Sequence rule double bonds

Scheme 2.8. Stereoisomeric Alkenes and Related Molecules with the Double-Bond Geometry Named According to the Sequence Rule...

Cahn-Ingold-Prelog sequence rules (Sections 6.5, 9.5) A series of rules for assigning relative priorities to substituent groups on a double-bond carbon atom or on a chirality center. 

Conformations about an sp2-sp2 bond may be similarly designated. It is known that in these cases the stable conformation involves the eclipsing of a double bond by a single bond (25). Therefore, the following symbols are given if A>B>C and X>Y in the Sequence Rule (Scheme 2). 

In Lipmann s original scheme group Y was visualized as adding to a carbon-carbon double bond to initiate the sequence. Isotopic exchange reactions ruled out the possibility that either ADP or P might serve as Y, but it was attractive to think that a bound phosphate ion, e.g., in a phospholipid or coenzyme, could be involved. Y B of Fig. 18-12 would be similar in reactivity to an acyl phosphate or thio-ester. However, whatever the nature of Y B, part of group Y would be left attached to B after the transfer of Y to X. For example, if Y were Y OH... 

An order of precedence is established for the two atoms or groups attached to each end of the double bond according to the sequence rules of Section 19-6. When these rules are applied to l-fluoro-l-chloro-2-bromo-2-iodoethene, the priority sequence is ... 

Sequence Rules for Geometric Isomers and Chiral Compounds. Although cis and trans designations have been used for many years, this approach becomes useless in complex systems. To eliminate confusion when each carbon of a double bond or a chiral center is... 

The following points should be made here a) In the sequence rules, isotopic differences are to be considered only after all other material differences are exhausted, thus s > OP > OC > lsO= but, 8(>s >, 60=e. b) Resonating P=0 double bonds and negative charges (and sometimes... 

Priorities are assigned to groups by a series of rules known as the Cahn-Ingold-Prelog sequence rules, named after the three chemists who developed them. These rules use the atomic numbers of the atoms attached to the carbons of the double bond. 

Double and triple bonds are split by the sequence rule into two and three single bonds, respectively. The duplicated or triplicated atoms are considered carrying no substituents and are drawn in brackets. Diagrams XII give a few examples. Aromatic... 

A molecular structure such as the one shown in diagram XLII is achiral (presence of a plane of symmetry), but when a b and c = d it can exist in two diastereoisomeric forms. When the two largest or remarkable substituents are on the same side of the double bond, the isomer is cis, and it is designated trans in the other case. Ambiguities have been encountered, and it is recommended to designate as (Z) the isomer with the two sequence rule-preferred substituents on the same side of the double bond (usually the cis-form), and as (E) the other isomer (usually the Wwts-form) (XLIII a>H,c>H) ([53], and refs, therein). 

When the difference between substituents is in configuration, then in general Z> E and R> S. However, the formal definition is that an olefinic ligand in which the substituent of higher sequence priority is on the same side of the alkene double bond as the chiral centre takes priority. This definition does not correspond with either E,Z- or cis/trans- (change to the rules in 1982), The subscript n (for new ) is used in cases of doubt. 

The direction of addition of the B-H bond to atrisubstituted double bond seems to be determined by the addition of the more electronegative hydrogen atom to the more highly substituted carbon atom of the double bond. Thus, with the B H bond assumed to be polarised in the sense B " —the process may be considered in terms of the electronic arguments summarised in the modern interpretation of Markownikoff s rule for ionic additions to double bonds [15]. The outcome of the hydroboration/oxidation sequence corresponds to an "anti-Markownikoff hydration of the double bond, which has many synthetic applications. If steric factors are also important, these of course operate in the same sense, generally favouring attachment of boron to the less-hindered secondary carbon atom. 

Priorities are assigned on the basis of atomic number for the two atoms or groups attached to each carbon of the carbon-carbon double bond. The same priorities apply as in the Cahn-Ingold-Prelog sequence rules. 

Ei/Ti (from entgegen/zusammen = on opposite sides/on the same side) Affixes to the name of a compound designating steric relations around a double bond and assigned by use of the sequence-rule procedure. For example, for a C=C double bond, the priorities of the two subtituents on the carbon atoms of the double bond are assigned (see R/S system). If the higher priority on each carbon is on the same side of the double bond the specification is Z, if it is on the opposite side it is E. 

Sequence Rule 3. (The student should defer study of this rule until he needs it,) Where there is a double or triple bond, both atoms are considered to be duplicated or triplicated. Thus,... 

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