Axial or planar chirality in spiro[3.3]hepta-1,5-diene

The two spirocyclic compounds drawn below are enantiomers. My question is, do they exhibit axial or planar chirality?

3D representations:

• Note that, according to IUPAC recommendations concerning stereogenic centers at spiro fusion atoms, the two bonds associated with one of the rings should be drawn as plain bonds, while the two bonds associated with the other ring should be drawn with one solid wedged bond and one hashed wedged bond.
– user7951
Sep 1, 2015 at 20:05
• Look here a detailed explanation of 2,6-dichlorospiro[3.3]heptane. ursula.chem.yale.edu/~chem220/chem220js/STUDYAIDS/isomers/… See Figs. 5 & 6 Nov 19, 2017 at 16:19

Useful Definitions (from the IUPAC Gold Book)

• chirality plane - "A planar unit connected to an adjacent part of the structure by a bond which results in restricted torsion so that the plane cannot lie in a symmetry plane. For example with (E)-cyclooctene the chiral plane includes the double bond carbon atoms and all four atoms attached to the double bond; with a monosubstituted paracyclophane the chiral plane includes the monosubstituted benzene ring with its three hydrogen atoms and the three other atoms linked to the ring (i.e. from the substituent and the two chains linking the two benzene rings)."
• planar chirality - chirality resulting from a chirality plane.
• chirality axis - "An axis about which a set of ligands is held so that it results in a spatial arrangement which is not superposable on its mirror image. For example with an allene abC=C=Ccd the chiral axis is defined by the C=C=C bonds; and with an ortho-substituted biphenyl the atoms C-1, C-1', C-4 and C-4' lie on the chiral axis."
• axial chirality - chirality resulting from a chirality axis

Interpretation

• According to the above definition, a chirality plane must involve restricted torsion (restricted rotation) about a bond.
• A molecule can contain both a chirality plane and a chirality axis; an example would be an ortho-substituted biphenyl where the restricted rotation is responsible for chirality (atropisomers).

Conclusion

Axial or planar chirality in spiro[3.3]hepta-1,5-diene

There is no restricted rotation that brings about chirality in this molecule. Unlike trans-cyclooctene or a biphenyl, this molecule cannot be made to lie in a plane by torsional motion (at least not as long as we maintain the tetrahedral nature about the spiro carbon). Therefore, this molecule does not possess a chirality plane.

Rather, this molecule is analogous to the allenes. It does have a chirality axis passing through the $\ce{CH}$ group in each ring furthest from the spiro carbon along with the spiro carbon. The molecule can be said to possess axial chirality.

With regard to nomenclature of chiral spiro compounds, the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) distinguishes three cases:

• stereogenic spiro atoms of the type ‘Xabcd’, where ‘a’ > ‘b’ > ‘c’ > ‘d’
• stereogenic spiro atoms of the type ‘Xabab’, where ‘a’ > ‘b’
• axial chirality of spiro compounds

The given example spiro[3.3]hepta-1,5-diene contains a stereogenic spiro atom of the type ‘Xabab’. The stereodescriptors ‘R’ and ‘S’ are used when the spiro atom ‘X’ is surrounded by four atoms arranged as equivalent pairs ‘a’/‘a'’ and ‘b’/‘b'’, where ‘a’ > ‘b’. Thus, the two stereosiomers are (R)-spiro[3.3]hepta-1,5-diene and (S)-spiro[3.3]hepta-1,5-diene.

(Note that no stereodescriptor ‘E’ or ‘Z’ is needed to describe a double bond when the stereogenic unit is located in a ring having less than eight members.)

By way of comparison, (2​R,4​S,6​R)-2,6-dichlorospiro[3.3]heptane is a spiro compound with axial chirality.

(Note that the sterodescriptors ‘R’ and ‘S’ are used to describe the chirality centers. The stereodescriptors ‘M’ and ‘P’ can also be used to describe axial chirality of spiro compounds; however, sterodescriptors ‘R’ and ‘S’ are used in preferred IUPAC names.)

