Dissymmetric atom

The use of S and R to specify the absolute configuration at a dissymmetric carbon atom is a fairly recent innovation (Cahn et al., 1956). This system depends upon no particular orientation of a projection formula, as does D and l configurational nomenclature, yet it unambiguously describes the three-dimensional representation of a dissymmetric atom. 

Atomic Inversion, Pyramidal (Lambert) Axially and Planar Dissymmetric Molecules, Absolute Configuration 6 19... 

When only one heteroatom of the dinucleophile possesses a hydrogen substituent, the reactions lead instead to alkenyl complexes rather than carbene compounds. Effectively, treatment of diphenylallenylidenes 1 and 6 with pyrazoles yields the heterocyclic derivatives 61 (Scheme 2.25) [76]. Interestingly, the dissymmetric 3-methylpyrazole (R=H, R = Me) provides only one regioisomer, in which the methyl group points towards the metal. This process, which formally corresponds to the addition of two nitrogen nuclei at C and Cy and a hydrogen atom at Cp, is assumed to take place through an initial nucleophilic attack at the Ca position. 

Finally a very important fact arising from our results is the occurrence of the asymmetric hydroformylation in olefins having C2v symmetry like d -butene. In this case the two faces of the olefin are identical, and the catalyst is evidently able to dissymmetrize the double bond favoring the attack of hydrogen to the re carbon atom and of carbon monoxide to the si carbon atom. A very simple way of looking at this dissymmetriza-tion of the double bond is to assume that the face of the olefin attacked... 

In principle, any cycloaddition involving two dissymmetric compounds can give head-to-head or head-to-tail products. These two compounds are usually obtained in nonequal proportions whose ratio cannot be adequately explained by steric or coulombic factors.1 For example, Reaction (5.1) favors the more hindered product. In Reaction (5.2), the product is formed by creating bonds between atoms having the same charge in the starting material. 

Is it better to place a substituent at each end of the bond, or to place both substituents upon the same atom A stepwise approach again provides the answer. If a donor D is present, the o bonding orbital becomes dissymmetric, with the larger coefficient at the nonsubstituted atom ... 

In recent years there has been much interest in homogeneous hydrogenations catalyzed by transition metal complexes (7). One facet of research in this area is the search for chiral catalysts (catalysts that are dissymmetric, i.e., optically active) that can be used to produce chiral compounds via asymmetric reactions. In this review, we survey asymmetric homogeneous hydrogenation reactions, that is reactions that create asymmetric carbon atoms by the addition of hydrogen across multiple bonds under the influence of soluble chiral catalysts. 

Molecules without reflection symmetry (no a plane) are called dissymmetric or chiral. Chirality (from the Greek cheir , hand) is the property displayed by any object (e.g., a hand) which is nonsuperimposable on its mirror image. If aCB( > 1) is also absent the structure lacks all elements of symmetry and is called asymmetric (point group C,). A carbon atom bearing four different substituents (asymmetric carbon atom) is a classical example of this point group. 

Generally, optical activity is observed when electrons are displaced along chiral paths by an applied electric field. This does not necessarily require the involvement of dissymmetric molecules. Quartz crystals, in which the first observation < of optical activity was achieved, have no chiral molecules or ions, and their optical activity arises simply from the helical placement of atoms in the crystal. Likewise, a crystal composed of achiral untwistable molecules may become optically active if there are strong helical interactions between neighboring molecules. 

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