Thermal Cyclization of Phenylallenes That Contain ortho‐1,3‐Dioxolan‐2‐yl Groups: New Cascade Reactions Initiated by 1,5‐Hydride Shifts of Acetalic H Atoms

A series of 2‐(1,3‐dioxolan‐2‐yl)phenylallenes that contained a range of substituents (alkyl, aryl, phosphinyl, alkoxycarbonyl, sulfonyl) at the cumulenic C3 position were prepared by using a diverse range of synthetic strategies and converted into their respective 1‐(2‐hydroxy)‐ethoxy‐2‐substituted naphthalenes by smooth thermal activation in toluene solution. Electron‐withdrawing groups at the C3 position accelerated these tandem processes, which consisted of 1) an initial hydride‐like [1,5]‐H shift of the acetalic H atom onto the central cumulene carbon atom; 2) a subsequent 6π‐electrocyclic ring‐closure of the resulting reactive ortho‐xylylenes; and 3) a final aromatization step with concomitant ring‐opening of the 1,3‐dioxolane fragment. If the 1,3‐dioxolane ring of the starting allenes was replaced by a dimethoxymethyl group, the reactions led to mixtures of two disubstituted naphthalenes, which were formed by the migration of either the acetalic H atom or the methoxy group, with the latter migration occurring to a lesser extent. Two of the final 1,2‐disubstituted naphthalenes were converted into their corresponding naphtho‐fused dioxaphosphepine or dioxepinone through an intramolecular transesterification reaction. A DFT computational study accounted for the beneficial influence of the 1,3‐dioxolane fragment on the carbon atom from which the H‐shift took place and also of the electron‐withdrawing substituents on the allene terminus. Remarkably, in the processes that contained a sulfonyl substituent, the conrotatory 6π‐electrocyclization step was of lower activation energy than the alternative disrotatory mode.     

Methyloxy Substituted Heteroleptic Bis(phthalocyaninato) Yttrium Complexes: Density Functional Calculations

The effects of alkyloxy substituents attached to one phthalocyanine ligand of three heteroleptic bis(phthalocyaninato) yttrium complexes Y(Pc)[Pc(α‐OCH₃)₄] ( 1 ), Y(Pc)[Pc(α‐OCH₃)₈] ( 2 ), and Y(Pc)[Pc(β‐OCH₃)₈] ( 3 ), as well as their reduction products {Y(Pc)[Pc(α‐OCH₃)₄]}^? ( 4 ), {Y(Pc)[Pc(α‐OCH₃)₈]}^? ( 5 ), and {Y(Pc)[Pc(β‐OCH₃)₈]}^? ( 6 ) [H₂Pc(α‐OCH₃)₄=1,8,15,22‐tetrakis(methyloxy)phthalocyanine; H₂Pc(α‐OCH₃)₈=1,4,8,11,15,18,22,25‐octakis(methyloxy)phthalocyanine; H₂Pc(β‐OCH₃)₈=2,3,9,10,16,17,23,24‐octakis(methyloxy)phthalocyanine] are studied by DFT calculations. Good consistency is found between the calculated results and experimental data for the electronic absorption, IR, and Raman spectra of 1 and 3 . Introduction of electron‐donating methyloxy groups on one phthalocyanine ring of the heteroleptic double‐deckers induces structural deformation in both phthalocyanine ligands, electron transfer between the two phthalocyanine rings, changes in orbital energy and composition, shift of electronic absorption bands, and different vibrational modes of the unsubstituted and substituted phthalocyanine ligands in the IR and Raman spectra in comparison with the unsubstituted homoleptic counterpart Y(Pc)₂. The calculations reveal that incorporation of methyloxy substituents at the nonperipheral positions has greater influence on the structure and spectroscopic properties of bis(phthalocyaninato) yttrium double‐deckers than at the peripheral positions, which increases with increasing number of substituents. Nevertheless, the substituent effect of alkyloxy substituents at one phthalocyanine ligand of the double‐decker on the unsubstituted phthalocyanine ring and on the whole molecule and the importance of the position and number of alkyloxy substituents are discussed. In addition, the effect of reducing 1 – 3 to 4 – 6 on the structure and spectroscopic properties of the bis(phthalocyaninato) yttrium compounds is also discussed. This systemic DFT study is not only useful for understanding the structure and spectroscopic properties of bis(phthalocyaninato) rare earth metal complexes but also helpful in designing and preparing double‐deckers with tunable structure and properties.     