The compound (1​R)-5'H-spiro[indene-1,2'-(1,3)oxazole] is an example for a stereogenic spiro atom of the type ‘Xabcd’, where ‘a’ > ‘b’ > ‘c’ > ‘d’.

• I don't understand the nomenclature of 2,6-dichlorospiro[3.3]heptane. Only two possible sereoisomers should exist, but in the name three stereodescriptors are represented. Shouldn't the compound be given just an R or S descriptor by the means of assigning configuration to an axially chiral compound: en.wikipedia.org/wiki/Axial_chirality#/media/…?
– EJC
Sep 2, 2015 at 12:13
• @Marko That is correct. 2,6-dichlorospiro[3.3]heptane has two stereoisomers: (2​R,4S,6R)-2,6-dichlorospiro[3.3]heptane = (2​P)-2,6-dichlorospiro[3.3]heptane = (2​S​a)-2,6-dichlorospiro[3.3]heptane and (2​S,4R,6S)-2,6-dichlorospiro[3.3]heptane = (2​M)-2,6-dichlorospiro[3.3]heptane = (2​R​a)-2,6-dichlorospiro[3.3]heptane. ‘R’ and ‘S’ describe the individual chirality centers. ‘M’ and ‘P’ or ‘R​a’ and ‘S​a’ describe the axial chirality. (‘R​a’ and ‘S​a’ are not used in preferred IUPAC names.)
– user7951
Sep 2, 2015 at 13:12
• Then the spiro carbon atom is a pseudoasymmetric center and should be written with a small letter, r or s, right?
– EJC
Sep 2, 2015 at 14:22
• @Marko I suppose, your question is justified. By way of comparison, ChemDraw actually calls it ‘(2R,4s,6R)-…’, whereas ChemSketch fails to generate any stereodescriptors at all. However, the example in the current IUPAC recommendations (Blue Book) explicitly reads ‘(2R,4S,6R)-…’. I think, the IUPAC recommendations are correct and that this is not a pseudoasymmetric center since reflection of (2R,4S,6R)-… yields (2S,4R,6S)-…, whereas the stereodescriptors ‘r’ and ‘s’ describing a pseudoasymmetric stereogenic unit are invariant on reflection in a mirror (‘r’ remains ‘r’ and ‘s’ remains ‘s’).
– user7951
Sep 2, 2015 at 15:57

Now that I have had some experience on this site, I would like to get into the good graces of Glorfindel by expanding on how the assignments to (2R,4S,6R)-2,6-dichlorospiro[3.3]heptane presented by Loong are made. I will use the (2S,4R,6R)-enantiomer. The method requires the use of digraphs as shown in illustrations A and C below. To create digraph A, each bond to the spiro atom (C4) is “cut” and methylene carbon is terminated with a phantom group (red dot, fake C4). What is left is a carbon atom with two pairs of identical groups of the type C(a,a’,b,b’) where a>a’>b>b’. The chirality of all “four” chlorine-bearing carbon atoms (C2 and C6 ) are assigned temporary descriptors (R0>S0) where Cl>CH2-C4>CH2-phantom>H. The R0 groups are assigned the top priorities with the proviso that the first and third priority groups are derived from the same ring. Accordingly, the CIP method assigns the spiro carbon as having the R-configuration.

Another approach is represented in illustration B that doesn’t require dissection of the rings. The methylene groups attached to both chlorine-bearing carbon are assigned temporary descriptors knowing that chlorine is the top priority and hydrogen the lowest. Both digraphs A and B give the same R-configuration for the spiro carbon independent of which ring is assigned the first and third priorities.

The assignments to C2 and C6 utilizes digraph C. To determine the configuration of C6, one dissects C2 and C4. The temporary configurations at C2 are assigned as seen in digraph A. Assignment at C4 has the priorities C6-chain>R0>S0>phantom. The chirality at C6 follows the priorities Cl>R0>S0>H leading to an S-configuration at C6. Owing to symmetry, the same analysis applies to C2.