Cis and trans descriptors

For olefins and related structures we use the same priority rules, but we divide the double bond in half and compare the two sides. For each carbon of an olefin, assign one ligand high priority and one low priority according to the rules above. If the two high priority ligands lie on the same side of the double bond, the system is Z (zusammen) if they are on opposite sides, the system is E (entgegen). If an H atom is on each carbon of the double bond, however, we can also use the traditional "cis" and "trans" descriptors. 

A third type of configurational interdependence exists if two elements are so interrelated that a change in the configuration of one automatically alters that of the other. This characterization applies to the two centers of 1,4-cyclohexanediol of the type Cg+g hi (5,51). Consequently only two isomers exist and a single pair of descriptors suffices for their distinction. We can remove the mutual dependence of the two elements by waiving the requirement that a line of stereoisomerism be occupied by bonds. The H and OH ligands have different distributions in the isomers about the line between C(l) and C(4), and the usual terms cis and trans express this relationship. Undoubtedly this is the most convenient description and the only one now available, but should we go further and say that the proper element of stereoisomerism in this case is this achiral line of torsion, and that its further factorization into two graphochiral centers is unwarranted ... 

With simple compounds, like the isomers of but-2-ene, the descriptors cis and trans are quite satisfactory, but a compound such as 3-methylpent-2-ene causes problems. Do we call the isomer below cis because the methyls are on the same side, or trans because the main chain goes across the bond ... 

Since the descriptors cis and trans are not generally applicable for octahedral coordination compounds, systematic descriptors based on the CIP system should be used in their place. These consist of the polyhedral symbol, in... 

When a compound has several substituents, list the substituents in alphabetical order disregarding the presence of N, O or S prefixes and the descriptors sec-, tert-, cis- and trans- but see rule 2.5 below. 

Chardonnay wines in contact with Hungarian and Russian oak wood with low oak lactone contents scored well for oak wood sensory attributes. GC-sniffing analysis of these woods concluded that such other components as eugenol and the cis and trans isomers of isoeugenol, together with guaiacol and its derivatives, also contributed to the spicy , clove , cinnamon , and woody/oaky aromas. Oak lactones, on the other hand, elicited the odour descriptors sweet , vanilla , and oaky (Dfaz-Maroto et al. 2008). 

The stereochemical outcome is known for many epoxide-opening reactions. Examination of the effects of substituents and conditions on stereochemistry can provide insight into the mechanisms of the rearrangements which are the focus of this section. The words syn and anti are used here to describe mechanistic features, with the terms cis and trans retained as structural descriptors. The vast majority of epoxide-opening reactions occur with anti stereospecificity, i.e. backside displacement of the cleaved C—O bond. Anti preference dominates sufficiently to warrant calling this the normal stereochemical outcome. 

Aside this confusion, there is a principal argument, not to use Evans convention, because the hard descriptors E andZ must not be redefined. The soft descriptors cis and trans, however, can be used without violation of the strict definitions of the unequivocal E and Z. Therefore, in this book, the recommendation of Eliel etal. [18] is followed using the soft descriptors cis and raKS, if a series or a class of enolates are addressed [19]. Thereby, cis means that the OM substituent is on the same side as the higher-priority group at the a-carbon atom, and trans means that the OM substituent is on the opposite side. Only in those cases, where an individual enolate is concerned, EtZ nomenclature is used according to its strict definition. 

For vicinally disubstituted olefins (and similar double-bond systems) the traditional prefixes cis and trans can be retained, although for all higher substituted systems the descriptors (Z) and ( ) as defined on the basis of the CIP sequence rules must be applied. 

A second problem that has repeatedly concerned us is the inability of the Sequence Rule to provide descriptors for some elements of stereoisomerism. When Cahn et al. (16) first encountered this problem with the all-cis and all-trans isomers of inositol, they attributed it to the fact that the symmetry has become so high that they have no asymmetric, nor even a pseudo-asymmetric atom. This interpretation, we believe, is incorrect. If the two ring ligands of any carbon atom of m-inositol were not heteromorphic, their exchange could not yield an isomer, as it clearly does. Each atom is a center of stereoisomerism with a pair of enantiomorphic ligands (Cg+g hi) and indistinguishable from the traditional pseudoasymmetric atom. The description of cu-inositol as all-5 could be accomplished by the same device that would allow one to specify the configurations of C(l) and C(4) of 4-methylcyclohexanol. 

ZjE-Isomers - These are considered here configurational rather than conformational isomers (see Section 1.1.3.6.). The descriptors Z and E must not be applied to characterize cis or trans relationships of substituents of rings. 

The complex can exist with both a cis and a trans configuration. Since the compound has two bidentate ethylenediamine ligands, the isomers with both chlorine atoms in the cis configuration will exist as a pair of enantiomers. These will have the descriptors OC-6-2 2-A and OC-6-2 2-A, respectively, and belong to the symmetry point group C2. The trans isomer has a plane of symmetry and three mutually perpendicular C2 axes and therefore belongs... 

If other elements of chirality are present in the molecule, the descriptors should be arranged in the order central > carbocationic > axial > planar > torsional (note that Sokolov [6] uses a different order for the descriptors). The effects caused by such other types of chirality are discussed elsewhere in some detail [41], e.g., S-cis/S-trans-isomerism in ferrocenyl ketones and alkenes, as well as the chemistry of biferrocenyl (containing a direct bond between two ferrocenes). 

GAl was found to be a useful index for the discrimination of cis/trans isomerism (- cis/trans descriptors) and to model the chromatographic behaviour of phosphorus derivatives. 

It specifies the presence of a para-chlorotoluene moiety and excludes any orthosubstituents. Bond qualifiers define a particular bond configuration, cis or trans for a double bond. Descriptor qualifiers can be used to restrict values of, for instance, physicochemical descriptors of an atom a qualifier like (-0.2 < q) identifies a charge descriptor with a partial atomic charge less than 0.2. In addition to predefined descriptors, reserved key can be used for dynamic descriptors that depend on the screen context. Examples are the reserved key enumerate, which denotes the frequency of the preceding substructure occurring in a molecule, and distance, which denotes the distance between the geometric centers of two substructures. 

Note the negative oxidation state and the 17 (hapto) connectivity symbol in the last two examples. Where appropriate, stereochemical descriptors, such as cis-, trans-, mer-, and c-, polyhedral descriptors, and chirafity descriptors may be added to give structural information, but these are more often used in names, except for the simplest formulae. Polynuclear species may be described using the appropriate multiplicative suffixes, and bridging ligands can also be shown. The bridging symbol is useful for this pur-... 

S-cis and s-trans. Descriptors for the conformation about a single bond, such as the C2-C3 bond in 1,3-butadiene, or the C-N bond of an amide. If the substituents are synperi-planar, they are termed s-cis ("s" for "single") if they are antiperiplanar, they are termed s-trans. 

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