a photochemical route to inorganic nanosheets

ZnS-CHA (CHA = cyclohexylamine) nanocomposite, a unique inorganic-organic hybrid semiconductor, has been prepared from a mild solvothermal reaction system. This material contains 2 nm-sized ZnS nanoparticles, and is photoactive toward UV light (≤300 nm). Under UV-irradiation, the ZnS-CHA nanocomposite is transformed to crystalline zinc hydroxysulfate nanosheets. The driving force of the light-driven transformation reaction is the photogeneration of charges (electrons and holes) in the ZnS nanoparticles, and these photogenerated charges interact with the CHA molecules and the inorganic S(2-) species, leading to decomposition of the organic component and self-oxidation of the inorganic ZnS nanoparticles to form zinc hydroxysulfate. Through simple thermal treatment, the as-formed zinc hydroxysulfate nanosheets are converted to ultrathin ZnO nanosheets with a porous feature, which exhibit high sensitivity and fast response and recovery for ethanol detection when used as an electrical sensing material.     

Enantioselective organocatalytic synthesis of the chiral chromenes by domino oxa-Michael-aldol reaction

The proline based chiral organocatalyst has been found to be an efficient catalyst for enantioselective domino oxa-Michael-aldol reaction. This catalytic system provided the synthesis of substituted 2-aryl-2H-chromenes-3-carbaldehyde in good to high yields (73%–97%) with excellent enantioselectivity (up to 97%) and reasonable reaction times. The atom economy, high yield and mild reaction conditions are some of the important features of this protocol.     

Palladium-catalyzed ring-opening reaction of methyleneaziridines with carboxylic acids: synthesis of alpha-amidoketones

In the presence of palladium catalysts, the reaction of methyleneaziridines 1 with carboxylic acids 2proceeded smoothly to give the corresponding alpha-amidoketones 3 in good to high yields.     

Next-generation quantum theory of atoms in molecules for the photochemical ring-opening reactions of oxirane

The conical intersections corresponding to the C─O and C─C ring opening were optimized and the reaction paths traversing these intersections were obtained. Investigation of the C─O ring opening revealed that when traversing the lowest energy conical intersection, the reaction path returns to the closed ring geometry. The C─O path traversing the intersection featuring torsion of terminal CH₂ group however, led to a ring-opened geometry, an H-shift and the formation of acetaldehyde that can undergo further dissociation. The observation of different reaction paths was explained by the 3-D paths from quantum theory of atoms in molecules (QTAIM) that defined the most preferred direction of electronic motion that precisely tracked the mechanisms of bond breaking and formation throughout the photo-reactions. The size, orientation, and location of these most preferred 3-D paths indicated the extent and direction of motion of atoms, bonds, and the degree of torsion or planarity of a bond indicating a predictive ability.     

Re-examination of the photochemical reaction of octaisopropylcyclotetragermane

The photochemistry of octaisopropylcyclotetragermane was studied by laser flash photolysis and trapping experiments. Upon irradiation of the cyclotetragermane, the main reaction was a ring opening to form octaisopropyltetragermane-1,4-diyl biradical (λ_max = 310 and 550 nm), but generations of diisopropylgermylene and tetraisopropyldigermene are negligible.     

Triple domino reaction for the synthesis of pyrazole/indoline linked chromenes

A simple and convenient method towards the synthesis of highly diversified chromenopyrazole/indoline frameworks in excellent yields via iodine promoted triple domino reaction involving Michael addition followed by intramolecular cyclization and dehydrogenation sequence has been described for the first time.     

β-Lactams via photochemical reaction of monothioimides

Photolysis of acyclic monothioimides gave β-lactams and thioamides via Type II reaction involving γ-hydrogen abstraction by thiocarbonyl group.     

Chirality transfer to iron in a photochemical reaction of a glucopyranosyliron compound

Photolysis (350 nm) of 2,3,4,6-tetra-O-methyl-β-d-glucopyranosyl(η⁵-cyclopentadienyl)dicarbonyliron with triphenylphosphine in benzene results in ligand substitution affording 2,3,4,6-tetra-O-methyl-β-d-glucopyranosyl(η⁵-cyclopentadienyl)(triphenylphosphine)carbonyliron in 85% yield. The process occurs with partial chirality transfer to iron giving a $\text{70}\text{30}$ mixture of diastereomers at that center. Circular dichroism spectroscopy was used to assign the (R) configuration to the iron center in the predominant diastereomer.