Theoretical Study of Remote Substituent Effects on X-H （X=CH2, NH,O） Bond Dissociation Energies of Azulene

In the study, the X-H （X=CH2, NH, O） bond dissociation energies （BDE） of para-substituted azulene （Y-C10H8X-H） were predicted theoretically for the first time using Density Functronal Theory （DFT） methods at UB3LYP/6-311 ＋ ＋g（2df,2p）//UB3LYP/6-31 ＋g（d） level. It was found that the substituents exerted similar effects on the X-H BDE of azulene as those on benzene, except for 6-substituted 2-methylazulene. Owing to the substituent-dipole interaction, the reaction constants （ρ^＋） of 2- and 6-Y-CIoHsX-H （X=NH and O only） varied violently. The origin of the substituent effects on the X-H BDE of azulene was found, by both GE/RE and SIE theory, to be directly associated with variation of the radical effects, although the ground effects also played a modest role in determining the net. substituent effects.     

Ground and Electronically Excited Singlet‐State Structures of 5‐Fluoroindole Deduced from Rotationally Resolved Electronic Spectroscopy and ab Initio Theory

The structure and electronic properties of the electronic ground state and the lowest excited singlet state (S₁) of 5‐fluoroindole (5FI) were determined by using rotationally resolved spectroscopy of the vibration‐less electronic origin of 5FI. From the parameters of the axis reorientation Hamiltonian, the absolute orientation of the transition dipole moment in the molecular frame was determined and the character of the excited state was identified as L_b.     

A DFT Study of the Modulation of the Antiaromatic and Open-Shell Character of Dibenzo[a,f]pentalene by Employing Three Strategies: Additional Benzoannulation,BN/CC Isosterism,and Substitution

Dibenzo[a,f]pentalene ( [a , f ]DBP ) is a highly antiaromatic molecule having appreciable open-shell singlet character in its ground state. In this work, DFT calculations at the B3LYP/6-311+G(d,p) level of theory were performed to explore the efficiency of three strategies, that is, BN/CC isosterism, substitution, and (di)benzoannulation of [a , f ]DBP , in controlling its electronic state and (anti)aromaticity. To evaluate the type and extent of the latter, the harmonic oscillator model of aromaticity (HOMA) and aromatic fluctuation (FLU) indices were used, along with the nucleus-independent chemical shift NICS-XY-scan procedure. The results suggest that all three strategies could be employed to produce either the closed-shell system or open-shell species, which may be in the singlet or triplet ground state. Triplet states have been characterized as aromatic, which is in accordance with Baird's rule. All the singlet states were found to have weaker global paratropicity than [a , f ]DBP . Additional (di)benzo fusion adds local aromatic subunit(s) and mainly retains the topology of the paratropic ring currents of the basic molecule. The substitution of two carbon atoms by the isoelectronic BN pair, or the introduction of substituents, results either in the same type and very similar topology of ring currents as in the parent compound, or leads to (anti)aromatic and nonaromatic subunits. The triplet states of all the examined compounds are also discussed.     

Nanoswitches based on DNA base pairs: why adenine-thymine is less suitable than guanine-cytosine.

