1
$\begingroup$

Is it possible to name compounds with lone pairs on the double bonded species using the cis-trans nomenclature? For instance, can't you name $\ce{Ph-N=N-Ph}$ by the relative position of the nitrogen atoms' electron pairs?

$\endgroup$
3

2 Answers 2

4
$\begingroup$

Actually, most of linear imines and exocyclic imines display $E/Z$-isomerism (commonly known as $cis/trans$-isomerism) with a lone pair on the nitrogen, which is a part of the double bond. The isomerism follows various mechanisms. For instances, $cis/trans$-isomerism of azobenzene a noted elsewhere is operated by UV/vis-irradiation is a good example. The mechanism of this transformation has been depicted as follows (Ref.1):

Mechanism of isomerism of azobenzene

Similar to cis/trans-isomerism in alkenes, extensive studies has been done on azo-compounds and imines. For example, acid-catalyzed $E/Z$-isomerization of imines has been reported frequently (Ref.2,3). The abstract of Ref.2 states that:

The kinetics and mechanism of acid-catalyzed Z/E isomerization of O-methylbenzohydroximoyl chloride (1Za and 1Ea), methyl O-methylbenzohydroximate (1Zb and 1Eb), ethyl O-methylbenzohydroximate (1Zc and 1Ec and five para and meta substituted derivatives), O-methylcinnamohydroximoyl chloride (2Za and 2Ea), and methyl O-methylcinnamohydroximate (2Zb and 2Zb) have been investigated. The kinetics of Z/E isomerization of these imines have been studied in glacial acetic acid (1Ea and 1Zc) and in dioxane solutions containing $\ce{HCl}$, trifluoromethanesulfonic acid, or tetrafluoroboric acid (1Ea, 1Zb, 2Ea, and 2Zb). The isomerization takes place by either (a) rotation about the carbon−nitrogen double bond of the protonated imine (iminium ion rotation) or (b) nucleophilic attack on the protonated imine to form a tetrahedral intermediate that undergoes stereomutation and loss of the nucleophile (nucleophilic catalysis). The hydroximoyl chlorides 1Ea and 2Ea only isomerize by the nucleophilic catalysis mechanism. The hydroximate 1Zb appears to be capable of isomerizing by either mechanism. The hydroximate 2Zb may be isomerizing only by iminium ion rotation. Theoretical calculations support the notion that increased conjugation in the protonated imine increases the rate of iminium ion rotation.

cis-trans isomerism of imines

Photochromic properties of a series of novel pyrrolidene imines bearing functionalized aryl or naphthyl moieties was studied by UV spectroscopy (Ref.4):

cis-trans isomerism of pyrrolidene imines

UV irradiation of these heterocyclic Schiff bases at room temperature promotes the trans–cis photoisomerization of the $\ce{C=N}$ double bond with formation of a variable amount of the cis-isomer that, in the absence of light, returns thermally to the original form in few seconds. The thermal cis–trans re-isomerization of these molecular switches is much more slower than for the common benzylidene aniline-type imines, allowing the observation of the photochromic phenomena at room temperature.

Another interesting studies have been done with indigo derivatives (Ref.5) noting that:

Substituted indigo derivatives undergo photoisomerization of the central double bond if both nitrogen atoms are functionalized. Indigo itself however does not photoisomerize because of a competing and highly efficient excited‐state proton transfer...

isomerism of substituted indigo dyes


References:

  1. H. M. Dhammika Bandara, Shawn C. Burdette, "Photoisomerization in different classes of azobenzene," Chem. Soc. Rev. 2012, 41(5), 1809-1825 (https://doi.org/10.1039/C1CS15179G).
  2. James E. Johnson, Nora M. Morales, Andrea M. Gorczyca, Debra D. Dolliver, Michael A. McAllister, "Mechanisms of Acid-Catalyzed Z/E Isomerization of Imines," J. Org. Chem. 2001, 66(24), 7979–7985 (https://doi.org/10.1021/jo010067k).
  3. James E. Johnson, Nancy M. Silk, Elizabeth Ann Nalley, Mohammed Arfan, "Mechanism of an acid-catalyzed geometric isomerization about a carbon-nitrogen double bond," J. Org. Chem. 1981, 46(3), 546–552 (https://doi.org/10.1021/jo00316a013).
  4. Paulo J. Coelho, M. Cidália R. Castro, M. Manuela M. Raposo, "Reversible trans–cis photoisomerization of new pyrrolidene heterocyclic imines," Journal of Photochemistry and Photobiology A: Chemistry 2013, 259, 59-65 (https://doi.org/10.1016/j.jphotochem.2013.03.004).
  5. Ludwig A. Huber, Peter Mayer, Henry Dube, "Photoisomerization of Mono‐Arylated Indigo and Water‐Induced Acceleration of Thermal cis‐to‐trans Isomerization," ChemPhotoChem 2018, 2(6), 458-464 (https://doi.org/10.1002/cptc.201700228).
$\endgroup$
2
  • $\begingroup$ is the fact that electron pairs and alkyl groups are the high-priority groups only because they're just...defined that way? Also, since you can't really use CIP to classify isomers with electron pairs, can you drop a link to a resource specifying the relative priorities of electron pairs and other such groups, if there's any list of that sort? $\endgroup$
    – harry
    Apr 24, 2021 at 3:24
  • $\begingroup$ if available, please respond. $\endgroup$
    – harry
    Apr 24, 2021 at 16:29
3
$\begingroup$

Yes.

Azobenzene displays cis-trans isomerism. The two isomers can be interconverted using UV/Visible-irradiation. The less stable cis-isomer will relax back to the more stable trans-isomer, but the lifetime of the cis-isomer is long enough to allow observation and characterization.

$\endgroup$
1
  • $\begingroup$ is the fact that electron pairs and alkyl groups are the high-priority groups only because they're just...defined that way? Also, since you can't really use CIP to classify isomers with electron pairs, can you drop a link to a resource specifying the relative priorities of electron pairs and other such groups, if there's any list of that sort? $\endgroup$
    – harry
    Apr 23, 2021 at 5:59

Not the answer you're looking for? Browse other questions tagged or ask your own question.