ABSTRACT

CBS-Q and G3 methods were used to generate a large number of reliable Si–H, P–H and S–H bond dissociation energies (BDEs) for the first time. It was found that the Si–H BDE displayed dramatically different substituent effects compared with the C–H BDE. On the other hand, the P–H and S–H BDE exhibited patterns of substituent effects similar to those of the N–H and O–H BDE. Further analysis indicated that increasing the positive charge on Si of XSiH₃ would strengthen the Si–H bond whereas increasing the positive charge on P and S of XPH₂ and XSH would weaken the P–H and S–H bonds. Meanwhile, increasing the positive charge on Si of XSiH₂^· stabilized the silyl radical whereas increasing the positive charge on P and S in XPH^· and XS^· destabilized P- and S-centered radicals. These behaviors could be reasonalized by the fact that Si is less electronegative than H while P and S are not. Finally, it was demonstrated that the spin-delocalization effect was valid for the Si-, P- and S-centered radicals.

A series of N-bonded donor-acceptor derivatives of phenothiazine containing phenyl (PHPZ), anisyl (ANPZ), pyridyl (PYPZ), naphthyl (NAPZ), acetylphenyl (APPZ), and cyanophenyl (CPPZ) as an electron acceptor have been synthesized. Their photophysical properties were investigated in solvents of different polarities by absorption and emission techniques. These studies clearly revealed the existence of an intramolecular charge transfer (ICT) excited state in the latter four compounds. The solvent dependent Stokes shift values were analyzed by the modified Lippert-Mataga equation to obtain the excited state dipole moment values. The large excited state dipole moment suggests that the full (or nearly full) electron transfer take place in the A-D systems. In the system of A-D phenothiazine derivatives, the transition dipole moments M_flu were determined mainly by direct interactions between the solvent-equilibrated fluorescence ¹CT state and ground state because of their lack of significant change with increase of the solvent polarity. The electron structure and molecular conformation of phenothiazine derivatives will be significantly changed with the increase of the electron affinity of the N-10 substituent.

Five- and six-membered cyclic allylic halides were found to be much less reactive than the acyclic allylic halides in aqueous allylation reactions. Nevertheless, it was found that SnCl₂/Cu was powerful enough to mediate the aqueous allylation reactions involving cyclic allylic halides. Both the aliphatic and aromatic aldehydes could beefficiently allylated and the reaction condition was mild, simple and safe. The yields were usually in 75%–97% and the reaction was erythro selective.

The performance of the newly developed G3B3 and CBS-QB3 methods in calculating absolute bond dissociation energy (BDE) was assessed. It was found that these two methods could predict the BDE with an accuracy of about 8.4 kJ/mol and therefore, they exhibited similar performance as the standard G3 and CBS-Q methods. On the other hand, it was demonstrated that the B3LYP method significantly underestimated the absolute BDE by 16.7–20.9 kJ/mol. This finding was valuable and timely because many researchers could use this relatively cheap method in studying radical reactions. Finally, 38 compounds were showed for which the theoretical BDE seriously deviated from the experimental data.

The performances of the density functional theory (DFT) methods in calculating XH bond dissociation energies (BDE, XC, N, O, Si, P, S) were evaluated. It was found that most DFT methods including B3LYP, B3PW91, G96LYP, PBE1PBE and BH&HLYP significantly underestimated the XH BDE by as much as 13–24 kJ/mol. The underestimation is not due to the use of finite basis set, because the DFT methods still significantly underestimate the XH BDE with the complete basis set. Therefore, these DFT methods can not be used to calculate the BDE directly. Nevertheless, the B3P86 method shows very small underestimation for the XH BDE. Further analysis suggests that there be no advantage for using the restricted open-shell DFT methods. The unrestricted DFT methods actually perform slightly better than the restricted open-shell DFT methods in most cases. Finally, it was found that the underestimation by the DFT methods was largely systematic. The use of the calibrated UDFT/6-311++G(d, p) method was recommended to calculate the XH BDE.

In this article, a general, simple, and inexpensive catalyst system was developed for the amidation of aryl bromide by using CuI as catalyst, amino acid as ligand, and K₃PO₄ as base.