Substituted Watson–Crick guanine–cytosine (GC) base pairs were recently shown to yield robust three‐state nanoswitches. Here, we address the question: Can such supramolecular switches also be based on Watson–Crick adenine‐thymine (AT) base pairs? We have theoretically analyzed AT pairs in which purine‐C8 and/or pyrimidine‐C6 positions carry a substituent X=NH^?, NH₂, NH₃⁺ (N series), O^?, OH or OH₂⁺ (O series), using the generalized gradient approximation (GGA) of density functional theory at the BP86/TZ2P level. Thus, we explore the trend in geometrical shape and hydrogen bond strengths in AT pairs along a series of stepwise protonations of the substituents. Introducing a charge on the substituents leads to substantial and characteristic changes in the individual hydrogen bond lengths when compared to the neutral AT pair. However, the trends along the series of negative, neutral, and positive substituents are less systematic and less pronounced than for GC. In certain instances, internal proton transfer from thymine to adenine occurs. Our results suggest that AT is a less suitable candidate than GC in the quest for chemically controlled nanoswitches.     

Supramolecular switches based on the guanine-cytosine (GC) Watson-Crick pair: effect of neutral and ionic substituents

We have theoretically analyzed Watson-Crick guanine-cytosine (GC) base pairs in which purine-C8 and/or pyrimidine-C6 positions carry a substituent X = NH(-), NH(2), NH(3) (+) (N series), O(-), OH, or OH(2) (+) (O series), using the generalized gradient approximation (GGA) of density functional theory at the BP86/TZ2P level. The purpose is to study the effects on structure and hydrogen-bond strength if X= H is substituted by an anionic, neutral, or cationic substituent. We found that replacing X = H by a neutral substituent has relatively small effects. Introducing a charged substituent, on the other hand, led to substantial and characteristic changes in hydrogen-bond lengths, strengths, and hydrogen-bonding mechanism. In general, introducing an anionic substituent reduces the hydrogen-bond-donating and increases the hydrogen-bond-accepting capabilities of a DNA base, and vice versa for a cationic substituent. Thus, along both the N and O series of substituents, the geometric shape and bond strength of our DNA base pair can be chemically switched between three states, thus yielding a chemically controlled supramolecular switch. Interestingly, the orbital-interaction component in some of these hydrogen bonds was found to contribute to more than 49 % of the attractive interactions and is thus virtually equal in magnitude to the electrostatic component, which provides the other (somewhat less than) 51 % of the attraction.     

Photoinduced Cyclization of (o‐Alkylbenzoyl)phosphonates to Benzocyclobutenols

(o ‐Alkylbenzoyl)phosphonates readily cyclize to highly strained benzocyclobutenols simply upon irradiation with UV light. The remarkable efficiency is ascribed to the electron‐accepting character of the phosphonate substituent, which facilitates thermal ring closing of the o ‐quinodimethane intermediate and suppresses reversion to the starting carbonyl compound.     

Structures and electronic absorption spectra of a recently synthesised class of photodynamic therapy agents

A theoretical study was performed on a novel class of boron-containing molecules (various substituted tetraarylazadipyrromethenes), which show in vitro activity for application in photodynamic therapy. Geometric optimisation of the structures for the singlet and triplet electronic states was carried out on compounds in vacuo at the density functional level of theory, by employing the PBE0 hybrid functional and the split-valence plus polarisation basis set. The absorbance properties in the UV-visible region were examined by means of time-dependent density functional response theory, using the same functional as mentioned above. To evaluate the influence of the solvent on the excitation energies, the continuum polarisable model was applied. Calculated electronic excitations, such as those regarding the Q-like band, were found to be in good agreement (within 0.01-0.1 eV) with experimental values and experimental trends on changing both the substituents and solvent.     

