Double bond: defined, 6 examples

This is an example of a Diels-Alder reaction. This is an electrocyclic reaction where no charges are involved. While no charges are involved, electron pairs do move and their movement can be illustrated using arrow pushing. The mechanism, illustrated below, involves aligning cyclopentadiene (a diene) with methyl vinyl ketone (a dienophile) such that all three double bonds define a six-membered... 

Once the atoms arc defined, the bonds between them arc specified in a bond block. Each line of this block specifies which two atoms are bonded, the multiplicity of the bond (the bond type entry) and the stereo configuration of the bond (there arc also three additional fields that arc unused in Molfiles and usually set to 0). The indices of the atoms reflect the order of their appearance in the atom block. In the example analyzed, V relates to the first carbon atom (see also Figure 2-24). "2" to the second one, 3" to oxygen atom, etc. Then the two first lines of the bond block of the analyzed file (Figure 2-29) describe the single bond between the two carbon atoms C1-C2 and the double bond C2=0-5, respectively. 

Similarly the stereobonds" can be defined and added to the bond list in the fourth column of the CT. A single bond acquires the value of 0 if it is not a "stereobond, 1 for np (a wedged bond). 4 for either up or down, and 6 for down (a basbed bond), The cisjtrans or E[Z configuration of a double bond is determined by the x,y.2 coordinates of the atom block if the value is 0, Tf it is 3, the double bond is either cis or tmns. In the bond block of our example (Figure 2-76), the stereocenter is set to 1 (up) at atom 6 (row 6, column 4 in the bond block), whereas the configurations of the double bonds are determined by the x,y coordinates of the atom block. 

Most parent structures consist essentially of an assembly of rings and/or chains, the degree of hydrogenation of which is defined (usually completely saturated or containing the maximum number of non-cumulative double bonds in cyclic portions), and having no attached functional substituents (the carbohydrates are a notable exception to this). The stereochemistry at all (or most) chiral centres is defined thus such parent structures are sometimes referred to as stereoparents . Some examples are shown (77)-(83). 

The butadiene polymers represent another cornerstone of macromolecular stereochemistry. Butadiene gives rise to four different types of stereoregular polymers two with 1,2 linkage and two with 1,4. The first two, isotactic (62) and syndiotactic (25), conform to the definitions given for vinyl polymers, while the latter have, for eveiy monomer unit, a disubstituted double bond that can exist in the two different, cis and trans, configurations (these terms are defined with reference to the polymer chain). If the monomer units all have the same cis or trans configuration the polymers are called cis- or trans-tactic (30 and 31). The first examples of these stereoisomers were cited in the patent literature as early as 1955-1956 (63). Structural and mechanistic studies in the field have been made by Natta, Porri, Corradini, and associates (65-68). 

If X or Y is a trigonal center in a double bond, then the selection of the preferred substituent requires application of the standard duplicate atom procedure. Of course, the magnitude of the torsional angle is defined by the original atom (examples 5, 6). 

In unsaturated heterocycles, if the double bonds can be arranged in more than one way, their positions are defined by indicating the N or C atoms that are not multiply bonded, and consequently carry an extra hydrogen atom, by 1H-, 2H- and so on, for example IH-azepine and 2H-azepine. 

But E2 reactions are stereospecific—the H and the LG must be antiperiplanar during the reaction, which defines w hich stereoisomeric double bond you get. This is best illustrated using Newman projections. If you draw a Newman projection, you can then easily rotate into the conformation that has the H and the LG antiperiplanar. This conformation then show s you which stereoisomer you get. Consider the following example. 

No reaction is observed when 50 and 1,1-dimethoxyethene are allowed to stand in the dark for an equivalent period of time. Irradiation of 50 with 4 equiv. of cyclohexene in methylene chloride solvent gives a 51% isolated yield of the (2 + 2)-cyclo-adduct 52 assigned cis-anti-cis stereochemistry. These two cycloadditions were the first two well defined examples of the (2 + 2)-photocycloaddition reaction of an olefin to a carbon-nitrogen double bond. With furan the (2 + 2)-photocycloadduct 53 is formed, again regiospecifically. 

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