Level Of Steric Hindrance Research Articles

Highly Efficient Cationic Palladium Catalyzed Acetylation of Alcohols and Carbohydrate-Derived Polyols

The development of a new facile method for the acetylation of alcohols and carbohydrate-derived polyols is described. This method relies on the nature of the cationic palladium catalyst, Pd(PhCN)2(OTf)2 which is generated in situ from Pd(PhCN)2Cl2 and AgOTf to catalyze the acetylation reaction. This new acetylation protocol is very rapid and proceeds under mild conditions with only 1 mol% of catalyst loading at room temperature. This new method has been applied to a variety of different alcohols with different levels of steric hindrance, as well as carbohydrate-derived polyols to provide the corresponding fully acetylated products in excellent yields.

Electronic structure of covalently linked zinc bacteriochlorin molecular arrays: insights into molecular design for NIR light harvesting.

Pigment-based molecular arrays, especially those based on porphyrins, have been extensively studied as viable components of artificial light harvesting devices. Unlike porphyrins, bacteriochlorins absorb strongly in the NIR, yet little is known of the applicability of covalently linked bacteriochlorin-based arrays in this arena. To lay the foundation for future studies of excited state properties of such arrays, we present a systematic study of the ground state electronic structure of zinc bacteriochlorin (ZnBC) molecular arrays with various linkers and linker attachment sites (meso vs β) employing density functional theory in combination with the energy-based fragmentation (EBF) method, and the EBF with molecular orbitals (EBF-MO) method. We find that the level of steric hindrance between the ZnBC and the linker is directly correlated with the amount of ground sate electronic interactions between the ZnBCs. Low steric hindrance between the ZnBC and the linker found in alkyne-linked arrays results in strongly interacting arrays that are characterized by a decrease in the HOMO-LUMO energy gaps, large orbital energy dispersion in the frontier region, and low ZnBC-linker rotational barriers. In contrast, sterically hindered linkers, such as aryl-based linkers, result in weakly interacting arrays characterized by increased orbital energy degeneracy in the frontier region and high ZnBC-linker rotational barriers. For all linkers studied, the level of steric hindrance decreases when the ZnBCs are linked at the β position. Hence, ZnBC arrays that exhibit strong, weak, or intermediate ground-state electronic interactions can be realized by adjusting the level of steric hindrance with a judicious choice of the linker type and linker attachment site. Such tuning may be essential for design of light harvesting arrays with desired spectral properties.

Selective Monoarylation of Benzenediols via Dianion Intermediates

Application of the SNAr reaction on benzenediols with arylfluorides revealed an enhancement in monosubstitution selectivity over bis substitution when excess base was used. This trend was studied using symmetrical and nonsymmetrical benzenediols with various levels of steric hindrance. The effect of various bases was examined. A 100% monoselectivity was achieved in the presence of 2.5 equiv. of Cs2CO3. The methodology was employed in the synthesis of an important pharmaceutical product, 1, an arylether.

Electronic state of push-pull alkenes: an experimental dynamic NMR and theoretical ab initio MO study.

The (1)H and (13)C NMR spectra of a number of push-pull alkenes were recorded and the (13)C chemical shifts calculated employing the GIAO perturbation method. Of the various levels of theory tried, MP2 calculations with a triple-zeta-valence basis set were found to be the most effective for providing reliable results. The effect of the solvent was also considered but only by single-point calculations. Generally, the agreement between the experimental and theoretically calculated (13)C chemical shifts was good with only the carbons of the carbonyl, thiocarbonyl, and cyano groups deviating significantly. The substituents on the different sides of the central C=C partial double bond were classified qualitatively with respect to their donor (S,S < S,N < N,N) and acceptor properties (C identical with N < C=O < C=S) and according to the ring size on the donor side (6 < 7 < 5). The geometries of both the ground (GS) and transition states (TS) of the restricted rotation about the central C=C partial double bond were also calculated at the HF and MP2 levels of theory and the free energy differences compared with the barriers to rotation determined experimentally by dynamic NMR spectroscopy. Structural differences between the various push-pull alkenes were reproduced well, but the barriers to rotation were generally overestimated theoretically. Nevertheless, by correlating the barriers to rotation and the length of the central C=C partial double bonds, the push-pull alkenes could be classified with respect to the amount of hydrogen bonding present, the extent of donor-acceptor interactions (the push-pull effect), and the level of steric hindrance within the molecules. Finally, by means of NBO analysis of a set of model push-pull alkenes (acceptors: -C identical with N, -CH=O, and -CH=S; donors: S, O, and NH), the occupation numbers of the bonding pi orbitals of the central C=C partial double bond were shown to quantitatively describe the acceptor powers of the substituents and the corresponding occupation numbers of the antibonding pi orbital the donor powers of the substituents. Thus, for the first time an estimation of both the acceptor and the donor properties of the substituents attached to the push-pull double bond have been separately quantified. Furthermore, both the balance between strong donor/weak acceptor substituents (and vice versa) and the additional influences on the barriers to rotation (hydrogen bonding and steric hindrance in the GSs and TSs) could be differentiated.

Determination of mechanisms via computational chemistry for xylene and hydroxynaphthalene separations on beta-cyclodextrin

Equilibrium and dynamic separations for the three xylene and two hydroxynaphthalenes (naphthol) isomers were experimentally obtained with beta-cyclodextrin (β-CD) on mesoporous glass bead and silica gel supports. β-CD–support attachment was either by direct covalent bonding or through tethering with 3-glycidoxypropyltrimethoxysilane. The obtained guest/host separation selectivity ratios were as high as 4.1/1.0 for 1-naphthol from 2-naphthol. Maximum separation ratios were 2.4/1.0 for m-xylene from p-xylene and 0.75/1.0 for o-xylene from p-xylene. β-CD tethering decreased the separation efficiencies of the xylene and naphthol isomers over those obtained by β-CD covalent bonding. Molecular mechanics and semi-empirical methods were used to determine the mechanisms for the above guest–host separation efficiencies. The three mechanisms evaluated were guest–host inclusion energy, charge transfer and steric hindrance. Computational results show that the steric hindrance of the guest entering the host was the controlling mechanism. Levels of steric hindrance were determined by guest–host overlap of electrostatic potential surfaces. A key component concept was used to develop a model for predicting separation selectivities for other guests with β-CD and other guest–host combinations. This model is based on a linear correlation between component/key component selectivity ratios and component/key component electrostatic potential overlap ratios.