Computational Study of the Interactions between Benzene and Crystalline Ice Ih: Ground and Excited States

Ground‐state geometries of benzene on crystalline ice cluster model surfaces (I_h) are investigated. It is found that the binding energies of benzene‐bound ice complexes are sensitive to the dangling features of the binding sites. We used time‐dependent DFT to study the UV spectroscopy of benzene, ice clusters, and benzene–ice complexes, by employing the M06‐2X functional. It is observed that the size of the ice cluster and the dangling features have minor effects on the UV spectral characteristics. Benzene‐mediated electronic excitations of water towards longer wavelengths (above 170 nm) are noted in benzene‐bound ice clusters, where the cross‐section of photon absorption by water is negligible, in good agreement with recent experimental results (Thrower et al., J. Vac. Sci. Technol. A, 2008, 26 , 919–924). The intensities of peaks associated with water excitations in benzene–ice complexes are found to be higher than in isolated ice clusters. The π→π* electronic transition of benzene in benzene–ice complexes undergoes a small redshift compared with the isolated benzene molecule, and this holds for all benzene‐bound ice complexes.     

Electrostatic calculation of the substituent effect: an efficient test on isolated molecules

The energy of a disubstituted molecule has often been approximated by simple electrostatic formulas that represent the substituents as poles or dipoles. Herein, we test this approach on a new model system that is more direct and more efficient than testing on acid-base properties. The energies of 27 1,4-derivatives of bicyclo[2.2.2]octane were calculated within the framework of the density functional theory at the B3LYP/6-311+G(d,p) level; interaction of the two substituents was evaluated in terms of isodesmic homodesmotic reactions. This interaction energy, checked previously on some experimental gas-phase acidities, was considered to be accurate and served as reference to test the electrostatic approximation. This approximation works well in the qualitative sense as far as the sign and the order of magnitude are concerned: beginning with the strongest interaction between two poles, a weaker interaction between pole and dipole, and the weakest between two dipoles. However, all the electrostatic calculations yield energies that are too small, particularly for weak interaction, and this fundamental defect is not remedied by some possible improvements. In particular, variation of the effective permittivity would require a physically impossible value less than unity. The explanation must lie in a more complex distribution of electron density than anticipated in the electrostatic model. It also follows that possible conclusions about the transmission of substituent effects "through space" have little validity.     

Control over the Chemoselectivity of Pd‐Catalyzed Cyclization Reactions of (2‐Iodoanilino)carbonyl Compounds

The factors that control the chemoselectivity of palladium‐catalyzed cyclization reactions of (2‐iodoanilino)carbonyl compounds have been explored by an extensive experimental computational (DFT) study. It was found that the selectivity of the process, that is, the formation of fused six‐ versus five‐membered rings, can be controlled by the proper selection of the initial reactant, reaction conditions, and additives. Thus, esters or amides produce ketones by a nucleophilic addition process, whereas the addition of PhO^? ions leads to the formation of indolines by an α‐arylation reaction. In contrast, the corresponding ketone reactants yield a mixture of both reaction products, the ratio of which depends on the base used, in the presence of phenol. The outcome of the processes can be explained by the formation of a common four‐membered palladacycle intermediate from which the competitive nucleophilic addition and α‐arylation reactions occur. The remarkable effect of phenol in the process, which makes the α‐arylation reaction easier, favored the formation of enol complexes, which are stabilized by an intramolecular hydrogen bond between the hydroxy group of the enol moiety and the oxygen atom of the phenoxy ligand. Moreover, the chemoselectivy of the process can be also controlled by the addition of bidendate ligands that lead to the almost exclusive formation of indoles at expenses of the corresponding alcohols.     

Prins Cyclization Catalyzed by a FeIII/Trimethylsilyl Halide System: The Oxocarbenium Ion Pathway versus the [2+2] Cycloaddition

The different factors that control the alkene Prins cyclization catalyzed by iron(III) salts have been explored by means of a joint experimental–computational study. The iron(III) salt/trimethylsilyl halide system has proved to be an excellent promoter in the synthesis of crossed all‐cis disubstituted tetrahydropyrans, minimizing the formation of products derived from side‐chain exchange. In this iron(III)‐catalyzed Prins cyclization reaction between homoallylic alcohols and non‐activated alkenes, two mechanistic pathways can be envisaged, namely the classical oxocarbenium route and the alternative [2+2] cycloaddition‐based pathway. It is found that the [2+2] pathway is disfavored for those alcohols having non‐activated and non‐substituted alkenes. In these cases, the classical pathway, via the key oxocarbenium ion, is preferred. In addition, the final product distribution strongly depends upon the nature of the substituent adjacent to the hydroxy group in the homoallylic alcohol, which can favor or hamper a side 2‐oxonia‐Cope rearrangement.     