The inductive/field effects on homolytic bond dissociation energies (BDEs) were studied for the first time using substituted 4-X-bicyclo[2.2.2]octane-Y—Z systems. A variety of very different chemical bonds (i.e. Z—Y: Z = CH₂, NH, O, SiH₂, PH, S; Y = H, F, Li) were considered, and popular substituents including H, CH₃, F, OH, NH₂, SH, CN and NO₂ were utilized. High-quality BDE values were obtained for the first time for many bicyclo[2.2.2]octane systems from carefully calibrated G3B3/B3LYP calculations. Significant effects of the substituents at the 4-position of bicyclooctane were found for the Z—Y BDEs of bicyclooctanyl-Z—Y systems. Nice Hammett-type correlations were obtained for these substituent effects using the inductive/field F constants. It was found that the reaction constants (i.e. ρ values) of the Hammett correlations varied dramatically from −1.96 to +23.01 kJ mol⁻¹ for different Z—Y systems: The ρ values for the Z—H BDEs were about ∼1.0–5.0 kJ mol⁻¹; the ρ values for the Z—F BDEs were about −2.0 to −1.0 kJ mol⁻¹; the ρ values for the Z—Li BDEs were ∼13.0–23.0 kJ mol⁻¹. The substituent effects on both the stability of the parent molecules before homolysis and the stability of the radical products after homolysis were demonstrated to be important for the BDEs. It was shown that the inductive/field substituent effects on BDEs could not be explained by the electronegativity or bond polarity theories. Nevertheless, we developed a theoretical model on the basis of the classic electrostatic theories for the inductive/field effects. This model successfully explained the intriguing inductive/field substituent effects on BDEs. Copyright © 2004 John Wiley & Sons, Ltd.

The CH bond dissociation energies (BDEs) of the methyl groups attached to a large number of heterocyclic compounds are calculated using a carefully calibrated B3LYP method. These CH bond dissociation energies are important for evaluating the metabolic stability of the methyl groups in heterocyclic compounds that may be used as drug candidates. It is found that the CH BDEs of the methyl groups attached to diverse heterocycles can dramatically vary from ca 80 to ca 100 kcal mol⁻¹ (1 kcal = 4.184 kJ). Therefore, the benzylic positions of different heterocycles may have remarkably different metabolic stabilities varying by ∼10¹²-fold. The heteroatoms in the aromatic rings vary the benzylic BDEs either by delocalizing the spin or by changing the charge carried by the radical center. N-Methyl groups have systematically higher CH BDEs than C-methyl groups. NH, O and S groups have similar effects on the benzylic CH BDEs. A methyl group at the α-position relative to the NH, O and S groups usually has a lower BDE than that at the β-position. On the other hand, the N group has a different effect on the benzylic CH BDEs. A methyl group at the β-position relative to N has a lower CH BDE than that at the α-position. There is a special aromatization effect associated with 1-methyl-2H-isoindole, 1-methylisobenzofuran, 1-methylbenzo[c]thiophene and related compounds. This aromatization effect dramatically decreases the benzylic CH BDEs. Finally, an interesting QSAR model has been developed. This model not only can successfully predict the benzylic CH BDEs of diverse heterocyclic compounds, but also can clearly and quantitatively reveal the mechanisms for the variation of the CH BDEs. Copyright © 2004 John Wiley & Sons, Ltd.

An efficient and environmentally friendly procedure for the one-pot synthesis of tetrahydropyrimidinones from aldehydes, β-diketones and urea/thiourea by using magnesium bromide as an inexpensive and easily available catalyst under solvent-free conditions was described. Compared with the classical Biginelli reaction conditions, this new method has the advantage of good to excellent yields (74%–94%) and short reaction time (45–90 min). The structure of the Biginelli reaction product from β-diketone, salicylaldehyde and urea has been proposed to possess an oxygen-bridge by cyclization (intramolecular Michael-addition).

A number of naphthalene donor compounds that possess an adamantanamine binding moiety and an (OCH₂CH₂)_n (n=1, 2, 3, 4, 6, 8) spacer were synthesized. The fluorescence quenching between these donor substrates and mono-6-O-p-nitrobenzoyl-β-cyclodextrin (pNBCD) and mono-6-O-m-nitrobenzoyl-β-cyclodextrin (mNBCD) was studied in detail. It was found that very efficient fluorescence quenching could occur in these supramolecular systems. This quenching was attributed to the photoinduced electron transfer inside the supramolecular assembly between the naphthalene donors and cyclodextrin acceptors. Detailed Stern-Volmer constants were measured and they were partitioned into dynamic Stern-Volmer quenching constants and static binding constants. It was demonstrated that the binding constants between all the naphthalene compounds and cyclodextrins are the same as they possess the same binding site, i.e., adamantanamine.

