Application of the shape group method to conformational processes: Shape and conjugation changes in the conformers of 2-phenyl pyrimidine

The shape group method (SGM) and the associated (a,b)-parameter maps provide a detailed shape characterization of molecular charge distributions. This method is applied to the study of the variations of shape and conjugation of conformers of 2-phenyl pyrimidine in their electronic ground state. Within the SGM framework, the method of (a,b)-parameter maps provides a concise, nonvisual, algorithmic technique for shape characterization of molecules with fixed nuclear geometries. Moreover, shape codes derived from the (a,b)-parameter maps afford a practical means for efficiently storing the shape properties of molecules in an electronic database. The shape codes of two or more charge distributions can be compared directly, and numerical measures of molecular shape similarity can be computed using a technique that is simple, fast, and inexpensive, especially in relation to direct, pairwise comparisons of electronic charge densities. The quantitative and automated nature of the method suggests applications in the field of computer-aided molecular design. In this study, the method is used for the first time to determine detailed numerical shape codes and shape similarity measures for a nontrivial conformational problem involving changes in energy and in conjugation. Numerical shape similarity measures of eight conformers of 2-phenyl pyrimidine are determined and correlated with variations in conformational energy and conjugation. The competing effects of steric repulsion and conjugation lead to important correlations between conformational energy and shape. © 1995 John Wiley & Sons, Inc.     

A novel strategy for the asymmetric synthesis of chiral cyclopropane carboxaldehydes

A new way of combining chiral auxiliaries and substrate-directable reactions for asymmetric synthesis is described that employs a three-step sequence of aldol-cyclopropanation-retro-aldol reactions for the stereoselective synthesis of enantiopure cyclopropane carboxaldehydes.     

Phase relationships and some magnetic properties of spinels in the systems M1−xNixCr2S4 where M = Mn,Fe, Co

Phase relationships between spinel and defect NiAs structures in the systems M_1?xNi_xCr₂S₄ (where M = Mn, Fe, Co) were investigated. It was found that the spinel structure is stable between x = 0 and x = 0.3 when M = Mn or Fe. When M = Co the spinel is formed in the region x = 0 to x ～ 0.4. The apparent stabilization of the defect NiAs phase by Ni²⁺ may be related to the strong sixfold site preference of Ni²⁺. Curie temperatures of all three ferrimagnetic systems increases with increasing Ni²⁺ substitution. This is probably due to higher NiS covalency.     

The preparation of bis- and tris-(trimethylsilyl)acetic acid

Contrary to an earlier report, lithiation of tris(trimethylsilyl)methane followed by carbonation does not yield the acid (Me₃Si)₃CCO₂H, (I), if an alkali extraction step is included in the work up; the product is actually the acid (Me₃Si)₂CHCO₂H, (II). The acid (I) can be obtained if the alkali extraction is omitted. Separate experiments have confirmed that (I) is rapidly converted into (II) by aqueous or methanolic base.