Cyclization Triggered by Deprotonation: The Gas‐Phase Acidity of 1,8‐Chalcogen‐Bridged Naphthalenes

High‐level density functional theory computations have been used to estimate the gas‐phase (intrinsic) acidities of the complete series of 1,8‐chalcogen‐bridged naphthalene derivatives. The existence of a chalcogen? chalcogen bond in chalcogen‐bridged naphthalene derivatives plays a crucial role in the intrinsic acidity of the system. For 1,8‐naphthalenediylbis(oxy), where this bond does not exist, the para C? H group is the most acidic site, whereas for the remaining compounds, deprotonation of the ortho CH groups is the most favorable process. Deprotonation of the aromatic rings has a large effect on the strength of the bonds of the five‐membered ring. These effects depend on the nature of the heteroatoms forming the X? Y bridge, and modulate the acidity of the molecule. Also importantly, when one of the heteroatoms is oxygen, ortho and para deprotonation lead to cleavage of the X? Y bridge. This bond fission favors the formation of a CYC (Y=S, Se, Te) three‐membered ring that enhances the stability of the anion and, therefore, increases the acidity of these compounds. We have shown that, whereas this cyclization process is energetically favorable for oxygen‐containing compounds, it is not favorable for the remaining derivatives.     

P‐Type Photochromism of New Helical Naphthopyrans: Synthesis and Photochemical,Photophysical and Theoretical Study

Two novel helical naphthopyrans have been synthesised. The helical scaffold has the interesting effect of increasing the thermal stability of the transoid‐trans (TT) open isomer formed upon UV irradiation of the closed form (CF), which transforms these naphthopyrans from thermal to photochemical photochromes. The photochromic performance is excellent in both polar and apolar solvents and the conversion percentage from the CF to the TT form can be as high as 92.8 %. We propose a new method to determine the quantum yields of the photochemical processes that lead to transoid‐cis (TC) and TT isomers, and their molar absorption coefficients. The thermal stability of the TT and TC isomers has been studied in different solvents. The quantum yields of fluorescence before and after irradiation, along with the decay lifetimes, have also been measured. TD‐DFT calculations have been performed to determine the relative thermodynamic stability of the species involved in the photochromic mechanism and to rationalise their spectral properties.     

Theoretical Study of the Substituent Effects on the Nonlinear Optical Properties of a Room‐Temperature‐Stable Organic Electride

Excess‐electron compounds can be considered as novel candidates for nonlinear optical (NLO) materials because of their large static first hyperpolarizabilities (β₀). A room‐temperature‐stable, excess‐electron compound, that is, the organic electride Na@(TriPip222), was successfully synthesized by the Dye group (J. Am. Chem. Soc. 2005 , 127, 12416). In this work, the β₀ of this electride was first evaluated to be 1.13×10⁶ au, which revealed its potential as a high‐performance NLO material. In particular, the substituent effects of different substituents on the structure, electride character, and NLO response of this electride were systemically studied for the first time by density functional theory calculations. The results revealed that the β₀ of Na@(TriPip222) could be further increased to 8.30×10⁶ au by introducing a fluoro substituent, whereas its NLO response completely disappeared if one nitryl group was introduced because the nitro‐group substitution deprived the material of its electride identity. Moreover, herein the dependence of the NLO properties on the number of substituents and their relative positions was also detected in multifluoro‐substituted Na@(TriPip222) compounds.