Blue-shifted dihydrogen bonds were predicted to be present in a number of non-covalent complexes including F₃C—H…H—Be—X, F₃C—H…H—Mg—X, F₃C—H…H₄Si, F₃Si—H…H—Li, F₃Si—H…H—Be—X, F₃Si—H…H—Mg—X, and F₃Si—H…H₄Si (X = H, F, Cl and CH₃). Pauli and nuclei–nuclei repulsions between the protonic hydrogen and hydridic hydrogen are proposed as the cause of the blue shift. Copyright © 2004 John Wiley & Sons, Ltd.

Solvation effects on the O—H homolytic bond dissociation energies (BDEs) of substituted phenols were studied. It was demonstrated that the BDEs measured in solution in general do not equal the BDEs in the gas phase. Detailed theoretical analyses indicated that a long-range solvation effect (i.e. the interaction between the solvent and the overall dipole moment of the solute) and a short-range solvation effect (i.e. the hydrogen bonding between the solute and solvent) were both important for the O—H BDEs in water and in DMSO. Neither one of these two factors by itself could fully explain the experimentally observed solvation effect. However, a combination of these two factors, estimated through a semi-continuum solvation model, was shown to be reasonably successful in explaining the experimental results. Copyright © 2004 John Wiley & Sons, Ltd.

Catalyzed by AgNO₃, Mg was found for the first time to be able to mediate the coupling reaction between aromatic aldehydes and benzyl bromide or chloride in water. The yields were slightly higher than the recent results for Mg-mediated allegation despite the fact that aqueous benzylation is intrinsically much harder than allegation. It was also found that the coupling reaction was chemoselective for aromatic aldehydes over aliphatic aldehydes, and chemoselective for aromatic aldehydes over aromatic ketones.

CH and NH bond dissociation energies (BDEs) of various five- and six-membered ring aromatic compounds were calculated using composite ab initio CBS-Q, G3 and G3B3 methods. It was found that all these composite ab initio methods provided very similar BDEs, despite the fact that different geometries and different procedures in the extrapolation to complete incorporation of electron correlation and complete basis set limit were used. Therefore, the calculated BDEs should be reliable. In addition, we found interesting dependences of the CH BDEs on the bond angles, spins and charges. A good quantitative structure–activity relationship (QSAR) model for the CH BDEs of aromatic compounds was also established. Copyright © 2003 John Wiley & Sons, Ltd.

Density function theory UB3LYP/6–31 + g(d) calculations were performed to study the hydrogen bonds between para-substituted phenols and HF, H₂O, or NH₃. It revealed that many properties of the non-covalent complexes, such as the interaction energies, donor-acceptor distances, bond lengths and vibration frequencies, showed well-defined substituent effects. Therefore, from the substituent effects not only the mechanism of a certain non-covalent interaction can be better understood, but also the interaction energies and structures of a certain non-covalent complex, which otherwise might be very hard or resource-consuming to estimate, can be easily predicted.

The exact structure of an arginine-carboxylate salt bridge in different chemical environments remains a controversial problem. In the present work, the zwitterionic and neutral forms of arginine-carboxylate salt bridge were studied by the B3LYP/6-311G (d, p)//PM3 method. It turns out that the neutral forms are more stable than the zwitterionic counterparts in gas phase. However, when bound by α-cyclodextrin, the zwitterionic forms become more stable than the corresponding neutral ones. It is suggested that the hydrophobic environment provided by the cydodextrin cavity leads to such behavior. Therefore, the salt bridge still could be in a zwitterionic form in the hydrophobic interior of the real proteins.

PM3 and B3LYP/3–21g* calculations were performed on cucurbit[7]uril and its complex with protonated 2, 6-bis (4, 5-dihydro-1 H-imidazot-2-yl) naphthalene. The results agreed well with the experimental observations. It indicated that in addition to van der Waals interaction and hydrophobic effect, hydrogen bonding is also an important driving force for the molecular recognition of cucurbiturils.